Instruments Used To Measure Precipitation 6i6y1x

1. INSTRUMENTS USED TO MEASURE PRECIPITATION

Standard Rain Gauge The standard NWS rain gauge, developed at the start of the 20th century, consists of a funnel emptying into a graduated cylinder, 2 cm in diameter, which fits inside a larger container which is 20 cm in diameter and 50 cm tall. If the rainwater overflows the graduated inner cylinder, the larger outer container will catch it. When measurements are taken, the height of the water in the small graduated cylinder is measured, and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall. Sometimes a cone meter is used to prevent leakage that can result in alteration of the data. In locations using the metric system, the cylinder is usually marked in mm and will measure up to 250 millimetres (9.8 in) of rainfall. Each horizontal line on the cylinder is 0.5 millimetres (0.02 in). In areas using Imperial units each horizontal line represents 0.01 inch.

Tipping Bucket Rain Gauges This type of rain gauge generates an electric signal (i.e., a pulse) for each unit of precipitation collected and allows automatic or remote observation with a recorder or a counter. The only requirement for the instrument connected to the rain gauge is that it must be able to count pulses. Thus, a wide selection of configurations and applications is possible for this measuring system. Solid precipitation can also be measured if a heater is set at the receptacle.

(1) Structure and Operation This type of rain gauge consists of a receiver and a measuring part, with the receiver serving as the container for the device (Figures 6.8). The measuring part consists of a tipping bucket and a pulse-generating reed switch (or mercury switch) assembled within the receiver. The tipping buckets consist of two triangular vessels attached to the left and right of a rotation shaft, each with a capacity equivalent to a specific amount of precipitation. The reed or mercury switch is connected to these tipping buckets to generate an electrical signal (i.e., a pulse) each time the buckets tip. (2) Operation Rainwater collected in the receptacle is channeled through the funnel and poured into a tipping bucket. When it reaches a predetermined amount, the bucket tips and dumps the water into a drain cylinder, causing the reed switch to generate a pulse. Subsequent rainwater is poured into the other bucket. As long as precipitation continues, this operation is repeated and a pulse is generated each time a bucket tips.

Weighing Precipitation Gauge A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass. Certain models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger. The advantages of this type of gauge over tipping buckets are that it does not underestimate intense rain, and it can measure other forms of precipitation, including rain, hail and snow. These gauges are, however, more expensive and require more maintenance than tipping bucket gauges. The weighing-type recording gauge may also contain a device to measure the quantity of chemicals contained in the location's atmosphere. This is extremely helpful for scientists studying the effects of greenhouse gases released into the atmosphere and their effects on the levels of the acid rain. Some Automated Surface Observing System (ASOS) units use an automated weighing gauge called the AWPAG (All Weather Precipitation Accumulation Gauge).

Optical Rain Gauge These have a row of collection funnels. In an enclosed space below each is a laser diode and a photo transistor detector. When enough water is collected to make a single drop, it drops from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of light. The flashes from these photo detectors are then read and transmitted or recorded.

Acoustic Rain Gauge The acoustic disdrometer developed by Stijn de Jong is an acoustic rain gauge. Also referred to as a hydrophone, it is able to sense the sound signatures for each drop size as rain strikes a water surface within the gauge. Since each sound signature is unique, it is possible to invert the underwater sound field to estimate the drop-size distribution within the rain. Selected moments of the drop-size distribution yield rainfall rate, rainfall accumulation, and other rainfall properties.

2. PAGASA RAINFALL ADVISORY

3. INSTRUMENTS USED TO MEASURE HUMIDITY HYGROMETER - instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation (the dew point), or changes in electrical capacitance or resistance to measure humidity differences. The first crude hygrometer was invented by Leonardo da Vinci in 1480 and a more modern version was created by polymath Johann Heinrich Lambert in 1755. 

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Metal-paper coil type The metal-paper coil hygrometer is very useful for dial indication of humidity changes. It appears most often in very inexpensive devices, and its accuracy limited, with variations of 10% or more. In these water vapour is absorbed by a salt-impregnated strip attached to a metal coil, causing the coil to shape. These changes (analogous to those in a bimetallic thermometer) cause an indication on a

giving a is devices, paper change dial.

Hair tension hygrometers These devices use a human or animal hair under tension. The hair is hygroscopic (tending toward retaining moisture); its length changes with humidity, and the length change may be mag nified by a mechanism and indicated on a dial or scale. In the late 1700s, such devices were called by some scientists hygroscopes; that word is no longer in current use, but hygroscopic and hygroscopy, which derive from it, still are. The traditional folk art device known as a weather house works on this principle. Whale bone and other materials may be used in place of hair. In 1783, Swiss physicist and geologist Horace Bénédict de Saussure built the first hair-tension hygrometer using human hair. It consists of a human hair eight to

ten inches[1] long, b c, Fig. 37, fastened at one extremity to a screw, a, and at the other ing over a pulley, c, being strained tight by a silk thread and weight, d. 

Sling psychrometer A sling psychrometer for outdoor use The sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes, is sometimes used for field measurements, but is being replaced by more convenient electronic sensors. Alternatively a whirling psychrometer uses the same principle, however the two thermometers are fitted into a device that resembles a ratchet or football rattle.

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Psychrometers (Wet and Dry Bulb thermometer) 1861 diagram of a psychrometer with wet bulb (a) and dry bulb (b). The wet bulb is connected to a reservoir of water. The interior of a Stevenson screen showing a motorized psychrometer A psychrometer, or wet-and-dry-bulb thermometer, consists of two thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb is covered with a thin coating of ice and may be warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wetbulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart. The two thermometers coincide when the air is fully saturated, and the greater the difference the drier the air. Psychrometers are commonly used in meteorology, and in the HVAC industry for proper refrigerant charging of residential and commercial air conditioning systems.

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Electronic hygrometer Chilled mirror dew point Hygrometers Dewpoint is the temperature at which a sample of moist air (or any other water vapor) at constant pressure reaches water vapor saturation. At this saturation temperature, further cooling results in condensation of water. Chilled mirror

dewpoint hygrometers are some of the most precise instruments commonly available. These use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface. The temperature of the mirror is controlled by electronic to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dew point temperature. An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±1.2%. These devices need frequent cleaning, a skilled operator and periodic calibration to attain these levels of accuracy. Even so, they are prone to heavy drifting in environments where smoke or otherwise impure air may be present.

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Modern hygrometers Capacitive For applications where cost, space, or fragility are relevant, other types of electronic sensors are used, at the price of a lower accuracy. In capacitive hygrometers, the effect of humidity on the dielectric constant of a polymer or metal oxide material is measured. With calibration, these sensors have an accuracy of ±2% RH in the range 5–95% RH. Without calibration, the accuracy is 2 to 3 times worse. Capacitive sensors are robust against effects such as condensation and temporary high temperatures. Capacitive sensors are subject to contamination, drift and aging effects, but are suitable for many applications. Resistive In resistive hygrometers, the change in electrical resistance of a material due to humidity is measured. [3] Typical materials are salts and conductive polymers. Resistive sensors are less sensitive than capacitive sensors – the change in material properties is less, so they require more complex circuitry. The material properties also tend to depend both on humidity and temperature, which means in practice that the sensor must be combined with a temperature sensor. The accuracy and robustness against condensation vary depending on the chosen resistive

material. Robust, condensation-resistant sensors exist with an accuracy of up to ±3% RH. 

Thermal In thermal hygrometers, the change in thermal conductivity of air due to humidity is measured. These sensors measure absolute humidity rather than relative humidity

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Gravimetric A gravimetric hygrometer measures the mass of an air sample compared an equal volume of dry air. This is considered the most accurate primary method to determine the moisture content of the air.[4] National standards based on this type of measurement have been developed in US, UK, EU and Japan. The inconvenience of using this device is that it is usually only used to calibrate less accurate instruments, called transfer standards.

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