This Q&A is a followup to my June 8 post What Causes Wind. The answer there was “high and low pressure systems”, which begs the titular question. We should start with a brief review of Barometric Air Pressure. Follow that link if you need more review than what follows.
The average atmospheric pressure at sea level is 14.7 lb/ft2 or 704 N/m2. As shown in the graphic, and as the units suggest, this pressure is the weight of a column of atmosphere over a specified area. Meteorologists typically report pressure in millibars (mbar), where average atmospheric pressure at sea level is 1013 mbar.
The weight of a column of atmosphere (of specified cross-sectional area) depends on two factors: the density of that column of air, and the height of that column. Density depends on the air’s temperature, pressure, and humidity, all of which vary with altitude.
The height of the column is shown as 100 km (62 miles) because this is the arbitrary definition of “where outer space starts.” In reality, Earth’s atmosphere extends far beyond that altitude, through the thermosphere and exosphere. But 99.99997% of the air is below 100 km, so little error is introduced by this definition.
When high and low pressure systems form they change the height of the atmosphere as shown below. This cross section of two systems shows the air flows involved. Note that both upper atmosphere and surface level winds participate.
You can see that at ground level in a high pressure system there’s literally more atmosphere above you. That’s where the high pressure comes from. The opposite occurs in a low pressure system. These variations in atmospheric height are driven by upper atmosphere winds. Where winds converge, air piles up. Where winds diverge they leave behind a depression. So where do those winds come from?
High pressure systems are spawned when air is cooled by contact with a land or water mass, or by radiation to space on a clear night. Cooler air is denser air and that raises the air pressure. Cooler air falling through the atmosphere, and/or cooler air flowing outward at ground level, drives the circulation pattern shown above. Highs can also be created by the chance convergence of upper atmosphere winds, or chance divergence of surface level winds, driven by contiguous systems.
Low pressure systems are spawned when air is heated by contact with a land or water mass, or by direct heating from the Sun. Warmer air is less dense and that lowers the air pressure. Warm air rising through the atmosphere is what drives the circulation pattern shown above. Lows can also be created by the chance divergence of upper atmosphere winds, or chance convergence of surface level winds, driven by contiguous systems.
In my June 8 post I explained how high and low pressure systems cause winds. Now I’m saying that winds can cause high and low pressure systems. That might seem like circular reasoning, but its an accurate reflection of the complex feedback loops operating in planetary atmospheres. This why weather evolves in a nonlinear manner and, consequently, is so difficult to predict.
The bottom line is: All weather phenomena are driven by heat energy from the Sun, and the Sun can only warm one side of the Earth at a time. Even on the warm side there are variations in cloud cover, surface reflectivity, land and water masses, and Sun angle (latitude). The result is irregular heating, and that’s where all weather gets its start.
Next Week in Sky Lights ⇒ The Unearthly Moons of Mars