Located squarely in the mid-continent,
hundreds upon hundreds of miles away from wet maritime air masses, South Valley
Park’s landlocked position very much determines its climate and the varieties
of its weather, notably its large diurnal and seasonal differences in
temperature, its persistent aridity, its sunshine and lack of clouds.
Three maritime sources and two continental
sources influence our mid-continental climate: the maritime polar (the wet and
cool Pacific Northwest); maritime tropical (the warm Pacific from the Baja area
of Southern California); maritime tropical (the Gulf of Mexico); the
continental tropical (Mexico); and the polar continental (north-central Canada).
The driving force behind the
movement is air is a term called atmospheric convection. When air is heated, it increases in volume
and then rises because of its greater buoyancy.
These vertical air currents deplete the surface of some its air, thus
creating regions of slightly lower pressure at the bottom of the rising air
column. As air is forced to move from
areas of higher pressure toward the newly created low-pressure area, a
horizontal pressure gradient develops.
These air movements vary in size and in their significance for the
climate, ranging from local winds, such as dust devils and warm southerly chinook
winds, to large-scale air currents in the upper levels of the atmosphere.
The largest-scale wind patterns are sometimes called
“the winds aloft." These are initiated
by pressure systems set up by latitudinal variations in insolation (incoming
solar radiation). In the Northern
Hemisphere, the winds aloft are deflected to the right by the Earth’s rotation,
spiraling counterclockwise around the lows and clockwise around the highs. Centers of low pressure are called cyclones;
centers of high pressure are called anticyclones. As a result of the Coriolis effect,
large-scale air motions move perpendicular to the pressure of gradient, that
is, around regions of low and high pressure, but not directly to or from
them.
The sharp north-to-south temperature
gradient that exists in the Northern Hemisphere during the winter months causes
the upper-air westerlies to intensify seasonally and with increasing
altitude. Consequently, winds in the
southern Rockies are nearly twice as strong in the winter as in summer. During the summer, when westerly airflow is
weak, the main air current and its associated polar front jet stream stray far
to the north, typically over Canada. By
winter, the westerlies and the main axis of the polar front jet tend to be
positioned over the Southern Rockies, guiding Pacific storm systems through the
region fairly regularly. However, during those winters when a large ridge in
the upper atmosphere keeps the polar front jet along or to the north of the
Canadian border, the southern Rockies experience a lack of snow, severe
drought, and unseasonably warm weather.
