Click the picture for more about the Troposphere!

If a meteorologist wants to make an accurate weather forecast for a place like Alabama, a lot of work goes into analyzing the atmosphere overhead, across the United States, and across the entire Northern Hemisphere.

So how do you actually describe the atmosphere? You can't see it, you cannot contain it in a laboratory, but you can put weather tools out there to try and get a good sample of it! As with anything, the more you know about the what the atmosphere is doing, the better our forecast will be. That's why our weather instruments are scattered all over the nation and the world.

Measuring Earth’s Atmosphere

When we start to make a forecast, analyzing the atmosphere is the very first step! On the Current Weather page, you can see how we read surface maps, and how we look at the atmosphere above us with weather balloons!

Click here for the NOAA JetStream Atmosphere Page!

Weather balloons are launched twice each day, and the one for Alabama is always sent up from the Shelby County Airport, located near Alabaster.

Weather Balloon

Weather Balloon Data (Called a Skew-T)

There are a lot of other tools that meteorologists use to measure the atmosphere that do not require a big balloon. Some of those, like thermometers and barometers, you may have at home.

Student Weather Center's Weather Toolbox Click the toolbox to learn more about the kinds of tools and instruments that give us a “picture” of what the atmosphere is doing!

So those are the basics about Earth’s atmosphere and how we measure it, but there’s much more to it than that! The atmosphere that we live and breathe in every day is made of more than the oxygen that keeps us alive. In fact, oxygen (O2) makes up only about 21% of the whole thing! Another element called (N2) accounts for a little over 78% percent of the atmosphere’s volume.

That’s 99% between oxygen and nitrogen, so 1% is made up of argon (Ar), carbon dioxide (CO2), methane, and other rare gases.

Water vapor (H2O) exists in the atmosphere, too. The temperature of the air, the amount of water vapor in that air (how humid the air is), and how the wind is moving it all are probably the three most important measurements we can have of the lower atmosphere, or troposphere. With the temperature and the humidity of the air, meteorologists can see a lot about what kind of weather the atmosphere can deliver.

 Details about Temperature, Humidity, and Wind

One very important thing to know about the atmosphere is where its source of heat is: the ground! That’s right, our atmosphere is heated from below! Here’s how it works:

The Sun’s energy (called solar radiation, or solar rays) cause the ground to heat up, and that in turn warms the air close to the ground first. It is like how a heater heats a cold room; the air nearest the heater warms first and best, and the air farthest away is heated last if at all.

Temperature

Why is it colder at the top of a 10,000 foot mountain that it is in the valley below it on a normal day? Temperature decreases as you go up away from the ground toward the sky in the troposphere.

The temperature either goes up or down through the stratosphere, mesosphere, thermosphere, and beyond based on what kinds of gases are in a certain layer.

So, normally, the higher in the sky you go, the colder the temperature will be.  It normally cools at a constant rate. But that is not always the case.  Sometimes, the air gets mixed up, and it actually gets warmer for a few hundred feet above the ground. This is called an inversion.   Sometimes the inversion can lead to a big difference in temperature across Alabama. Occasionally when the air is very cold, and the wind is calm, the temperature might be as much as 20 to 30 degrees warmer on top of a mountain like Mount Cheaha (elevation 2,407 ft.) than Anniston (elevation 719 ft.) which sits in a valley about 10 miles away. That means people in Anniston could need a heavy coat and have a thick frost while visitors on the mountain 1,688 feet above might not even need a light jacket!

Moisture in the Air

Ask any visitor from the northern part of the United States, and they will probably tell you that Alabama is about as hot and humid as it can get, especially in the summertime! That is because we are close to the Gulf of Mexico which supplies most of the moisture for Alabama’s weather. In fact, over 90% of Alabama’s rain begins as water in the Gulf. That liquid water gets into the weather mix because of the water cycle.

Humidity is just the amount of water vapor in the air, and the dewpoint is one of the easiest ways to describe it. The dewpoint is the temperature at which dew forms (condensation). If the dewpoint is high (above 55 degrees), the air usually feels humid. If the dewpoint is lower than that it usually feels dry. A very humid air mass has different weather than a very dry one. High humidity leads a smaller range in temperature from the morning low to the afternoon high. A very hot day with high humidity feels even hotter because of the heat index. Days with low humidity usually start cooler and end up warmer than more humid days.

The Wind that Makes it Move

Alabama’s weather would not change much if it were not for the wind. Wind is moving air, and as long as air is on the move, air masses are moving, temperatures are changing, and the humidity levels go up and down. When the wind is from the south here, it usually gets warm and humid. When the wind blows from the north, it normally gets colder and drier. Without the wind to move the weather, you would never see any changes!

The wind is named for the direction it comes FROM. A north wind blows from Cullman to Birmingham. A south wind blows from Clanton to Birmingham. A east wind gets to Anniston before it gets to Birmingham, and a west wind will come through Fayette before Birmingham.

Click the links below to learn more about how wind, temperature, and moisture are plotted on a weather map:

Reading a station plot (HPC)
Current Alabama Weather Conditions
Current Alabama Surface Map

The wind can change direction above the ground. Don’t think that just because the wind is out of the south that it is that way through the whole troposphere! Most of the time there is some change in wind direction with height.