MIL-HDBK-1011/2
APPENDIX A (continued)
strategies, where the elimination of solar gains is a precondition to the
analysis. It is therefore beyond the scope of this handbook. Use
Architectural Graphic Standards or the ASHRAE Handbook of Fundamentals to
determine the shading from trees, buildings and building features. Check the
documentation of the building loads program being considered for the computer
simulation. If a loads program cannot handle external obstructions, it is
probably better to pick a more comprehensive computer program than to
preprocess the weather data to correct for local site shading effects.
2.2.3
Temperature. Temperature varies geographically with elevation and
surface type. Urban areas may have higher temperatures than the surrounding
rural terrain. Temperature data are most commonly available from
second-order local weather stations. The data are usually in the form of
monthly averages of daily maximum and minimum temperatures, obtained from
daily readings from a max-min thermometer. Such averages may be used to
adjust the bin data or hourly data from the weather station, by adding to each
bin value or hourly value the difference between the monthly averages at the
two locations. This technique might also be used to approximate an urban heat
island of estimated magnitude. Be aware that if the daily temperature range
(as described by the daily maximum minus the daily minimum) differs for the
two sites, this technique will not be accurate. Such differences may occur
between coastal and dry inland locations. Finally, each bin value or hourly
value may be adjusted for altitude differences at the adiabatic lapse rate of
5.4deg.F per 1,000 ft elevation (1deg.C per 100 m), with temperature
decreasing with
elevation.
2.2.4
Wind. The most important climate data extrapolations occur with
the wind. As with temperature, local records may be used to adjust the bin or
hourly data. Local records of wind are however far less common than local
temperature records, and are often of dubious accuracy due to poorly
positioned or maintained instruments. The most likely adjustment will be due
to local site influences. These could be assessed by setting up short-term
monitoring on site to obtain a local record, or by estimating the wind effects
of the local site based on some of the principles described below. For major
projects, a meteorologist should be consulted to make such estimates.
2.2.4.1
Topography. Topography has a pronounced effect on the wind at the
surface. Wind flow conforms to terrain, changing its strength, steadiness,
and direction as it passes over the uneven ground. Figure A-2 shows the
velocity profiles of wind approaching a hill or ridge, at the crest, and on
its leeward side. A strong acceleration is seen near the surface at the top
of the hill, and a flow reversal due to an eddy at low levels in its lee. In
general, the wind acceleration on the windward side of hills and ridges is
fairly predictable, but the extent of shelter in the lee is highly variable,
depending on the roughness of the hill and the stability of the atmosphere.
Wind may also be extensively channeled by topography. Figure A-3
shows two typical wind flow patterns identified in the San Francisco Bay
region. High ground is noted in gray. Local areas of wind turning in excess
of 90 degrees to the gradient wind may be noted. This type of channeling
occurs primarily when the atmosphere is stable, and the flows depicted extend
roughly to the height of the surrounding terrain. Similar flow turning and
channeling has been observed in street canyons.
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