Lesson 3

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.