tual and modeled ice accretion shapes in freezing
four present weather code fields in the meteoro-
rain are discussed in section 3. The factors affect-
logical records. The present weather code is as-
ing the collision efficiency of freezing rain drops,
sumed to apply for the entire hour it is reported.
in comparison with cloud droplets, are explained
Only freezing rain or drizzle, rain with the air
in section 7. The calculation of the wind load on an
temperature below freezing, or mixed precipita-
ice-covered structure using modeled ice loads is
tion freeze to the structure. In particular, the mod-
presented in section 8. Accreted ice thicknesses
els do not attempt to calculate the accretion of
and masses predicted by the two models are com-
snow.
pared in section 9.
ZRAIN also requires the history of the anemom-
eter height above ground at the weather station.
The wind speed at a specified height above ground
is obtained from the measured wind speed, using
2. HOURLY WEATHER DATA
the anemometer height (ft) and assuming a 1/7
Weather elements required in the detailed heat-
power law and exposure C (ASCE 1994). Typi-
balance model are
cally, anemometer heights in the United States
have varied between 15 and 100 ft above ground,
Present weather code
so the correction of the wind speed to 10 m (the
Wind speed
worldwide default anemometer height) or 30 m
Precipitation rate
(typical height of a transmission-line wire) may be
Air temperature
significant (Fig. 1). Anemometer height histories
Dew-point temperature
for first-order weather stations are available in
Solar radiation
Local Climatological Data Annual Summary (NOAA,
Atmospheric pressure.
yearly), and for military stations from ETAC in
electronic format.
Only present weather code, precipitation rate, and
Solar radiation data are used to determine the
wind speed are used in the simple model. Conve-
solar heat flux at the accretion surface. Typically,
nient, but not consistent, units are used in ZRAIN:
there is little solar radiation during freezing rain
wind speed is in m/s, precipitation rate in mm/hr,
because of the complete cloud cover and the rela-
solar radiation in W/m2, atmospheric pressure in
tively low solar angle in winter. During the day,
mbar, and air temperature and dew-point tempera-
solar radiation fluxes in these conditions range up
to about 200 W/m2, with values of about 80 W/m2
ture in C. The dew-point temperature, along with
the air temperature, is used to determine the rela-
more common. If solar radiation information is
tive humidity of the air:
not available, then it can be assumed that there is
no incoming solar radiation. This is true at night,
Rh = edewT/eT ,
(1)
but this assumption will reduce the total heat flux
from the actual amount during the day and may
where eT and edewT are the saturation vapor pres-
slightly increase the hindcast ice load.
sures of air over water at temperatures T and dewT,
Atmospheric pressure is used in calculating the
density ρa and saturation vapor pressure eT of air
respectively. The diameter of the structural ele-
and the diffusivity of water vapor in air κm. The
ment on which the ice load is to be determined is
specified in centimeters. Appropriate factors are
dependence of these parameters on atmospheric
incorporated in the model to convert to a consis-
pressure is not great. For a 50-mbar pressure change
from 1000 mbar, ρa and κm change by 5% and eT by
tent set of units.
Neither model predicts freezing rain. They both
only 0.02%. If atmospheric pressure data are not
rely on the weather observer having indicated by
available, then the pressure appropriate for the
the present weather code when freezing rain occur-
elevation of the site may be assumed, with little
red. A number of present weather codes are used
effect on the hindcast ice load.
to record various intensities of freezing drizzle,
Most weather elements are measured every
freezing rain, and mixed precipitation. ZRAIN
hour and are reported as an instantaneous value,
assumes that the user has screened the weather
rather than as an average over the hour. In the
data, extracted the freezing-rain events, and sum-
heat-balance model I assume that temperature,
marized the occurrence of precipitation in each
dew-point temperature, atmospheric pressure, and
hour as freezing rain or drizzle (Z), mixed precipi-
wind speed vary linearly between the reported
tation (+), rain (R), or snow (S) by examining the
values. In each hour there are six 10-min time
2
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