Chapter 7 (continued) Weather instruments

7.4 Accurate weather forecasting requires details about present weather conditions over a wide region. The National Weather Service has established about 300 weather stations to supply data (information) to construct weather maps. There are 5 weather conditions we must understand related to forecasting. They are temperature, pressure, humidity, wind direction, and wind speed.

7.4 -Measuring Temperature Thermometers are devices used for measuring temperature Many thermometers have contained mercury. Mercury is the only liquid metal, and it is the heaviest liquid with a density 13.6 times greater than water.

7.4 -Measuring Temperature Most thermometers used today are alcohol thermometers. Alcohol thermometers are less accurate than mercury thermometers and contain a red liquid. Many meteorologists use a maximum-minimum thermometer that measures both the highest and lowest temperatures during a time period.

7.4 -Measuring Temperature and Pressure Thermographs are recording thermometers that create a chart with a continuous recording of the temperature. Meteorologists also use barometers. Barometers are tools used to measure air pressure. Mercury barometers consist of a long tube sitting in a dish of mercury. Downward air pressure would cause the mercury in the tube to rise. Normal air pressure at sea level is recorded at 29.92 inches of mercury.

7.4 -Measuring Pressure essay question (hint, hint) Many meteorologists do not use a mercury barometer because of a few disadvantages. Some are: An instrument over 30” high is awkward to use. A mercury barometer must be kept upright to prevent spillage. The preferred instrument to measure air pressure is an aneroid barometer. Aneroid barometers are small, portable instruments without liquid that measure air pressure.

7.4 -Measuring air pressure and humidity Standard pressure is the average air pressure at sea level. It is expressed in several ways. 29.92” of mercury 760mm of mercury 1,013 mbar (millibars) Humidity In order to make a forecast, meteorologists must know the humidity. Humidity is the percent of water vapor in the air. Relative humidity is the percent of the maximum amount of humidity possible at the current temperature. Hygrometers and psychrometers are used to measure humidity.

7.4 -Measuring humidity A hygrometer is any instrument used to measure humidity. One of the most commonly used hygrometers is a psychrometer. Psychrometers consist of two identical thermometers mounted side by side with the bulb of one thermometer being dry and the other thermometer being wet. The difference in the two recorded temperatures can be used to find the relative humidity. Wet-bulb depression is the difference in temperature between the two bulbs of a psychrometer.

7.4 -Measuring humidity A hair hygrometer uses actual hair which shortens and lengthens to measure humidity. A hygrograph gives a continuous recording of relative humidity on a chart. A hygrothermograph gives a continuous recording of temperature and humidity on a chart.

7.4 -Measuring wind direction Knowing wind direction is very important because it gives understanding of the direction of approaching storms. A wind vane is an arrow that points in one of the directions of the compass because of the movement of the wind.

7.4 -Measuring wind speed Wind speed and direction have been very important throughout time, especially for sailors. Sailors commonly express speed in knots. A knot is 1 nautical mile per hour. A nautical mile is equal to about 1.15 statute miles (ordinary miles)

Measuring wind speed and precipitation Scientists must also be able to measure wind speed. A tool that is used to measure wind speed is an anemometer. The preferred tool to measure wind speed is a Robinson cup anemometer. A Robinson cup anemometer has 3 or 4 hemispherical cups that catch the wind blowing in any direction and cause the shaft to spin faster as wind speed increases. Scientists also use a wind sock. Wind socks show both wind speed and wind direction. Sailors often use the Beaufort scale to estimate wind speed. Measuring Precipitation A rain gauge can be used to measure the amount of precipitation.