why is there clouds in the sky

Douglas Wesley, a senior meteorologist in the Cooperative Program for Operational Meteorology, Education and Training (COMET) at the University Corporation for Atmospheric Research, explains:
Clouds are composed primarily of small water droplets and, if it's cold enough, ice crystals. The vast majority of clouds you see contain droplets and/or crystals that are too small to have any appreciable fall velocity. So the particles continue to float with the surrounding air. For an analogy closer to the ground, think of tiny dust particles that, when viewed against a shaft of sunlight, appear to float in the air. Indeed, the distance from the center of a typical water droplet to its edge--its radius--ranges from a few microns (thousandths of a millimeter) to a few tens of microns (ice crystals are often a bit larger). And the speed with which any object falls is related to its mass and surface area--which is why a feather falls more slowly than a pebble of the same weight. For particles that are roughly spherical, mass is proportional to the radius cubed (r ). Thus, as a tiny water droplet grows, its mass becomes more important than its shape and the droplet falls faster. Even a large droplet having a radius of 100 microns has a fall velocity of only about 27 centimeters per second (cm/s). And because ice crystals have more irregular shapes, their fall velocities are relatively smaller. Upward vertical motions, or updrafts, in the atmosphere also contribute to the floating appearance of clouds by offsetting the small fall velocities of their constituent particles. Clouds generally form, survive and grow in air that is moving upward.


Rising air expands as the pressure on it decreases, and that expansion into thinner, high-altitude air causes cooling. Enough cooling eventually makes water vapor condense, which contributes to the survival and growth of the clouds. Stratiform clouds (those producing steady rain) typically form in an environment with widespread but weak upward motion (say, a few cm/s); convective clouds (those causing showers and thunderstorms) are associated with updrafts that exceed a few meters per second. In both cases, though, the atmospheric ascent is sufficient to negate the small fall velocities of cloud particles. Another way to illustrate the relative lightness of clouds is to compare the total mass of a cloud to the mass of the air in which it resides. Consider a hypothetical but typical small cloud at an altitude of 10,000 feet, comprising one cubic kilometer and having a liquid water content of 1. 0 gram per cubic meter. The total mass of the cloud particles is about 1 million kilograms, which is roughly equivalent to the weight of 500 automobiles. But the total mass of the air in that same cubic kilometer is about 1 billion kilograms--1,000 times heavier than the liquid! So, even though typical clouds do contain a lot of water, this water is spread out for miles in the form of tiny water droplets or crystals, which are so small that the effect of gravity on them is negligible. Thus, from our vantage on the ground, clouds seem to float in the sky. Answer originally posted May 31, 1999 What are clouds and how do they form? Ever wondered what clouds are, the different types and how they form?


Clouds are made of tiny drops of water or ice crystals that settle on dust particles in the atmosphere. The droplets are so small - a diameter of about a hundredth of a millimetre - that each cubic metre of air will contain 100 million droplets. Clouds will either be composed of ice or water droplets depending on the height of the cloud and the temperature of the atmosphere. Because the droplets are so small, they can remain in liquid form in temperatures as low as -30 `C. Extremely high clouds at temperatures below -30 `C are composed of ice crystals. How do clouds form? Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. There is water around us all the time in the form of tiny gas particles, also known as water vapour. There are also tiny particles floating around in the air - such as salt and dust - these are called aerosols. The water vapour and the aerosols are constantly bumping into each other. When the air is cooled, some of the water vapour sticks to the aerosols when they collide - this is condensation. Eventually, bigger water droplets form around the aerosol particles, and these water droplets start sticking together with other droplets, forming clouds. Clouds form when the air is saturated and cannot hold any more water vapour, this can happen in two ways: The amount of water in the air has increased - for example through evaporation - to the point that the air cannot hold any more water. The air is cooled to its dew point - the point where condensation occurs - and the air is unable to hold any more water.


The warmer the air is, the more water vapour it can hold. Clouds are usually produced through condensation - as the air rises it will cool, and reducing the temperature of the air decreases its ability to hold water vapour so that condensation occurs. The height at which the dew point is reached and clouds form is called the condensation level. What causes clouds to form? 1. Surface heating - This happens when the ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals. Surface heating tends to produce cumulus clouds. 2. Topography or orographic forcing - The topography - or shape and features of the area - can cause clouds to be formed. When air is forced to rise over a barrier of mountains or hills it cools as it rises. Layered clouds are often produced this way. 3. Frontal - Clouds are formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts. A front is the boundary between warm, moist air and cooler, drier air. 4. Convergence - Streams of air flowing from different directions are forced to rise where they flow together, or converge. This can cause cumulus cloud and showery conditions. 5. Turbulence - A sudden change in wind speed with height creating turbulent eddies in the air. The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds. To find out more about different types of clouds and how you can identify them, read our cloud spotting guide.

  • Views: 20

why does it hail instead of snow
why does a small pebble sink in water
why does descending air not allow for clouds to form
why do rain bearing clouds look black
why is there a halo around the moon tonight
why is there a circle around the sun
why fog droplets appear to be suspended in air