- Atmospheric moistude:
Air circulating over large bodies of water picks up large amounts of moisture that evaporates from the surface of the water.
This effect is stronger when the water is warm (during the summer, or over the tropics).
Air circulating over large forests also picks up moisture that plants give off by a process called transpiration.
As the air rises, it cools, and the moisture condenses into billions of tiny droplets (mostly microscopic in size) to form clouds.
- Humidity:
The amount of moisture in the air is called humidity.
Absolute humidity can be measured in liters of moisture per thousands of cubic meters of air.
More often, we measure relative humidity.
At any given temperature, the air is capable of holding a certain maximum amount of moisture, at which point it is said to be saturated.
Relative humidity, expressed as a percent, is the ratio between the moisture in the air and the maximum that the air could hold if saturated.
Air below 50% relative humidity generally feels dry because your sweat evaporates rapidly and cools you off.
Air above 80% relative humidity feels moist,
and air above 90% humidity feels very moist.
Relative humidity can be measured by an apparatus consisting of two thermometers, one of them partially wrapped in fabric that has been dipped in water.
The apparatus is swung around in circles
to evaporate the water from the wet bulb, and then the wet-bulb and dry-bulb thermometers are both read.
A table is then used to calculate the relative humidity.
Dew point: If moist air is cooled, the microscopic droplets of moisture will begin to collect on each other or on any available dust particle.
These droplets will then condense and form liquid water in the form of rain or ice crystals in the form of snow.
The temperature at which this occurs is called the dew point.
Dew points can also be read from charts using the information from wet-bulb and dry-bulb thermometers.
Rain shadows:
If winds blow moist air against mountains, the air is forced to rise. The temperature at higher altitudes is lower, so the air may reach
its dew point, and deposit the moisture on the upwind ("windward") slopes of the mountains.
Once the air crosses the mountains, it is much drier, so it picks up and absorbs a lot of moisture as it descends down the downwind (or "lee") side.
This creates a "rain shadow" on the windward side of the mountains, and often a desert on the downwind or lee side,
especially in places where the prevailing winds consistently blow in the same direction all the time.
Hawaii, for example, receives Trade Winds that blow quite consistently from East to West, so the Eastern (windward) slopes of its mountains
are heavily forested with moist, tropical forests, while the downwind (lee) sides of the same mountains
are covered by desert or semidesert regions.
In California, the prevailing winds blow West to East, and moisture from the Pacific Ocean is deposited as heavy snowfall on the
western (windward) slopes of the Sierra Nevada mountain range, while most of Nevada, just beyond these mountains, is a desert.
- STORMS:
Storms generally begin as low pressure areas that gather up moisture over large bodies of water.
Storms traveling over open water keep picking up more moisture.
If a storm travels over land, or towards a colder air mass or a higher altitude or latitude, it may cool below its dew point and begin to fall as Precipitation.
Liquid precipitation is called rain; frozen precipitation is called snow.
Raindrops or snowflakes tend to form around microscopic dust particles or other "seed crystals", and they tend to grow larger once they form.
Electrical discharges from cloud to cloud (less often from cloud to ground) emit both bright light (lightning) and loud sound thunder).
CYCLONES:
These large, tropical storms are called Hurricanes in the Atlantic and Eastern Pacific, and Typhoons in Asia.
Hurricanes grow in strength and size as they linger over warmer waters. They lose strength over colder water or over land.
Hurricanes generally lose their water once they come onto land. The massive rainfall often causes flooding.
Hurricanes are hundreds (sometimes over a thousand) miles wide. Most of the damage they cause is by flooding or especially by storm surge.
A storm surge is a large mass of ocean water, tens of feet high, driven ashore (and far inland) by the hurricane.
TORNADOES are very intense, fast-moving low-pressure cells that occur in the late spring and early summer in certain inland locations.
Kansas, Missouri, Oklahoma, and Arkansas receive the most tornadoes and are sometimes called "Tornado Alley" for this reason.
Tornadoes can also spin off the advancing margins of hurricanes.
Air circulation within a tornado is rotational and cyclonic (counter-clockwise in the Northern Hemisphere).
Because of their extremely low pressure inside, tornadoes can rip off roofs and pick up loose objects as large as cars and house trailers.
Tornadoes are very intense but very local, seldom more than a mile wide (or just a few city blocks), and usua;lly no longer than 50 miles.
Tornadoes can completely demolish houses, but leave houses untouched a block or two away.
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