Download 08. Weather Part 3 – Humidity

Humidity
Remember:
 Water takes longer to warm and cool because of its
high heat capacity.
 Lots of solar energy is needed to change the state
of water!
 Solid
 Ice


Liquid
Water


Gas
Vapour
Consider This:
Consider This:
 When liquid water from the lake evaporates, it
exists as vapour.
 We cannot see it.
 It is in the space
between the lake
and the clouds.
Consider This:
 But the air cannot hold all of the vapour.
 Some will condense
back into a liquid,
in the form of clouds,
fog, mist, or rain.
Consider This:
 As the distance from the lake increases, the air
temperature cools.
 Warm air can hold
more water vapour
that cool air.
Saturated Air
 When the air contains the maximum amount of
water vapour possible, at that temperature.
 Water will continue to evaporate until the air is
saturated.
 Example:
 At 20°C
 1 kg of saturated air has 15g of water vapour.
Saturated Air
 If air becomes cooler or if more water evaporates:
 Some of the vapour will condense.
 Water droplets form around particles in the air.
 Ex: dust, salt
 Droplets also form on the surface
of solids.
 Ex: dew on grass
Humidity
 The amount of water vapour in the air.
 Coastal regions tend to have a higher humidity than
regions inland.
 Summer tends to have higher humidity.
 Seems “sticky” when it is hot and humid.
 Does bad things to your hair!! (haha)
Humidity
 Warm air can hold more vapour than cool air.
Absolute Humidity
 The actual amount of water vapour in the air.
 Expressed in grams of vapour per kilogram of air.
Relative Humidity
 The percentage of water vapour in the air
compared with the amount the air can hold (at that
temperature) if it was saturated.
Relative Humidity
 Relative Humidity Demo
 https://www.youtube.com/watch?v=CL5cgXwKUXc
Think About It:
 What does a weather reporter mean by saying “the
relative humidity is 62 percent”?
 The amount of water vapour in the air is 62% of the
amount that would be in the air at that temperature
if the air was saturated.
Consider This:
 The relative humidity inside the house is low during
the winter.
 Why?
 The air is much warmer in the house than outside.
 Warm air can hold more.
 With a low relative humidity, no wonder your skin
feels dry!
Dew Point
 The temperature at which the air is saturated so
vapour condenses into liquid.
 Condensation begins to occur.
 Clouds form, precipitation may happen.
Consider This:
 Cold cans of pop get condensation on them (dew)
because temp of can is cooler than temp of air.
 As air becomes cooler around the can, the vapour is
the air begins to condense.
 This creates the drops on the can or the puddle
underneath it.
Humidex
 The humidex scale was created by Environment
Canada.
 It takes into account both the temperature and the
relative humidity.
 It indicates how warm a given temperature will feel
to the average person when relative humidity is
taken into account.
Consider This:
 On a summer day when the relative humidity is
high, you feel much hotter than when the
temperature is the same, but it is less humid.
 A weather forecast may indicate that the
temperature is 35°C but feels like 40°C.
Consider This:
 Why does it feel warmer?
 An increase in relative humidity reduces the amount
of perspiration (sweat) from evaporating and in
return cooling your body, causing you to feel hotter!