Composition of the Atmosphere

Oxygen is a vital gas in the air that we need to breathe and survive. However, the most abundant gas in the atmosphere is actually Nitrogen at 78%, Oxygen at 21%, Argon at 0.9% and other trace gases that make up a very small percentage of the atmosphere. The most important gas that influences our daily weather is water vapor. Water vapor is an invisible gas made up of one oxygen atom (Remember: an atoms are subatomic particles of a singular element while molecules make up two or more atoms). and two hydrogen atoms. It is highly variable thanks to the water cycle, which is a continuous cycle in how water is moved throughout earth above and below the surface and in different phases. 

The Water Cycle

Initially, the water cycle begins through a process called evaporation, where a liquid turns into a gas through heating, which in this case is daytime heating from the sun and from any large body of water. As the air rises up into the atmosphere, it cools and expands until it reaches saturation, or when water vapor transitions into a liquid known as condensation. When condensation occurs, this is typically where you get clouds in the sky. Eventually, once a cloud accumulates billions of tiny water droplets and grow large enough to fall, the liquid will fall as precipitation. There are several forms of precipitation that can fall, which depend on the weather patterns, which is discussed later in this course. For now, once that precipitation falls, the water drains away back onto earth into creeks, streams, rivers, and lakes called runoff, which eventually makes it back out to the ocean. A secondary source of water comes from plants and trees as well, which release water vapor into the air called transpiration. The loss of water from both plants/trees and moisture from soils/ bodies of water is known as evapotranspiration. Then, through daytime heating, the water cycle will start all over again. Through all of these variables in the water cycle, it is evident that the concentration of water vapor can be highly variable in the atmosphere. 

Permanent vs. Variable Gases

Gases that don't change composition in the atmosphere are called Permanent Gases. Among these gases include nitrogen, oxygen and argon as well as other trace gases such as neon, helium, krypton, xenon, and hydrogen. Therefore, the 78% concentration of nitrogen will remain the same. However, water vapor is not a permanent gas. Gases that change composition in the atmosphere are known as Variable Gases. Among these gases include carbon dioxide, methane, water vapor, nitrous oxide, and ozone. These gases may sound familiar as they are also known as the Greenhouse Gases. These greenhouse gases trap heat and act as a blanket for the atmosphere keeping our atmosphere warm and habitable. However, if these greenhouse gases increase in concentrations, then the planet may become too warm. Carbon dioxide is one of the leading gases in increasing concentrations and these concentrations are normally expressed in parts per million, billion, or even trillion. Carbon dioxide has been rising ever since the Industrial Revolution and has exponentially increased through today. However, believe it or not, carbon dioxide is actually not the most abundant greenhouse gas; it is actually water vapor. Water vapor varies between 0 and 4% where warmer air can hold more water vapor and colder air can hold less. If greenhouse gases continue to increase and the planet warms, then that gives the air the ability to hold more water vapor. Therefore, carbon dioxide, while not the most abundant greenhouse gas, can directly influence water vapor through increasing temperatures. 

Any object that has mass has matter and as discussed with density above, most mass appears at Earth's surface where molecules are near in proximity to one another. When any object has mass, if an external force is acted upon an object, there would be an acceleration, and depending on the mass, is whether that acceleration increases or decreases (F=m*a). This is called Newton's 2nd Law of Motion. For example, you go to the grocery store and are pushing an empty cart. Notice how the empty cart pushes very easily. Two hours later, and the cart is completely full and harder to push. This is because the cart has more mass. Essentially, the more force you exert on the cart, the more acceleration there is and the faster the cart moves. As mass increases, acceleration decreases. Now, how does this even relate to pressure? 

Pressure is the force exerted over an area: 

P= F/A OR P= mg/A

F= P*A

Pressure is inversely proportional to area and proportional to the force exerted. If area increases, pressure decreases and vice versa. Rearraigning the equation above by multiplying area, gives the force equal to pressure multiplied by the area. That force comes from molecules colliding with any surface. Newton's second law is the sum of forces on the left-hand side. The atmosphere has many forces, but for now pressure acts as one of those real-world forces. If the pressure of an object exerted on one side of the object is larger than the other side, the net force would cause an acceleration. This is in fact what causes the wind. Another force is gravity, which is another form of acceleration, so replacing F with mg gives P=mg/A. Putting it all together, as mass decreases with height, density decreases, and so does pressure. Therefore, pressure decreases as you increase in altitude. Think of density, pressure, and force as one family; all interconnected with all depending on mass, volume, or area. 

Ideal Gas Law

Pressure and density are not the only quantities that can change with altitude in the atmosphere. Temperature also plays a major role, the average amount of kinetic energy in a substance. It is warmest at the surface and coldest aloft, which makes sense. Think about hiking up a mountain. You notice how the air becomes thin, less dense with molecules becoming sparser with pressure decreasing due to fewer molecules or mass. The temperature also becomes colder as expected, especially with snowpack higher in the atmosphere. Relating all of these quantities (density i.e. in terms of volume, pressure, and temperature) is called The Ideal Gas Law: 

PV= nRT

Where the following is: P=pressure, V=volume, n= number of moles, R= the universal gas constant, and T=temperature. A mole is a unit chemists use to count the number of particles (atoms, molecules, ions) that exist, which would be tedious to count each individual atom of a substance, so a mole is used. This law is a universal concept of atmospheric physics that describe the ideal behavior of a gas of dry air. If temperature increases, pressure or volume must change. If you compress a gas, pressure rises. If you add more gas (moles), pressure or volume increases. Think of a clown car. Each clown represents a mole or one particle of mass. Increasing the number of clowns in a space increases the amount of gas or moles in this analogy. Therefore, the system must respond as more and more clowns squeeze into the tiny clown car. The car's space or container represents the volume. It is a small car, so there is a small volume. However, any car does not change size, so the volume is fixed or held constant; it doesn't change. Pressure represents how squished the clowns feel. While pressure is defined as the force exerted over an area, in a gas sense, it's the collective effect of particles colliding with the walls, or in this case, the clowns. Therefore, the more clowns there are, the higher the pressure. Remember, temperature is based on the amount of kinetic energy or movement. Therefore, temperature would increase if the clowns were moving a lot in the car but would remain lower if everyone is held still. The Ideal Gas Law's core idea is particles in a confined space create pressure, and changing the number of particles, their energy, or container size changes the system's behavior. If the pressure gets too high, the doors pop open like a container venting. If you exert too much pressure on yourself, it is okay to talk to a friend to relieve some of that pressure. Now that's science!

Parcel Theory of Dry Air

Now, let's relate the Ideal Gas Law to one of the basic fundamental properties of dry air; which is Parcel Theory. First, a parcel is defined as a small volume of air (like a sample) that does not mix with the surrounding environment (at least for the short time we study it). moves vertically like a balloon, carries its own temperature and pressure, and adjusts its pressure to instantly match the surrounding environment. Pressure inside the parcel always equals the pressure outside the parcel. As learned previously, pressure decreases with height. So, as the parcel of air rises, like a hot air balloon, the surrounding pressure is lower than that of the parcel. In order to match the surrounding environment, the parcel responds by expansion (increasing the volume). Expansion requires energy, so the parcel uses its internal energy, so the parcel cools. As long as the surrounding environment is colder than the parcel, then the parcel will keep rising at the dry adiabatic lapse rate, which is the rate at which dry air cools with height (9.8C/ kilometer OR 5.4F/1000 feet). However, if the parcel is cooler than the surrounding environment (i.e. cold air is more dense while warm air is less dense), then the parcel will sink. As the parcel sinks, the pressure of the surrounding environment is now increasing as altitude decreases. Thus, the parcel compresses to match the pressure of its environment; decreasing its volume. Compression adds energy to the parcel, so the parcel warms at the dry adiabatic lapse rate. So, putting it all together, when an air parcel rises, it cools and expands and when it sinks, it warms and compresses. This fundamental principle is the backbone to how thunderstorms form minus moisture. Now, try thinking of parcel theory in terms of a hot air balloon. 

The burner heats the air inside the balloon. Warm air expands and becomes less dense. The balloon then becomes buoyant and lifts off of the ground. The balloon expands as it rises into lower pressure and expands. Expansion cools the air. If the pilot doesn't add more heat, then the balloon will eventually sink back to the ground. As the hot air balloon sinks into higher pressure, the air compresses slightly. Compression warms the air. The balloon becomes less buoyant and descends unless it is reheated. 

Above describes the ideal behavior of a gas, but many scientists treat the behavior of the atmosphere like a fluid, it is very similar. The Venturi Effect is what happens when a fluid flows through a narrower section or channel allowing the fluid to speed up but also drop in pressure. This effect is based upon two principles: The Conservation of Mass and Bernoulli's Principle: The Conservation of Mass means that mass cannot be created nor destroyed. In other words, what comes in must come out. The amount of fluid entering a pipe must equal the amount of fluid leaving the pipe. Thus, mass does not change or is held constant in this case. The second principle is Bernoulli's Principle, which describes that when a fluid moves faster, its pressure decreases. When it moves slower, its pressure increases. This counteracts the idea as when area increases, pressure decreases and vice versa as discussed earlier. Why is that? 

Well, the concept of pressure exerted over an area is the idea of a static change. Pressure changes because of the change in area, not because the fluid is moving. The Venturi effect is due to a moving fluid (dynamic fluid), which is very different than a static fluid (not moving). Taking the example from earlier, imagine clowns running through a hallway. A wider hallway, they jog comfortably. A narrower hallway, they must squeeze together and speed up. Since they are rushing, they push less on the walls and therefore creates a lower pressure. For example, in the real-world, faster flow over the curved top surface of an airplane leads to lower pressure, and therefore lift off. This is also why you experience wind gusts in between buildings as winds accelerate through the narrow passageway between buildings. Another example is when air spirals into a smaller radius from the formation of a tornado, creating a sudden drop in pressure in a very localized location. Think of the Mount Washington Observatory, which regularly sees wind speeds in excess of 100 mph in the winter. This is due to the Venturi Effect as air aloft is forced to squeeze between the mountain's peak and the tropopause. Air is unable to penetrate above the tropopause as it acts as a lid for the atmosphere, which we will learn later in the course in the discussion of the Layers of the Atmosphere. 

Dry Air vs. Moist Air

So far, we have discussed the concept of dry air. Many would think that dry air is more dense than moist air, but it is in fact the opposite. Why? Well, dry air, as discussed in previously about the composition of the atmosphere, is mostly made up of nitrogen and oxygen. When the air contains moisture, water vapor displaces some of those nitrogen and oxygen molecules. Density depends on the mass per unit of volume, so the molecular weights of the molecules matter. Nitrogen has a molecular weight of 28 g/mol while oxygen is 32 g/mol as both are diatomic molecules, which are molecules made up of two atoms. On the other hand, water vapor is made up of two hydrogen atoms with a molecular weight of about 1g/mol each plus 16g/mol for oxygen. So, water vapor's total molecular weight is 18g/mol, which is much lighter than nitrogen and oxygen. Therefore, a fundamental concept of meteorology is that warm, moist air is less dense while cold, dry air is denser. This general concept aids the development of the dryline, which is essential for severe weather formation. 

Dryline Formation

A dryline is a frontal boundary between hot, dry air to the west and warm, moist air to the east. Sometimes it is called a moisture or dewpoint front, where dewpoint is the temperature the air cools to reach saturation. Typically, west of a dryline comes from the Mexican Plateau with hot, dry southwesterly flow. Temperatures typically in the 90s with sunny skies. East of the dryline, is a warm, moist airmass with temperatures in the 70s or 80s and partly cloudy skies and possible thunderstorms. The warm, moist air comes from southerly flow transporting from the Gulf of Mexico. Topography is much higher in elevation to the west due to the Rocky Mountains. So, since dry air is denser than moist air, the dry air sinks to the bottom while the moist air rises, which allows for thunderstorms to develop. The dryline progresses eastward as the dry air mixes out the moist air, but the moist air deepens the further east you go. This is why drylines can't go further east than maybe Arkansas and Missouri. Once the sun goes down, and the ground cools, the cool, moist air is now denser and the dryline retreats back west. 

While differences in air density can cause air parcels to rise, this describes the vertical movement of air. However, the differences in pressure in the horizontal is related to vertical movements of air. As air converges or comes together meaning adding more mass into a region, the air has no place to go but upward, so it is forced to rise. This is called convergence. The opposite occurs when air diverges, or spreads apart meaning mass is leaving the region, and now air must fill the void. So, when air diverges, air is forced to sink from aloft. This is called divergence. At the surface, as air comes together or converges, air is forced to rise. Since air is rising and leaving the surface, the surface pressure drops creating an area of low-pressure. When low-pressure forms, this means bad weather with clouds, rain, and possible thunderstorms as air parcels cools and expands and reaches saturation. When there is high-pressure at the surface, air diverges or spreads apart at the surface, meaning air sinks from aloft to fill the void. Sinking air causes air parcels to warm and compress, which is why you experience dry and sunny conditions. Moral of the story, the atmosphere is always seeking to restore balance. What comes in, must come out. Remember, mass cannot be created nor destroyed, so the air must move somewhere in order to fill the void. This is why air flows inward and counterclockwise (cyclonic) around an area of low-pressure and outward and clockwise (anti-cyclonic) around an area of high-pressure. High-pressure always wants to fill in the area of low-pressure to restore balance, which is why air tends to always flow from high-pressure to low-pressure. This is called the Pressure Gradient Force. These differences in pressure is what creates the wind, which is one of the building blocks to steering weather systems. 

While there are variations in pressure, density, temperature, and humidity due to the unequal heating of the Earth, several factors can impact how solar radiation interacts with the Earth and the weather. Solar energy is the key driver in our global weather patterns, which leads to the influence of the four seasons that Earth experiences. The Earth tilts on its axis at about 23.5 degrees but is not always the case. The tilt on Earth's axis causes the seasons because the sun faces away from the sun or towards the sun certain times of the year. The four seasons are as follows: 

1. Winter Solstice for Northern Hemisphere (Summer Solstice for Southern Hemisphere) around December 21-22nd. 

2. Spring Equinox for Northern Hemisphere (Fall Equinox for Southern Hemisphere) around March 20-21st.

3. Summer Solstice for Northern Hemisphere (Summer Solstice for Southern Hemisphere) around June 21-22nd. 

4. Fall Equinox for Northern Hemisphere (Spring Equinox for Southern Hemisphere) around September 22-23rd. 

When a hemisphere is tilted towards the sun, the sunlight hits more directly with longer days, more concentrated days, and rising temperatures, which leads to the summer solstice. When a hemisphere tilts away from the sun, sunlight arrives at a lower angle and energy is more spread out with shorter days and decreasing temperatures known as the winter solstice. The equinox is when the sun is directly over the equator and creates equal day and night during the fall and spring. This is especially important the Polar Jetstream is more active in the winter and less active in the summer as one example described in lesson 2. It's all one revolving cycle. 

Some may say that the Earth's distance from the sun can also have an influence on solar energy, which is true to some degree. The earth's average distance from the sun is 93 million miles. However, since earth has an elliptical orbit, the distance from the sun can vary. When earth is closest to the sun at about 91,400,405 million miles is called the Perihelion. This occurs in January. On the contrary, when the earth is furthest from the sun at 94,512,258 million miles is called the Aphelion, which is in July. That may seem counterintuitive where during the winter the Earth is closer to the sun while in the summer it is further from the sun. This is why Earth's tilt on its axis needs to be considered the primary factor, for which causes the seasons. 

The sole reason why we have jetstreams, which steers our weather systems at the surface, in the first place is that Earth is in the shape of a sphere. Earth's sphere-like shape allows most solar energy to be directed at the equator while less sunlight reaches the poles due to the angle it poses. This is why there are large temperature contrasts, which causes jetstreams to form. Another influence on temperature is land versus water where land heats and cools faster than water, which drives local temperature differences. Another factor is the diurnal cycle of temperature swings between radiational cooling at night and warming during the day due to daytime heating from the sun. 

Thermal Vs. Dynamic Pressure Systems

Pressure systems that form due to temperature contrasts in the atmosphere are called Thermal Pressure Systems. When warm air expands, it becomes less dense, and it rises forming an area of low-pressure. When cold air compresses, it becomes denser and it sinks forming an area of high-pressure. These systems are very shallow and are formed from heating and cooling and not from upper-level winds. Think of deserts in the summer. It is very hot and dry, so the air will expand, and a thermal low will form. Whereas in the winter when it's cold, a thermal high will form over Siberia since it is snow-covered and cold air at the surface compresses. Dynamic Pressure Systems form from upper-level winds as described in lesson 1. Surface convergence leads to air coming together and forcing it to rise causing air to spread apart aloft or divergence aloft forming an area of low-pressure. High-pressure is the opposite with divergence at the surface and convergence aloft causing air to sink and the pressure to rise at the surface. These pressure systems are your typical areas of highs and lows on a weather map that are steered by the jetstream or upper-level wind flow. Regardless of the type, if there is a difference in temperature and pressure, then wind will exist thanks to the pressure gradient force. An important concept when introducing the world global circulation wind patterns. 

Global Circulation Pattern

The global circulation pattern is the planet-scale movement of air that redistributes heat from the equator towards the poles. It's the engine behind the trade winds, jet streams, deserts, monsoons, and storm tracks. NOAA describes it as "the movement of air around the planet... explaining how thermal energy and storm systems move over Earth's surface." First, let's start from the beginning. 

The One-Cell Model (Single-Cell Model)

The one-cell model is the simplest theoretical model of Earth's atmospheric circulation. Not exactly how the real atmosphere behaves but is a steppingstone in understanding the global circulation pattern. Several key assumptions arise from this theory such as the Earth does not rotate, the sun is always directly over the equator, the surface is uniform meaning as all water, no continents, no seasons, and no Coriolis effect. Due to these assumptions, it is very simplified. 

First, the equator absorbs direct sunlight creating strong heating. As the air warms, it becomes less dense and rises creating a belt of low-pressure along the equator. The rising air spreads out toward the poles in the upper atmosphere. As the air moves poleward, it cools. At the poles, it becomes dense and sinks, forming a high-pressure region. The cold, dense air flows back toward the equator at the surface. This completes one giant convective loop in each hemisphere. However, this concept is unrealistic as in the real atmosphere, the Earth rotates producing the Coriolis effect. The Coriolis deflects winds, which prevents a single cell having flow from the equator all of the way towards the poles. In the real world, we also have seasons and continents. Instead, the circulation splits into three cells per hemisphere. 

The Three-Cell Model (Real-World Circulation)

A. Hadley Cell (Equator to 30 Degrees North and South)

At the equator, the sun heats the surface causing air to expand and become less dense and allowing it to rise forming a thermal area of low-pressure. That part doesn't change. A belt of thermal lows forms the Intertropical Convergence Zone or ITCZ here where showers and thunderstorms form near the equator. Air moves poleward aloft, cools, and sinks around 30 degrees forming subtropical highs where most world deserts from such as the Sahara Desert. Surface air returns towards the equator forming the trade winds. In the northern hemisphere, they are the northeast trade winds and in the southern hemisphere, they are the southeast trade winds. The clashes of air masses in the northern and southern hemisphere allows air to converge at the equator, which is how the ITCZ exists. The ITCZ follows a shift in the maximum solar heating, so it can shift north during the northern hemisphere summer in July and south in the southern hemisphere summer in December. This variation causes a wet and dry season in the tropical regions and is the sole reason why many tropical rainforests exist near the equator. 

B. Ferrel Cell (30-60 Degrees North and South)

Air at the surface flows poleward from the subtropical high. It meets cold, polar air at about 60 degrees latitude, which is the location of the polar front at the surface and Polar Jetstream aloft and where many midlatitude storm systems form and clash across the United States. Depending on the season, this will shift north or south. Rising motion and the Coriolis effect cause the westerly winds in the midlatitudes and is why most weather systems steer from west to east. This is counterintuitive as you may think that since there are sub-tropical high-pressure systems, the air should not rise. This is because the Ferrel cell is an indirect circulation. A direct circulation, like the Hadley and Polar Cells, is when warm air rises and cold air sinks as expected. The indirect circulation is the opposite in which cold air rises and warm air sinks. This is because the Ferrell cell is not thermally driven, but rather dynamically driven. Dynamically driven since lows and highs act to transfer momentum and energy in the midlatitudes. Remember, the atmosphere always wants to achieve and restore balance, and since there is a clash of temperature contrasts in the midlatitudes, these eddies may force air to rise and sink in ways that oppose what pure heating would do. Therefore, this cell is driven by upper-level winds. 

C. Polar Cell (60-90 Degrees North and South)

Cold, dense air sinks at the poles due to a polar high at the surface from cold, snow-covered lands. Surface air flows equatorward as polar easterlies since the Coriolis force turns winds to the right in the NH. It rises again at the polar front at about 60 degrees latitude. It is worth noting that the Polar Easterlies are weaker than the westerlies since the temperature contrasts are not as steep as in the midlatitudes. This is the cell with your cool, polar air and cold, arctic air. Thus, differential heating plus rotation from the Earth creates the global wind and pressure patterns we see in the real atmosphere and influences our jet streams, shifting storm tracks, and various climate zones around the globe. 

One haunting phenomenon created by winds are known as Aeolian sounds. Aeolian sounds are tones generated when wind flows past an object, creating oscillating vortices that shed rhythmically from either side of the obstacle. These vortices create pressure fluctuations that your ears interpret as sound, which is often the sound of a hum, whistle, or low drone. When this wind encounters an object like a wire, pole, twig, or even a blade of grass, the airflow separates and forms alternating swirling eddies downstream, which is called vortex shredding. Think of when there is construction and the two lanes on the road diverge as there is a barrier preventing the left lane from going straight. This is vortex shredding where swirls of winds, or eddies take different paths creating an oscillating area of low-pressure in which surrounding higher-pressure tries to fill.  

The following wind described below are downslope winds, which are called katabatic winds. Katabatic winds are driven by the force of gravity. On the contrary, anabatic winds are upslope winds and are driven by surface heating. The two winds below are examples of katabatic winds. Another example are mountain-valley circulations where nighttime cooling causes air to sink causing a cool, katabatic wind. During the day, the surface heats up creating upslope winds up the mountain, which forms a warm, anabatic wind. 

Chinook Winds

One well-known winds are called the Chinook Winds. The Chinook winds are warm, dry, westerly winds that descend on the leeward side of the Rocky Mountains. As discussed in lesson 1 with Parcel Theory, when air descends, the parcels warm and compress at the dry adiabatic lapse rate (9.8 C/km). Since this is a warm wind, as the downslope winds; especially in the winter, can cause snow to rapidly melt as temperatures can rise upwards of 20 to 40 degrees in the matter of minutes to hours. This is why it is sometimes referred to as the "Snow Eater". This is also a dry wind because on the windward side of the Rockies, air is forced upward through orographic lift causing parcels to expand and cool and condense into clouds and precipitation. Most of the moisture is lost on the windward side due to it precipitating out, so the leeward side leads to warm, dry air descending the slopes of the Rocky Mountains. If conditions are dry at the surface on the leeward side with low relative humidity, these downslope winds can also cause a wildfire risk. 

Santa Ana Winds

Another type of downslope winds is the Santa Ana Winds. The Santa Ana winds are strong, dry, downslope winds that blow from the Great Basin toward coastal Southern California, often producing gusts of 40-65 mph and extremely low humidity. They come from a high-pressure system over the Great Basin (Nevada/Utah region), which creates a pressure gradient that pushes air from the interior toward the coast. As the air descends the mountains toward sea level, it warms and compresses, which can cause downed trees and powerlines and elevated fire weather concerns. 

A dust storm is a wall of dust and debris lifted into the air by strong winds, often miles long and thousands of feet high. They are most common in arid and semi-arid regions, especially North Africa, the Middle East, Central Asia, and China. They often form from strong winds over dry, loose soil. This can be either from the outflow of a thunderstorm or strong surface winds blowing across a dry surface, lifting sand and dirt into the air. Many dust storms, especially in the U.S. Southwest, are triggered by thunderstorm outflow boundaries, producing a fast-moving wall of dust called a haboob. Dust storms have significant impacts and can reduce visbility to near zero causing dangerous driving conditions and multi-vehicle pileups. Fine dust can cause respiratory issues, eye irritation, and worsen existing conditions. This massive broom that is the atmosphere sweeping across the dusty ground is important to watch out for and can be particularly dangerous. 

Monsoons are one of the most powerful examples of seasonal atmospheric circulations, which are driven by global wind patterns. Many people associate "monsoons" with torrential rainfall, which certainly can happen, but that is only half of the story. A monsoon is a seasonal reversal of wind direction that produces distinct wet and dry seasons around the world. They are indeed not just rain, but a seasonal wind system that shifts direction for months at a time. Before diving into how a monsoon forms, a fundamental concept of heat capacity is important to note. Heat capacity is the amount of energy required to raise the temperature of an object or substance by one degree. Let's say summer just started and your neighborhood pool just opened up. While it is certainly hot outside, the water is certainly cold. In fact, you dip your foot into the water and it's freezing. Now you just want to lay out and sunbathe. A heat wave comes a few days later with temperatures heating into the 90s to near 100 degrees. After a few days of the hot weather, you return back to the pool and notice that the water is warm like bath water. In an essence, this is heat capacity. Water has a higher heat capacity than air as it takes way more energy to heat up the water and the surrounding air. This is why it takes the pool a lot longer to heat up than the surrounding air. 

Now, there are two kinds of monsoons. The summer monsoon, known as the wet season, allows land to heat up faster than the ocean. This creates a thermal area of low-pressure on land and moist, ocean air flows inward. Air rises, cools, and condenses and produces heavy rainfall. The other half of the story is the winter monsoon, known as the dry season. During the winter, land cools much faster than the ocean. High-pressure forms over land causing dry, continental air to flow offshore toward the ocean. As a result, dry conditions form over land. Monsoons are tied to the annual migration of the Intertropical Convergence Zone (ITCZ), which shifts north and south with the seasons depending on where the most sunlight and daytime heating occur. As the ITCZ moves, it drags the monsoon circulation with it. Thus, monsoons occur in many tropical and subtropical regions including South Asia (India, Bangladesh, Pakistan), Southeast Asia, West Africa, Australia, and the Southwestern United States (Arizona and New Mexico). Several locations rely on these monsoonal patterns as part of their annual rainfall, but if too strong, a summer monsoon can bring flooding, landslides. On the contrary, a strong winter monsoon can bring drought conditions and damage agriculture, water supply, and ecosystems. 

A considerable amount of precipitation evaporates between the cloud base and the surface. Streaks of rain evaporating and not reaching the ground is called virga. This is especially common in the desert southwest United States due to low relative humidity. Very hot air would cause intense daytime heating and rising bubbles of air, but since the air is dry, the thunderstorms become more elevated and may produce lightning, with very little rain due to evaporation. These dry thunderstorms pose a risk for wildfires in the west as lightning can ignite dry fuels. 

As more evaporation takes place, the air cools and becomes denser. Evaporation is a cooling process because water needs to absorb heat from its surroundings in order to change from a liquid into a vapor. Evaporative cooling can cause a downdraft, winds moving down and out from of a thunderstorm, to strengthen. When rain is heavy enough combined with evaporative cooling and winds aloft reaching the surface, an intense bust of winds can plow to the surface known as a downburst, which is hazardous to aircraft preventing further lift and can cause crashes to be fatal when hitting a tailwind. Two types of a downburst are a microburst, which affects a smaller area than 4 km, and a macroburst, which are larger than an area of 4 km. Both are just different in size, duration, and damage footprint, but both can create very damaging straight-line winds from a few seconds to several minutes in a microburst and anywhere from 5 to 20 minutes in a macroburst. If it is primarily virga within the thunderstorm and no rain makes it to the surface and there are intense, damaging straight line winds, then that is a dry microburst. 

Tides are the regular rise and fall of sea level caused by the gravitational pull of the moon and the sun, combined with Earth's rotation. Gravity is the key player. As the moon pulls on Earth's oceans, water closest to the moon feels a stronger pool, so it bulges outward forming high tide, but high tides also occur at the opposite side of the Earth. Why? Well, the moon pulls more strongly on the near side of the Earth. There is less pull on the far side, so water there is "left behind" as Earth moves forming another high tide. The area between bulges experiences low tide. As the Earth spins, coastlines move into and out of these bulges where most places get two high tides and two low tides per day known as a semidiurnal tide. However, some locations can have diurnal tides where there is only one high tide and one low tide. Although when the sun gets involved, it can become concerning as there could be possible coastal flooding as a result. 

The sun is very huge compared to the moon, but much further away. Thus, it's tidal pool is about half as strong as the moon's. When there is a new moon or full moon, their gravity adds together between the sun and the moon, which produces extra high-tides and extra low-tides known as spring tides. When the sun and the moon are at right angles during the first or third quarter moon, their gravity partially cancels, which weakens tidal range and produces neap tides. Essentially, think of the Earth as wearing a loose sweater with the moon tugging on the sweater, which stretches the fabric outward on both sides. If the sun joins in on the party, then that sweater stretches even more if aligned, but if at a right angle with one another, then there's an occasional tug, but they don't seem to bother as much. An important concept to coastal communities as high tides add more to the already dangerous storm surge that hurricanes may bring when making landfall. 

It is well known that the freezing point of water is 32F, but it is important to note that is only for pure, fresh water. Salt water actually freezes at a lower temperature. This is because as salt dissolves in water, made up of sodium and chloride (NaCl), the sodium and chloride ions split. These ions hold an electrical charge from gaining or losing electrons and disrupt water molecules from bonding and if water molecules are unable to bond, then the crystalline structure of ice is unable to form. Therefore, as salinity, concentration of dissolved salts, increases, then the freezing point of water decreases and vice versa. Very similar to when adding salt or a dissolved substance into a cloud droplet from the solute effect, it decreases the relative humidity needed to reach saturation for precipitation to form. This is why typically in the winter; salt is used on roadways to lower the freezing point at which snow sticks to help create better travel. The downside to using salt on roadways is the damage to the environment and corrosion on your car due to an electrolyte solution. It is highly recommended to frequently give your car a car wash in the winter to minimize any corrosion and damage. 

Recall that density is measured as mass divided by volume (g/cubic meter). It is fundamental that the colder the water is, the more dense it becomes, and while that is true, it is true to a certain extent. It turns out that water is actually most dense at 38F. As the temperature cools below 38F, the water starts to become lighter. Why? This may seem backwards, but at 38F, water starts arranging its molecules in an open, hexagonal structure as they approach freezing. The open, hexagonal structure takes up more space (volume), so the density decreases even though the temperature is dropping. This is very typical in the winter where the bottom of a lake is warmer and denser at 38F while the surface waters may be 32F but lighter. This concept reverses in the summer where the coldest, denser waters are at the bottom and the warmest, lighter waters are at the surface. In the summer, daytime heating from the sun will warm the surface of the ocean more efficiently than the deep ocean. Therefore, lakes freeze top-down and not bottom-up or else many fish would die in the winter. Additionally, this is why ice typically floats on water as it is less dense. 

Now, recall that the atmosphere's layers are divided based off of temperature. Well, the ocean also has layers based off of temperature. In a lake or ocean, the warmer waters are typically layered at the surface followed by colder waters in the deep ocean. The rapid temperature decreases between the surface layer, and the deep ocean is called The Thermocline. It acts like a "boundary" separating warm, surface water from cold, deeper water. The surface layer is typically turbulent and forms waves due to the surface winds while the deep ocean is relatively stable. It forms because warm water is less dense and stays at the surface while cold water is deser and sinks. Since the sun only heats the upper ocean and mixing doesn't reach very deep, a sharp temperature gradient naturally forms between the two layers. If the thermocline is shallow, then it is easier for wind-driven surface currents to bring cold, nutrient efficient waters to the surface known as upwelling. However, if the thermocline is deeper, then it suppresses upwelling placing colder, nutrient waters further from the surface. Therefore, the thermocline introduces a temperature gradient, which controls how heat is stored and distributed globally and how surface currents interact with deeper currents.

On a much smaller scale, the ocean influences the atmosphere from the dispersion of salt aerosols. Aerosols are tiny solid or liquid particles suspended in the atmosphere, ranging from a few nanometers to tens of micrometers, and they play major roles in visibility, air quality, cloud formation, and climate. Let's say you're at the beach and as always, the waves are breaking upon the shoreline. It makes the air smell like salt. These waves that break disperse salt aerosols into the air, which can act as a particle for cloud droplets to form. Since salt reduces the freezing point, sea ice forms at 28F, which rejects the salt called brine rejection. This increases the salinity of nearby water. Sea ice has gaps or holes when it forms, so when wind blows through them, it forms what we like to call sea spray that disperses more salt particles in the air. Sea ice is purely fresh water, which forms ice crystals first and thus rejecting the salt and flushing away. 

Sea ice is important due to climate regulation much like the thermohaline circulation. It has a high albedo, meaning it reflects a lot of the incoming solar energy. When sea ice melts, darker ocean water absorbs more heat from the sunlight, which amplifies warming. When the sea ice disperses salt to the surrounding water, the dense water sinks, aiding the drive of the thermohaline circulation. It is especially important because if sea ice disappeared, Earth's climate would warm dramatically, ocean circulation would weaken, sea levels would rise faster, and global weather patterns would become more extreme. 

One of the most powerful climate regulators, which also aids in the thermohaline global circulation, is latent heat. Latent heat is the "hidden energy" stored or released when water changes phase. It quietly moves vast amounts of energy between the ocean and atmosphere, shaping global temperature patterns, storm intensity and long-term climate stability. In other words, latent heat is the energy absorbed or released when water changes phase without changing temperature. For example, when the sun heats surface of the ocean, it warms and eventually evaporation (from liquid to vapor) occurs. In order for evaporation to occur, energy is absorbed from the environment. In this case, the sun's energy. As the parcel rises, it cools and condenses in the atmosphere and once reaching saturation at 100% relative humidity, clouds form through condensation (from water vapor to liquid). In order for water vapor to change to a liquid, the internal system needs to cool down, so in return, it releases that stored energy into the atmosphere. This is why the moist adiabatic lapse rate cools at a rate slower than the dry adiabatic lapse rate is because of heat that is released through condensation. This can be very explosive in stronger thunderstorms. Let's say it is winter, and after the cloud droplets grow through collision and coalescence and falls as precipitation, raindrops freeze through a deep surface layer after a shallow warm layer forming sleet. Thus, heat is released through freezing as energy needs to be released transitioning from a liquid to a solid. Melting is the opposite, in which energy is absorbed from the surrounding atmosphere in order for the ice to melt. Sometimes, a solid would go directly from a solid to a gas known as sublimation, like dry ice, so heat from the surrounding environment is absorbed in order to change phase. The opposite from a gas to a solid occurs through deposition where a gas goes straight to a solid, like frost on a bitterly cold, clear morning. 

Essentially, latent heat moves heat around the planet. When water evaporates from warm oceans, it stores energy as latent heat. Winds transport that vapor thousands of miles. When it condenses in clouds, the energy is released into the atmosphere. This process redistributes heat from the tropics toward the poles, drives major circulation patterns, and moderates temperature extremes. Condensation releases huge amounts of heat into the atmosphere, fueling thunderstorms, monsoons, and tropical cyclones. The released energy warms the surrounding air, causing it to rise faster and intensify storms. Since water can absorb so much energy during evaporation without warming, latent heat acts like a planetary thermostat. These effects include cooler ocean surfaces during strong evaporation, milder coastal climates, and reduced day-night and seasonal temperature swings. This buffering is a key reason Earth's climate is more stable than that of planets with little water like Mars. Furthermore, land surfaces also exchange latent heat through evaporation. When soils dry out, less latent heat flux occurs, and more energy goes into sensible heat, essentially heat energy that changes the temperature of an object, causing temperatures to spike. Climate models show that soil moisture and latent heat coupling is crucial for predicting heat waves and drought behavior. Therefore, the Earth as a whole is all one feedback loop. 

Cyclones are areas of low-pressure centers that spin counterclockwise or cyclonic in the northern hemisphere and clockwise or anticyclonic in the southern hemisphere. There are three types of cyclones: Extratropical cyclones, which originate in the midlatitudes and are cold-core lows, Tropical Cyclones, which originate in the tropics and are warm-core lows, and subtropical cyclones, which is a hybrid of both warm-core and cold-core lows. Cyclones that are most associated with clashes of air masses that form fronts are midlatitude cyclones (also called extratropical cyclones) where cold, polar air meets warm subtropical air, which is a zone called the polar front. This boundary is inherently unstable because the two air masses have very different densities and temperatures. When a disturbance occurs along this front, the atmosphere begins to reorganize itself into a rotating low-pressure system. 

Polar Front Theory (Norwegian Cyclone Model)

Polar Front Theory was developed by the Bergen School, which describes how cyclones originate, intensify, and decay along the polar front. It is the foundational model for understanding midlatitude weather systems. There are six stages in the formation and decay of a midlatitude cyclone: 

1. Stationary Front (Initial Stage)

Recall, that a stationary front is when neither air masses are stronger than one another and therefore, remain stagnant or stationary. There is cold, polar air and warm, subtropical air lying side by side waiting for a signal. Winds blow parallel to the front in opposite directions and typically high-pressure is situated north and south of the boundary. There is no movement yet, but the boundary is primed for instability forming clouds and precipitation. 

2. Frontal Wave

In order for the cyclone to form, there needs to be a small disturbance. Usually, this is accompanied by a shortwave trough aloft, a wave of energy that allows the two stagnant air masses beginning to clash. This creates a kink in the front where waves of air begin to form and a low-pressure begins to deepen from the temperature differences, and is usually supportive of upper-level divergence, for air to rise and the cyclone to deepen further. Warm air pushes poleward while cold air pushes equatorward and hence form the birth of a cyclone. 

3. Open Wave

The open wave stage of a cyclone begins to form distinct warm and cold fronts along the area of low-pressure where the cold front orients from north to south of the low and the warm front extends eastward of the low, forming an open wave. The airmass between the warm front and the cold front is called the warm sector. The warm sector is warm, humid airmass that feeds energy to several thunderstorms and possible severe weather outbreaks depending on the time of year and the characteristics present. Precipitation forms ahead of the warm front and is widespread stratiform rain while along the cold front, a narrow band of heavier rain or storms develop. The cyclone now has a recognizable comma-shaped cloud pattern. 

4. Mature Stage

The low-pressure center deepens as upper-level divergence strengthens called cyclogenesis due to the Jetstream. Winds increase due to the tightening pressure gradient and fronts sharpen as temperature contrasts strengthen. The cold front starts to move faster and begins catching up to the warm front. This stage marks when the storm is the strongest. 

5. Occlusion

In this stage, the cold front overtakes the warm front, lifting warm air completely off the ground forming an occluded front. The cyclone reaches maximum intensity but begins losing its temperature contrast or its fuel. As a cyclone occludes, the original surface low begins to lose its instability because the warm sector is lifted off the ground. The temperature gradient weakens, and the primary low starts to fill. However, if there is still strong upper-level forcing and residual instability along the triple point, the point where all three fronts connect, a secondary low-pressure may develop and become the dominant area of low-pressure. The triple point is often the last place where surface warm air still exists, a strong temperature gradient remains, and deep lifting is maximized making it a prime location for secondary cyclogenesis. This secondary low can deepen rapidly, become the new dominant center, and shift the storm's track. The parent low often drifts and fills while the secondary low takes over. 

6. Dissipation 

Now, that the warm air is aloft and cold air at the surface wraps around the area of low-pressure, the cyclone lost its energy source and begins to dissipate known as cycloysis. Pressure begins to rise, winds weaken, and precipitation starts to fade. High-pressure fills in behind the low, but if there is still a residual temperature gradient and energy, a stationary front may form and initiate new cyclogenesis. If there is another shortwave trough that brings energy to the wave, then the process could restart all over again. Therefore, cyclones are the atmosphere's way of balancing temperature contrasts. They transport warm air poleward and cold air equatorward, helping to redistribute energy across the planet. Keep in mind Polar Front Theory only explains part of the story as it only describes surface characteristics and not the entire atmosphere as it is a 3-D structure, not a 2-D structure. This will be explained in a later course, but for now, we'll stick with the basics. 

While the global conveyor belt and midlatitude cyclones redistribute heat and modulate climate on a much larger scale, there are very localized regions in the world that have climates of their own based off of the terrain, proximity to the ocean, and local water sources. For example, in San Francisco, on a summer day, it is not uncommon to see the shoreline bombarded with by fog and drizzle on the Pacific Ocean side with temperatures in the upper-50s. While just across the bay, it can be above 100 degrees with sunny skies. This is an example of a microclimate, which is a sharp disparity in temperature, cloud cover, and precipitation due to the proximity of the ocean or complex mountainous terrain. Along the west coast, typically storm systems can hit San Francisco, especially during the wet season in the winter months from November through March, which comes from the Pacific Ocean. These storms channel plumes of water vapor from the tropics. These long, narrow bands of concentrated water vapor in the atmosphere that transports huge amounts of moisture are called Atmospheric Rivers or The Pineapple Express. While these events denote most of the west's annual snowpack and rainfall, if they become too intense, it can lead to heavy rainfall and flooding, mudslides and landslides, and increased runoff. While they are most common along the west coast, these atmospheric rivers can also occur from the Gulf of Mexico. 

One primary example of a microclimate is the rain shadow effect, which forms due to mountainous terrain. The rain shadow effect is the process where mountains force moist air to rise, cool, and drop its precipitation on the windward side; leaving the leeward side warm, dry, and often desert-like. This creates dramatic climate contrasts over short distances, such as lush forests on one side and arid valleys on the other. Essentially, prevailing wind carries moist air from the oceans or large lakes toward the land. When this air encounters a mountain, it is forced upward known as orographic lift. As it rises, the air expands and cools, causing water vapor to condense into clouds, release latent heat into the atmosphere, and form precipitation. The windward side of the mountain becomes wet, cool, and vegetated. On the contrary, on the leeward side of the mountain, after losing most of its moisture, the air crosses the peak and sinks down the leeward slopes. Sinking air compresses and warms, which inhibits cloud formation. The result is a dry, warm region knwon as the rain shadow. Therefore, mountains act as a physical barrier, stripping moisture from air masses. 

An important concept of climate is temperature, and most meteorologists often refer to the average low and high temperatures when comparing it to new record low or high temperatures. The Diurnal Temperature Range explains how far the gap is between a low and high temperature throughout a given year and is entirely dependent on location. For example, in San Diego, California, throughout the year, the high and low temperatures are only 10-degrees apart throughout the entire year. Whereas in Topeka, Kansas, the temperature range is 20 degrees. Why is this? Well, it's because Topeka, Kansas is in the middle of North America while San Diego is along the California coast, in proximity to near water. This is because as mentioned before, water has a higher heat capacity, the amount of energy required to raise the temperature of an object 1 degree Celsius. Therefore, in the summer, the land heats up much faster while the Pacific Ocean is still cool thanks to the cold California current. Onshore wind helps cool the land close to the ocean down whereas further inland, that cool air becomes hot, dry desert air in the valleys. In the winter, the water would act to keep the coast warmer since it takes longer for the water to cool down. Since Topeka, Kansas is in the middle of land and is not near any source of water, the air has a lower heat capacity, and therefore heats and cools much faster and causes wider temperature swings. This is why many world deserts tend to have very large diurnal temperature ranges with cold nights and hot days since the air is so dry.