# Weather Measurements What’s up with the weather?.

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Weather Measurements What’s up with the weather?

Weather is the state or condition of atmospheric variables at any given location over a short period of time. Variables include: temperature, air pressure, wind, moisture conditions, cloud cover, precipitation and storms. Weather occurs in the troposphere.

Atmospheric Pressure & Density Air pressure can also be called barometric pressure and vapor pressure. Air pressure can also be called barometric pressure and vapor pressure. Air pressure and density are directly related. Air pressure and density are directly related. The denser the air the more it weighs so the greater pressure it exerts. The denser the air the more it weighs so the greater pressure it exerts.

Measuring Air Pressure The instrument used to measure air pressure is a barometer. The instrument used to measure air pressure is a barometer. There are two kinds, mercury and aneroid. There are two kinds, mercury and aneroid.

Mercury Aneroid

Temperature and Air Pressure  Temperature  Air Pressure  Temperature  Air Pressure  Temperature  Air Pressure  Temperature  Air Pressure This is all due to changes in density. This is all due to changes in density. As temperature increases, air expands. Therefore density decreases. Just the opposite with decreasing temp. As temperature increases, air expands. Therefore density decreases. Just the opposite with decreasing temp.

Effect of Water Vapor on Air Pressure N 2 molecular weight = 14 N 2 molecular weight = 14 O 2 molecular weight = 16 O 2 molecular weight = 16 H 2 O molecular weight = 10 H 2 O molecular weight = 10

v N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 N2 O2 H2OO2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 H2ON2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 O2 N2 N2H2O O2 H2O AIR WITH WATER VAPOR DRY AIR LESS AIR PRESSURE MORE AIR PRESSURE LIGHTER HEAVIER

Altitude and Air Pressure  Altitude  Air Pressure & Density  Altitude  Air Pressure & Density  Altitude  Air Pressure & Density  Altitude  Air Pressure & Density  So what’s the BIG idea? As Altitude, Temperature & Moisture content  - Air Pressure & Density  As Altitude, Temperature & Moisture content  - Air Pressure & Density 

Isobar Map

Wind Horizontal movement of air parallel to Earth’s surface. Horizontal movement of air parallel to Earth’s surface. Caused by differences in air pressure. Caused by differences in air pressure. The closer the isobars, the steeper the gradient and faster the wind speed. The closer the isobars, the steeper the gradient and faster the wind speed.

Wind Speed Measured with an anemometer. Measured with an anemometer. Units are mph or knots. Units are mph or knots. One knot = 1.15mph One knot = 1.15mph

Wind Direction Due to the Coriolis effect, winds generally move from SW to NE in the United States. Due to the Coriolis effect, winds generally move from SW to NE in the United States.

Pressure differences due to storms will change that direction over a relatively small area. Pressure differences due to storms will change that direction over a relatively small area. Wind is always named by the direction it comes from. Wind is always named by the direction it comes from. Ex., Wind, NW 17 – means winds from the NW at 17mph. Ex., Wind, NW 17 – means winds from the NW at 17mph.

Atmospheric Moisture Gaseous water in the atmosphere is called water vapor. Gaseous water in the atmosphere is called water vapor. The oceans are the source of most atmospheric moisture. The oceans are the source of most atmospheric moisture. Moisture enters the atmosphere in the form of water vapor by evaporation and transpiration. Moisture enters the atmosphere in the form of water vapor by evaporation and transpiration. Collectively these are called evapotranspiration. Collectively these are called evapotranspiration.

Energy of Evaporation Also Called Vaporization Large amounts of energy are needed to change liquid water to water vapor. (540 cal./g) Large amounts of energy are needed to change liquid water to water vapor. (540 cal./g) Almost all this energy comes from insolation. Almost all this energy comes from insolation. Incoming Solar Radiation Incoming Solar Radiation

Factors Affecting Evaporation Rate Higher temperature. Higher temperature. –Higher temp. = faster evaporation Larger surface area. Larger surface area. –Larger surface area = faster evaporation Degree of saturation of air with water vapor. (Humidity) Degree of saturation of air with water vapor. (Humidity) –More water vapor = slower evaporation Wind speed. Wind speed. –Higher wind speed = faster evaporation

Humidity, Temperature Absolute humidity – The total amount of water vapor a volume of air can hold at a certain temperature. Increases with an increase in temperature. Absolute humidity – The total amount of water vapor a volume of air can hold at a certain temperature. Increases with an increase in temperature. Relative humidity – The amount of water vapor a volume of air is holding now. Usually expressed as a percent. Relative humidity – The amount of water vapor a volume of air is holding now. Usually expressed as a percent. If humidity is stable and temp.  then relative humidity . If humidity is stable and temp.  then relative humidity .

Dew Point If a volume of air has a certain relative humidity and the temperature begins to go down, the air will reach a point where it is at 100% humidity. If a volume of air has a certain relative humidity and the temperature begins to go down, the air will reach a point where it is at 100% humidity. The air is at a temperature where it can not hold any more moisture. That is the dew point. The air is at a temperature where it can not hold any more moisture. That is the dew point. Water vapor actually condenses on structures such as: grass, leaves and car windshields. Water vapor actually condenses on structures such as: grass, leaves and car windshields.

Finding Humidity and Dew Point Both are measured with a sling psychrometer (hand held) or hygrometer (electronic). Both are measured with a sling psychrometer (hand held) or hygrometer (electronic).  Take the temp. of both wet and dry bulbs. Find the difference. Go to the charts on page 12 of ESRT.