Weather

Weather

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Weather

All weather is a result of humidity, condensation

and pressure.

Water Cycle

Humidity—Water Vapor in the Air

Specific humidity: actual amount of water vapor in the air at a given time & place (grams of vapor per kilogram of air)

Warmer air can hold more water vapor

Humidity

Relative humidity: how close the air is to reaching its maximum capacity of water vapor expressed as a

percentage 100% means it has

reached maximum capacity

Humidity

Measured with a psychrometer

A psychrometer has two thermometers to determine humidity. The dry bulb thermometer measures the room temperature, and wet bulb thermometer is wrapped in a wet cloth. Air is passed through the psychrometer to evaporate moisture on the wet bulb. The readings on the dry bulb thermometer and the wet bulb thermometer are then compared to determine the actual humidity.

CondensationWhen water changes from

a gas to a liquid

Dew point: the temperature at which the air becomes saturated with water vapor & condensation occursIf air cools below the dew point, water starts condensing into a liquid, forming dew or cloud droplets

Condensation

Fog & clouds form only when there are condensation nuclei (like dust particles) for the water to condense on

Air must cool below its dew point

Condensation Nuclei

NASA View of Windblown Sands off Coast of Africa

NASA Views of Ash from Volcanic Eruption

NASA Views of Smoke and

Ash from California

Fires

Smog over LA

from car exhaust

Cloud Formation

When warm, wet air from the surface rises, it begins to cool. Eventually, the temperature drops to its dew point, and the water vapor can condense onto the condensation nuclei

Condensation level: the atmospheric level at which condensation occurs

Warm air rises taking the water vapor along with it. Once it cools, it condenses

forming a cloud.

Cloud Types

Classified by altitude

and shape

Explain the process of cloud formation

UFOs or what?What happened?

Cloud Types

Stratus: clouds that form in layers

Cirrus: high, feathery ice clouds

Cumulus: fluffy clouds with flat bases

Nimbus: dark rainclouds

Cloud types

PrecipitationAny form of water that falls

from a cloud to Earth’s surface

Types of Precipitation

Rain

Snow

Sleet: raindrops that refroze on their way to the surface

Freezing Rain: raindrops that only freeze when they hit the surface

Hail: when frozen raindrops are bounced up & down in the cloud until they fall in a huge ball of ice

Types of Precipitationdepend on the temperature of the atmosphere, both at the

surface & on the way down

snow sleet freezing rain

Measuring Precipitation

Rain gauge: measures liquid precipitation

Measuring stick: measures frozen precipitation

Where is Precipitation?

Where is Precipitation?Rain Shadow effect: near a mountain range, the

windward side gets lots of rain and the leeward side gets little/no rain – the rain shadow

Explain the Rain Shadow Effect

Extreme Weather1 minute video

http://science.discovery.com/videos/against-the-elements-mashups-devastating-winds.html

Air Mass

A large body of air in the lower troposphere that has similar characteristics throughout

Temperature & humidity depend on origin and move with the air mass

Types of Air Masses

Continental: dry Polar: cold

Maritime: wet Tropical: warm

Continental polar (cP): cold & dry

Maritime polar (mP): cold & wet

Continental tropical (cT): warm & dry

Maritime tropical (mT): warm & wet

Types of Air Masses

Types of Air Masses

Fronts—boundary between two air masses

FrontsCold Front: boundary between advancing cold air mass & a warmer air mass it is displacing Rising warm air usually produces precipitation if wet Air becomes colder after front passes

FrontsWarm Front: boundary between advancing hot air mass & a colder air mass it is displacing 1st clouds days in advance, then RAIN Air becomes warmer after front passes

FrontsOccluded Front: when cold front ‘catches up’ to a warm front, producing clouds & precipitation

Fronts

Stationary front: when a front stops moving forward, producing clouds & precipitation – causes floods if stationary too long

Station Model

Surface Weather Map

Locating a Front

Wind direction changes

Temperature changes sharply

Dew Point changes sharply

Weather Forecasting

Satellites

Radiosondes

Surface observations

Forecasting

Computer models take current data & plug it into equations to predict weather

Meteorologists take computer models & tweak them to fit their experience with local conditions

ForecastingTrend Method: using past movement of a front &

precipitation to predict future movement

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