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How Condensation Occur
Scientists refer to rain, sleet, snow, hail, and mist as precipitation. Precipitation can occur only when large masses of air are go1ng through steady drop in temperature.
But how does an air mass lose enough heat to bring about condensation?
An air mass loses a certain amount of heat during the night. It radiates heat to outer space. But its greatest loss of heat occurs when it rises to a higher elevation. Rising air drops in temperature.
As the air rises, it expands. It expands because there is a decrease in air pressure at high altitude. The pressure is great enough to compress the air, or to keep it small space.
The gas molecules in the expanding air spread out. The spread-out molecules do not strike each other so frequently. They no longer bounce off one another at great speed. ` Heat, remember, is energy that comes about through the motion of molecules. A piece of iron or a quantity of Water becomes hot when its molecules begin to move faster than normally. Temperature is a measure of this molecular motion.
As the molecules slowdown, iron or Water loses heat. It is the same with air. Rising, expanding air gives up heat. Its g as molecule slows down. The temperature of the air falls.
Now, we come to another important question. What causes air m to rise? Why do we have an upward movement of air in the first place? Why does a pocket of air near the ground not stay at the surface?
Convection and Up-Drafts
Warm air seeks higher elevation because it IS lighter than surrounding air. On a warm morning, the sun Shines down. It heats up the surface of the earth. But different parts of the earth are heated to different temperature.
A field bare ground, for example, heat up more rapidly than a wood land. The bare field warms the overlying air. The air over the patch of land begins to rise in a tall column.
As the air rises, it cools. Eventually the rising air reaches the same temperature as the surrounding air. It comes to a rest.
Sometimes the rising air cools to the dew point before it comes to a rest. The dew point is the temperature at which water vapour begins to condense from air. as the vapour condenses, clouds begin to form. In time there may be precipitation. Even as clouds form the air may continue to rise. Sometimes the convection column gives birth to a cumulonimbus cloud. A cumulonimbus cloud is a thunderhead. Heavy rain falls from such a cloud; you can do an experiment to observe a convection up-draft, or rising current of Warm air.
The air above the hot plate is warmed. Cold air is heavier than Warm air. The cold air beyond than hot plate sinks and moves into the area of the hot plate. The warm air above the hot plate rises and produces an up-draft. The up-draft turns the paper spiral.
Mountains to Climb
Some moving air masses must flow over mountains. The side of a mountain next to the approaching air is called the windward side. The opposite side of the mountain is called the lee side.
Suppose an air mass crosses an ocean before it comes to the mountain. Such a mass contain a great deal of moisture. As it rises to get over the mountain, it cools. With this drop in temperature, the air mass gives up moisture. The precipitation falls on the windward side of the mountain. The air mass passes over the mountain top. It moves down to the lee side.
As it falls to a lower elevation, tie air warms up. This warm air becomes dry. It now can pick up moisture. It pied up moisture from the surrounding landscape. In north-western United States, these drying winds are called Chinook winds.
Air tends to move across the earth in large piles known as air -masses. An air mass takes shape over a speciﬁc geographical area; It can form because the atmosphere above this region is remaining quiet for a time. The huge mass of air takes on the temperature and humidity typical of the region over which it forms. Scientists classify air masses according to their source, or to the region over which they form. Thus, we have cold polar (P) air masses and warm tropical (T) air masses. The letters “c” and “m” are used along with the symbols “P” and “T” to show whether the air mass forms over land or water. For example, if a polar air mass forms over land, the symbol is cP. The “c” stands, f continental.” If the air mass forms over water, its symbol is mP. The “m” stands for “maritime,” Which signiﬁes water.
There are also continental tropical air masses. Their symbol is cT. Then, too, some tropical air masses are formed over water. Their symbol is mT". In all, there are four principal kinds of air masses, with the symbols mP, mT, cP, and c'T. An air mass seldom remains over the area in which it is formed. Instead, it is carried along by the general movement of the atmosphere. As it moves into a different part of the earth, the air mass often changes.
Suppose a cold polar air mass moves over a warm sea. Its lower air becomes warm. The Warm air moves up within the mass. Clouds form. Rain or snow falls.
Over North America
Seven great air masses move across North America. These air masses have a great inﬂuence on the weather in the United States.
The seven North American air masses (listed with their classiﬁcation symbols) are known as Polar Canadian (cP), Polar Paciﬁc (mP), Polar Atlantic (mP), Tropical Continental (cT), Tropical Gulf (mT), Tropical Atlantic (mT), and Tropical Paciﬁc (mT).
Each of these air masses partially determines the weather pattern within an area of the United States. The Polar Canadian air mass brings wintertime cold waves to northern states. Polar Paciﬁc air masses spread rain and fog along the west coast during the summer.
At times polar air masses and tropical air masses run into each other. Cold air and Warm air masses, you might say, meet in conﬂict. These unlike air masses do not mix freely. Each air mass remains separate, apart, and clearly deﬁned. The points at which the air masses touch each other are called fronts. At the fronts there is condensation. Frequently there is rain or some other form of precipitation.
There are two kinds of fronts:
(1) a cold front and
(2) a Warm front.
Both kinds of fronts bring stormy weather they differ from each other in the way the air flows.
In cold fronts, a cold air mass pushes actively forward against a warm air mass. Or, you might say, the cold air move against the warm air zone. The air is heavy. It hugs the ground. The cold air forces the warm air up.
A cold front usually has a steep slope to it. Condensation occurs as the warm air is forced up the steep slope. Cumulonimbus clouds form. Violent thunderstorms sometimes develop. Precipitation may cover more than 50 miles.
In a warm from! The warm air mass pushes actively against a cold air mass. Or, to put it another Way, the warm air moves into a region of cold air. Again the cold air remains close to the ground. But the rise of the warm air mass is along a gentle slope. You might say the warm air goes up a ramp. This ramp might be 600 to 700 miles long. As the warm air rises, it cools. There is condensation.
In all probability, there will be some form of precipitation. Rain or snow often falls over an area of more than 150 miles. High, feathery clouds often foretell the approach of a warm front. Such clouds are known as cirrus clouds. They are high in the sky. Cirrus clouds often soar at altitudes of 40,000 feet. As the front moves in.