The figure to the left shows the temperatures (degrees F on the left, degrees C on the right) observed in Boulder during a downslope windstorm. Notice the enormous and sudden temperature jump around 5 a.m., several hours before sunrise in December. Click on the figure to see a larger version.
The figure to the right shows the wind speed measured in Boulder during the same windstorm. At 5 a.m. the winds, which before that time stayed below 10 knots, began gusting to 50 to 60 knots (approaching hurricane force). These winds were blowing from the west, which, in Boulder, means the winds were blowing down hill ("downslope").
As the air descends the foothills to the west of Boulder, it is compressed by the higher pressure at lower elevations. This raises the temperature of the air. The compression also dries the air, lowering its relative humidity. Conversely, air that rises or is lifted will expand, and therefore cool to a lower temperature.)
For more details, check pp. 113-144 and 166-168 of Ahrens.
Figure 4 (below right) shows what happens if Boulder happens to be in a pool of cold air (caused by nighttime cooling or due to a cold air mass sitting over the plains of eastern Colorado) when a Chinook arrives, the temperature can rise suddenly and dramatically. Conversely, if chinook winds weaken or cease, the cold air can move back in from the east, and the temperature will drop just as suddenly.
Chinook Winds and Pressure
Since chinook winds blow from the west across the mountains, they are caused by High pressure west of Boulder, Low pressure over or east of Boulder, and strong westerly winds observed at mountain top level. This is because it is the pressure difference across Colorado that moves the air, with the wind blowing from High to Low pressure, and with the speed depending on the magnitude of the pressure difference. The pressure difference causes the Pressure Gradient Force (PGF), the force that drives the wind.
On a surface weather map, the pressure pattern along the Front Range would look like the figure to the left. The lines are isobars (lines of equal pressure) labelled in millibars. The pressure is "reduced to sea-level pressure" (see Ahrens, Fifth Edition, pp. 224-225, or last week's lab). There is a "trough" of low pressure over northeastern Colorado, just east of the mountains, with high pressure situated over western Colorado, west of the mountains.
On days when you suspect that a chinook occurred, you can look at the surface pressure map (on the "Composite" or "Pressure" maps on the PAOS weather center) to see if this pattern actually occurred. Also check the 700 mb or 500 mb upper-air chart to see if there are strong westerly winds over Denver or Grand Junction (in western Colorado).
On days when westerly winds blow down the east side of the Rockies into Boulder, the air flow can produce waves over and east of the mountains and, beneath the waves, turbulent swirls called "rotors" (see Ahrens, Fifth Edition, p. 252). When the "trough", or bottom, of the first wave settles into Boulder, wind speeds can reach extreme values and do quite a bit of damage. The lee wave clouds are also known as lenticular clouds, and are a common sight over Boulder on windy days.
Bora Winds
Bora winds, downslope winds that replace relatively warm light wind conditions with cold temperatures and strong wind gusts, may also be observed in Boulder. Bora winds that strike Boulder blow from the west (down the eastern slopes of the Rockies), are relatively dry (due to the descending air), but are also cold (since the air is very cold to begin with before it crosses the Rockies). The arrival of a Bora in Boulder is in some ways similar to the onset of a Chinook, with strong westerly winds. However, a Bora brings colder and drier air (both temperature and dew point decrease), whereas a Chinook brings warmer and drier air (temperature increases while dew point decreases). In Boulder, Bora winds are usually caused by a cold front passing over Colorado from the northwest. Figure 10.30 and related discussion appears in Meteorology Today, Fifth Edition, by C. D. Ahrens on p. 271.