Page 395 - Fundamentals of Air Pollution 3E

II. Representativeness 349

(1/16 of the compass), performing a calculation for each wind speed-stabil-
ity combination occurring for that sector, and weighting the calculation by
the frequency for this combination. For annual concentrations, this saves
a considerable number of calculations compared with simulating the period
hour by hour.
Mixing heights for each day can be calculated from the radiosonce data.
Such data for a 5-year period of record, 1960-1964, were calculated and
used in a study by Holzworth (2). Figure 21-2 shows the mean annual
afternoon mixing height variation across the contiguous United States,

II. REPRESENTATIVENESS

The term representativeness in air pollution meteorology usually means
the extent to which a particular parameter is measured by instrumentation
sited in such a way and with sensitivity and accuracy such that it is useful
for the designated purpose. For normal climatological purposes, wind mea-
surements are made at locations relatively free from observation; thus,
they are not influenced in different ways by winds coming from different
directions and consequently present an unbiased record. A parameter such
as wind, which varies with height above ground, must have the height of
the measurement reported along with the data.

Fig. 21-2. Mean annual afternoon mixing height, in hundreds of meters. Source: Adapted
from Holzworth (2).