Soil moisture

The amount of water held in the top layer of soil, usually given as the fraction of the soil’s volume that is water. Satellites estimate it for the upper few centimeters of ground.

Why it matters

Soil moisture links the water, energy, and carbon cycles — it controls how much rain runs off versus soaks in, drives drought and flood risk, and tells farmers when crops are stressed.

Where you’ll meet it

• SMAP (Soil Moisture Active Passive) is NASA’s dedicated soil-moisture mission, using a passive-microwave radiometer to map global surface moisture every 2-3 days.
• AMSR-E / AMSR2 passive-microwave sensors also retrieve soil moisture as one of several land and ocean products.
• ESA’s SMOS mission, often used alongside NASA data, measures soil moisture from the natural microwave glow of the land.

In plain terms

It’s like checking a sponge — a dry sponge soaks up a spill, but a soaked one just lets the water run off the counter.