Using clouds to spy during wartime
Contrail forecasting was important in wartime when highflying aircraft wished to avoid detection. Contrails or Condensation Trails form to the rear of high flying aircraft from condensation. Sometimes they can be seen on satellite pictures like criss crossing roads.

  Clouds - the rules
Low clouds are officially specified by international convention as being those based below 2.5km (8000 ft); high clouds have their bases from 6km (20,000 ft) upwards, and medium clouds are all of those between the two.