The intensity of the charring varies according to the condition of the air as to cleanliness and freedom from moisture. If the air is clear and dry, the focal rays burn a long gap in the sheet. On the other hand, a moist or a polluted air may absorb
Sunshine differential thermometer, electrically connected with the recording apparatus.
so much heat that the surface of the recording chart is merely discolored. The record for the first half hour or more after sunrise is usually indistinct; this is usually true of the time just before sunset. If a cloud obscures the sun, even for a few minutes, the burning process is arrested.
The charts used in this type of sunshine recorder may be changed at any time between sunset and sunrise of the following
