8 10 Describing Weather Objectives Define weather Identify

8. 10 Describing Weather Objectives: • Define weather. • Identify and describe weather variables. • Explain how weather is related to the water cycle.

What is Weather? �The atmospheric conditions, along with short-term changes, at a certain place at a certain time is called weather.

Weather Variables �A weather map can help you plan for the day.

�A variable is a quantity that can change. �Scientists that study and predict weather are called meteorologists. �Specific variables to describe atmospheric conditions: �Air temperature �Air pressure �Wind speed and direction �Humidity �Cloud coverage �Precipitation.

�Air temperature • A measure of the average kinetic energy of molecules in the air. • Air molecules move faster the warmer the air is. • Air temperature varies with time of day, season, location and altitude.

�Air pressure

� Air pressure (cont. ) • The force that a column of air applies on the air or surface below. • Decreases with altitude. • Instrument: Barometer [measures barometric pressure in millibars (mb)]

� Wind • Air moves from areas of high pressure to areas of low pressure, which creates wind. • Direction is reported from which the wind if blowing. �For example: 10 mi/h, SE, is coming from the southeast and moving toward the northwest. • Instrument: anemometer (measured in mph or km/h)

�Humidity • The amount of water vapor in the air. • Measured in g/cm 3. • High humidity means more water vapor in the air.

�Relative humidity • Air can hold only so much water vapor. • Depends on temperature. �Warm air can contain more water vapor than cold air. • The amount of water vapor present in the air compared to the maximum amount of water vapor the air can contain at a specific temperature is called relative humidity. • Instrument: Psychrometer

� Dew Point • Temperature at which air is saturated and condensation can occur. • When air near the ground becomes saturated, water vapor in the air will condense. �If temperature is above 0 o. C, dew forms. �If temperature is below 0 o. C, frost forms.

Clouds and Fog �What happens to your breath when you exhale on a cold day? �When warm air containing water vapor rises and cools, water vapor condenses into tiny droplets. �Clouds are water droplets or ice crystals suspended in the atmosphere. �A cloud near the Earth’s surface is called fog.

Types of Clouds have different shapes and can be found at different altitudes.

Precipitation �Precipitation occurs when cloud droplets combine and become large enough to fall back to Earth’s surface. �Precipitation is water, in liquid or solid form, that falls from the atmosphere.

Types of Precipitation. Rain, freezing rain, sleet, and snow are forms of precipitation.

The Water Cycle The Sun’s energy powers the water cycle, which is the continual movement of water between the ocean, the land, and the atmosphere.