Humidity and Condensation After completing this section, students will compare the physical characteristics of the three states of water (Standard PI – 053)

After completing this section students will: Compare the physical characteristics of the three states of water (PI Standard – 053) Explain the formation of the different types of clouds and the type of weather associated with those clouds (PI Standard – 055)

Molecules of water are moving through the air around us This water vapor strongly affects the weather

Water is unique. It is the only substance that exists commonly in all three states The state of matter depends on the temperature of water

Water is in solid state at temperatures of zero degrees Celsius or below It can occur as ice, snow, sleet or ice crystals

Water is in liquid state between zero degrees and 100 degrees Celsius It occurs as cloud droplets or rain

Water occurs as a gas at 100 degrees Celsius or above, as water vapor It occurs as clouds and steam

Water changes state in the atmosphere when it absorbs or gives off heat

Condensation is the change from water vapor (gas) to a liquid Condensation can form fog, dew and clouds

Evaporation is when a liquid changes into a gas (water vapor)

Humidity

The amount of water vapor in the atmosphere varies The actual amount of water vapor in the atmosphere at a given time and place is called specific humidity

The amount of water in the air depends on temperature The warmer the air, the more water vapor it can hold

Relative humidity compares the actual amount of water in the air to how much it can hold at a given temperature Measured with a psychrometer and written as a percentage

Condensation occurs when the air temperature cools past the point of saturation Dew point – the temperature at which saturation occurs

For condensation to occur, two conditions are necessary There must be material for water vapor to condense onto (condensation nuclei) The air must cool below its dew point

When condensation occurs, dew or frost can form Dew occurs when the air temperature is above freezing, frost occurs below zero

Radiation fog occurs when the ground loses heat rapidly and cools the air above it Occurs near rivers and lakes and usually occur in late fall and winter

Advection fog occurs when warm moist air moves over a cool surface Usually thickest in early morning and ‘burns off’ in the late morning