Understanding Weather

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

5 Factors that affect Weather Air Temperature Wind Humidity Relative Humidity Dew Point

Sun Provides almost all of Earth’s energy Causes evaporation It is a source of heat energy Earth’s surface absorbs the Sun’s heat The Sun provides the energy that drives Earth’s weather

Air Temperature High 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 Air Low Pressure Cold Air High Pressure

Thermometer – instrument used to measure temperature

Wind Air 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 Wind Wind Direction is measured with a wind vane Wind Speed is measured with an anemometer Wind Vane Anemometer

Humidity The amount of water vapor in the air More water vapor can be present in warmer temperatures Less water vapor is present in cooler temperatures When enough water vapor is present in air for condensation to take place, the air is Saturated

Relative Humidity Is 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 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 Point The 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)

Clouds Form as warm air is forced upward, expands, and cools When 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.

Stratus Forms at low altitudes Clouds of layers, or smooth, even sheets Stratus clouds near the ground are called FOG Associated with fair weather or rain or snow

Cumulus Clouds are masses of puffy, white clouds, often flat bases Associated with fair weather or thunderstorms

Cumulonimbus = Thunderstorms

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

Precipitation Water falling from the clouds 4 different forms: Rain, Snow, Sleet, and Hail Air temperatures determine which it will be.

Rain and Snow Rain drops falling in Temps Above Freezing. Snow forms When the Temps are So cold that Water vapor Turns into A solid.

Sleet Sleet forms when Raindrops pass through A layer of freezing Air near the surface.

Hail Hailstones are pellets Of ice that form inside A cloud.

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