Objectives:Water vapor and the atmosphere

How the system worksDifferent ways to measure water vaporWhat makes it important

Humidity, dew point, frostFog and other cloudsTypes of precipitationInstruments used to measure moisture

I. Earth’s Hydrologic CycleThe 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 & ClimateEvaporation – 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 plantsPrecipitation – 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 & WaterWhen 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:

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 ThreholdsDewpoint temperature≤ 10°F Significant snowfall is inhibited≥ 55°F Minimum for severe thunderstorms to form ≥ 65°F Considered humid by most people≥ 70°F Typical of the rainy tropics≥ 75°F Considered oppressive by most

Impossible to count the number of water vapor molecules in the air!

Dew PointTemperature

Saturation occurs

Condensation and/or precipitation

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, MN14°F

Little water vapor100% R.H.

Phoenix, AZ77°F

Lots of water vapor20% 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 CondensationA. 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

Types of fog:1. Radiation - thin2. Advection - thick3. Upslope – mountains4. Steam – over water5. Frontal – where precipitation occurs

Need a slight wind to stir particles into air

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

Decrease temperature

100% R.H.D.P. Temp

Clouds form

Growth of precip. begins

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.

C. Condenstion nuclei – particles in which water vapor molecules can attach to

D. Four ways to form clouds by lifting air: 1. Orographic lifting 2. Frontal wedging 3. Convergence 4. Convection c

c c csalt

dust

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 CloudsA. 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!)

Fun fact: An average sized cloud contains 13 million gallons of water- enough to fill a small pond!

10,000 km

3,000 km

VII. Types of PrecipitationA. 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

850mb: 1490m

700mb: 3013m

500mb: 5590m

ALL SNOW

Temperature curves are below

freezing

X

X = Probable location of cloud formation (dew point = air temperature)

Height Values:

1000mb: 189m

850mb: 1514m

700mb: 3098m

500mb: 5730m

FREEZING RAIN

Snow

Rain

Shallow freezing layer

X

X = Probable location of cloud formation (dew point = air temperature)

Height Values:

1000mb: 205m

850mb: 1481m

700mb: 3057m

500mb: 5660m

SLEETDeep freezing layer

Rain

Snow ?

X

X = Probable location of cloud formation (dew point = air temperature)

Height Values:

1000mb: 156m

850mb: 1525m

700mb: 3128m

500mb: 5790m

ALL RAINX

X = Probable location of cloud formation (dew point = air temperature)

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 WarningsA. 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