Everyone knows that in summer it’s hot, and in winter it’s cold. But what makes the weather change from day to day or week to week?

High and low pressure areas affect weather a lot. Atmospheric or barometric pressure is measured in inches of mercury or in pounds per square inch. “Normal” sea level pressure is 29.9 inches or 14.7 psi. Because air has weight, that pressure decreases as altitude increases (an inverse proportion). For example, the air pressure here in the mountains at 2,500 feet would be about 27.3 inches or 13.4 psi. Generally, the air temperature also decreases as altitude increases.

All weather, such as storms, snow, rain or wind, is the result of uneven heating of the atmosphere by the sun. These temperature differences cause air currents to move heat from cooler areas to warmer ones. Also, generally speaking, areas that are warm have lower air pressure then colder ones. This is because warm air weighs less than cold air. The result is air moving from high pressure to low, causing wind.

This constant heat and pressure redistribution results in ever changing weather patterns. For instance, a cold air mass moving south from Canada colliding with warm moist air can cause severe storms, rain or snow. This is because warm air is capable of containing more moisture than cold air, so the advancing cold “squeezes” the water vapor out. Also, because it is heavier, the colder air pushes the warm air up, called override, and even more moisture is forced out. This process is known as adiabatic cooling.

Dew point is the term given to the temperature of the air at which moisture condenses out. That point is always less than the temperature of the air. The water vapor forced out can be in the form of a cloud, fog or dew. At the point where the dew point equals the air temperature, the relative humidity is 100 percent. If the air temperature falls even more, then precipitation occurs in the form of rain or snow. Ice forms when rain falls through freezing air. Large amounts of ice (ice storms) can cause severe damage to trees and power lines.

The sun’s heating of the Earth also makes the atmosphere unstable, causing vertical air currents. This makes puffy-looking clouds (cumulus), showers, and thunderstorms in warm air masses. Vertical heat transport also results in wind flowing over the surface causing evaporation of water, cooling the surface and then releasing the heat farther up in the atmosphere where the water vapor condenses. This mode of vertical heat transport is called “moist convection.”

Because the Earth is a sphere, sunlight is more perpendicular in the tropics than at the Poles. This causes horizontal temperature variations, which in turn makes air pressure differentials, resulting in wind which redistributes heat from the tropics toward the poles.

All of this vertical and horizontal heat redistribution produces atmospheric circulation “cells.” When the air moves from higher to lower temperatures, there has to be a return flow of air in the opposite direction.

For instance, for all of the air rapidly rising within a thunderstorm, there must be an equal amount of air sinking elsewhere. This sinking can occur over a much broader area than the strong updrafts in the thunderstorm, and the rate of sinking is slower. That’s why there is turbulence when flying through clouds, but not so much in the clear areas around the clouds. When the sinking occurs rapidly, there can be violent straight-line winds, called a microburst. Stronger and larger cells can develop into hurricanes or tornadoes.

In the descending branch of a circulation cell, the weather will likely be clear and stable (high pressure). That descending air is being forced downward by rising air in systems hundreds or even thousands of miles away (low pressure). Then, because these circulation cells tend to travel, in a day or two the weather will change, and the ascending branch of a circulation cell will move in bringing clouds, rain or snow.

Together, this uneven heating in both the horizontal and vertical directions in the atmosphere causes everything that we perceive as “weather.”

The Earth and the surrounding air as a whole is getting warmer. This is causing extreme weather rarely seen before, such as droughts, floods, “super storms,” record-breaking heat and, ironically, cold. Most meteorologists and scientists are predicting that this trend will continue because of elevated levels of carbon dioxide in the atmosphere. This “greenhouse effect” caused by the CO2 blanket traps heat in the atmosphere that would normally radiate into space.

As the atmosphere heats up, the oceans get warmer, the water expands and sea levels rise. The warmer air and water is also causing melting of the icecaps and glaciers at the poles. All of this will eventually, maybe sooner than we think, threaten many coastal areas with flooding.

Those who would like to ask me questions about any other science topic may e-mail inquiries to lynn@thesylvaherald.com.

Marcus Goodkind of Tuckasegee, a retired aerospace engineer, worked with the National Aeronautics and Space Administration as a manager at Kennedy Space Center on all the manned programs from Mercury to Shuttle, including Apollo 11, the first manned lunar landing.