|HOW DOES RADAR DETECT|
METEOROLOGIST JEFF HABY
Radar sends out a certain wavelength of electromagnetic radiation. This electromagnetic radiation is of the
longwave variety and therefore will not damage the environment. The energy emitted from the radar travels at
about the speed of light as does all electromagnetic radiation. The speed of light is 299,800,000 m/s. With
this speed, radar can sample hydrometeors during one pulse in a small fraction of a second. Radar can
typically send and receive radiation between 200 and 3,000 times in one second. The number of pulses radar
sends out is called the Pulse Repetition Frequency.
Radar sends out electromagnetic radiation that strikes hydrometeors in the atmosphere. Some of this radiation
reflects back directly toward the radar set. This energy that is reflected directly back is called
backscattered radiation. The amount of energy the radar receives (compared to the amount it sends out) is
very small. An example of this is to think about the amount of light reflected off Mars and seen on Earth
compared to the total energy emitted from the Sun. The Earth only receives a very small amount of the total
energy that the sun gives off that is reflected off Mars and toward the Earth.
The amount of backscattered radiation the radar receives depends on the number of hydrometeors, the size of
hydrometeors and absorption qualities of the hydrometeors such as density, shape and other unique properties.
An object with a higher density, a larger surface area facing perpendicular to the radar's energy and
composition of a more reflective material will send relatively more backscattered energy to the radar set.
Radar is an
active remote sensor. It sends out as well as receives electromagnetic radiation. The
backscattered energy it receives is processed by the computer and put into a graphical form for the
radar user. Higher amounts of backscattered energy correlate with higher
VIP values on the graphical
display. Because electromagnetic radiation travels at a known speed and the radar can process the
time between radiation being emitted and received, a known distance to the hydrometeors can be
calculated. Velocity (m/s) * Time (s) = Distance (meters). With the information of distance and
amount of backscattered radiation, the data can be plotted on a graph showing intensity of
precipitation and the direction / distance the precipitation is from the radar set.