SYS11000OM Rev G
[UF 350-HD, Generic Template]
FLIR Systems Polytech AB
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1.12.4
Cloud Cover
Cloud cover impacts the diurnal heating and cooling in two ways. Firstly, increased cloud
cover decreases the amount of radiation striking the earth’s surface. Secondly, the
clouds form a kind of insulating blanket over the Earth which reduces the loss of heat
from the Earth into space by reflecting, scattering, absorbing, and re-radiating IR
radiation back to the Earth.
The net effect is to decrease the amplitude of diurnal cycle induced temperature
excursions by keeping days cooler and nights warmer.
1.12.5
High Humidity
High humidity like clouds tends to dampen diurnal temperature excursion. Unlike clouds,
humidity does not noticeably reduce radiation to the earth’s surface. But like clouds, the
water molecules do absorb and re-radiate IR radiation being emitted by the objects on
the Earth at night, thus tending to keep the nights warmer. In other words, high humidity
is synonymous with warmer nights and a reduced diurnal contrast swing at night.
1.12.6
Precipitation
Precipitation in the form of rain tends to make all objects it falls on a similar temperature.
Two mechanisms cause this phenomenon. First, since rain is usually cooler than either
the surface air or objects upon which it falls, it tends to drive all objects towards cooler
more uniform temperatures. This is further amplified by the eventual evaporation of the
water, which provides an additional cooling mechanism. Additionally, as rain wets the
surface of objects, it moves their emissivity toward (if not to) that of water. Thus, rain
tends to make the scene target and background objects similar temperatures,
substantially reducing contrasts.
1.12.7
Wind
Convective heat transfer is a function of the degree to which the convective medium (in
this case air) is in motion. The more wind, the more the air is able to transfer heat from
objects to the air. Wind tends to be an additional contrast-reducing mechanism. Unlike
some contrast reducers however wind not only lowers contrast but also tends to modify
thermal shapes due to its directional effects.
