How do clouds form when it's not above 100 C in the sky?

Question

As water condenses below 100 C and it is really cold high up in the sky why don't all clouds just condense as rain the second they form?

Answer 1

You misunderstand several things. Condensation does not mean rain - when water vapor condenses it does so on tiny particles (nuclei) and forms small cloud droplets. At virtually all locations there are sufficient nuclei for this to occur. These droplets grow a bit more via diffusion of vapor onto them, and collide with others and perhaps ice crystals to form the much larger raindrops.

Regarding why clouds form below 100 °C:

Air consists of a mixture of gases - one of which includes water vapor. Saturation occurs when vapor reaches a certain pressure, as can be seen here:

http://www.decatur.de/javascript/dew/index.html

So whether or not a cloud forms depends on whether the actual vapor pressure in the air reaches saturation vapor pressure. That is what relative humidity refers to. For example, suppose the temperature of air is 20 °C. The calculator shows that the saturation vapor pressure at that temperature is 2.339 kPa (23.39 mb). Suppose the dewpoint is 14 °C. The claculator shows that the actual vapor pressure is 1.599 kPa (15.99 mb). Thus the relative humidity is 1.599/2.339 = 68% - the % of water vapor relative to saturation. To saturate air, the relative humidity must reach 100% - i.e., the actual vapor pressure must reach the saturation vapor pressure (actually a tiny bit greater to form cloud droplets primarily because of their curvature). This is accomplished raising the air. As air rises it cools, and cooler air becomes saturated at a lower vapor pressure - so it eventually reaches that point and a cloud is formed - called the lifted condensation level:

http://www.shodor.org/os411/courses/_master/tools/calculators/lcl/lcl1calc.html

In the case above, the temperature at which that would occur depends on what pressure it started rising at (assuming no mixing with the environment), but would be 12.6 °C from a typical ground level of 970 mb. Note the dewpoint also decreases as air rises, but not as much (they become equal at saturation).

100 °C is the boiling point of water at standard atmospheric pressure of 101.325 kPa (1013.25 mb). What that means is that all water (raindrops, etc.) boils to vapor at that temperature and cannot condense - so clouds can't form.

Answer 2

See the link to learn more
http://www.srh.noaa.gov/srh/jetstream/synoptic/clouds.htm

Clouds form when air is cooled to its dewpoint or the temperature, if the air is cooled, it reaches saturation. Air can reach saturation in a number of ways. The most common way is through lifting. As a bubble or parcel of air rises it moves into an area of lower pressure (pressure decreases with height). As this occurs the parcel expands. This requires energy, or work, which takes heat away from the parcel. So as air rises it cools. This is called an adiabatic process.

The rate at which the parcel cools with increasing elevation is called the "lapse rate". The lapse rate of unsaturated air (air with relative humidity <100%) is 5.4°F per 1000 feet (9.8°C per kilometer). This is called the dry lapse rate. This means for each 1000 feet increase in elevation, the air temperature will decrease 5.4°F.

Since cold air can hold less water vapor than warm air, some of the vapor will condense onto tiny clay and salt particles called condensation nuclei. The reverse is also true. As a parcel of air sinks it encounters increasing pressure so it is squeezed inward. This adds heat to the parcel so it warms as it sinks. Warm air can hold more water vapor than cold air, so clouds tend to evaporate as air sinks.

Answer 3

Sometime wator vapor won't condense unless it has a surface on which to condense. Think of your breath on a cold day. Does the vapor condense and fall on your shoes? Of course not! Also, take a look at a water phase diagram. The temperature where vapor turns to liquid varies due the air pressure.

Answer 4

Water vapor rises as it is ligher than air and unless there are enough nuclei (like dust particles or some other solid particles) condensation may not take place. Hot air currents keep the clouds from condensing till the appropriate atmospheric conditions exist.

Answer 5

Temperature isn't the only cause of changes in state of matter- pressure also plays a major role.