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Let's first start with some definitions that will help you understand precipitation and rain.

Evaporation - is when a liquid becomes a gas. For example, a water left in a bowl will slowly disappear. The water evaporates into water vapor, the gas phase of water. The water vapor mixes with the air.

Condensation-Condensation is the process in which water vapor changes into a water droplet or ice crystal. It is the opposite of evaporation.

Transpiration - Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants (similar to sweating)

Weather Fronts - A weather front is a boundary separating two masses of air of different densities.

Cold Front -- A cold front is defined as the leading edge of a cooler mass of air, replacing (at ground level) a warmer mass of air.
Warm Front -- A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass.
Stationary Front -is a boundary between two different air masses, neither of which is strong enough to replace the other.

Precipitation -any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, and hail.

Dew Point - The dew point is the temperature where water vapor condenses into liquid water. Condensation of water vapor starts when the temperature of air is lowered to its dew point and beyond.

The Water cycle - The water cycle is the cycle water goes through on Earth. See the image below for a clear description of the water cycle.


The water cycle, also known as the hydrologic cycle, refers to the continuous movement of water between the earth and the atmosphere. The main components of the water cycle are:
1. Evaporation and Transpiration
2. Condensation
3. Precipitation
4. Runoff and Groundwater

Image and further discussion at: http://www.noaa.gov/

What causes precipitation as rain or snow?

1- Water from evaporation and transpiration rises (see schematic above)
2- As warm air rises it cools and causes condensation of water vapor into tiny droplets.

NOTE: Warm air can hold more water vapor than cold air. Clouds will form when air becomes supersaturated, forcing water vapor in the atmosphere to condense into visible liquid cloud droplets.

3- Water droplets increase in size until it can no longer be supported by an upward movement of air. It then falls to the ground.


To the left is a diagram showing two air masses in proximity with their fronts designated Cold (top panel) where the cold air mass is steep and Warm where the front is more gentle. Note that the cold, heavier air mass, associated with a High, is typically below the lighter warm air.

Image: Nasa_gsfc



What causes dew on the ground?

Dew is a thin film of water that has condensed on the surface of objects near the ground in the morning or evening. These objects cool in the night. When they cool, the thin layer of air around them cools too. This makes some water vapor condense on the object. The condensation happens because the air is able to hold less water when it is cooled. The temperature when condensation begins is called the dew point. If the dew point temperature becomes lower than 32 F (0 C), the dew turns into frost.

Test your Understanding:
1. Liquid water changing to water vapor is an example of --
a) condensation
b) evaporation
c) transpiration
d) conduction

2. The loss of water vapor from plants is called --
a) Condensation
b) Transport
c) Transpiration
d) Evaporation

3. What is most likely to occur when a cold air mass moves into a warm air mass?
a) the two air masses will mix forming one air mass
b) A warm front will form as cold air is forced upward.
c) A cold front will form, as warm air is forced upward.
d) there will be a sudden drop in air pressure on the ground.

4. What happens when the temperature of the air is lower than the dew point?
a) water can easily evaporate
b) nothing will happen
c) the air temperature will increase
d) there will be condensation

5. Which of these is not part of the Water Cycle?
a) evaporation
b) erosion
c) condensation
d) transpiration


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