METEOROLOGIST JEFF HABY
Other terms that mean the same as orographic lifting are
upslope flow, topographic uplift and
forced land lifting. When this type of lifting occurs the land forces the air to move
to a higher elevation. Several thermodynamic variables change when air rises. The temperature
of the air cools as air is lifted. In this situation it is common to notice cooler temperatures
in the higher elevation regions and warmer temperatures in the lower elevation regions. As unsaturated
air rises the
relative humidity of the air will increase. This occurs because the temperature
of the air is decreasing. As air cools the saturation vapor pressure of the air decreases. Since
relative humidity is (vapor pressure / saturation vapor pressure) * 100%, the relative humidity will
increase as saturation vapor pressure decreases toward the vapor pressure. Another way to think of this
is that more
moisture can evaporate into warmer air than cooler air. As temperature cools, not as much
moisture has to be evaporated into the air to reach the saturation vapor pressure. As air cools,
eventually the saturation vapor pressure will drop to the actual vapor pressure and the relative
humidity will reach 100%. When this occurs clouds and precipitation can develop. It is common
for an orographic lifting situation to produce cooler and damp weather especially if the
air has to rise a significant vertical distance.
The wind direction will determine which side of a high elevation region experiences orographic
lifting. Since weather in the mid-latitudes tends to move from west to east it is common
for the western side of high elevation regions to have significant orographic lifting. Examples
of such locations are the west side of the Cascades and Sierra Nevada range in the western U.S. In some
situations the wind will blow from another direction. Orographic lifting will occur in the
Rocky Mountains on the east side of the range when the wind is from the east. Since the flow
is counterclockwise around low pressure, winds will flow from the east on the north side of the
low pressure system. This can bring upslope rain and snow on the east side of the Rocky mountains
and out into the higher elevations of the plains.
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