From your basic flight training, you probably remember that density altitude is the pressure altitude corrected for temperature. Since increasing temperature makes the air less dense, an airplane will perform as if it is at a higher altitude than on the colder day, given that the airplane is at the same height above sea level.
You should also remember that the standard lapse rate (the rate at which temperature normally decreases with increasing altitude in unsaturated air) compounds the problem somewhat. At sea level, the standard temperature is 59 degrees Fahrenheit, however, at 10,000 feet MSL, the standard temperature is only 23 degrees. This means that at Leadville, Colorado (elevation 9,927 feet) when the temperature is only 24 degrees, the density altitude is already above the field elevation. We will discuss density altitude in more detail later.
Winds Aloft Reports
The winds aloft report are very important to your mountain flight planning process. You should pay close attention to the forecasts at and above the mountain ridges of the terrain you will be flying into. In the west, that usually means the 9,000 and 12,000 foot forecasts. In the easy, you will look at lower winds. Winds above 25 knots at these levels should be a warning sign that should cause you to think about delaying your trip.
High and Low Pressure Patterns
As you analyze the mountain weather before your flight, pay special attention to the posistion of the higs and lows to give cles to the wind speed potential. For example, in the winter, a high often sets up over the Four Corners area in southwest Colorado. This is often coupled with a low pressure centered in eastern Colorado or western Kansas. The result can be very strong westerly winds and dangerous turbulance in the high terrain for days at a time.
When the wind speed is above about 25 knots and flowing perpendicular to the ridge lines, the air flow can form waves, much like water flowing over rocks in a stream bed. The waves form down wind from the ridge line and will be composed of very strong up and down drafts, plus dangerous rotor action under the crests of the waves. If enough moisture is present, lenticular clouds can form to give a visual indication of the wave action. These clouds are reported in the remarks section of hourly sequence reports as ACSL (altocumulus standing lenticular) or CCSL (cirrocumulus standing lenticular).
Winds Through Passes
Just as the flow through a carburetor speeds up in the restriction of the throat, wind flowing through the narrow restriction of a mountain pass will also speed up. When the winds are forecasted above 20 knots, be aware that the speed in passes may cause turbulence and drafts that should be avoided.
As the wind blows moist air upslope, it will cool and may form clouds. If, as is often the case in winter, the air is stable, the clouds will stay close to the mountain, forming a "cap" cloud. However, if the air is unstable, as is usually the case in the summer, this initial lifting will be enough to start convection and result in thunderstorms forming.
Microbursts have received much space in the aviation and popular press in the past several years because of their implication in many serious airline accidents. Many light aircraft accidents have also been caused by these events. The wet microburst is found in the middle of an active thunderstorm or intense rain shower and avoiding the strong downdraft is relatively easy. The dry microburst, however, is somewhat more insidious because it occurs with little or no warning in the clear air beneath virga. Dry microbursts are common in and near the Rockies and other mountainous areas of the western U.S. in the summer. The formation of the dry microburst is likely when thunderstorms with bases above about 3,000 to 5,000 feet AGL exist and the temperature / due point spread on the surface is more than about 40 degrees. If dust is blowing underneath one of these highly based thunderstorms, stay clear until the event passes (usually only a few minutes).
Mountain valleys are often conductive to the formation of temperature inversions and valley fog at night. This should play a part in your arrival and departure planning. Since the inversion breaks and the fog dissipates by late morning, you may have to delay slightly at some airports.