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WIND, HUMIDITY, AND AIR PRESSURE Many factors such as temperature, wind, air pressure, and moisture make up what is called weather. The following are a few very general suggestions for obtaining information and making predictions about weather from wind: Use a wind vane to find patterns of wind direction and sub- sequent changes in weather. For example, consider the west coast of the United States. Winds from the south, southwest, west, and northwest usually bring rain because they blow from the ocean. Winds from the northwest, north, northeast, and east usually bring fair and cooler weather because they come from cooler land areas. On the east coast of the United States, winds coming from the west, northwest, or north usually bring fair and cooler weather because they come from cooler land farther north or from drier land farther west. Winds from the northeast, east, southeast, and south usually bring rain because they blow from the ocean. You might also use an anemometer ("ann-e-MOM-et-ter": an instrument for measuring wind direction and speed), or the Beaufort Scale to determine how long it takes for certain types of weather to move into your locale. The ("BO-fort") Scale is a scale of wind velocity (speed). The U.S. National Weather Service uses an adaptation of this scale. It employs numbers from 0 to 12, representing calm, light air, light breeze, gentle breeze, moderate breeze, fresh breeze, strong breeze, moderate gale, fresh gale, strong gale, whole gale, storm, and hurricane. Zero (calm) is a wind velocity of less than 1 mile per hour and 12 (hurricane) represents a velocity of over 75 miles per hour (Concise Columbia Encyclopedia). We'll have to find out if velocity is (or is not) a factor in tornadoes. Are cyclones and tornadoes the same things? Cyclones, according to Concise Columbia Encyclopedia, are masses of rotating winds, spinning clockwise in the southern hemisphere and counter-clockwise in the northern hemisphere. Tornadoes, are dark, funnel-shaped clouds containing violently rotating air that develops below a heavy cumulo- nimbus** cloud mass and extends toward the earth. The diameter of a tornado varies from a few feet to a mile; the rotating winds reach velocities of 200 to 300 miles per hour, and the updraft at the center may reach 200 miles per hour. In comparison with a cyclone, a tornado covers a much smaller area but is much more violent and destructive. The atmo- spheric conditions required for the formation of a tornado include great thermal instability, high humidity, and the convergence of warm, moist air at low levers with cooler, drier air above. Tornadoes occurring over water are called "waterspouts." (C.C. Encyclopedia) ** ("CEUM-you-low-NIM-bus"; an extremely dense, vertically developed cumulus cloud with a relatively hazy outline and a glaciated top, usually producing heavy rain, thunderstorm, or hailstorm. American Heritage Dictionary.) We can make some kinds of predictions about weather from air pressure: Until weather satellite photos became available, the barometer was the major instrument used to forecast long term changes in the weather. You can use a barometer to identify patterns between readings and subsequent weather changes. You will find that a rising barometer usually means the approach of a high pressure air mass that generally brings increasingly cool, heavy air and better weather. A falling barometer usually means the approach of a low pressure air mass that generally brings increasingly warm, light air and stormy weather. A barometer is an instrument for measuring atmospheric pressure. The mercurial barometer consists of a mercury- filled glass tube, that is sealed at one end and inverted in a cup of mercury. Pressure on the surface of the mercury in the cup supports the mercury in the tube, which varies in height depending on variations in atmospheric pressure. At 32o F, standard sea-level pressure (1 standard atmosphere) is 14.7 lb/in.2 (14.7 pounds per square inch, which is equivalent to a column of mercury 29.92 inches in height. The aneroid barometer contains a sealed, partially evacuated metallic box. As the air pressure on it varies, one of its surfaces expands or contracts; this motion is transmitted by a train of levers to a pointer, which shows the pressure on a graduated scale. (C.C. Encyclopedia) Will Rogers said something like "everybody talks about the weather, but nobody does anything about it." We can't do anything about it, but storm warnings are helpful, in that they give us a chance to survive. Here's what Mark Twain had to say about barometers and weather forecasting in general, in his speech to the New England Society on December 22, 1987: "Probable nor'east to sou'west winds, varying to the southard and westard and eastard and points between; high and low barometer, sweeping round from place to place; probable areas of rain, snow, hail, and drought, succeeded or preceded by earthquakes with thunder and lightning." Frequently, people complain that changes in barometric pressure has made their bones ache. They also complain that wind makes people irritable, because it puts electricity into the air. Then there are the hair-dressers who excuse their poor work by saying the weather caused hair to be unmanageable or frizzy, or something. We don't know whether or not this stuff is true, but when one person says it, the other person nods and makes knowing, agreeing comments. Weather predictions are often made from moisture information. An hygrometer ("hi-GROMM-itter") can be used to measure the amount of water (humidity) found in the air. The most common type of hygrometer is the dry- and wet-bulb psychrometer ("sigh-CHROMM-itter") It consists of two mercury thermometers, one of which has a wet wick around its bulb. The sling type of psychrometer is swung around in the air. Water evaporating from the wick absorbs heat from the bulb, causing the thermometer reading to drop. The observer, after reading the dry-bulb temperature and the drop in the wet-bulb temperature, can determine the relative humidity from appropriate tables. Among other kinds of hygrometers are ones that use human hair or electrical resistance, rather than thermometers, to determine moisture content. (CC Encyc) In general, you will find that the humidity of the air increases before a rain and decreases afterwards. In terms of approaching weather fronts, the humidity will usually change as shown. We have seen advertisements for a glass swan, cut in half and mounted on a well-finished and highly polished board. One puts colored water into the swan until it is a little more than half full, and hangs this arrangement on the living room wall, and calls it a "barometer." It is said that when the pressure rises, the colored water goes up into the swan's neck; when the pressure falls, the water goes back down into the body of the swan. We don't know if it really works. Folk "instruments," just like folk remedies, often function very well. You could buy one for your mom, and find out if it works or not. (Moms really hate things like that, but they love their kids, and treasure the kids' gifts.) Any problems with this page? Send URL to
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