Geoscience Reference
In-Depth Information
7
Measuring Surface
Heat Fluxes
Introduction
This chapter provides an overview of some of the most commonly used methods
by means of which important terms in the surface energy budget are measured.
Most measurement methods provide an estimate of the average rate of flow of
energy into or out of the Earth's surface over a specified period. However, latent
heat flux is also frequently measured in the form of the net evaporative water loss
from sample areas of the terrestrial surface.
Measuring solar radiation
The most common early approach used to derive surface solar radiation was to
estimate how much of the calculable solar energy entering the top of the atmosphere
was absorbed before it reached the ground. This required estimates or
measurements of cloud cover. Subsequently, instruments were devised which
measured the incident solar energy from the warming it induced when incident
on a near blackbody surface or from the number of electrons it mobilized in a
semiconductor.
Daily estimates of cloud cover
Estimates of cloud cover are made with the human eye by trained observers at
meteorological stations looking upward at the sky overhead. When estimates are
made in this way, they are usually expressed either in
oktas
(eighths of the sky) or
in
tenths
and given to the closest whole number value. A value of 0 refers to clear
