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
