Have vapor, so for every water molecule

Have you ever wondered what happens to air as it rises? Well, you’ve come to the right place.

This lesson examines lapse rates, or the speed at which air cools as it rises in the atmosphere.

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Lapse Rate

Get ready, you’re about to go up, up, up into the atmosphere!But why, you ask? Well, you are about to explore lapse rates, which are the rates at which the temperature changes with altitude.3-2-1, here we go! You are going to be traveling within a parcel of air, which just means a clump of air that shares similar properties. As you go up, you might notice it gets colder, so go ahead and put on that scarf and hat!We are going to examine the three types of lapse rates: dry adiabatic, wet adiabatic and environmental. But before you ascend any further, why don’t you hover for a minute so we can get some background information under our belt.

Adiabatic Lapse Rates

When air is saturated, it means the air cannot hold any more water vapor at that pressure and temperature. Water is evaporating, or changing into water vapor, and condensing, or changing from a gas to a liquid, all the time.

When the atmosphere is saturated, it can’t hold any more water vapor, so for every water molecule that evaporates, one must condense. So, the dry adiabatic lapse rate is dealing with air that is not saturated. The name ‘dry’ is a little misleading. Just because the air isn’t saturated doesn’t mean it’s dry.OK, let’s start to ascend with the parcel of air. You’ll notice, the higher you go, the pressure decreases and your ears might pop!As the pressure decreases, the parcel of air is able to spread out and cool.

But why? Well, with less pressure, the molecules are able to take up more space because the pressure isn’t keeping them contained. If you had a balloon with you, you’d notice the balloon would get bigger and bigger as the pressure decreased because there is less pressure pushing inwards on it. Eventually it would pop!Back to those molecules. As they spread out, they are less likely to collide with one another, thus reducing the amount of heat energy exchanged between molecules. This causes the parcel of air to cool.The dry adiabatic lapse rate is the rate at which unsaturated air cools or warms as it changes altitude.

And here is that rate:

  • For every 1,000 feet the air ascends, the temperature will decrease by about 5.5 degrees F.
  • This is the same for descending, too. Meaning for every 1,000 feet the air descends, the temperature will go up by 5.5 degrees F.
  • And for you metric folk: for every 100 meters, it changes by 1 degree C.

But let’s keep going up and see what happens.

Man, I bet you are getting cold! It looks like you are about to reach the atmospheric dew point, where the air has cooled enough to become saturated and will start to condense. You might notice this by seeing some clouds forming.So now that the air is saturated, the lapse rate will change, hence the wet adiabatic lapse rate. This lapse rate is the rate at which saturated air cools or warms as it changes altitude.

Because the air is condensing (which releases heat), the rate of cooling will slow. Unlike the dry lapse rate, the wet adiabatic lapse rate can vary. This is dependent upon how much water vapor the air parcel had before it started to rise.

But here are some ranges:

  • For every 1,000 feet ascended, there’s a 3.2 to 2.2 degree F decrease in temperature.

  • And for every 1,000 feet the air descends, there’s a 3.2 to 2.2 degree F rise in temperature.

  • And for you metric folk: the wet adiabatic lapse rate average is about 0.5 degrees C for every 100 meters.

You might be wondering why the air became saturated when it cooled. Why don’t you hover again, while we go over the details. Warm air can hold more water vapor without becoming saturated, whereas colder air holds less before becoming saturated.

Now technically, it’s a little more complicated than this and air doesn’t ‘hold’ water vapor, but for the scope of this lesson, just know warmer air has more water vapor without becoming saturated.OK, keep going up, up, up. Eventually all of the water inside your air parcel will condense out and the air will revert back to the dry adiabatic rate! We don’t want you to float away too far, so why don’t you come back down to earth for now.There are some things you need to know about adiabatic before we go into the environmental lapse rate. The adiabatic lapse rates are theoretical, meaning there are certain criteria that must occur in order for those lapse rates to hold true.

Let’s briefly go over those criteria.

  • It stipulates that there is no mixing between air parcels.
  • And there is no heat exchange, meaning no heat leaves or enters the parcel.

Environmental Lapse Rate

Even though parcels of air are often large so mixing or heat exchange may be minimal, the air parcels aren’t sealed off from one another. So, there is an environmental lapse rate. This is the rate change in temperature with altitude at any given location and time, meaning a real-world lapse rate without the stipulations seen in the adiabatic lapse rates.Both the dry and wet adiabatic rates are theoretical and can be calculated mathematically, so how do scientists determine the environmental lapse rates? This is determined by releasing weather balloons and measuring the temperature at different altitudes.

Standard Atmosphere

In 1953, a bunch of organizations and scientists in the United States developed the U.S. Standard Atmosphere, which gives data for temperatures, pressures and densities at different altitudes. It was calculated by using environmental lapse rates, satellite and rocket data, as well as looking at mathematical expressions. The U.S.

Standard Atmosphere was revised several times, the last in 1976. The U.S. Standard Atmosphere changes depending on the altitude, but up until about 6.8 miles, the rate is a change of 3.57 degrees F for every 1,000 feet.The international community also developed a standard atmosphere, sometimes called the International Standard Atmosphere, which agrees with the U.

S. Standard Atmosphere until an altitude of about 19.8 miles.

Lesson Summary

Now that you’re back on the ground, why don’t we take a moment to review everything we learned about lapse rates, or the rates at which temperatures change with altitude.

There are dry adiabatic lapse rates and wet adiabatic lapse rates. The first is the rate at which temperature changes when the air is unsaturated, meaning the air can still hold some more water vapor. The latter is the rate at which temperature changes when the air is saturated, meaning the air can’t hold any more water vapor. Both of the adiabatic lapse rates are theoretical.The environmental lapse rate is the rate at which temperature changes with altitude at any given place at any given time. Scientists came up with the Standard Atmosphere, which is the temperatures, pressures and densities at different altitudes.

There is a U.S. Standard Atmosphere and an International Standard Atmosphere, which are the same until about 19.8 miles.

Who knew so much happened way up there? Air expanding, cooling and condensing! How exciting!

Learning Outcomes

Once you’ve finished with this lesson, you will have the ability to:

  • Define lapse rates
  • Differentiate between dry adiabatic and wet adiabatic lapse rates
  • Explain what the environmental lapse rate is and how it’s different from the adiabatic lapse rates
  • Compare the U.S. Standard Atmosphere to the International Standard Atmosphere
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