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Weather and Climate
Chapter 2
February, 2011
Bell Work
 What is condensation?
 What is dew point?
 Can you find dew in the air?
What is weather?
 Weather
– A condition in the atmosphere at a particular
time and place
Water in the Air
 1. Water in the air effects weather
 2. Forms of water in the air
– A. Gas
 As water vapor
– B. Solid
 Ice (snowflakes)
– C. Liquid
 rain
Water Cycle
 1. Constant movement of water from land to
air and back to land again
 2. Precipitation– “Let it Snow” lab
“Let it Snow Lab”
 Follow Directions
 Equipment needed is at the lab stations
 Lab Safety?
“Let it Snow”
 Graduated Cylinder
“Let it Snow”
 Beaker
“Let it Snow”
Rules




1. No throwing snow
2. No walking in the snow
3. No eating snow……….yuck!!!!!!!!
4. Act like MATURE 7th graders.
Water Cycle Review
 1. Condensation
– Water vapor cools and changes back into liquid droplets
(how clouds form)
 2. Evaporation
– Liquid water changes into water vapor
 3. Transpiration
– Plants release water vapor into the air
 4. Precipitation
– Rain, snow, sleet, or hail falls from the clouds to Earth’s
surface
Humidity
 1. Humidity (pg. 37) in book
– Amount of water vapor or moisture in the air
 2. As water evaporates
– Humidity of the air increases
 3. Warmer air holds more water vapor
 4. Colder air holds less water vapor
Relative Humidity
 1. Relative Humidity
– Amount of moisture the air holds compared to the
maximum amount it can hold at that temperature
Amount of water in the air ÷
Amount of water the air can hold x 100
 2. Saturated
– When air holds all the water that it can hold at that
temperature
– Saturated air has a relative humidity of 100%
Water Vapor Versus Temperature
 1. If the temperature stays the same– Relative humidity changes as water vapor
enters and leaves the air
 2. The more water vapor that is in the air
– The higher the relative humidity is
 3. Relative humidity is affected by changes
in the temperature
– If water vapor in the air stays the same,
 Relative humidity ↓ as temperature ↑
 Relative humidity ↑ as temperature ↓
Bell Work
 What is relative humidity?
 Does warm air hold (more or less) water
vapor than cold air?
Measuring Relative Humidity
 1. Psychrometer
 2. Instrument used to
measure relative
humidity
Measuring Relative Humidity
 1. Sling Psychrometer
 2. Has 2 thermometers
– A. Dry bulb measures
temperature
Measuring Relative Humidity
 Sling Psychrometer
– B. Wet bulb is covered with
a damp cloth
– C. As air passes over the
wet-bulb, the water in the
cloth begins to evaporate,
and the thermometer begins
to cool
– D. If there is less humidity in
the air, the water in the cloth
will evaporate more quickly
and the temperature will
drop
– E. If there is more humidity
in the air, the water in the
cloth will evaporate less and
there will be little
temperature change
Measuring Relative Humidity
Amount of water in the air ÷
Amount of water the air can hold x 100
Measuring Relative Humidity
1. Psychrometer
-a. Find the dry bulb
reading
-b. Find the wet bulb
reading
-c. Find the difference
-d. Find RH
Bell Work
 1. Which
Bell Work
 Fill in the blanks
 If the humidity is low, a ______amount of
water will evaporate from a wet-bulb
thermometer and the _____ between the
wet-bulb reading and the dry-bulb reading of
the psychrometer will be high.
 If the dry bulb reads 10 C, and the
difference between the thermometers is 8 C,
the relative humidity is_______.
Condensation
 1. Process when a gas,
like water vapor, becomes
a liquid
 2. Before condensation
can occur, the air must be
saturated: it must have a
relative humidity of
___100%_____
 3. Condensation occurs
when saturated air cools
Dew Point
 1. Air can become saturated when water
vapor is added to the air……
or when it cools to its dew point
2. Dew Pointtemperature to which air must cool to
become saturated
Example……
Dew Point
 3. Before it can
condense,
water vapor must have
a surface to condense
on
Bell Work
 What is condensation?
 What is dew point?
 Can you find dew in the air?
Making Dew
1. The ice brought the temperature of the can
down to dew point and the moisture in the
air condensed.
2. It formed from the water vapor in the air.
3. The other items in the room were not at
dew point
4. It is the temperature at which dew forms.
5. Lower
Making Dew
6. All the items would be wet.
7. When you take a hot shower or bath, you
add moisture to the air and it condense on
the cold mirror.
8.At night, these things cool down to the dew
point temperature and the water vapor in the
condenses on them.
9. No, not all things can cool down to dew
point.
Making Dew
10. Dew does not fall, it forms
11. It evaporates
Making Frost
 1. It formed from water vapor in the air
 2. Yes
 3. The temperature of the can was below
freezing point
 4. Both are formed from water vapor in the
air. They both form on cool surfaces.
 5.Dew is a liquid. Frost is a solid.
Making Frost
6. No. Frozen dew would look like frozen drops of
water. Frost is made of tiny crystals. When dew
forms and then freezes, frozen dew is created.
Frost is formed when water vapor turns directly
into a solid, skipping the liquid phase.
(sublimation)
7. Dew is formed when water vapor in the air
condenses of surfaces that have cooled down to
dew point. Frost is formed at freezing point
Making Frost
 8. Frost forms on objects at Earth’s surface.
Snow forms in the atmosphere and falls to
the ground.
 9. Snow
Clouds
 Clouds– Are a collection of millions of tiny water droplets
or ice crystals