Download Moisture, Clouds, Precipitation

Moisture, Clouds, Precipitation
Chapter 18
Objectives
• Understand how heat energy is gained or
lost as water changes state
• Know how to apply the changes in latent
heat to humidity measurements.
Changes in State of Water
Latent Heat Exchange (page 505 in your book)
Humidity-Terms and Definitions
• Humidity - the amount of water vapor in
the air
• Saturation –the maximum amount of
water vapor air can hold at a specific
temperature
• Relative Humidity – the amount of water
vapor in the air relative to (measured
against) the maximum amount (saturated)
of water the air could hold at the current
temperature.
Water Saturation of Air
C degrees
-40
g/kg of water
vapor in air
0.1
0
3.5
5
5
10
7
20
14
30
26.5
40
47
Water vapor needed to reach
saturation
As air
temperature
increases, the
amount of water
vapor that air can
hold also
increases.
As air
temperature
decreases, the air
can no longer
hold as much
water vapor.
What happens to
the excess water
vapor?
www.atmos.umd.edu
Humidity Summary:
• If the water vapor content of air is held
constant:
– Decreasing the air temperature will cause an
increase in relative humidity (the amount of
moisture the cooler air can hold goes down)
– Increasing the air temperature will cause a
decrease in relative humidity (the amount of
moisture the warmer air can hold goes up)
Dew Point
• Think about it – what is dew?
• The Dew Point Temperature is the temperature
to which air must be cooled to reach saturation.
• If air is cooled below the dew point temperature,
condensation forms.
• For every 10° C increase in temperature, the
amount of water vapor needed for saturation
doubles. (Saturated air at 0° C has ½ the water
vapor of saturated air at 10° C)
Practice Understanding Humidity
and Dew Point
• Brown lab book – p 206, #’s 9-12. Figure
14.5 has been reproduced for your use.
Do not write in the lab book – use your
paper
• Brown lab book – p 207-208, #’s 13-23
Do not write in the lab book – use your
paper
Measuring Relative Humidity
• Text book pages 524 and 525. Read the
entire lab before you begin.
Understanding How the Relative
Humidity Measurement Works
1. Why does the temperature of the wet bulb
thermometer decrease as you fan it?
2. Why does the temperature of the wet bulb stop
decreasing at a certain point?
3. Should you expect a greater wet bulb
temperature depression when the relative
humidity is higher or lower?
4. Does 25° c air at 90% relative humidity contain
the same amount of water as 0° c air at 90%
relative humidity?
1. Why does the wet bulb
temperature decease?
• The temperature of the web bulb
thermometer decreases because
converting water to vapor requires an input
of heat energy. The removal of heat from
the air and the thermometer evaporates
the water and causes the temperature (a
measure of heat in the system) to
decrease.
2. Limits to temperature change of
wet bulb thermometer
•
The wet bulb stops decreasing at a
certain point because the wet cloth can
only lose as much water as the humidity
in the surrounding air allows it to. In
other words it can only lose water until it
matches the humidity in the air – no
more. At that point no more energy is
being used to evaporate water, so the
temperature stops dropping.
3. How can you predict wet bulb
temperature depression?
• Wet bulb temperature depression depends
on how much water can be evaporated
from the cloth to the air. As long as
evaporation can continue to occur, the
temperature will continue dropping. The
lower the relative humidity, the more water
that can evaporate from the cloth. In other
words, if the humidity is very low, the wet
bulb temperature decrease will be greater
than when the relative humidity is high.
4. Is 90% humidity the same in
warm and cool air?
• 90% relative humidity at a high
temperature indicates more water in the
air than 90% relative humidity at a low
temperature because:
– Warm air has a much higher capacity to hold
water, therefore 90% of a large number (the
amount of water in the warm air) is greater
than 90% of a smaller number (the amount of
water in cool air)
C degrees
of air
-40
0
5
10
20
30
g/kg of
water
vapor in
saturated
air
0.1
3.5
5
7
14
26.5
g of water
in the air at
90% Rel.
Humidity
0.09 g
3.15 g
4.5 g
6.3 g
12.6 g
23.85 g
Supplement for #4 of Understanding
Relative Humidity
Sections 18.2, 18.3
18.2 Cloud Formation
•
Four Learning Objectives:
1. What happens to air as it changes volume
(becomes compressed or expands)?
2. Know the four mechanisms that cause air to
rise
3. Be able to describe he differences between
stable and unstable air
4. Know the conditions that help water in the
air to condense.
What happens to air as it changes
volume?
• Compressing air masses causes it to
warm up
– Molecules are closer together and generate
heat
• Expanding air masses will cool off
– Molecules are farther apart and lose heat
Changes in Air Pressure
How does air pressure change?
decreases as you go up
• Air pressure _____________
increases as
in the atmosphere and _____________
you get closer to Earths Surface.
• You can predict the effect of pressure
change on air temperature.
increases
Increasing pressure ______________
temperature and lowering pressure
decreases temperature
________________
Wet Adiabatic Rate
5°C / 1000 meters
Dry Adiabatic Rate
10°C / 1000 meters
Adiabatic clip
Orographic Lifting/Adiabatic Processes
Adiabatic Processes
Practice – Calculate Adiabatic
change
•
The dry adiabatic rate is 10° Celsius per
1000 meters.
1. If a dry air mass rises from sea level to 3000
meters, how much will it have cooled off?
2. If a dry air mass descends from 5000 meters
to 3000 meters, how will its temperature
change?
3. You are driving up a road in the mountains.
At the base of the mountain it is 12° C.
Later you notice it is 7 ° C. How much
higher are you?
Practice – Change in Temperature
•
The wet adiabatic rate is 5°C per 1000 m.
1. You started at sea level where the temperature
is 25 ° C and drove up a mountain. You enter
clouds at 1000 m. What is the temperature?
2. You continue to drive up. When you stop the
temperature is 10 ° C. What is your elevation?
Adiabatic Change
What are the processes that cause
air to rise?
• There are four main processes that lift
air
• When an air mass rises (is lifted), it can
cause a change in the temperature of
the air due to adiabatic cooling or
heating.
Lifting Processes
Caused by the mountain
Caused by density differences
Lifting Processes
Convergence - Caused by the
winds meeting
Local Uplift - Caused by local
heat rising up
Understand what stable air is
• Stable air remains in its original position
(vertically)
• Unstable air tends to rise, often at a very
fast rate.
• Cloud formation is more common in
unstable air.
• Unstable air may lead to stormy weather.
What do these clouds tell you?
Know what conditions favor
condensation of the water vapor in
air.
• Condensation requires
saturation
AND
a surface or particle for the condensation
to occur on (dust particle, blade of grass,
etc.)
Precipitation
• Precipitation includes:
– Rain
– Snow
– sleet, hail, and glaze (what we call ice storms)
• The type of precipitation to fall is the result
of the temperature in the lower few
kilometers of Earths atmosphere.
Precipitation
• Know the mechanisms that cause air to
rise and how they work (this cools the air
and allows moisture to condense).
• Water Vapor droplets are EXTREMELY
small.
• Vapor must “grow” to larger droplets
before it was enough mass to fall due to
earths gravitational pull.