why does descending air not allow for clouds to form

Clouds
are made of water droplets or ice crystals that are so small and light they are
able to stay in the air. But how does the water and ice that makes up clouds get
into the sky? And why do different types of clouds form? The
water or ice that make up clouds travels into the sky within air as water
vapor, the gas form of water. Water vapor gets into air mainly by evaporation в
some of the liquid water from the ocean, lakes, and rivers turns into water
vapor and travels in the air. When air rises in the atmosphere it gets cooler
and is under less pressure. When air cools, itвs not able to hold all of the
water vapor it once was. Air also canвt hold as much water when air pressure
drops. The vapor becomes small water droplets or ice crystals and a cloud is formed. Itвs
easier for water vapor to condense into water droplets when it has a particle
to condense upon. These particles, such as dust and pollen, are called
condensation nuclei. Eventually, enough water vapor condenses upon pieces of
dust, pollen or other condensation nuclei to form a cloud. Some
clouds form as air warms up near the ground and rises. Heated by sunshine, the
ground heats the air just above it. That warmed air starts to rise because,
when warm, it is lighter and less dense than the air around it. As it rises,
its pressure and temperature drop causing water vapor to condense. Eventually,
enough moisture will condense out of the air to form a cloud. Several types of
clouds form in this way including cumulus, cumulonimbus, mammatus, and
stratocumulus clouds.


Some clouds, such as lenticular and stratus clouds, form when wind blows
into the side of a mountain range or other terrain and is forced upward, higher
in the atmosphere. The side of the mountains that the wind blows towards is
called the windward side. The side of the mountains where the wind blows away
is called the leeward side. This can also happen without a dramatic mountain
range, just when air travels over land that slopes upward and is forced to
rise. The air cools as it rises, and eventually clouds form. Other types of
clouds, such as cumulus clouds, form above mountains too as air is warmed at
the ground and rises. Air is also forced upward at areas of low pressure. Winds meet at the center
of the low pressure system and have nowhere to go but up. Air is also forced
upward at weather fronts - where two large masses of air collide at the Earthвs
surface. At a warm front, where a warm air mass slides
above a cold air mass, the warm air is pushed upward forming many different types
of clouds в from low stratus clouds to midlevel altocumulus and altostratus
clouds, to high cirrus, cirrocumulus and cirrostratus clouds. Clouds that
produce rain like nimbostratus and cumulonimbus are also common at warm fronts. At a cold front, where heavy cold air mass
pushes a warm air mass upward, cumulous clouds are common. They often grow into
cumulonimbus clouds, which produce thunderstorms. Nimbostratus, stratocumulus,
and stratus clouds can also form at cold front.
he wind blows because of differences in air pressure from one location to another.


Wind blows from areas of high pressure toward areas of low pressure. If the high pressure area is very close to the low pressure area, or if the pressure difference is very great, the wind can blow very fast. What is Air Pressure? Imagine a group of acrobats at the circus. One climbs up and stands on another's shoulders. The weight of the acrobat on top puts more pressure on the one below. Then another acrobat climbs up and stands on the second acrobat's shoulders. Now there's even more pressure on the acrobat on the bottom because he is under the weight of the two acrobats above him. It's the same with air. Yes, air has weight, and probably more than you think. In fact, the weight of the air on your desk at school weighs about 11,000 pounds. That's about the same weight as a school bus! Since air pressure pushes in all directions, the air pressure pushing up from under your desk balances out the air pushing down on it, so the desk doesn't collapse under the weight. Just like an acrobat with two people stacked on his shoulders would want to move to where there wasn't so much pressure on him, air moves from areas where the pressure is higher to where it is lower. What causes Air Pressure? Air pressure depends on the density of the air, or how close together its molecules are. You know that a hard rubber ball is more dense than a Styrofoam ball and that ice cream is more dense than whipped cream.


Air lower in the atmosphere is more dense than air above, so air pressure down low is greater than air pressure higher up. (Remember those acrobats; there's a lot more pressure on the one on bottom than on the one on top. ) Temperature also makes changes in air pressure. In cold air, the molecules are more closely packed together than in warm air, so cold air is more dense than warm air. Since warm air is less dense and creates less air pressure, it will rise; cold air is denser and creates greater air pressure, and so it will sink. When warm air rises, cooler air will often move in to replace it, so wind often moves from areas where it's colder to areas where it's warmer. The greater the difference between the high and low pressure or the shorter the distance between the high and low pressure areas, the faster the wind will blow. Wind also blows faster if there's nothing in its way, so winds are usually stronger over oceans or flat ground. Meteorologists can forecast the speed and direction of wind by measuring air pressure with a barometer. Although wind blows from areas of high pressure to areas of low pressure, it doesn't blow in a straight line. That's because the earth is rotating. In the northern hemisphere, the spin of the earth causes winds to curve to the right (to the left in the southern hemisphere). This is called the coriolis effect. So in the northern hemisphere, winds blow clockwise around an area of high pressure and counter-clockwise around low pressure.

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