Mountain Terrain, Weather, and Hazards

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Local weather patterns force air currents up and over mountaintops. Air is cooled on the
windward side of the mountain as it gains altitude, but more slowly (3.2 degrees Fahrenheit per
1,000 feet) if clouds are forming due to heat release when water vapor becomes liquid. On the
leeward side of the mountain, this heat gained from the condensation on the windward side is
added to the normal heating that occurs as the air descends and air pressure increases. Therefore,
air and winds on the leeward slope are considerably warmer than on the windward slope, which
is referred to as Chinook winds. The heating and cooling of the air affects planning
considerations primarily with regard to the clothing and equipment needed for an operation.

WEATHER FORECASTING
1-92. The use of a portable aneroid barometer, thermometer, wind meter, and hygrometer help in making
local weather forecasts. Reports from other localities and from any weather service, including USAF, USN,
or the National Weather Bureau, are also helpful. Weather reports should be used in conjunction with the
locally observed current weather situation to forecast future weather patterns. Weather at various elevations
may be quite different because cloud height, temperature, and barometric pressure will all be different.
There may be overcast and rain in a lower area, with mountains rising above the low overcast into warmer
clear weather. To be effective, a forecast must reach the small-unit leaders who are expected to utilize
weather conditions for assigned missions. Several different methods can be used to create a forecast. The
method a forecaster chooses depends upon the forecaster’s experience, the amount of data available, the
level of difficulty that the forecast situation presents, and the degree of accuracy needed to make the
forecast. The five ways to forecast weather are—

PERSISTENCE METHOD
1-93. “Today equals tomorrow” is the simplest way of producing a forecast. This method assumes that the
conditions at the time of the forecast will not change; for example, if today was hot and dry, the persistence
method predicts that tomorrow will be the same.

TRENDS METHOD
1-94. “Nowcasting” involves determining the speed and direction of fronts, high- and low-pressure centers,
and clouds and precipitation. For example, if a cold front moves 300 miles during a 24-hour period, we can
predict that it will travel 300 miles in another 24-hours.

CLIMATOLOGY METHOD
1-95. This method averages weather statistics accumulated over many years. This only works well when
the pattern is similar to the following years.

ANALOG METHOD
1-96. This method examines a day’s forecast and recalls a day in the past when the weather looked similar
(an analogy). This method is difficult to use because finding a perfect analogy is difficult.

NUMERICAL WEATHER PREDICTION
1-97. This method uses computers to analyze all weather conditions and is the most accurate of the five
methods.

RECORDING DATA
1-98. An accurate observation is essential in noting trends in weather patterns. Ideally, under changing
conditions, trends will be noted in some weather parameters. However, this may not always be the case. A
minor shift in the winds may signal an approaching storm.

26 July 2012

TC 3-97.61

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