THE WATER CYCLE

• Water cycle the continuous movement of water from the ocean to the atmosphere to the land and back to the ocean
• Water is continuously being moved, primarily between the oceans and the continents.
• Evaporated water vapor condenses to form precipitation.
• Evaporation occurs when solar energy heats water molecules, and they rise as gaseous water vapor
• Transpiration the process by which plants release water vapor into the air through their leaves
• Precipitation any form of water that falls to Earth’s surface from the clouds
• Air contains varying quantities of water vapor.
• Humidity the amount of water vapor in the air.
• Relative humidity is the actual amount of vapor in the air compared to the maximum amount the air could hold at that temperature.
• Air that has a relative humidity of 100% is said to be saturated.
• Warmer temperatures evaporate more water.

Warm air can hold more water vapor than cold air can

• Water vapor becomes liquid at dew point.
• Dew point is the temperature at which air or a gas begins to condense to a liquid
• When humidity is high, there are more molecules of water in the air and it is easier to form liquid.
• The higher the humidity, the higher the dew point.
• Clouds form as warm, moist air rises.
• Clouds form when warm air rises and water vapor condenses into tiny droplets of liquid as it cools.
• This process usually occurs in the troposphere.
• Cloud names describe their shape and altitude.
• Clouds are named with combinations of three root words:
• cirrus
• stratus
• cumulus
• Cirrus clouds are thin, wispy, and occur at high altitudes
• Stratus clouds are sheetlike and layered
• Cumulus clouds are white and fluffy with somewhat flat bottoms

Cloud names reflect combined characteristics.

• Cirrostratus clouds are high, layered clouds that form a thin white veil.
• Altostratus and altocumulus clouds are stratus and cumulus clouds that occur at middle altitudes.
• Cumulonimbus clouds are towering rain clouds that often produce thunderstorms.
• Nimbostratus clouds are large, gray clouds that often produce steady precipitation.
• Barometric pressure the pressure due to the weight of the atmosphere; also called air pressure or atmospheric pressure
• Changes in barometric pressure often accompany changes in the weather.
• Falling pressure may indicate that a large air mass is leaving the area.
• Rising air pressure may mean that an air mass is moving in.
• Instruments used to measure air pressure are called barometers.
• At sea level, the barometric pressure of air at 0 C is around 760 mm of mercury.
• 760 mm of mercury is defined as 1 atmosphere (1 atm) of pressure
• Differences in pressure create winds.
• When air pressure varies from one place to another, a pressure gradient exists.
• The air in a pressure gradient moves from areas of high pressure to areas of low pressure.
• This movement of air from a high-pressure area to a low-pressure area is called wind.
• Earth’s rotation affects the direction of winds.
• Coriolis effect the curving of the path of a moving object from an otherwise straight path due to Earth’s rotation.
• Points at different latitudes on Earth’s surface move at different speeds.
• Earth goes through a full rotation in 24 hours.
• Points on the equator travel the Earth’s full circumference in 24 hours.
• Points closer to the poles do not travel as far.
• Predictable air circulation forms wind patterns.
• Winds in the Northern Hemisphere curve clockwise.
• Winds in the Southern Hemisphere curve counterclockwise.
• The resulting patterns are very regular, and have been named by meteorologists.
• Polar easterlies
• Westerlies
• Northeast trade winds
• Southeast trade winds
• Global wind patterns form circulation cells.
• Because temperatures closer to the equator tend to be warmer, air traveling toward the equator tends to rise.
• Warm rising air tends to move toward the poles.
• As air moves closer to the poles it cools and sinks.
• Three loops of rising warm air and sinking cold are can be found in each hemisphere
• These loops are called cells.
• Meteorologists predict weather by tracking the movement of air pockets called air masses.
• Air mass a large body of air where temperature and moisture content are similar throughout
• Interactions between air masses have predictable effects on the weather in a given location.
• Front the boundary between air masses of different densities and usually different temperatures
• Clouds, rain, and sometime snow can occur at fronts.
• The three types of fronts are:
• warm front
• cold front
• stationary front
• Lightning is a discharge of atmospheric electrical energy.
• Water droplets and ice crystals in thunderclouds build up electrical charges.
• Sparks, called lightning, jump between clouds or between clouds and Earth to equalize the charge.

Lightning superheats the air so fast that the air expands faster than the speed of sound. The shockwave created is thunder

• Tornadoes are funnels of high-speed wind.
• Tornado winds are the most violent winds on Earth, with speeds up to 500 km/h.
• Tornadoes typically form along a front between cool, dry air and warm, humid air.
• Tornadoes are fast moving and unpredictable.
• Hurricanes are large storm systems.
• Hurricanes are large circulating masses of clouds, wind, and rain with diameters of about 600 km.
• Hurricanes are powered by energy released as water vapor condenses to form clouds.
• Hurricanes rotate around a center called the eye.
• The most violent winds in a hurricane are those surrounding the eye. The eye is usually calm.
• Hurricane winds can reach speeds greater than 250 km/h .
• Hurricanes weaken and die as they move over land or cooler water.

• Climate the average weather conditions in an area over a long period of time.
• Temperatures tend to be higher close to the equator.
• Sunlight strikes the earth more directly close to the equator.
• The suns rays are less concentrated at the poles, and do not warm the atmosphere as much.
• Earth’s tilt and rotation account for our seasons.
• When the North Pole is tilted toward the sun, the Northern Hemisphere experiences summer.
• When the South Pole is tilted toward the sun, the Southern Hemisphere experience summer.
• Summer solstice the longest day of the year
• Winter solstice the shortest day of the year
• Vernal equinox the point in the spring when the day and night are equal length
• Autumnal equinox the point in the fall when the day and night are equal length
• Earth’s surface features affect climate.
• Topography the configuration of a land surface, including its relief
• Variations in topography affect the climate of a region, for example:
• Mountains can trap moisture on one side
• Broad flat surfaces allow winds to reach great speeds
• Global climate changes over time.
• Many factors produce changes in Earth’s climate, such as:
• movement of continents
• slight changes in Earth’s tilt
• volcanic eruptions
• changes in the greenhouse effect
• Earth’s climate is likely to continue changing over the millennia to come.