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What is weather?
Chapter 2: Section 1
Weather Defined
• Weather refers to the state of the atmosphere at a specific time and place.
• Weather describes conditions such as air pressure, wind, temperature, and the amount of moisture in the air.
Air Temperature
• Temperature is the measure of the average amount of motion of molecules.
• When the temperature is high, the molecules in air move rapidly.
• When the temperature is low, the molecules in air move less rapidly.
• Air temperature is measured by a thermometer in Fahrenheit or Celsius.
Wind
• Air moving in a specific direction is called wind.
• Wind results because air moves from areas of high pressure to areas of low pressure.
• Wind direction can be measured using a wind vane.
• Wind speed can be measured using an anemometer in miles or kilometers per hour.
Wind Instruments
Wind Vane Anemometer
Humidity
• The amount of water vapor present in the air is called humidity.
• More water vapor can be present when the air is warm than when it is cool.
• When enough water vapor is present in air for condensation to take place, the air is saturated.
• Relative humidity is the measure of the amount of water vapor present in the air compared the amount needed for saturation at a specific temperature.
Measuring Humidity
• Humidity is measured using a psychrometer and is represented by a percentage.
Dew Point
• The temperature at which air is saturated and condensation forms is the dew point.
• The dew point changes with the amount of water in the air.
• Frost may form when temperatures are near 0°C.
Pressure
• Atmospheric pressure is an indication of weather systems.
• Low pressure areas are windy and can be associated with stormy weather.
• High pressure areas are associated with fair weather.
• A barometer measures atmospheric pressure in Millibars.