Moisture, Clouds, and Precipitation

Humidity

• Describes the amount of water vapor in the air• Relative humidity – ratio of actual water vapor

content compared to the amount of water vapor that the air can hold at a particular temperature and pressure– Can be changed by:• Adding or taking water vapor away• Temperature – higher temperatures can hold more water

Atmosphere Terms

• Dew point – temperature air should be cooled to in order for water vapor to condense

Cloud Formation• In order for clouds to form, air has to rise. It will

rise to meet air of the same temperature (stability)

Water vapor rises until it finds stable air, then it converts to liquid around a condensation nuclei to form a cloud

Dust, smoke, or salt or something for the water to attach and form a droplet

18.3 – Cloud types and precipitation

• Clouds are classified on the basis of form and height

• 4 types

Cirrus – whispy and high (curl of hair)

Cumulus – (pile or heap) look like cotton balls

Stratus – (layer) look like layers of clouds over sky

Nimbus – (gray) rain clouds

Bergeron Process (Cold cloud precipitation)

• Supercooling:– Water in the upper atmosphere turns solid at

------- 40ºC, not 0ºC– When supercooled water falls from the sky and hits

the ground, it freezes on impact = freezing rain• Supersaturation:– When air is saturated, the water vapor cannot

coexist with ice crystals, so the water vapor freezes and falls as snow flakes

Bergeron Process

Warm cloud precipitation

• Collision-coalescence process– As cloud droplets move through the cloud, more

water collects and the drops grow– When the drops get too large, they fall as rain

Collision-coalescence

Forms of precipitation• Sleet, glaze, and hail:– Sleet – small particles of ice– Glaze - freezing rain– Hail – supercooled water droplet that falls through a

cumulonimbus cloud, gaining water and freezes by the time it leaves the cloud

Atmospheric Conditions

• Weather – state of atmosphere at any point in time

• Both are driven by the Sun

Air Pressure

• Weight of the air above• Exerted in all directions, so objects do not

collapse from the weight• Measured with barometer• Caused by uneven heating of Earth’s surface

• Isobars - Lines that connect equal points of air pressure

• Difference shows how rapidly or slowly pressure is changing –pressure gradient

High Low• Called anticyclones• Rotate clockwise• Highest pressure in center• Tend to have calm, clear

weather• Winds flow outward away

from center

• Called cyclones• Rotate counterclockwise• Lowest pressure at

center• Associated with bad

weather• Winds flow inward

towards the center

Wind

• Caused by the uneven heating of Earth’s surface, resulting in horizontal differences in air pressure

• Flows from high to low• Wind name = the direction it blows from• 3 factors control wind:– Pressure difference (creates wind)– Coriolis Effect (gives direction)– Friction (speed)

Global Winds

• Warm air moves towards high latitudes (rises) while cool air travels towards the equator (sinks) creating a convection cell

Earth’s Wind Systems

• 0-30° N/S - Trade winds (east to west)• 30-60° N/S – Prevailing Westerlies (west to

east)• 60-90° N/S – Polar easterlies (east to west)

• Jet stream – high altitude belt of wind that travels west to east above the friction zone– Airplanes use this to travel places faster

Earth’s Wind Systems Jet Stream

Measuring Wind

• A wind vane measures direction• An anemometer measures wind speed

Air Masses

• Immense body of air with similar temperatures and moisture at any altitude

• Air masses move and they take on the characteristics of the new area they cover (air mass modification)

Classifying air masses• Named for source region• Characteristics:–m (maritime) - form over water (humid)– c (continental) – form over land (dry)

–T (tropical) – form at low latitudes (hot)–P (polar) – form at high latitudes (cold)

4 types of air masses:

• 1. continental polar (cP)– dry and cool– Canada and over Great Lakes

• 2. continental tropical (cT)– dry and warm/hot– Southwest U.S.

More on air masses• 3. maritime polar (mP)– humid and cold– northern Pacific

• 4. maritime tropical (mT)– humid and hot– Gulf of Mexico or Southern Pacific

Fronts

• The boundary between two air masses• 4 types – each with specific symbol• Front is moving the direction the symbol is

pointing

Warm Fronts• Definition: warm air

moves into an area that was covered by cold air

• Symbol: red line with red semicircles

• Weather associated with front:– Warmer

temperatures– Light-to-moderate

precipitation

Cold Fronts

• Definition: cold air moves into an area that had warmer air (forces it upward)

• Symbol: blue line with blue triangles,

• Weather Associated: – Heavy downpours – Gusty winds

Stationary Fronts• Definition: cold air mass

and warm air mass meet and the masses travel parallel to the line

• Symbol: blue triangles on one side, red semicircles on the other side

• Weather Associated:– Gentle to moderate

precipitation

Occluded Fronts

• Definition: active cold front overtakes an active warm front

• Symbol: purple triangles and purple semicircles on the same side

• Weather Associated:– Light precipitation

Weather Prediction and Station Models

Weather Instruments

Hygrometer –humidity Ceilometer – cloud height

Doppler Radar• Doppler Effect –wave frequency changes as energy is moved towards or away from a point

• Predicts wind speeds

Doppler Weather Radar• Locates storms (waves are scattered by

raindrops)– Tracks precipitation, not clouds– Can also measure wind speeds due to rain

scattering the signal

Weather Satellites

• Tracks clouds, not precipitation

Forecasting

SHORT TERM• Up to 3 days• More accurate

LONG TERM• Up to 10 days• Less accurate - too many

atmospheric variables

Severe Weather – Thunderstorms, Tornadoes, and Hurricanes

• Thunderstorm - storm that generates lightning and thunder– Produce gusty winds, heavy rain, and hail

• Form when warm, humid air rises in an unstable environment and form a cumulonimbus cloud

Air Movement within the Cloud – Key to formation

• Updrafts – strong upward movement of air (cause storm cloud to grow)

• Downdraft – strong downward movement of air (cool the cloud)

3 stages of development:

–1. Cumulus: strong updrafts cause warm air to rise, making the cloud grow taller

2. Mature: • Equal updrafts and

downdrafts make a convection cell

• Rain starts = cloud growth stops

3. Dissipation: • Warm air runs out• Precipitation no

longer forms• Downdrafts cause

the storm to fall apart

Lightning

• Circulating ice particles in a cumulonimbus cloud create areas of opposite charge within the cloud– Positive at the top; negative at the bottom

• To relieve the imbalance, ions travel to the ground to even out

Parts of Lightning1. Stepped leader- invisible channel of ions that

travels from the cloud to the ground2. Return stroke- the visible channel that rushes up to

meet the stepped leader

Thunder

• Gas around electron stream is superheated causing rapid expansion and contraction and makes a cracking sound

Tornadoes

• Violent windstorms that form a vortex (rotating column of air)

Occurrence

• Over 1,000 annually• Most frequent between April and June• Most tornadoes occur in the Mid-west US

How Tornadoes Form

– The attempt between balancing high pressure and low pressure pushes air up and creates a vortex (mesocyclone)

– Wind shear picks up vortex and connects it from ground to cloud

• When the mesocyclone touches down, it forms a tornado– Larger storms are capable of having multiple vortices

How you measure a tornado– The enhanced Fujita scale measures a tornado’s

intensity based from damage done – Has 6 steps (F0 – F5)• F0 – light damage (broken branches)• F1 – moderate (moving car blown off road) • F2 – Considerable (large trees snapped, cars picked up) • F3 – Severe (roofs torn off home, train overturned) • F4 – Devastating (homes leveled, cars thrown) • F5 – Incredible (Houses lifted off foundations and carried,

bark torn from trees)

Hurricanes

• A hurricane is a whirling tropical cyclone that produces winds of at least 74 mph

• Most powerful storms on Earth• Called a:– Hurricane - around the US (Atlantic and Pacific)– Cyclone – any ocean in the Southern Hemisphere

and in Indian Ocean in NH– Typhoon – NH in western Pacific near Asia

Occurrence of Hurricanes

• Hurricane season is from June 1 through December 1

Cross Section of a Hurricane

How Hurricanes Form

• Needs:– 1. Warm ocean water for energy and to make

humid air– 2. Ocean level winds blowing towards each other

to force wind up– 3. Winds flow outward at top of storm to allow

warm air to rise

• 4. Rising humid air to make storm clouds• 5. Winds outside storm to guide it and make it

grow

– Coriolis effect – to start spin

Stages of hurricane development

• Stages of hurricane development – Tropical disturbance – storm gets organized – Tropical depression – winds form and storm

begins to spin; first gets a name– Tropical storm – has winds of 39-73 mph – Hurricane – has eye and has wind speeds of 74+

mph

Parts of a Hurricane

• Eye wall – doughnut shaped wall that surrounds the core of storm– Most violent part

• Eye – center of storm– Calmest part

Weather brought by hurricanes

• Storm surges – walls of water blown inland that causes massive flooding– Most damaging aspect of a hurricane

• Tornadoes can form

How a hurricane falls apart

• When the warm water supply is taken away – Cold water

• When heat is taken away– Moving over land

• Friction from land

How do you measure a hurricane?

• Saffir-Simpson scale measures hurricane intensity

• 5 categories

Saffir-Simpson Hurricane Scale

El Nino

• Warming of Pacific Ocean near Peru due to stopped trade winds– Upwellings stop = massive fish death

• Causes worldwide changes in weather– Calmer Atlantic hurricanes– Southern US has wetter conditions than normal– West Coast has drier conditions

What is the difference between weather and climate?

Factors Affecting Climate• 1. Latitude – as latitude increases,

intensity of solar energy decreases• 2. Elevation – higher elevation = colder

climate• 3. Topography – influences precipitation

amounts (rain-shadow effect)

• 4. Bodies of water – water temp causes air temp to be cooler

• 5. Global winds – distribute heat and moisture

• 6. Vegetation – controls energy exchange through transpiration

Climate Classification

• Koeppen system– Based on monthly and annual precipitation and

temperature– 5 principal groups

• 1. Humid tropical – climates with no winters (temp – 64ºF, 200+cm of precipitation)

• 2. Humid mid-latitude – hot summers, mild winters

• 3. Dry – precipitation exceeded by evaporation rates (desert, steppe)

• 4. Polar – warmest month temp is below 50ºF + little precipitation

• 5. Highland – cooler and wetter than local area at lower elevation

The Greenhouse Effect

• Natural heating of Earth’s surface due to gases like water vapor, carbon dioxide, and methane trapping heat in the atmosphere

Natural Climate Changes• Volcanic Eruptions – causes a global cooling• Ocean Circulation – short term climate changes can

cause current circulations to change

Global Warming

• Rising of Earth’s average temperatures• Causes increased evaporation rates = more water

vapor in air = more heat is absorbed in atmosphere• Sea ice melts = rise in sea level• Shifts in weather patterns, such as increased

hurricane activity and strength