Measuring the Temperature of the Sky and Clouds

Web Id:P18
Measuring the temperature of the sky (27 F or -3 C).

Measuring the temperature of the sky (27 F or -3 C). Image courtesy of Forrest M. Mims III

Purpose: In this project you will learn about the greenhouse effect by measuring the temperature of the sky and clouds far overhead with an infrared thermometer.

Sunlight warms the Earth, which then emits the energy it absorbs as thermal (infrared) radiation that we perceive as heat. This process is important, for if the Earth did not release the energy it absorbed from the sun, the Earth would become excessively warm. Life as we know it would be unable to survive.

If the Earth did not have an atmospheric blanket, most of the radiation absorbed during the day would be radiated into space at night. The Earth would be so cold that the oceans would freeze! Fortunately, our atmosphere has plenty of water vapor, which strongly absorbs some of the infrared rays radiated from the soil, rocks, oceans and cities below. Thanks to water vapor, the Earth remains warm enough for life to exist.

Age Range: 14 to adult
Time Required: In under half an hour you can measure the temperature of the sky and clouds and learn how clouds tend to stabilize the temperature on the ground below. You can learn much more by spending a few hours reviewing this project, visiting the links it lists and performing some experiments of your own.

Sunlight warms soil, rocks, plants, roads, buildings and bodies of water, all of which then emit the solar energy they absorb in the form of thermal (infrared) radiation that we perceive as heat. If there were no atmosphere, the Earth surface would release as much energy to space as it receives from the sun. But that would keep the Earth much too cold for life as we know it to exist.

The atmosphere is why the Earth is warm enough to support life, for it contains water vapor, a gas that strongly absorbs infrared radiation. This water vapor is the key to why the Earth is warm enough to support life. According to the National Oceanic and Atmospheric Administration, removing water vapor from the atmosphere would reduce the average temperature of Earth to 0 degrees F! The oceans would be frozen solid. The Irish scientist John Tyndall was among the first to explain this, and in 1863 he wrote,

Aqueous vapour [water vapor] is a blanket, more necessary to the vegetable life of England than clothing is to man. Remove for a single summer night the aqueous vapour from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost.

You can find this and more of John Tyndall’s wisdom on pp. 417-418 of his book Heat.

You can use an infrared thermometer to see the impact of water vapor on warming the atmosphere. The temperature in outer space approaches absolute zero, which is -273 degrees Celsius. But you will measure a much warmer temperature if you point an infrared thermometer at the sky directly overhead (the zenith). Depending on the season and your location, the temperature will likely be near or below zero degrees Celsius. While this is very chilly, it’s far from being as cold as absolute zero. The difference is caused mainly by water vapor in the sky that has become warm by absorbing infrared radiation emitted by the Earth below. The warmed water vapor returns some of the infrared back to the Earth, and this helps keep the Earth warmer than space.

The ability of water vapor to warm the Earth is called the the greenhouse effect. Like a closed car on a hot day, a greenhouse warms the plants inside by trapping air heated by the sun. The natural greenhouse effect is based on a different phenomenon, for air is not trapped and is free to circulate. The sun warms the planet, which emits some of the heat as infrared radiation. Water vapor absorbs and traps some of this radiation, thus, warming the atmosphere. Carbon dioxide, methane and other gases that absorb infrared also contribute to the greenhouse effect, and they and water vapor are collectively known as greenhouse gases.

Photographs by Forrest M. Mims III.

Photographs by Forrest M. Mims III.

The two images above show an infrared thermometer pointed at the sky overhead (left) and a cumulus cloud (right) a few minutes apart on the same day. The sky temperature is just above freezing (34 degrees Fahrenheit or 1 degree Celsius). The cloud temperature is considerably warmer (65 degrees F or 18 degrees C). The cloud is warmer because it is much lower in the sky than the portion of the clear sky being measured at left.

The difference between the extreme cold of outer space and what you measure when you point an infrared thermometer at the sky is caused mainly by the water vapor in the atmosphere, which is warmed as it absorbs infrared emitted by the Earth. Water vapor strongly absorbs the infrared radiation emitted by the Earth.

Nearly all the water vapor in the atmosphere is found in the troposphere, the layer of the atmosphere between the surface and around 10 to 15 kilometers. The troposphere is where weather occurs. When an infrared thermometer is pointed straight up at the sky, it measures the temperature through a cone-shaped column of the troposphere. This means that the “sky temperature” indicated in the photo at left above represents the infrared brightness of the sky. For the purpose of this project, the “sky temperature” can be thought of as an average of the temperature between the ground and the upper troposphere where water vapor is much less abundant. Therefore, the “sky temperature” is really not the temperature of the sky but a number that indicates that the sky is much warmer than space and cooler than cumulus clouds. For this project, let’s call the temperature of the sky indicated by an infrared thermometer the “apparent temperature.”

The apparent sky temperature measurements in the photographs above were made in South-Central Texas on September 21, 2008. The temperature of the sky measured by a weather balloon launched from Del Rio, Texas, that morning (about 277 kilometers or 172 miles away) is shown by the blue line in the graph below.


The apparent temperature measured by the infrared thermometer is indicated by the circle, which corresponds to the temperature of the air at an elevation of about 4,000 meters. The temperature below 4,000 meters is warmer than the temperature above 4,000 meters. This explains why the infrared thermometer measures an “average” temperature that demonstrates the decline of temperature with elevation.

Note how the temperature falls sharply until an altitude of about 17,500 meters (57,415 feet). The temperature then begins to rise. This change marks the tropopause, the border between the troposphere, which is where most water vapor resides, and the very dry stratosphere above. The temperature increase in the stratosphere is caused by the ozone layer, which is warmed when it absorbs ultraviolet sunlight. View a lesson on radiosonde data that shows the troposphere.

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