How is air temperature, pressure and density related to cloud formation?

Answer 1

As air rises, it gets cooler due to abiabatic cooling. This would normally increase its density, because temperature is inversely related to density. But as air rises, its gases become subject to lower atmospheric pressure, which counteracts the effect of lower temperatures on density. Overall then, the density of the air decreases. As the air cools, its relative humidity increases; it cannot hold as much water as vapor, so the water condenses on any particles present (dust, etc.) to form clouds. The density within the clouds is greater than that of the surrounding air.

Answer 2

Although the answer above is correct, there is a bit more to it; the adiabatic effect. For example, your fridge pumps a coolant. The increase of pressure creates heat. Then it is released, inside the fridge, through a nozzle. The decrease of pressure creates cold. This is the adiabatic effect. As the sun heats the soil, it warms up a layer of air right above it. That air, being warmer is also lighter and it rises. As it comes into lesser pressure aloft, it cools down. The average adiabatic lapse rate on earth is 0.65 C per 100 meters (roughly 3 F per 1,000 ft). When the air reaches surrounding equal temperature, it stops rising. But here comes the interesting fact: As it reaches dew point temperature, moisture must condense as the colder it is, the less moisture the air can contain as vapor. This is how clouds are formed. But then comes an even more interesting fact: when moisture condenses, it releases heat energy. In fact, the so-called wet adiabatic lapse rate is half that of the dry one. Because of that, the air keeps rising in the cloud, thus forming the typical cumulus cloud where the base is at the altitude where you find the dew point temperature and each cauliflower-like top is one cell of warm air that has stopped rising. If there is enough heat and moisture near the ground, the cloud grows bigger and bigger until there is the droplets of water are big enough to overcome the rising by gravity on their mass and they come down as precipitation of rain or snow. Of course, rising air decreases the pressure and this is why low pressures are associated with clouds and rain. But it also creates wind as the surrounding air rushes in to even the pressure. The air that rose in a low pressure eventually comes down, creating regions of high pressure. Here, the adiabatic effect is inverted; the increase of pressure causes a warming of the air and clouds simply evaporate, giving place to a bright sunshine weather and very little wind. Sometimes the atmosphere is said to be unstable. This is when the measured adiabatic lapse rate is superior to the dry one. When it happpens, the air rises all the way to the top of the troposphere; what is called the tropopause. Because it rises so much, it becomes big cumulonimbus clouds and the friction of the ice crystals in the air causes static electricity. This is when we get thunderstorms. Sometimes the "falling" rain is taken so fast upward that the droplets freeze into pellets of ice. When it finally comes down, it is as hail.

Answer 3

Let's say you have a mass of air down near the ground. It will warm up from heat radiated from the Earth. Warm air spreads out and becomes less dense. Because it is less dense than air nearby, it will rise. As it rises, it cools off. When it cools, the amount of water the air can hold goes down, so the humidity in the air goes up. When the humidity is high enough, clouds form.