|All materials consist of many small particles, the molecules. Air, the earth - everything which surrounds us. The aggregation state of a material - solid, liquid or gaseous - depends upon how closely these molecules are bound together. Take the example water. Water freezes below 0°C, thus becomes solid. The molecules move closer and connect together. Starting from a temperature of 0 °C (fusion point) to 100°C (boiling point), water is liquid. The water molecules are in a somewhat looser grouping and move relative to each other. Starting from the boiling point, the water evaporates: it becomes gaseous. The molecules are now very far apart from each other and have nearly no more cohesion. They move about faster than in the liquid or solid state. This condition requires more energy. One calls this also heat energy, which we measure with the temperature.|
As previously mentioned, this change in material state changes the density and depends on „the tightness “ with which molecules stay and bind together. Solid bodies have thus a higher density than gaseous materials, because their molecules stay more closely together. In the atmosphere, most molecules are air. Air protects the earth from the sun and from space.
In the accompanying pictures one can see, how the Sunlight passes the atmosphere and reaches the earth surface. The molecules of the ozone layer filter a part of the sun light, which does therefore not arriveon the earth's surface. Most sun light reach the ground, however, the density of air is small.
On the earth surface, the sun light is to a large extent taken up (absorbed) by the ground. Thus, the ground warms up. A smaller part of the sun light is thrown back again into the atmosphere (reflected).
Air at ground level is now heated by the ground. Since the air molecules move apart when heating up, air expands and looses density - the heated air rises. Since the pressure at the ground is higher than in the atmosphere, ascending air can expand further with the altitude. During this expansion heat energy is lost. Ascending warm air cools down. It hardly continues to warm up due to the increased distance to the ground.
Also, the reflected and partially long-wave radiation has less energy and does not heat the air as much as the incoming radiation. Further, air contains fewer molecules by volume, which could be met by the sun light, due to the expansion.
Therefore, lower temperatures prevail the higher layers of air (see Meteogram to the right).
If it is cloudy, the water molecules in the cloud absorb a large part of the sun light. Only a small part of the sun light then reaches the ground. Therefore, it is often cool under the clouds, even with high sun radiation.
Air elevation by high temperature:
If the sun radiation is very strong, air warms up more rapidly at the ground and rises. It displaces cooler air from higher layers and expands at the same time, since the air pressure decreases with altitude. The rise of air temperature and warming of higher atmosphere layers during the day is shown in an animated AirMeteogramm to the right.
If more and more air warms up and rises, a kind of "air bubble" develops, which can expand the whole trophosphere (lower layer of the atmosphere) upwards. This is one of the reasons that the troposphere is thicker in the Tropics than in the Polar zones.
An inversion is a situation where air temperature in higher layers is higher than in lower layers of the atmosphere.
This is caused by the process of heating. The hot air rises and then stays above the cool air. For an inversion to happen, something must either prevent the heated air from cooling when it is in a higher layer, or something must cool the air below more rapidly than the air above. Causes for such an inversion are:
Examples of an inversion due to cold air flow can be seen in the sample meteogram (right): The flow of cold air from nearby mountains creates a cold air cushion above the valley bottom at 970 meters (green bar) in about 2000 meter altitude, above the surrounding terrain (brown line).