Instruments Used in Meterorology
Josie Lumbard and Annastasia Hulse
What does a barometer measure and how does it work?
A barometer is a instrument used in meterorology to measure the pressure of the atmosphere and it can also be used to measure altitude. Pressure is measured by the height of a column of mercury supported by the atmosphere. Weather forecasters use barometers to detect changes in air pressure. Barometers can also be used to measure altitude because air pressure decreases as altitude increases. There are two kinds of barometers, mercury and aneroid. A mercury barometer consists of a glass tube of mercury with a reservoir at the bottom. Changes in air pressure cause the mercury in the tube to rise and fall. An aneroid barometer measures the effect of air pressure on a metal chamber from which part of the air has been removed.The dial may be scaled in millibars, inches, or millimeters. These light and portable barometers are widely used in homes, offices, and schools, and on ships and airplanes. Scientists use a type of aneroid barometer called a barograph to record changes in atmospheric pressure. A barograph includes a pen that records the air pressure on a paper chart mounted on a rotating drum. Some uses of the barometer are that in weather forecasting, the barometer's chief function is to determine the sea-level pressure and its change. A change in pressure usually means the weather will change. In general, cloudy weather occurs in low-pressure areas, and clear weather occurs in high-pressure areas. The other major use of barometers is to measure altitude. Atmospheric pressure is lower at higher altitudes because the air there is thinner and has less air above to weigh it down. For example, the average air pressure drops from 1,013 millibars at sea level to about 700 millibars at an altitude of 10,000 feet (3,000 meters) and to about 300 millibars at 30,000 feet (9,100 meters). Pilots use a kind of barometer called a pressure altimeter to determine their altitude. Its scale shows altitude rather than pressure. In addition, hikers and mountain climbers may use barometers to measure altitude.
What does a psychrometer measure and how does it work?
A psychometer or most commonly called the hygrometer, measures the moisture. It is spun around in the air for a few minutes. Six general kinds of hygrometers can be differentiated based on the physical processes they use in assessing humidity; cooling by evaporation of water, a change of dimension of a substance through water- vapor absorption, condensation of vapor by the cooling of air, there could be a change of chemical or physical properties of a substance through absorption of vapor, there could be a diffusion of vapor through a porous membrane and absorption of radiation by vapor. The psychrometer consists of two thermometers mounted on the same frame. One type of psychrometer, called the sling psychrometer or the whirled psychrometer, has a frame that can be whirled in the air by hand. The bulb of one thermometer is covered with a tight-fitting muslin sack and wetted with water. This thermometer is known as the wet-bulb thermometer. The other is the dry-bulb thermometer. The psychrometer is whirled to provide ventilation for the bulbs. The dry bulb indicates the temperature of the air. The wet bulb helps determine the relative humidity. When the sling psychrometer whirls through the air, water from the muslin evaporates. The evaporating water cools the wet bulb. The amount of cooling that occurs depends on the relative humidity. The lower the humidity, the faster the water in the muslin will evaporate, and the more the bulb will cool. High humidity will cause less evaporation, slowing the cooling process. In air that has less than 100 per cent relative humidity, the wet bulb will record a lower temperature than the dry bulb. This difference in temperature is known as wet-bulb depression. A special chart is used to convert the wet-bulb depression to relative humidity. The Assman psychrometer, an extremely accurate type of psychrometer, has a built-in fan. The fan draws the proper amount of air through two stationary metal tubes. These tubes hold wet-bulb and dry-bulb thermometers. The hair hygrometer uses a bundle of human hairs to detect relative humidity. The length of the hair increases as it absorbs moisture from the air. One end of the bundle of hair is anchored. The other end of the bundle is attached to a lever that moves a pointer on a scale. As the humidity lengthens or shortens the hair, the lever moves the pointer on the scale. Other hygrometers measure relative humidity by different methods. An absorption hygrometer uses a chemical that absorbs water vapor from the air. The chemical gets heavier as it absorbs the moisture, and the change in weight is measured to determine relative humidity. An electrical hygrometer uses carbon or some other substance whose electrical resistance responds to changes in humidity. A dew-point hygrometer measures dew point, the temperature at which the air has 100 per cent relative humidity. Its polished surface is chilled until dew appears. A device on the surface measures the temperature at which the dew forms.The hair hygrometer uses a bundle of human hairs to detect relative humidity. The length of the hair increases as it absorbs moisture from the air. One end of the bundle of hair is anchored. The other end of the bundle is attached to a lever that moves a pointer on a scale. As the humidity lengthens or shortens the hair, the lever moves the pointer on the scale. Other hygrometers measure relative humidity by different methods. An absorption hygrometer uses a chemical that absorbs water vapor from the air. The chemical gets heavier as it absorbs the moisture, and the change in weight is measured to determine relative humidity. An electrical hygrometer uses carbon or some other substance whose electrical resistance responds to changes in humidity. A dew-point hygrometer measures dew point, the temperature at which the air has 100 per cent relative humidity. Its polished surface is chilled until dew appears. A device on the surface measures the temperature at which the dew forms.
What does an anemometer measure and how does it work?
An anemometer is an instrument that is used in meterorology that measures the wind speed. The faster the three cups the higher the wind speed. . The wind makes the cups rotate regardless of the wind direction. Anemometers are usually divided into two classes: one class is designed for measuring wind speed, and the other is designed to measure wind pressure. However, many anemometers will typically provide information for both. Weather stations use modern anemometers to gauge wind and how it relates to humidity, temperature and pressure. There are also anemometers available that help with other meteorological findings. The five basic types of anemometers available for are: Sonic, windmill, hotwire, cup and hotwire. Each uses their own particular method for providing measurements and testing the wind speed and one may be more beneficial than the other in certain situations. A handheld anemometer is typically very compact, measuring only a few inches, and extremely useful for gathering wind information. The readings include the speed of the wind in different measurements (mph, m/s, km/h or Knots), wind chill and temperature. Sudden changes in the wind speed or direction, as well as wind current, do not usually interfere with the accuracy of handheld anemometer readings. This information is often registered on a dial on the anemometer. However, it can be transmitted by electrical means to display devices located some distance from the anemometer itself.
What does a thermometer measure and does it work?
A thermometer is a graduated instrument that measures the intensity of heat. The way is works is that there is mercury in there and as it moves up it’s getting hotter or if it moves down it’s getting colder. Outdoor and clinical are two types of thermometers. A thermostat and a thermocouple are some uses for a thermometer. One thermometer that you are probably familiar with is the outdoor thermometer. It uses the expansion and contraction of a liquid to measure temperature. The liquid in the bulb (usually ethyl alcohol with colouring added) expands when it is warmed and is forced up the narrow bore on its temperature. The more the liquid cools. It contracts, dropping lower in the bore. Another thermometer that you are probably familiar with is the clinical thermometer. It also uses expansion and contraction of a liquid to measure temperature. But it must be sensitive to very small changes in temperature, and able to measure temperatures only within a few degrees of normal body temperatures (37ºC).
Explain how the temperature can be different from its measurement. (HINT: What can happen that makes it feel colder? hotter?
The temperature can be different from its measurement because of humidity. Humidity is a term that describes the amount of water vapor in the air. The warmer the air is, the more water vapor it can hold. When the air contains as much water vapor as it can hold at a certain temperature and pressure, the air becomes saturated. This is called relative humidity. If the air contains only half of the water vapor it can hold, the relative humidity is 50 percent. In clouds and fog, the air is saturated and the relative humidity is 100 percent. The relative humidity of an area may vary greatly during the day,. In such cases, the relative humidity changes as the temperature rises and falls. The relative humidity may be high in the morning, when the temperature is low and the air cannot hold much more than its present water vapor. But as the temperature rises during the day, the air can hold more water vapor and the relative humidity becomes lower. As air at a fixed pressure and with a constant amount of water vapor is cooled, it will reach a temperature where it becomes saturated. This temperature is called the dew point. If the temperature is lowered further, water vapor will begin to condense and form clouds, fog, or dew. The closer the temperature of the air is to its dew point, the higher the relative humidity will be.
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