I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.
By: Randy Bynon , 5:07 AM GMT on November 22, 2006
The podcast for this lesson can be found at
In our last lesson, I talked about how the earth is heated by the sun. But since the equator gets most of the heat and the poles get almost none, the heat has to spread poleward. To understand what happens when the heat tries to move poleward, you have to understand a few things about the atmosphere in order to know how it will behave.
There are three primary properties of our atmosphere that affect the weather. They are temperature, pressure, and water vapor. Each has a huge affect on the state of the atmosphere and on each other. Let’s look at these properties. Since we’re talking about the atmosphere, we’ll apply these definitions to the air around us.
Pressure is probably the simplest property of the three to understand. The pressure of a parcel of air in our atmosphere is affected by a number of factors including altitude, temperature, and water content. Air has mass and is affected by gravity meaning it has weight. The atmosphere is like an ocean of air. The deeper you go into the ocean of air, the greater the pressure of the air around you. The higher the pressure of the air, the greater it‘s density. At sea level, the average atmospheric pressure is 1013.2mb or 29.92 inches of mercury.
What is temperature? Most people would say it’s a measure of the amount of heat in the air (or any other substance for that matter). But in reality, the measure of heat is the calorie (and you thought that was the measure of fat around your waist). Temperature is actually the measure of the average energy in a substance. Physically, it’s a measure of the motion of the molecules in a substance. As molecules of air are heated, they speed up. As they speed up, they bounce off one another and they move farther apart. This results in a decrease in density. Less dense air is buoyant and tends to rise. But it also works the other way around. If you remove heat from molecules of air, they slow down and become more tightly packed. This results in an increase in the density of the air. Dense air tends to sink. So what affect does pressure have on temperature? It’s actually pretty easy to see in the atmosphere. As you rise up in the atmosphere, pressure decreases and therefore so does the density. This results in a lower temperature. As you increase pressure, molecules are forced closer together and begin to bounce off one another. This causes them to speed up resulting in an increase in temperature. Changing the temperature of the air by increasing or decreasing pressure rather than by adding or removing heat energy is referred to as adiabatic cooling or heating.
Water vapor occurs naturally in our atmosphere. It is water molecules suspended in the air and invisible to the eye. Water evaporates from our oceans, lakes, rivers, and streams into the air as water vapor. Under the right conditions, the water vapor condenses into visible water droplets that we call clouds. If the condensation process continues, the droplets get bigger until they can no longer be suspended in the atmosphere and they fall out as rain or snow and are added once again to the oceans, lakes, rivers, and streams. This entire process is called the water cycle.
The temperature of the air determines how much water vapor can be evaporated into it. It acts much like a suspension in fluids. Did you ever do the experiment in school where you dissolved sugar into a beaker of water until no more would dissolve into it? Then you would suspend a string in the beaker and place the beaker in the freezer or refrigerator. As the water cooled, it was not able to support as much sugar in suspension. So the excess sugar crystallized on the string as rock candy. The air works in much the same way but it doesn’t make rock candy! It is able to hold more water vapor at higher temperatures than it can at lower temperatures. Many people think that relative humidity tells you how much water vapor is in the air. In reality, it doesn’t. To meteorologists, RH means very little. What it actually tells you is how close the air is to holding all the water vapor it can hold. If you evaporate enough water into 40 degree F air to bring it to 100% RH (i.e. you saturate the air so that it can’t hold anymore water), then warm the air to 60 degrees F without adding or removing water vapor, the RH will decrease to near 50%. This is because air at 60 degrees F is capable of holding nearly twice as much water vapor as air at 40 degrees F. So what happens if you saturate the air at 60 degrees F and then cool it to 40 degrees F? Well, if the air is saturated at 60F, the moment it cools below 60F, condensation will begin to take place until water droplets form and excess water will precipitate out as rain.
An important property of water is that it is lighter than dry air and has a higher specific heat than dry air. What this means is that it takes more energy to warm up moist air than dry air. It also means that moist air contains more latent heat than dry air and is less dense. That becomes important in later lessons.
PUTTING IT ALL TOGETHER
One important thing to remember is that all these properties affect one another. In the next lesson we’ll see how these properties come into play as heat is moved from the equator to the poles in our atmosphere.
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