View Full Version : Clouds
2003-Sep-22, 02:50 PM
Well how do they do it, how can clouds have a shape when there is so much turbulence? The wind should be able to disperse any attempt at cloud formation. What keeps clouds together?
2003-Sep-26, 05:49 PM
Clouds exist becuase of turbulance, not in spite of it. They are the result of large scale updrafts and what not. There are many size scales to turbulance. A thunderhead cloud is a very large scale turbulent effect. A small white pufffy cloud is a smaller scale effect. A high sheet of cirus cloud is the effect of the interface of huge layers of air masses, which are forming cells tens if not hundereds of miles across.
Fractals are the attempt to quantify a phenomon seen in nature where basically you have a constant level of detail across a logrithmic space. That is, (in standard units), you have the same level of detail at the foot level as at the 10 foot, 100 foot, 1000 foot, 10,000 foot and so on level. That is why if you ever fly in an airliner over Arizona the moutains look exactly like a patch of mud that you sprayed a hose on. Turblent air has a similar phenomenon. There are the inch-scale detail you feel standing in the wind, and the hundred of mile scale detail of a hurricane. Clouds are an effect that lets you see some of this in the air.
2003-Sep-26, 10:49 PM
Clouds are formed because of turbulence you say, then how come that there is a lower level of cloud formation? They don't form below approximately 1000 meters at sea level (below that only mist exists). Why aren't clouds dispersed by the wind, that's what happens at ground level. At ground level there are no clouds, only layers of mist. There are also many patterns that you can see in clouds, they reshape time and again but they look the same. You talk about air cells, but what makes air behave like cells? What keeps these boundaries between cells intact? Air is a mass of molecules that can be moved around, water vapour can condense into droplets, but why the distinctive shapes? I don't see any reason why a cloud should form, to me everything should look more homogenous,like mist swirling around, driven by wind.
2004-Oct-23, 07:11 PM
Clouds form when water vapor (water that has evaporated from the surface of the Earth) condenses (turns into liquid water or solid ice) onto microscopic dust particles (or other tiny particles) floating in the air. This condensation (cloud formation) happens when warm and cold air meet, when warm air rises up the side of a mountain and cools as it rises, and when warm air flows over a colder area, like a cool body of water. This occurs because cool air can hold less water vapor than warm air, and excess water condenses into either liquid or ice.
Low clouds are of mostly composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters). However, when temperatures are cold enough, these clouds may also contain ice particles and snow. Fog is actually very very low Stratus clouds
The bases of mid-level clouds typically appear between 6,500 to 20,000 feet (2,000 to 6,000 meters). Because of their lower altitudes, they are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are cold enough.
High-level clouds form above 20,000 feet (6,000 meters) and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.
Check out my sources at http://www.enchantedlearning.com/subjects/...s/earth/clouds/ (http://www.enchantedlearning.com/subjects/astronomy/planets/earth/clouds/)
and my favorite http://www.sundog.clara.co.uk/atoptics/phenom.htm
2009-Nov-14, 05:19 PM
Well how do they do it, how can clouds have a shape when there is so much turbulence? The wind should be able to disperse any attempt at cloud formation. What keeps clouds together? Anyone?
Most water clouds do not quickly disperse like dust or smoke does. A cloud can remain intact in the sky when updrafts are negligible and the air mass around it is relatively unlayered with a near-uniform temperature. A cloud is kept together mainly due to the tendency of one water molecule to stick to another water molecule due to van der Waals forces. This is why a cloud has a temporary shape and structure. Van der Waals forces between water molecules are also responsible for cloud formation. Dipole-dipole intermolecular bonds between water molecules result in long curved strands of water molecules. A strand of molecules can branch off into multiple strands. When clouds get dark, the strands get bunched together tightly like spaghetti in a bowl. Eventually, the strands become wound so closely together that the water vapour condenses and it rains.
2009-Nov-16, 08:11 PM
You talk about air cells, but what makes air behave like cells?
PeterG and chuckb are generally right in their explanations. This topic is difficult to fully explain without giving some kind of crash course in meteorology.
These "cells" are warm parcels of air (thermals) which are formed due to differential heating of the earth's surface by the sun. The warmer sections (heated by conduction) will break away from the surface due to their higher buoyancy and rise. As they rise, they will cool at the dry adiabatic lapse rate of about 2 degrees Celsius per 1000 ft. When they reach the condensation level (which depends on their moisture content), clouds will form. Also, the latent heat of vaporization of water will slow the rate at which the cells cool and render them more buoyant relative to the surrounding air mass. If the surrounding air mass is unstable (temperature lapse rate greater than the dry adiabatic lapse rate) or marginally stable, it will continue to rise until it reaches a strong temperature inversion (very stable air) or until it becomes a thunderstorm. This naturally depends on the actual temperature lapse rate of the surrounding air mass which can vary drastically with altitude and also varies from day to day.
In fact, the real mystery should be why do deserts often have cloudless skies when there is clearly differential heating of the ground due to the sun.
The answer is because the rising thermals over deserts often reach a strong temperature inversion and stop rising long before they can get to the condensation level where they form clouds. This is what is happening whenever there are clear blue skies all day and is often referred to as a "blue thermal day".
Here (http://www.mike-the-strike.net/Soaring_Forecast/thermal_forecasting.htm) is a link that gives a very brief explanation.
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