Objectives:
 Water vapor and the atmosphere
 How the system works
 Different ways to measure water vapor
 What makes it important
 Humidity, dew point, frost
 Fog and other clouds
 Types of precipitation
 Instruments used to measure moisture
I. Earth’s Hydrologic Cycle
 The gigantic
system
powered by
energy from
the sun in
which the
atmosphere
that provides a
link between
the oceans and
continents.
Important Components
Affecting Weather & Climate
 Evaporation – change in state from gas to a liquid.
LOTS of energy gained (2260 J/g)
 Condensation – change in state from gas to a liquid.
LOTS of energy released (latent heat – 2260 J/g)
 Transpiration –water vapor released by plants
 Precipitation – liquid or solid water that returns back
to the Earth’s surface in the form of rain, snow, sleet, or
hail.
Distribution of Water on Earth
II. Principles of Air & Water
 When liquid water evaporates, it turns into a gas called
water vapor.
 Amount of water vapor in the atmosphere is limited by
temperature:
 warmer air holds more water vapor
 Saturation: when air can’t hold any more water
 Dry skin and chapped lips
in the winter
III. Measurement
A. Dew point:
 Impossible to count
the number of water
vapor molecules in the
air!
1. The amount of water vapor in the air
2. Often described as the temperature at which air
needs to cool to reach saturation (°F or °C)
Table 4-2 Dew Point Threholds
Dewpoint temperature
≤ 10°F
≥ 55°F
≥ 65°F
≥ 70°F
≥ 75°F
Significant snowfall is inhibited
Minimum for severe thunderstorms to form
Considered humid by most people
Typical of the rainy tropics
Considered oppressive by most
Condensation
Dew Point
and/or precipitation Temperature
occurs
Saturation
occurs
B. Relative Humidity
1. Measure of how close the air is to saturation
2. Saturation occurs at 100%
3. Dependent on temperature
and amount of moisture
International Falls, MN
14°F
Little water vapor
100% R.H.
Phoenix, AZ
77°F
Lots of water vapor
20% R.H.
Relative Humidity Activity
http://cimss.ssec.wisc.edu/satmet/modules/clouds/cloudformation_RH.html
Relative Humidity Activity: Answers
Scenerio
Controlling Factor
Relative Humidity
Raise thermostat
Warmer temps
Decrease
Lower thermostat
Cooler temps
Increase
Take a shower
Moisture added
Increase
Open the door
Cooler temps
Increase
Cook
Warmer temps
Decrease
Throw a BIG party
Moisture added
Increase
IV. Types of Condensation
A. Dew - when the air temperature cools beyond the dew
point temperature, water is squeezed out of the air
(condensation – gas to liquid).
 Usually happens in the
morning or evening when
the temperatures are
cooling.
B. Frost - same concept as dew, only with ice crystals
instead of liquid water (deposition – gas to solid)
 Dew point must be 32°F
or less
 Water vapor turns to ice
C. Fog - same process as dew forming, but the dew
forms on tiny particles in the air that you cannot see
 Need a slight wind
Types of fog:
to stir particles into air
1. Radiation - thin
2. Advection - thick
3. Upslope – mountains
4. Steam – over water
5. Frontal – where precipitation occurs
Radiation fog – Usually forms at night when there are clear skies
and fairly high relative humidity (condensation)
Advection fog – warm, moist air moves over a cold surface (condensation)
Upslope fog – air moving up the mountain is forced to cool (condensation)
Steam fog – cool air moves over warm water (evaporation)
Frontal fog – evaporation of rain after cold air moves through
Fun fact: The foggiest location in the U.S. is Cape Disappointment, WA
V. Formation of Clouds
A. Clouds are composed of tiny particles of liquid water
and ice crystals
B. They form when air cools to the dew point
temperature, and relative humidity increases to 100%.
Eventually the excess water is dumped and latent heat
is released.
100% R.H.
D.P. Temp
Clouds form
Growth of
precip. begins
Decrease temperature
C. Condenstion nuclei – particles in which water vapor
molecules can attach to
dust
salt
cc
c
c
D. Four ways to form clouds by lifting air:
1. Orographic lifting
2. Frontal wedging
3. Convergence
4. Convection
pollen
Orographic Lifting– air that is forced to rise over a mountain range
Frontal wedging – warmer air moving over a cold pocket of air
Convergence - winds blowing in different directions forces air up
Convection – warm parcels of air rise due to solar heating
VI. Types of Clouds
A. Basic Cloud Types:
1. Cirrus – high, wispy
2. Alto – middle
3. Stratus – low, layers
4. Cumulus – cotton-like
5. Nimbus – produce rain
B. Special clouds:
1. Lenticular – formed in mountainous areas
2. Contrails – forms when the humidity starts to
increase (bad weather is approaching!)
10,000 km
3,000 km
Fun fact: An average sized cloud contains 13 million gallons of waterenough to fill a small pond!
VII. Types of Precipitation
A. Rain - spherical liquid droplets
B. Snow – aggregates of ice crystals
 Ice crystals shapes are based on temperature in
which they form
Fun fact: Powdery snow is the result of very low temperatures because
the moisture content of air is small.
C. Sleet – frozen raindrops
D. Glaze – rain that freezes when it comes in contact
with the ground
Fun fact: The snowiest city in the U.S. is Rochester, NY (avg. of 239 cm
per year). Buffalo, NY is a close runner-up.
E. Hail – small to large chunks of ice that are composed
of several layers
Fun fact: Largest hailstone ever recorded fell in Aurora, Nebraska in June
2003. It measured 17.8 cm in diameter and weighed 1.5 pounds!
Another fun fact: People used to believe that strong noises- explosions,
cannon shots, or ringing church bells- prevented hail formation.
Height Values:
1000mb: 246m
X = Probable location of
cloud formation
(dew point = air temperature)
850mb: 1490m
700mb: 3013m
500mb: 5590m
Temperature
curves are below
freezing
X
ALL SNOW
Height Values:
1000mb: 189m
X = Probable location of
cloud formation
(dew point = air temperature)
850mb: 1514m
700mb: 3098m
500mb: 5730m
Snow
Rain
Shallow freezing layer
X
FREEZING RAIN
Height Values:
X = Probable location of
cloud formation
(dew point = air temperature)
1000mb: 205m
850mb: 1481m
700mb: 3057m
500mb: 5660m
Snow ?
Rain
Deep freezing layer
X
SLEET
Height Values:
1000mb: 156m
X = Probable location of
cloud formation
(dew point = air temperature)
850mb: 1525m
700mb: 3128m
500mb: 5790m
X
ALL RAIN
In case you wanted to know…
VIII. Precipitation Measurement
A. Rain gauge: measures rainfall by funneling liquid into
a cylinder or bucket below.
B. Snow: Difficult to measure because the ratio of snow
to liquid water varies. Weather spotters!
4:1
(wet snow)
30:1
(fluffy snow)
IX. Hazards & NWS Warnings
A. Rain Events:
1. Flood watch/warning –large rivers have the
potential or have exceeded flood stage
2. Urban/small stream watch/warning – local roads
may have or already have water running over them
3. Flash flood watch/warning – low-lying areas may
have or already have flooded
4. Drought watch/warning – Drought is a deficiency
in precipitation over an extended period of time,
usually a season or more, resulting in a water shortage
causing adverse impacts on vegetation, animals,
and/or people.
U.S. Drought Monitor
B. Snow Events:
1. Winter storm watch/warning – Potential/certainty:



6+ inches or snow (12 hrs) or 8+ inches of snow (24 hrs)
Reduced visibility (winds)
¼ inch of ice accumulation
2. Blizzard watch/warning – Potential/certainty:


Wind gusts sustained 35mph or greater for 3+ hrs
Falling or blowing snow