Lesson 1

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.