Background Lessons for Journey North

Up, Up and Away: Thermals and Updrafts

Eagle in flight

Imagine being able to float in the sky, held up by a soft breeze of rising air. That's what flying animals, from hawks to monarch butterflies, do every day.

How do they catch a "breeze" going UP? And how does a breeze going UPWARD help them to migrate ACROSS the continent? This lesson will help you figure it out as you discover:

  • What is a Thermal?
  • What is Air?
  • Try This! Proof That Hot Air Rises
  • What is an Updraft?
  • How Migrating Animals Take Advantage of Thermals and Updrafts
  • Try This! See How a Thermal Holds Up a Feather
  • Activity: Flight Plan Match-up

What Is a Thermal?

A thermal is a column of rising air, caused by uneven heating of the earth.


Cumulous clouds are tell-tale signs of thermals

Thermals can form wherever the air is just a couple of degrees warmer than the air next to it. There are thermals above mountains, next to rivers and lakes, above plowed fields, highways or other patches where dark ground is beside lighter, leafy areas. Thermals frequently form above mountains in the afternoons, when the sun warms a mountainside unevenly.

Cumulous clouds are tell-tale signs of thermals. They form when a column of air rises, and then cools at higher altitudes. The moisture in the air condenses and forms the cloud.

Thermals form when warm air is beside cooler air. Warm air rises (red) above cool air (blue).

Where air at two different temperatures meets, the faster-jumping warm air, being less dense than the slower-jumping cool air, floats above. This is just the way helium, which is less dense than air, floats. The cool air sinks and spreads below the warm air.

Warming molecules speeds them up, too. So hot air molecules move about very fast! They bounce against the ground, trees, plants, birds, butterflies, and each other-everything in their path. All that bouncing around spreads them out. Molecules of cold air move more slowly, so they bounce less, and don't spread out as much as molecules of hot air-which is another way of saying cold air is more dense.

In the cool areas next to where thermals form, the opposite happens. Cold air sinking below the rising thermal causes what is called a downdraft.

What Is Air?
Air is a gas. It is made up of molecules. Air contains molecules of oxygen, nitrogen, carbon dioxide, water vapor, and other gases. It also contains tiny solid particles such as pollen and dust. When it's foggy, water is also present in the liquid form, as drops.

Air is usually invisible. However, we CAN see air when it's foggy, or when the molecules in it are so disturbed that our eyes can detect the wiggly movements.

Our skin can feel air moving. Our bodies are so much denser than air that we don't get pushed around by it unless it is moving fast in a strong wind. Bodies that are much less dense than ours can feel that push much more easily. Bird and flying insect bodies are designed to get a push upward from even very tiny movements of air.

Try This! Proof That Hot Air Rises
  • Have you ever looked close up at a car on a hot, sunny day and seen the air wiggle? That heated air is rising, floating above the cooler air farther from the car, making the cooler air take its place, only to get heated by the car and rise. All this rising air is called a thermal, and though we can't really feel it, feather-light monarchs, and eagles with their enormous but feather-light wings and hollow bones can actually float on a thermal, making their migration very easy. Fortunately, migrating birds and butterflies don't need a hot car to find a thermal!
  • In your classroom, measure the temperatures gradient from the floor to the ceiling. How does the temperature change as you go higher? How much of a difference can you find in a single room? (For a fun experiment, try this in your gymnasium. Attach thermometers to helium baloons. Send them to the ceiling and measure the temperature at various points in between!

What Is an Updraft?
Wind is caused by differences in air pressure between different areas on the earth. When wind hits an object, a rock, a mountain, or anything else, the air can't go through the object, and it can't go down through the ground, so it has to go up. Some places where updrafts are common are in cities, next to ridges and mountains, and on islands and shorelines.
  • Think of as many reasons as you can why so many different kinds of birds migrate along lakes and rivers.

How Migrating Animals Take Advantage of Thermals and Updrafts
Birds, butterflies and dragonflies all take advantage of rising currents to migrate. Think about the shape of a soaring eagle, crane, hummingbird, and monarch butterfly. All these animals are so light for their large wings that even gently rising air can push them up when their wings are spread. Which one never soars? Do the following demonstration to help confirm your reasoning.

Note: This activity must be done under the supervision of an adult so you don't get burned.

If birds and monarchs were weightless, they could rise straight up on thermals (at least if the wind was absolutely calm). But these creatures are not weightless. In order to stay in the air at all when they're not flapping, they must be moving forward. (See our lesson about flight.) And in order to move forward AND stay over a thermal or updraft, they move in a circle. The rising air carries them upwards at the same time. This makes their overall movement a spiral.

In a thermal, eventually the rising warm air cools down to the same temperature as the air around it. And air rising in an updraft slows down little by little until it dies out. At that point, the animal starts heading in the direction it wants to move. If it is soaring or just not flapping much, it will lose altitude. It gets lower and lower until it discovers another thermal. Then up it goes again!

Try This! Flight Plan Match-up
The following graphs show four migration flight plans: a crane following an ultralight, an eagle, a hummingbird, and a monarch butterfly in an area with a lot of thermals.

First, discuss as a class which flight plan matches each animal. CLICK TO ENLARGE.

Then read our thoughts

National Science Education Standards

Physical Science
Materials can exist in different states?solid, liquid, and gas. Some common materials, such as water, can be changed from one state to another by heating or cooling. (K-4)

An object's motion can be described by tracing and measuring its position over time. (K-4)

Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature. (5-8)

Life Science
Each plant or animal has different structures that serve different functions in growth, survival, reproduction. (K-4)

Living systems at all levels of organization demonstrate the complementary nature of structure and function. (5-8)