Air Lift: Definition & Processes

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  • 0:07 Air Moves
  • 0:58 Orographic Lifting
  • 2:17 Frontal Lifting
  • 3:37 Convergence
  • 4:43 Convective Lifting
  • 6:06 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson, you will learn about air lift. You will also identify the different ways air is lifted, as well as some of the effects each mechanism has.

Air Moves

Believe it or not, air is actually a fluid. This is because a fluid is any substance that flows and has an indeterminate shape. This is different from a liquid, which is a specific phase of matter. All liquids are fluids, but not all fluids are liquids because gases are also fluids! Air, which is a mixture of various molecules in their gaseous phase, definitely flows. Think of something like wind - that's just air flowing around horizontally.

Air also flows vertically, which we call lift. But, air doesn't lift all by itself - it needs help. There are four main processes that lift air, known as lifting mechanisms. When air does flow upward, we get some pretty interesting weather processes, like cloud formation, precipitation, thunderstorms, tornadoes and hurricanes.

Orographic Lifting

The first way that air is lifted is through orographic lifting. This is when air is lifted over an obstacle, like a mountain. As air moves across an area, it sometimes runs into obstacles, like mountains, and of course, it has nowhere to go but up! As the air is forced up the side of the mountain, it cools and condenses, which means it gets more compact.

Most of the moisture in the air gets forced out as the air condenses, which forms a cloud over the side of the mountain shown below, known as the windward side. It's called the windward side because this is the side of the mountain the wind first encounters. The cloud is full of moisture, so it's also pretty rainy on the windward side.

Wind first encounters the windward side of the mountain.
windward side of mountain

Once the air goes over the top of the mountain, it sinks back down again, and as it sinks, it warms up. However, it left most of its moisture on the windward side, so this air flowing down the backside of the mountain, also known as the leeward side, is not only warm but very dry.

Areas like the Western U.S. know this phenomenon quite well. As the wet air coming from the Pacific blows into the mountains in this area, the side that the wet air first hits stays pretty wet, while the other side is dry and warm. So, Oregon and Washington are wet and rainy, but New Mexico and Nevada are warm and dry!

Frontal Lifting

Another way that air is lifted is through frontal lifting. A front is where two different air masses meet, so frontal lifting is when one air mass along a front gets lifted up over the other. A cold front occurs when a cold air mass advances onto a warm air mass. A warm front is the opposite: when a warm air mass advances onto a cold air mass.

A cold front is like jumping into a lake in the middle of winter. Your body is quickly shocked by the change in temperature, and the warm air experiences something similar - it's quickly and forcefully pushed upward. Because of this, we get clouds forming quickly as well, and they form vertically instead of horizontally. These make the best thunderstorm clouds, so cold fronts tend to bring dramatic changes in weather, too.

Warm air along a warm front is quite different - it gets slowly lifted up instead of the dramatic lift we see with cold fronts. It's more like getting into a hot tub, slowly stepping in and letting your body adjust to the change in temperature. Since air is lifted so slowly along a warm front, cloud formation is more horizontal than vertical. This means that, while the weather does change, it's much less dramatic than with a cold front, and you'll likely experience mild rain over a wide area.

Convergence

Next, we have convergence, which is when air converges in the center of a low-pressure area. As air flows horizontally and meets along the surface of Earth, it's forced upward since that's the only place it can go. (It can't go into the ground, right?) Convergence is responsible for some well-known weather events, like hurricanes and cyclones. In fact, these storms only survive if wet air continues to converge in the center. If it stops, they die out.

The 'eye' of a hurricane is the low-pressure center where the air comes together, and as the air rises, it brings up moisture from the ocean it formed over. This is also why a hurricane can't form over land - it needs water just like we need food. The wet air converges, rises up through the low-pressure center, and as you now know, it cools and condenses as it is lifted up. Condensation releases a lot of energy, and this is the very energy that drives the storm along its path. As long as there is moist air being drawn up into the eye, the storm continues to soldier on.

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