Clouds reflect light

The atmosphere has an influence on the light

As we learned in the first lesson on the topic 'Lower Atmosphere', air consists of different gases. In addition, particles float in the air, small particles that we also call aerosols. Last but not least, there is a lot of water in the air, which sometimes forms small droplets and turns into clouds.
During a sand storm, when the sun turns pale from the dust, or on a rainy day, when clouds cover the sky, it is much darker than a clear sky with no clouds and clean air. So it is obvious that the amount of light reaching the earth can change. But the invisible gases in the air also have an influence on the light that comes from the sun.


1. All energy comes from the sun.


Energy is in balance

When sunlight reaches the earth, it warms up. The water of the oceans gets warmer in summer. Roads can even heat up so much that they get too hot to walk on with bare feet. The warm earth also emits energy again, back into space. But this radiation is different from that which comes from the sun. Sunlight is visible or ultraviolet light. The radiation emitted by the earth is not visible. We call it infrared light or long-wave radiation, also known as thermal radiation. It has less energy than sunlight.

  • We need to keep one important rule in mind:

If the earth did not send back radiation as well, all energy coming from the sun would accumulate and the earth would become hotter and hotter. But this is not the case. Energy is in balance. This means: The earth gives back what it received.
Radiant energy comes from the sun. We call it light and show it in yellow in the following picture. But radiation is also emitted from the earth. We call them infrared light, infrared radiation or long-wave radiation and show them in red in the following picture.


2. What happens to the radiation? We explain it in ten points.
Author: Elmar Uherek

A flash animation shows the process step by step. Developed and made available by: Florian Thierfeldt.


What happens when radiation passes through the atmosphere?

First, let's look at what happens to sunlight:

(1) The sun is the source of all radiation and energy that reaches the earth from space.
(2) Part of the sunlight reaches the earth's surface with all its different landscapes: forests, oceans, deserts, savannahs, cities, ice and snow.
(3) The earth does not absorb all of the solar energy, but reflects a certain amount. Reflection means that the radiation is sent back without the earth warming up. Particularly very bright surfaces (e.g. ice and snow) are very good reflectors.
(4) But reflection does not only take place on the surface of the earth. Some of the light is already reflected on the top of the clouds, and smaller portions also from aerosols.
(5) The absorption of light (we call it absorption) does not only take place on the surface of the earth. The gas molecules and particles in the air also absorb sunlight.

The part of the sunlight that reaches the earth warms its surface. The earth sends back long wave rays. We now want to see what happens to this thermal radiation.

(6) The surface of the earth warmed up by solar radiation is a source of thermal radiation (long-wave infrared radiation).
(7) Some of this energy is needed to evaporate water. As we know from a kettle, for example, we need energy to bring water from a liquid state (e.g. in the oceans) to a gaseous state (water vapor in the air).
(8) Part of the infrared radiation goes straight back into space, but this part is small.
(9) Clouds don't just reflect sunlight. They also absorb the earth's radiation and radiate back again. A cloudy sky keeps the earth warmer like a blanket.
(10) After all, there are various gases and particles in the air that absorb infrared radiation. We call these gases greenhouse gases. They hold the energy of thermal radiation close to the earth's surface.

All of these interactions between light and atmosphere take place in our climate system and we have to take them into account in order to understand it. But why are we talking about a greenhouse effect?


3. The greenhouse effect - compare the interaction with light in a greenhouse and on earth!
Original source: NOAA

The role of greenhouse gases in the atmosphere can be compared to the glass in a greenhouse. The glass lets in the sunlight and the light warms the soil and plants in the greenhouse. These in turn emit thermal radiation. If this reaches the glass, however, it cannot penetrate like sunlight, but the glass * sends part of the radiation back into the greenhouse, in which it becomes warm. This is exactly what the greenhouse gases in the atmosphere do: They let the sunlight in, but the heat radiation of the earth no longer out.

* We have to note here that glass is a solid and therefore also a barrier for warm air currents. This is not so in the atmosphere, so the greenhouse is not a perfect model.

Response of the atmosphere to increased greenhouse effects

If the greenhouse effect is increased by an increase in greenhouse gases, then the climate adapts to it. The mean temperatures close to the ground increase compared to a natural, undisturbed system such as we had before the industrial age. In the stratosphere, however, the temperature drops.

4. The temperature profile of the atmosphere adapts to climate change. Source: modified from IPCC AR4 2007

About this page:
Author: Dr. Elmar Uherek - MPI for Chemistry, Mainz
scientific reviewer: Dr. Pascal Guyon - MPI for Chemistry, Mainz
educational reviewer: Roland Bergmann + students, Velbert comprehensive school
Last revised: 2007-08-20