3.8 - Wind

Wind

Watch: Instructor's Video Links to an external site.

The atmosphere is always in motion. Wind refers to the horizontal movement of air. All wind, from a sea breeze to the trade winds of Hawaii are the result of the same thing: the unequal heating of the Earth's surface which creates differences in atmospheric pressure which in turn, drives the wind. Wind is how imbalances in temperature and pressure are restored to a balance. Winds are nature's attempt to even out the uneven distribution of temperature and air pressure over the Earth.

Air generally flows from areas of high pressure to areas of low pressure. And if the Earth didn't rotate and there were no friction, that would be all we would have to know. But the Earth does rotate and there is friction so it gets a little more complicated.

Impacts on Wind Direction and Speed

The first and most obvious thing to impact wind speed and direction is the pressure gradient. If there is higher pressure in one area and lower pressure in another, the air will begin to move from the higher pressure toward the lower. I like to think about high pressure as a hill and low pressure as a valley and I imagine the air flowing down the pressure gradient just like a ball rolls down a hill.
The second impact on wind speed and direction is the Coriolis Effect. Because the Earth rotates, any object moving freely near Earth's surface appears to be deflected to the right in the Northern Hemisphere and the left in the Southern Hemisphere. The Coriolis Effect impacts the wind, ocean currents, rockets, planes etc.. Earth is, of course, wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race along at nearly 1000 miles per hour. Near the poles, Earth rotates at a snail's pace of 0.00005 miles per hour. This can be hard to understand so imagine you are standing on the Equator and you want to throw a ball to your friend in the middle of the US. If you throw in a straight line, it will appear to land to the right of your friend because he is moving slower and has not caught up. Now let's imagine you are standing at the North Pole. When you throw the ball to your friend, it will again appear to land to the right of him. But his time it's because he's moving faster than you are and has moved ahead of the ball. The Coriolis Effect does not exist right on the Equator so it wouldn't impact the path of your ball if you were standing on the Equator . The faster something moves the more significant the Coriolis Effect. Watch Instructor's Video Links to an external site.for more help with this concept.
The last impact on wind direction and speed is friction. Air moving over the surface of the Earth moves over mountains, hills, valleys, buildings, people, cars - all creating friction, slowing winds and altering their direction. As you move up in altitude (above 3300 feet) friction becomes less of an issue. By the time you're at 30,000 feet, friction is even more limited. What's the air going to encounter there? An occasional 747, a goose? Friction is primarily a factor over land near the surface of the Earth. At high altitudes and over the oceans, it is less of a factor.

Lets put all of this together and see what's really happening as the wind blows from high pressure to low pressure.

Cyclones and Anticyclones

high and low pressure winds with friction, the coriolis effect and the pressure gradient

We know that the pressure gradient causes the air to flow from areas of high pressure to areas of low pressure. The addition of friction and the Coriolis Effect mean that air does not move in a straight line. Instead, in the Northern Hemisphere the air diverges (moves away )from the center of the high pressure (anticyclone) and rotates clockwise. The air flows into (converges) the center of low pressure (cyclone) in a counterclockwise flow. In the Southern Hemisphere, the direction of rotation reverses for high and low pressure but everything else stays the same.

Three dimensional view of high and low pressure

As you can see in the graphic above, this is a three dimensional process. Lets summarize what is happening:

Unequal heating of the Earth's surface creates differences in atmospheric pressure. Over a warm surface, air will begin to rise moving counterclockwise as it rises, creating low pressure at the surface.
As the air rises, it expands and cools. Condensation may occur and clouds and precipitation become more likely
Air from an adjacent area with higher pressure begins to flow toward the low pressure, rotating clockwise as it does
Since nature is attempting to equalize the air pressure, air descends into the high pressure. As this air descends, it is being compressed and warming making condensation, clouds and precipitation unlikely.

**Comparing High and Low Pressure**
High (anticyclone)	Low (cyclone)
Diverging	Converging
cooler at the surface	hotter at the surface
air descends	air rises
compresses	expands
warms	cools
clear skies are likely	clouds and rain are likely
flow is clockwise in the N. Hemisphere	flow is counterclockwise in the N Hemisphere
flow is counterclockwise in the S. Hemisphere	flow is clockwise in the S. Hemisphere

Sea and Land Breezes/Mountain and Valley Breezes

Graphic of sea and land breezes To see how this might work in practice let's start small. Later we will look at the global scale. During the day, a beach will heat up quickly and to higher temperatures than the ocean. The air will start to rise as it heats up over the warm beach. The air over the ocean is colder and comparatively higher pressure. The wind moves from the higher pressure over the ocean to the lower pressure over land. This is a Sea Breeze. At night, the process reverses. The beach cools off quickly leaving the ocean comparatively warmer. The air rises over the ocean creating low pressure - air moves from the higher pressure over land toward the ocean. This is a Land Breeze. What set this wind in motion? The unequal heating of the Earth's Surface.

Mountain and valley breezes, like all winds, begin the same way.

Diagram showing mountain and valley breeze

The slopes of a mountain heat up quickly on a hot summer afternoon. The air rises creating localized low pressure.

Air from the cooler valley moves upslope. This is called a Valley Breeze. At night, the Sun dips below the mountains and the mountain top cools very quickly. The wind reverses and it becomes a Mountain breeze.

You will notice that winds are always named for the direction FROM which they blow (blows from sea to land = sea breeze, land to sea = land breeze)

The General Circulation of the Atmosphere

The trick is to apply all of this to the planet as a whole. There are four horizontal belts of high and low pressure north and south of the Equator, around the globe. Associated with the belts of high and low pressure are the prevailing winds. These are the dominant winds over regions of the Earth. Winds change direction often, but the wind direction will return to the prevailing wind direction over and over.

General circulation of the atmosphere It all starts with the unequal heating of the Earth's Surface. The Equator heats up and air begins to rise creating low pressure (creating the Equatorial Low). This is the Intertropical Convergence Zone (ITCZ) often called by sailors, the Doldrums. Here as the air rises, there is very little surface air motion as wind but because the air is rising and cooling, rain is a frequent occurrence. This is a common place for sailors to become becalmed. Not too bad a place to be stuck because at least you have drinking water provided by the rainfall.

40,000 feet above the Earth that rising air over the Equator begins to move north and south of the Equator. It has cooled off significantly, becoming more dense and begins to sink at about 30°N and S. These are the Subtropical Highs (STH). Because the air is descending here there is very little wind at the surface and because it is being compressed and warming there is very little rainfall. This was a place sailors did not want to end up. No wind and no water. This zone has been called the Horse Latitudes. This pattern of alternating high and low pressure continues all the way to the poles where the Polar High helps drive the Polar Easterlies.

The prevailing wind directions are the result of these bands of high and low pressure. Let's focus on two: our own Westerlies and the primary wind of the Tropics; the Trade winds. Winds are always named for the direction FROM which they blow. The Westerlies blow from the west in both the Northern and Southern Hemispheres from about 30° to 60° latitude and come from the poleward side of the Subtropical Highs. In the Northern Hemisphere, these winds are less constant and persistent than the Trade Winds of the Tropics. There is a lot of land in the Northern Hemisphere and friction slows these winds. In the Southern Hemisphere, there is more ocean and less land at this latitude. In the Southern Hemisphere, these winds are much more consistent and have much higher speeds. Sailors call them the Roaring 40's for that reason. Although the Westerlies at the surface in the N. Hemisphere may be a somewhat inconsistent, go up 30,000 to 40,000 feet and you hit two cores of high speed Westerlies call the Jet Streams.

The Trade Winds come from the equator ward sides of the Subtropical Highs. These winds affect latitudes from just outside the ITCZ to about 25°N and 25°S. They are particularly constant over oceans and in the Tropics where there are vast areas of ocean. The Trade Winds dominate more of the globe than any other winds. The Trade Winds blow from the Northeast in the Northern Hemisphere and the Southeast in the Southern Hemisphere. The Trade Winds are the most reliable of the prevailing winds and provided the power sailors needed to get from Europe to the "New World" and to sail across the Pacific Ocean.

Prevailing winds high and low pressure

The Seasonal Shift of the Subtropical High and the Dry California Summers

Because this whole system is due to the unequal heating of the Earth's surface, all of the elements of the General Circulation of the Atmosphere move north and south responding to seasonal changes in the temperature at the surface. In our summer, everything shifts north, including the STH. It parks right off California and keeps our summers bone dry. In the winter, everything shifts south and the STH moves away from California, allowing conditions to occur making it more likely to get rain.

January and the global circulation system

Summer Global circulation system

Spend some time looking at the diagram of the Earth's Circulation and check out the other prevailing winds, and persistent high and low pressure as you move north and south of the Sub Tropical highs. See how their position changes seasonally.

Regionally Important Winds

map showing monsoonal pattern in India There are many regional variations to this general circulation of the atmosphere. By far the most important variation from this basic pattern is the monsoons. Monsoons are a seasonal reversal of winds. There are monsoons in South Asia, East Asia, Northern Australia and West central Africa. In the summer, land masses heat up, air rises and low pressure dominates. Warm moist air from the comparatively higher pressure over the oceans blows ashore bringing the wet summer monsoons. In the winter the winds reverse, as the continents cool and low pressure develops over the warmer ocean. Cold dry air blows across these regions in winter.

santa ana winds graphic There are many other important regional and local wind patterns including Katabatic winds, foehn or chinook winds. One specific local wind that warrants special mention here in California is the Santa Ana winds (now called the Diablos in Northern California). The Santa Ana's are a type of katabatic wind. Often beginning in Fall or late Summer, the high elevation Great Basin in Nevada cools off compared to the warmer Pacific Ocean. The pressure increases over the interior. This creates a steep pressure gradient between the high pressure over the cold interior and the low pressure over the warmer Pacific. The air moves westward, downhill toward the coast. As it moves downslope it compresses and warms and its humidity drops. It is squeezed through passes in the mountains between Nevada and the ocean, driving wind speeds up. In the end very hot, dry easterly winds pounds Southern California bringing perfect conditions for fire. We see these conditions in Nor Cal too. Meteorologists have begun calling them the Diablo's. They have become a much more common feature as the climate has changed. The 1991 Oakland Hills fire started during such conditions, as did the 2017 Nuns and Tubbs fires and the 2020 Shady and Glass fires. The Paradise fire in Butte County in 2019 and the fires in California and Oregon in 2020 occurred during this easterly wind shift.