Water in the Atmosphere 18.1

•Water vapor is the source of all condensation and precipitation.•When it comes to understanding

atmospheric processes, water vapor is the most important gas in the atmosphere.

If you leave liquid water out in an open container, you will notice that the water level goes down over time. The liquid water is evaporating into the air—it is changing into an invisible gas called water vapor.

Evaporation • The process of converting a liquid to a gas.

• Evaporation happens at the surface of a liquid, where some molecules have enough kinetic energy to escape. As the gas molecules fly off, they take away some kinetic energy from the remaining liquid.

•When kinetic energy is taken away, the temperature of the remaining liquid decreases.

• Evaporation is a cooling process, because evaporation cools what is left behind.

Example: Moisture evaporating from the skin cools the human body.

Humidity •Humidity

is water vapor in the air.

• The amount of water vapor that the air can hold depends on the air temperature.

•When air gets warm, it expands. Warm air can hold more water vapor than cold air because of this expansion (there is more room for water molecules).

Relative Humidity • The amount of water vapor actually in the air

compared with the maximum amount of water the air can hold at that particular temperature.

• It can be measured with a psychrometer, a device with a dry and wet-bulb thermometer.

How can we measure the amount of water vapor in the air (humidity)?

•Air can only hold a certain amount of water.•Once the air is holding all the

water vapor it can hold, evaporation can not occur.

•With no evaporation:–the temperature on a wet thermometer would not change–the air is saturated (100% full) of water vapor

•Oppositely, in dry air with very little water vapor, evaporation will happen faster because there is more space for the water molecules in the air.

• The faster the evaporation:–the lower the temperature on a wet thermometer–the less water vapor in the air

Conclusion:Comparing the temperature of a wet thermometer with the temperature from a dry thermometer can allow us to determine how much water vapor is in the air (humidity). The greater the temperature difference, the less water vapor there is in the air.

Dew Point• Condensation is the opposite of

evaporation: gas turns into a liquid.

• Evaporation is a cooling process.

• Condensation is a warming process.

• Dew point is the temperature at which condensation occurs.

or in other words…

• Dew point is the temperature at which the water in the air will turn back into a liquid!