Wind, Weather, and You (Part 3)

We are on a Spinning Planet, Tilted on its Axis, Orbiting the Sun

 As we orbit the sun, different parts of the earth are warmed more intensely or cooled more than others. We grow a year older, and the cycle continues over and over again for 4 billion years. It is this basic concept that builds the earth as we know it. It creates forests, ocean currents, frozen tundras, summer thaws, and billions of species filling billions of niches. For us as humans it creates our food and spawns our disasters. There is nothing more primordial to us than weather. We owe all life and death to it. All because we live on a spinning planet, tilted on its axis, orbiting the sun.

In these next chapters we'll learn how these three factors of our planet (titled axis, spinning, orbiting the sun) create the world as we know it.

First a note:
This is where we hit the culmination of everything we've learned in the previous sections. Concepts in these chapters are more complicated, and I urge you if you've never had any background in Meterology to first read Part 1 and Part 2. 

Global Circulation and You

The earth is in a constant battle to equalize itself. We learned earlier heating in one place and cooling in another creates pressure gradients that try to equalize themselves.
Q: But why is it like this in the first place?
A: We live on a sphere.

The Hadley Cell

Lets assume a couple things for this section:

1. There is only water on Earth
2. We're in the northern hemisphere
3. The sun shines parallel to the equator (like it does on the equinox)
4. The sun does not move
5. The Earth does not rotate
6. The Earth is a relatively small system

The sun shines at the equator. This rising air creates a low at the surface and high aloft. As air escapes upwards it hits the upper part of the atmosphere spreading outwards towards the poles. The air aloft cools as it moves northward leading to a sinking motion that creates a low aloft and a high at the surface. This high pressure at the poles then moves back towards the low created at the equator. Called a Hadley Cell this simplistic model tries to explain global circulation. In this model instability is created at the equator, while stability is created at the poles. We'd expect high amount of thunderstorms at the equator and clear skies at the poles, as well as strong northern winds at the surface.

But wait a second. This doesn't explain anything. Winds are much more complicated than this, it doesn't match reality. Why not?

The Three Cell System

Lets take out the assumption the earths not rotating and that its a relatively small system. Leaving us with the assumptions:
1. There is only water on Earth
2. We're in the northern hemisphere
3. The sun shines parallel to the equator
4. The sun does not move

What happens now?

Air at the equator rises creating a low at the surface and a high aloft. This time as the air moves towards the poles it is deflected to the right by the Coriolis effect. Air traveling northwards in the upper parts of the atmosphere travel south westerly creating SW winds aloft. Bringing in to account a large Earth, the air moving north cools and begins to sink around 30* latitude. The high created is called the Sub-tropical High. This creates a stable atmosphere with clear skies. The wind moving back towards the poles is deflected to the right again, and returns coming from the north east. We call these the North-East trade winds. It moves back towards the equator to an area called the Inter-tropical Convergence Zone. Basically a zone where winds on the ground are converging as the meet at the equator.

Some of the wind at the surface of the sub-tropical region moves northwards instead of south. Deflected to the right it moves in a south western direction, creating another set of trade winds we know as the Westerlies.

Now lets start at the poles. Sinking air at the poles create a polar low aloft and high pressure at the surface moves the only direction it can, southward. Deflected again it moves from the north east. This cold air hits the incoming westerly wind from the sub-tropical high. The lighter warm sub-tropical air and heavy cold polar air do not mix easily creating what we call the Polar Front. This stationary front creates rising air with vast instability, completing the 3 cycles.

The other trade wind features created by this system are the Doldrums and the Horse Latitudes

The doldrums are the area right at the equator where very little wind is encountered. Most air at this latitude is moving upwards not laterally, as it rises from the heat and the pressure gradient. This feature is famous in sailing and literature as an dangerous area where ships can get stuck for long periods of time.

The Horse latitudes are the area below the sub-tropical high where quite opposite from the doldrums sinking air diverges from high creating only light winds.

Well what does all this tell us?

Too much infact that we'll have to break it down in the next section

This global circulation model predicts fairly accurately: wind patterns, areas of high instability, location of rain forests, and deserts. As we've talked in depth about winds let's move on to storms.

Storms

From here we'll take out more assumptions. That the sun is parallel to the equator and the sun does not move. Which leaves us with:
1. There is only water on Earth
2. We're in the northern hemisphere

The three cell system predicts instability (and thus storm formation) at two places. First at the Inter-tropical Convergence Zone (ITCZ) where heat from the sun and incoming area create rising air that feeds storm formation. Second at the Polar front. The warm sub-tropical air converges with the dense cold air. The tendency along this boundary is for the warm air to rocket upward and cool, priming the atmosphere for storm formation.

We have to remember though that we're now on a planet that rotates around the sun, and with it the suns zenith moves northwards as summer sets in and southwards again as winter starts. This movement moves the cells and boundaries with them. In summer the ITCZ moves towards the tropic of Cancer (23* North of the equator) while the polar front is pushed far into northern Canada. During the winter the ITCZ moves south of the equator towards the Tropic of Capricorn (23* south of the equator) and the polar front moves towards the northern and central parts of the US.

This movement of the polar front is particularly important for the United States and the South where the Horse Latitudes suppressing storms and wind move south and allow the polar front to move in. This brings much needed storms to the south as well as aids in bringing cold fronts to cool off the country. (The Jet Stream aids in this as well, we'll deal with it another time)

This movement throughout the year creates high rainfall totals between both tropics and highest at the equator.

Forests

With rain comes forests. As you compare the two pictures (rainfall totals and forest locations) you can clearly see the majority of the tropical forests are created by the ITCZ. But what is up with the eastern united states and western Europe? Chile? The Pacific Northwest? Asia? The Rockies?

We'll finally take away all of our assumptions and look at the Earth closer to what it really is. A spinning earth, tilted on its axis, rotating around the sun with land and water mixed in.

The boreal forests as well as South East Asia and North America can both be explain with wind patterns. Sitting above 30* latitude they receive ample southwestern wind from the oceans, creating lots of moisture for rain and forests. The boreal strips from Europe, Russia, and Canada are from the largely unstable zone between the Polar and Sub-Tropical zones. This unstable front creates lots of snow during the winter and ample rainy seasons. These are amplified by the jet streams created along the Polar Front.

The Rockies and southern Chile can be largely explained by the addition of mountains and physical geographys effect on weather.

The Valdivian rainforest in Chile

Lifting of any kind can result in precipitation, if a column of air is physically forced upward it meets cooler air. As it cools the moisture is squeezed out of the air. Wind pushing air towards large mountain ranges, act as ramps for the air column, forcing air upward and an accelerated rate, forcing most of the water out of the air as it ascends. The higher the mountain the more intense the effect. The Rocky Mountains (~15,000ft) are overall twice as tall as the older Appalachian mountains in the eastern united states. The Chilean Coastal Range in South America are even larger (highest peaks ~20,000ft). This added to the emergence near the southern polar and prevailing wind directions leads to intense rainfall (up to 160inches of rain) creating the Valdivian Rain Forests. In North America, these conditions create the Quinault rainforest in Washington.

Deserts

 

To understand deserts in relation to latitude and the 3 cells lets look at them together:

 From here you can see quite easily that most deserts are right on the tropic lines. The wind's push moisture away from these locations not towards it. This problem is exacerbated when over large areas of land. The middle east suffers further by receiving wind from currently dry locations, creating a desert stretching far into Asia. 

Without any new shot of moisture from the ocean humidity plummets. This creates the opposite effect of living near the ocean. Without waters buffering properties in the air, things heat up and cool off quickly creating very hot days and cool nights. Conversely sweating is much more efficient in a dry location allowing better cooling than humid areas. This is largely why a dry 120 feels the same as a humid 100. Both are dangerous but for different reasons. The dry heat will take water from you quickly leading to dehydration. The humidity never allows your body to cool down leading to heat exhaustion.

Joshua tree in the Mojave (CA)

Ironically many of these deserts are further hurt by their relation to extremely tall mountain ranges and their consequent wet areas. These include the American south west (particularly the Mojave) and the Atacama desert in Chile. Any wind that comes from the ocean side is squeezed of water from the extremely tall mountain ranges, leaving almost no moisture left in the atmosphere for the regions leeward (the side facing away from the wind) of the mountains.




The effect elevation has on rainfall is best exemplified on the island of Maui which houses one of the highest rainfall extremes (300in+ above Hana to 12.5in in Kihei). The windward side of the island is the N/NE. As very moist air comes into the island it rises 10,000 ft dropping intense amounts of rain on the windward side of the mountain. By the time the wind wraps around the mountain the moisture is gone. So you can go from a rainforest to a desert in 20 miles. The wind does come locally directly off the ocean in the south, but its met by sinking dry air, leading to a pleasant tropical vacation at the beach that never gets ruined by rain. Which is why all the resort towns are on the south side of the island (Lahaina, Kihei, Wailea).

The Hawai'ian island Maui