Chapter 5

5.1 Describing Weather Pages

Weather The short-term (minutes/days) atmospheric condition of a specific place at a specific time Studied and predicted by a meteorologist using a combination of variables

Weather Variables Temperature Air Pressure (barometric pressure) Wind speed/direction Humidity (relative humidity) Dew point Cloud Coverage Precipitation

Air Temperature a measure of the average kinetic energy of moving air molecules. cold air is dense, the molecules move slowly, therefore low K.E. warm air is less dense, the molecules move quickly, therefore a high K.E.

measured in Fahrenheit °F (standard) or Celsius °C (metric) using a THERMOMETER a closed glass tube containing liquids such as alcohol air around the tube heats the liquid, the liquid expands and moves up the tube indicating the temperature.

Air Pressure force exerted by air molecules, also known as barometric pressure measured in inches or millibars (mb) using a BAROMETER. air pressure is the greatest near Earth’s surface as altitude increases air pressure decreases

high barometer reading means high pressure with sunny and dry conditions. a low barometer reading means low pressure with stormy and wet conditions.

Wind Speed as air moves from an area of high pressure to an area of low pressure wind is created. measured in miles per hour (mph) using an ANEMOMETER. rotating cups catch the wind which turns a dial attached to the instrument and records the speed.

Wind Direction winds are named from the direction they are blowing a WIND VANE determines the direction from which the wind is blowing example: a westerly wind is coming from the west, so the arrow points west

Humidity % of water vapor in the air measured by a PSYCHROMETER high relative humidity makes your skin feel sticky and sweaty. usually reported as relative humidity

Temperature determines how much water air can hold. Warm air can hold more water than cooler air COLD CAN’T HOLD

When water vapor in the atmosphere it condenses and forms clouds. When water vapor near the ground condenses and forms either dew or frost.

Dew is water drops that form on cold surfaces when the air temperature is above 0°C Frost is ice crystals that form on cold surfaces when the temperature is below 0°C.

Cloud Coverage Stratus Clouds – found at an altitude of up to 2,000 meters, flat, layered, white/greyish in color that turn darker indicating bad weather.

Cumulus Clouds – found at altitudes between 2,000- 6,000 m, fluffy, heaped or piled, often called "fair- weather clouds".

Cirrus clouds – found above 6,000 m, thin, wispy clouds composed mostly of ice that usually predict fair weather.

Fog - a cloud that forms and suspends over Earth’s surface. It can be dangerous by reducing visibility.

Precipitation water in a liquid or solid state that falls to Earth as rain, snow, sleet, or hail depending on the temperature of the atmosphere

rain reaches Earth’s surface as a liquid. snow reaches Earth’s surface as frozen crystals (solid).

Sleet originates as snow, it passes through a layer of warm air and melts, then refreezes when it passes through another layer of freezing air. also known as freezing rain

Hail originates as small pieces of ice that begin to fall but get caught up in an “updraft” within a cloud. Another layer of ice is added. This continues to happen until the ice becomes too heavy to be lifted back up and falls as a large pellet of ice.