Download Humidity - Mrs. GM Earth Science 200

Moisture, Clouds,
& Precipitation
Chapter 18
200
Water in the Atmosphere ch. 18.1
• When it comes to understanding
atmospheric processes, water vapor is the
most important gas in the atmosphere.
Water’s Changes of State
Humidity
• What is humidity?
– general term for amount of water vapor in the air
• There are two “types” of humidity:
– absolute (specific) humidity
– relative humidity
(g)
(1 kg)
Humidity
• What is water vapor
capacity?
– amount of water air
can “hold” at a given
temperature
• How does temperature
affect the amount of
water vapor needed to
saturate the air?
– Warm air can hold more
water than cold air.
Humidity
• What is specific (absolute) humidity?
– the actual amount of water vapor in the air
(for a given time & place)
• How do we express it?
(g)
– specific humidity = mass of H2O vapor (g)
1 kg of air
What is the absolute
humidity of this 1 kg
parcel of air?
15 g
H20
(1 kg)
Humidity
• What is relative humidity (RH)?
– ratio of how close the air is to maximum
capacity (How “full” of water vapor the air is.)
• How do we express it?
– relative humidity = specific humidity
maximum capacity
X 100
RH = ?
100%
Humidity
• What is the relative humidity of each container?
– 50%
• How can they be the same????
– b/c both are ½ full to capacity
50%
50%
Humidity
• Relative humidity can be changed in two ways.
– adding or removing water vapor
– changing the air temperature
Relative Humidity & Temperature
• How does temp. affect relative humidity?
– As air temp goes down, RH goes up.
• Why?
– b/c cold air can hold less water than warm
– As temp goes up, RH goes down.
• Why?
– b/c warm air can hold more water than cold
Humidity
• When air has 100% relative humidity it is
also called…
– saturated
• & air is “full” of water vapor (has reached capacity)
Humidity
• What is the special name for the temp.
at which saturation (100% RH) occurs?
– dew point (temperature)
• What happens at the dew point temp?
– condensation (& precipitation)
» gas  liquid
Saturation & Temperature
• How does temp. affect saturation?
– It takes more water to saturate warm
air & less to saturate cold air.
• Why?
– b/c warm air can hold more water & cold
air can hold less.
What information can we gather from
these figures?
Measuring Relative Humidity
• Using a sling psychrometer:
– 1. Wet the wick on the wet bulb.
– 2. Spin the psychrometer for the specified amount
of time.
– 3. Read temperatures on both the wet & dry bulbs.
• The wet bulb temp. should always be equal or cooler than
the dry bulb temp. due to the evaporation of the water.
– 4. Record your data.
Measuring Relative Humidity
• Using the RH chart
– 1. Calculate
difference between
wet & dry bulb
temperatures
– 2. Locate
difference on the
(top of) RH chart
– 3. Locate dry bulb
temp. on the (left
of) RH chart
– 4. Where they
intersect gives the
RH (in %)
Interactive Relative
Humidity Chart
Example: What is the RH?
• If the wet bulb temp. = 16°C & dry bulb temp.
= 26°C, what is the Relative Humidity?
o 34%
Interactive
Relative
Humidity
Chart
Determining Dew Point Temperature
• Same method as determining relative
humidity, except use dew point chart
Interactive
Dew Point
Chart
Example: What is the Dew Point?
• If the wet bulb temp. = 16°C & dry bulb
temp. = 26°C, what is the dew point?
o 9° Celsius
Interactive Dew
Point Chart
Practice Determining Dew Point
& Relative Humidity Using
Psychrometer Readings
EX.
Dry Bulb
Temp.
Wet Bulb
Temp.
24 C
17 °C
EX.
20 C
16 °C
Diff.
Between
Dew point
Temp. in °C
% Relative
Humidity
7
12 °C
49%
4
14 °C
66%
Cloud Formation
ch. 18 sec. 2
• Where, in the atmosphere, can clouds form?
– anywhere in troposphere
• Why?
– b/c only layer w/ water vapor
• What are clouds made of?
– water (liquid or ice)
– condensation nuclei
Cloud Formation
• How do clouds form?
– warm ground heats air above it
• air rises & cools until reaches dew point &
condenses (gas  liquid) or undergoes
deposition (gas  solid)
– cloud forms (LIQUID or solid water NOT gas)
– What is the name for the atmospheric level
where condensation occurs?
• condensation level
condensation
level
Processes that Lift Air
• What is the orographic effect?
– rainfall that results from the “lifting” of air over
mountains
• different effects on windward & leeward sides of
mountain
• windward  moist air forced over mountain & rises…
the air expands and cools  precipitation
• leeward  air is now dry & cool at top  air sinks &
warms  less rain/cloud cover = “rain shadow desert”
Internet Investigation ES1806
Which Way Does the Wind Blow?
Orographic Lifting & the
Orographic Effect
The Wind Blew Over the Mountain
Tune: For He's a Jolly Good Fellow
Written By: Unknown/Copyright Unknown
The wind blew over the mountain,
The wind blew over the mountain,
The wind blew over the mountain,
And it was wet on the windward side
wet on the windward side
wet on the windward side
The leeward side of the mountain,
The leeward side of the mountain,
The leeward side of the mountain,
Was as dry as it could be
Was as dry as it could be,
Was as dry as it could be,
The leeward side of the mountain,
Was as dry as it could be!
Processes that Lift Air
• frontal wedging
– masses of warm & cold air collide producing a front
• warmer, less dense air rises over cooler, more dense air
Processes that Lift Air
• convergence
– air flows
horizontally,
collides, & gets
pushed upward
Processes that Lift Air
• localized convective lifting
– air is warmed more than surrounding air,
becomes less dense, rises
Condensation
• What causes condensation?
– as air is cooled, water vapor
capacity decreases (& RH ↑)
• When air reaches dew point
temperature, air becomes
saturated (100% RH)
– condensation occurs (gas  liquid)
» forms dew, clouds, fog, precipitation
Condensation
• Besides cooling air to the dew point, what
else is needed for condensation to occur?
– material for water vapor to condense onto
•
condensation nuclei
–
dust, sand, salt, aerosol particles
Cloud Types & Precipitation ch. 18 sec. 3
• Where, in the atmosphere, can clouds form?
– anywhere in troposphere
• Why?
– b/c only layer w/ water vapor
• What are clouds made of?
– water (liquid or ice)
– condensation nuclei
Cloud Types
• Names formed from
one or more of 5
words/word parts
• Classified according
to:
1. shape
ch. 18 sec. 3
2. altitude (height in atm)
• low
– stratus, nimbostratus,
cumulus, stratocumulus
• alto (middle)
– altostratus, altocumulus
• cirrus or cirro- (high)
– cirrus, cirrostratus,
cirrocumulus
• stratus or strato– layers
• cumulus or cumulo– upward puffs/heaps
3. dark, rain clouds
• nimbus or nimbo– nimbostratus,
cumulonimbus
» vertical development
Low Clouds
Middle Clouds
High Clouds
Dark, Rain Clouds
a.k.a. thunderheads
shows vertical development
also considered a low-altitude cloud
cirrocumulus
Precipitation
• What is precipitation?
– any form of water that falls
to Earth from a cloud
• H2O droplets/ice crystals have
to be heavy enough to fall
– examples:
» drizzle
» rain
» glaze/freezing rain
» snow
» sleet
» hail
Formation of Hail Animation
Measuring Precipitation
• National Weather Service (NWS)
measures in hundredths of an inch
• What instruments are used to
measure precipitation?
• rain?
–rain gauge
• snow?
–measuring stick