7.8: Precipitation Measurement
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The simplest precipitation instrument is a rain gauge, which is a cylindrical bucket into which the rain falls. By using a measuring stick to manually read the water depth in the bucket at successive times such as every hour, you can determine rainfall rate. For greater sensitivity, a funnel can be placed over the bucket to collect rain faster, but the depth of water in the bucket must be reduced by the ratio of the horizontal cross-section areas of the bucket to the funnel opening. To get the liquid-water equivalent of the snowfall, some rain gauges are heated to melt snow, and others are painted black to passively melt snow by absorbing sunlight. Some gauges are surrounded by a segmented wind shield to reduce errors due to blowing precipitation.
Automated rain gauges exist. Weighing rain gauges weigh the rain-filled bucket over successive intervals, inferring rain accumulation by weight increase, knowing the density of liquid water.
Tipping-bucket rain gauges direct the captured rain into a tiny bucket on one side of a lever. When the bucket gets full, it tips the lever like a seesaw (teeter-totter), emptying that bucket while simultaneously moving under the funnel an empty bucket from the other end of the lever. Each tip can be counted digitally, and the frequency of tips during an hour gives the rainfall rate.
Drip rain gauges funnel rainwater into a small orifice, under which individual drips of water form by surface tension. As each drip separates from the funnel orifice, it touches two conductors as it falls toward a drain, allowing each drip to be counted.
An evaporative rain gauge has two metal plates, one above the other, each oriented horizontally (one facing up, and the other facing down). Each plate is heated electrically to maintain the same specified temperature warmer than ambient air. Precipitation falling on the hot top plate evaporates quickly, thereby removing heat from that plate. By measuring the amount of extra electricity needed to keep the top plate at the same temperature as the bottom, and knowing the latent heat of vaporization, the rainfall rate can be determined.
Attenuation rain gauges have a light beam that shines horizontally across an open air path exposed to precipitation. The attenuation of the light beam is related to precipitation intensity, but errors can be due to air pollution, fog, and different absorption cross-sections of liquid vs. solid precipitation.
A disdrometer measures size distribution of rain drops via the momentum imparted to a horizontal plate by each falling drop. Another method is a particle imager that sends light from an array of light-emitting diodes to an array of tiny photodetectors. Each hydrometeor casts a shadow that can be detected, where the size and the shape of the shadows are used together to estimate precipitation rate, hydrometeor type and size. A Knollenberg probe uses this imaging method, and can be mounted on aircraft flying through clouds and precipitation.
A liquid-water content (LWC) probe consists of an electrically heated wire. When mounted on an aircraft flying through a cloud, the rain and cloud droplets evaporate upon hitting the hot wire. By measuring the electrical power needed to maintain a constant wire temperature against the evaporative cooling, the LWC can be inferred.
Snow amount on the ground can be measured by placing a liquid antifreeze-filled thin-skin metal snow pillow on the ground before the winter snow season. As snow accumulates during the season, the weight of the snow squeezes the pillow and displaces some of the fluid. Pressure sensors measure the weight of the displaced fluid to infer snow weight.
Downward-pointing ultrasonic snow-depth sensors mounted on a tall pole measure the travel time for an emitted pulse of sound to reach the ground and echo back to the sensor on the pole. This gives the distance between the top of the snow and the sensor, which can be subtracted from the sensor height above bare ground to give the snow depth. Similar sensors use travel time for IR or visible light pulses.
Remote sensors (see the Satellites & Radar chapter) can also be used to measure rain rate or accumulation. Ground-based weather radar actively emits microwaves, and can estimate rainfall rate from the echo intensity and polarization characteristics of the microwave signal that is scattered back to the radar from the precipitation particles. Passive microwave sensors on some weather satellites can measure the brightness temperature of the minute amounts of microwaves emitted from the Earth’s surface and atmosphere. With this info one can infer the atmospheric total water content in a column of the atmosphere (used to estimate tropical rainfall over the oceans), and can infer snow depth on the ground (over high-latitude regions).