What Causes Differences in Air Pressure?

a) Differential Heating

The earth's surface receives varying amounts of solar radiation due to the curvature and tilt of the planet. This results in cold temperatures at the poles and hot conditions near the equator. Temperature extremes produce their own air pressure patterns. For instance, extreme cold produces high air pressure. Why? Think about if you were to open a window in your bedroom on a cold and calm winter's night and you stood away from the window in the middle of the room. On what part of your body would you first feel the cold? Near your feet. This is because cold air is dense and the air molecules are sinking. On a larger scale, a cold air mass pushing down on the earth's surface creates an area of high air pressure.

On the other hand, extreme heat produces low air pressure. Think about the air rising above a camp fire or the heat radiating above an asphalt road on a hot summer's day. The air molecules as they are heated, begin to expand and leave the earth's surface putting less pressure on it. On a larger scale, this hot air creates an area of low air pressure.

An experiment that clearly demonstrates these concepts is as follows:

Step 1: Get a large, wide mouth jar and cut up some strips of newspaper.



Step 2: Blow up a balloon so that it is slightly larger than the mouth of the jar. Tie up the balloon so that the air does not escape.



Step 3: Light the strips of newspaper on fire and place them in the jar.



Step 4: As soon as the fire goes out, place the balloon in the mouth of the jar. The balloon should be drawn into the jar.



b) Processes of Precipitation - Generate Low Air Pressure

Extreme heat is only one mechanism to produce low air pressure. This is the case with convectional precipitation. The heat of the sun warms the ground and the ground then heats the air above it. This causes the air molecules to rise leaving less pressure on the earth's surface.

However, with orographic precipitation it is not extreme heat that causes the air to rise. Rather, the presence of mountains on the windward side forces the air to rise and create lower air pressure at the surface. In contrast, on the rainshadow or leeward side of the mountains the air is descending and pushing down the earth's surface producing higher air pressure in comparison to the windward side.

With frontal precipitation, it is the meeting of two different air masses that causes the air to rise. The warmer air mass is forced above the colder air mass leaving lower air pressure at the surface.

c) Descending Branch of a Convection Current - Results in High Air Pressure.

The deserts of the world are associated with high air pressure whether they are hot such as the Sahara or cold such as the Canadian Tundra. Recall that high air pressure is consistently associated with dry conditions because the air is descending. Air that is descending is not rising, cooling, condensing, and forming clouds readily.

But you may ask, should not the heat of the Sahara desert produce low air pressure? Yes, there will be localized low pressure created with the heat of the day but generally it is not enough to alter the much larger pattern at work. That pattern involves a convection current of air rising at the equator where the average annual temperature is even hotter than the Sahara desert. Deserts, such as the Sahara, can get quite cool at night due to the loss of heat with clear skies. The air that rises over the equator, cools, condenses, and forms clouds and considerable precipitation via convective processes. This helps to create the tropical rainforest biome. After releasing its moisture, this air proceeds northward and descends over the Sahara desert generating high air pressure and dry conditions.



Hence, most hot deserts of the world are formed due to being on the descending branch of a convection current or on the rainshadow / leeward side of the mountains (i.e. Death Valley, California).

 

Low Pressure Areas

Low pressure areas are also known as depressions or mid-latitude cyclones. In the northern hemisphere, low pressure areas rotate counterclockwise versus clockwise in the southern hemisphere. This is similar for the direction water goes down a drain; counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. This is all due to the rotation of the earth about its axis. On the satellite photo below, the low pressure areas have been labeled with an L.

Courtesy: The Weather Network

Notice that the low pressure areas are associated with clouds and precipitation which is evidence of unstable conditions.

Local Scale Winds - Sea Breeze and Land Breeze

Sea Breeze

Land heats up faster than water. Consequently, as the land heats up on a clear, hot summer's day, localized low air pressure develops as the air molecules expand and rise. In contrast, over the ocean the temperature is cooler and consequently the air pressure is relatively higher. Since wind is simply air moving from a place of higher air pressure to a place of lower air pressure, a local sea breeze develops often in the afternoon.

Land Breeze

Land cools down faster than water. Consequently, as the land cools down on a clear, summer's night, localized high air pressure develops as the air molecules descend towards the earth's surface. In contrast, over the ocean the temperature remains warmer and consequently the air pressure is relatively lower. The net result is a local land breeze as the air moves from the land to the sea.

Global Pattern of Prevailing Surface Winds

On a global scale, the prevailing winds at the surface follow the same principles described above namely that
- wind is simply air moving from a place of high air pressure to a place of low air pressure
- extreme temperatures produce their own pressure patterns
- rising air is associated with low air pressure, and
- subsiding air is associated with high air pressure

Furthermore, notice with the monsoon diagrams above that the air does not flow straight from high air pressure to low air pressure, but rather it is deflected slightly to the right as it moves towards the low. This is due to the rotation of the earth and is called the Coriolis Effect or Ferrel's Law. In the northern hemisphere, the wind is deflected slightly to the right whereas it is deflected slightly to the left in the southern hemisphere.

Knowing this information, one can generate a diagram of the world's prevailing winds. Start by placing in key lines of latitude that have dominant air pressure patterns. At the two poles, the extreme cold produces high air pressure whereas at the equator, extreme heat produces low air pressure. At thirty degrees north and south, the air is subsiding on a downward branch of a convection current as described earlier. Therefore, high air pressure dominates at those locations. At sixty degrees north and south, the air moving away from the poles encounters a larger area to flow into therefore it expands generating low air pressure.

Then insert arrows from high to low remembering to apply the Coriolis Effect for the respective hemispheres. In terms of naming the winds, a wind is always named for the direction that it is coming from. The winds near the equator are often relatively light, hence the area has been named the doldrums by sailors. Similarly, light winds under stable high air pressure near thirty degrees north and south have resulted in the regions being nicknamed the horse latitudes. Some sailors, frustrated by their lack of progress under such conditions, would resort to throwing their horses overboard to lighten the load and presumably increase their speed; hence the nickname.