Understanding Weather

Weather FactorsWeather – refers to the state of the atmosphere at a specific time and place. It occurs in the troposphere.

5 Factors that affect WeatherAir TemperatureWindHumidityRelative HumidityDew Point

SunProvides almost all of Earth’s energy

Causes evaporationIt is a source of heat energyEarth’s surface absorbs the Sun’s heat

The Sun provides the energy that drives Earth’s weather

Air TemperatureHigh Temp. – air molecules move fast and it feels warm

Temperature is a measure of the average amount of motion of molecules.

Low Temp. – air molecules move slower and it feels cold

Warm AirLow Pressure

Cold AirHigh Pressure

Thermometer – instrument used to measure temperature

WindAir moving in a specific direction

Wind results because air moves from areas of high pressure( cold air) to areas of low pressure(warm air).

Instruments to Measure WindWind Direction is measured with a wind vaneWind Speed is measured with an anemometer

Wind Vane Anemometer

HumidityThe amount of water vapor in the airMore water vapor can be present in

warmer temperaturesLess water vapor is present in cooler

temperaturesWhen enough water vapor is present

in air for condensation to take place, the air is Saturated

Relative HumidityIs a measure of the amount of water vapor

present in the air compared to the amount needed for saturation at a specific temperature.

Weather forecasters report the amount of moisture in the air as Relative Humidity

If you hear the forecaster say that the relative humidity is 50%, it means that the air contains 50% of the water needed for the air to be saturated.

Psychrometer – instrument to measure humidity. Uses the difference in readings between two thermometers, one having a wet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air

Dew PointThe temperature at which air is

saturated and condensations forms.Dew point changes with the amount

of water vapor in the air.Air near the ground cools to its dew

point, water vapor condenses and forms dew,

Frost may form when temperatures are near 0⁰ C (32⁰ F)

CloudsForm as warm air is forced upward,

expands, and coolsWhen relative humidity reaches

100% tiny water droplets begin to form(condensation)

Billions of droplets attach to particles in the air to form a cloud

3 main types of clouds: Stratus, Cumulus, and Cirrus.

StratusForms at low altitudesClouds of layers, or smooth, even sheetsStratus clouds near the ground are called FOGAssociated with fair weather or rain or snow

CumulusClouds are masses of puffy, white clouds,

often flat basesAssociated with fair weather or

thunderstorms

Cumulonimbus = Thunderstorms

CirrusCirrus clouds are the highest clouds. They usually mean fair weather. They look white and feathery and made of

ice

PrecipitationWater falling from the clouds4 different forms: Rain, Snow, Sleet, and Hail

Air temperatures determine which it will be.

Rain and SnowRain dropsfalling in Temps Above Freezing.

Snow formsWhen theTemps areSo cold thatWater vaporTurns intoA solid.

SleetSleet forms whenRaindrops pass throughA layer of freezing Air near the surface.

HailHailstones are pellets Of ice that form insideA cloud.

The relative humidity of air helps determine whether a location will have a dry day or experience some form of precipitation.