# Microwaves and Water Molecules: Radar vs Cooking

I am studying microwave radiometry and confused over this apparent contradiction: Microwaves emitted at 2.45 GHz (~15 cm) are absorbed by water molecules, causing the molecules to rotate under the influence of the electric field component so producing kinetic (thermal) energy that cooks food. Going out to 3 GHz, (~10 cm), we see that weather radar pulses are scattered by water molecules in rain drops (and other airborne particulates), which is the source of the "echo" used to create radar maps. Both types of microwaves are in the S-band, so what explains the difference in absorption by water molecules at 2.45 GHz vs. scattering (reflection) at 3 GHz? Are microwave echoes detected via weather radar actually "scattered" or are they being absorbed and re-emitted according to Kirchoff's radiation law?

In a radar system much of the incident radiation reflects or scatters off of the higher effective dielectric permitivity of the air mass containing water vapor and droplets. There is some absorption, but the reflected energy is easily detected, providing information about positon and velocity of the clouds. Since power in the transmitted beam falls off as $$1/r^2$$, and the radar is usually pulsed, energy density is low compared to that in a microwave oven.