The Weather Guys

Radar, an acronym for Radio Detection and Ranging, was invented during World War II to detect aircraft, but precipitation frequently got in the way. The military’s noise is meteorology’s signal.

A radar consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern. Precipitation scatters these radio waves, sending some energy back to the transmitting point where it is detected by the radar’s receiver.

The intensity of this received signal, called the radar echo, indicates the intensity of the precipitation. Measuring the time it takes for the radio wave to leave the radar and return tells us how far away the storm is. The direction the radar is pointing locates the storm.

Uniquely, Doppler radar can measure the velocity of precipitation particles (and thus, the wind) in precipitating regions. A Doppler radar receiver “hears” waves of a higher frequency if precipitation particles are moving toward the radar and a lower frequency if particles are moving away. This allows Doppler radars to identify the detailed wind structure within severe thunderstorms. For example, if particles switch from moving toward and then away from the Doppler radar over a small distance, then a tornado is possible.

The National Weather Service is currently replacing its older radars with dual-polarization radars to improve observations of the interior of storm systems. A radio wave is an electromagnetic wave and therefore has electric and magnetic fields that are oriented perpendicular to one another. The orientation of these oscillations is referred to as polarization.

A polarizing filter for a camera, or polarizing sunglasses, can be used to observe the effects of polarization of light in a cloud-free sky. Rotate the filter, or glasses, while looking through them at a portion of the sky away from the sun — at a certain orientation, the intensity of the sky-light will be reduced. The filter is removing polarized light that is not aligned with the filter.

The additional information on polarization improves the precipitation rate measurement as well as the determining of the type of precipitation (snow, rain, freezing rain and possibly hail). The polarization radars can also measure information about both the horizontal and vertical dimensions of precipitation sized particles.

The typical speed of a falling raindrop depends on the size of the drop. Gravity pulls everything downward. As an object falls, it experiences a frictional drag that counters the downward force of gravity. When the gravity and frictional drag are balanced, we have an equilibrium fall speed that is known as the terminal velocity of the object. The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Thus, it is worth talking a bit about the shape and size of raindrops.

While cartoonists typically draw raindrops in a teardrop or pear shape, raindrops are not shaped in those forms. They are drawn as teardrops to give the image of falling through the atmosphere, which they do.

As the raindrops fall they are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through. Raindrops are at least 0.5 millimeters (or 0.02 inches) in diameter. You will not find a raindrop any bigger than about one-quarter of an inch in diameter; larger than that, the drop will break apart into smaller drops because of the air resistance. Precipitation drops smaller than 0.02 inches in diameter are collectively called drizzle, which is often associated with stratus clouds.

The terminal velocity of cloud droplets, which are typically about 10 microns in radius or 0.0004 inches, is about 1 centimeter per second, or about 0.02 miles per hour. Tiny cloud droplets can stay in the atmosphere because there is upward moving air that overcomes the force of gravity and keeps them suspended in the cloud. Only a very gentle upward movement of air is required to keep them afloat.

Raindrops are larger. A large raindrop, about one-quarter of an inch across or about the size of a house fly, has terminal fall speeds of about 10 meters per second or about 20 mph. That kind of speed can cause compaction and erosion of the soil by their force of impact. Raindrops are of different sizes, and the smaller raindrops are traveling about 2 mph.

The National Weather Service, or NWS, is a part of National Oceanic and Atmospheric Administration (NOAA). The NWS provides “weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy.”

The NWS makes and collects surface, marine and atmospheric observations and distributes them nationally and internationally. Professional meteorologists and private forecasting companies often interpret this information provided by the NWS in their weather analysis. In addition to issuing severe weather and marine watches and warnings, the NWS is responsible for computer weather model forecasts, which many forecasters rely on in making their local forecast.

The NWS formed in 1870 through a joint resolution in Congress. It was originally operated by the U.S. Army Signal Corps in the Department of War and made meteorological observations at military stations.

The organization was moved to the Department of Agriculture and renamed the U.S. Weather Bureau in 1891. In 1940 the Weather Bureau became part of the Department of Commerce.

Today the NWS is headed by Dr. Louis Uccellini, a graduate of the University of Wisconsin-Madison, continuing a strong connection between the organization and the state of Wisconsin. The first public storm warning was issued for a Great Lakes storm on Nov. 8, 1870 by Professor Increase Lapham of Milwaukee. On Jan. 3, 1921, UW-Madison’s experimental radio station made the first media weather forecast. Professor Verner Suomi of UW-Madison is known as the Father of Satellite Meteorology; weather satellites are a critical component of the various NWS activities.