Weather radars

The Finnish Meteorological Institute produces weather radar images for the public, its own and clients use. Radars are used in the Research Sector's activities, and they are important forecasting tools for meteorologists . It's convenient to know, whether it is raining or not, and especially, whether the rain is over.

Radar images help in precipitation forecasting

By watching a series of radar images from couple of hours, one can estimate where the precipitation areas are moving, and make short forecasts. Weather forecasting is not so simple: rain showers develop, grow and dissolve. A large precipitation area can be followed up to six hours, while the summertime afternoon showers live only half an hour to two hours.

 

How a radar works

Weather radars send pulses of microwave radiation. When a pulse meets obstacles, say raindrops, part of the energy of each pulse will scatter back in the direction of the radar station. The horizontal distance from station to target is calculated simply from the amount of time that lapses from the initiation of the pulse to the detection of the return signal.

A weather radar must be rather sensitive equipment, because the return signal is much weaker than the transmitted signal. Typically the transmitted peak power is typically 250 000 W, while return signal is only 0.000 000 000 000 1 - 0.000 000 01 Watts.

Because millimetre-sized raindrops fall typically 10 cm from each other, the microwaves can easily penetrate the precipitation area and bring information even from the inner parts and rear end of the precipitation area. Microwaves are easily scattered by raindrops and snowflakes, so a weather radar helps to see where precipitation occurs and how heavy it is. Heavier rain gives stronger echoes which are represented with different colours.