Clouds can take on all sorts of shapes and sizes, ranging from thin wispy clouds (cirrus) to large, dark menacing clouds (cumulonimbus). While there are several factors that influence and affect the formation of clouds, the sun plays a major role in producing clouds.
To help understand basic cloud formation, let’s take a look at a field at sunrise. In the morning, the field is relatively cool. The sun begins to heat up the field, and throughout the day, the field becomes warmer and warmer. Certain areas of the field may begin to heat up more quickly than others due to the terrain or surrounding conditions (for example, bare soil heats up more quickly than vegetation). When this happens, a thermal (also known as an updraft) can form. A thermal can be thought of as a rising “blob” of warm air due to unequal heating of the earth’s surface. When the thermal forms at the surface, it is warmer than the surrounding air. Warm air has a tendency to rise while cold air sinks, and since the thermal is warmer than the air around it, the air in the thermal will rise. As it rises, it will begin to expand and cool, and will continue to do so until its temperature is the same as the surrounding air temperature
Although we can’t see thermals with our eyes, we can feel and observe their effects. For example, many birds will use thermals to fly higher in the air. By catching a thermal, they do not have to expend as much energy to gain altitude since the rising air will carry them upwards. Hang gliders also use thermals to gain altitude and fly while gliding in the air.
The process of thermals forming in the atmosphere is a form of convection. Convection is basically the transfer of heat (in this case, through thermals) from one area to another. In our example, the heat was transferred from the surface into the atmosphere.
So how does convection help in the formation of clouds? As mentioned earlier, when a thermal rises, it begins to cool and expand. But why does it cool and expand as it rises?
Have you ever tried boiling water on a mountain? You may have noticed that the water boils faster on a mountain than as compared to boiling water at sea level. The air pressure is lighter on a mountain than at sea level, so water actually starts to boil below 212 °F! Air pressure decreases with height, and as the thermal rises, there is less pressure on it. Additionally, the internal energy inside the thermal wants to expand it. So as the pressure decreases while the thermal is rising, the thermal is able to expand more easily. However, by doing this, the thermal will begin to cool as its temperature is proportional to its internal energy.