DENVER, Colo. -- Many of the Colorado ski areas picked up a foot of snow or more over the weekend, but in Denver and across the plains, only a few flurries fell.

Much of the weather in Colorado is determined by the topography. Our spine of tall mountains that divide the state can create all kinds of havoc with our weather.

The winds that blow across Colorado cannot go under the mountains or through them, so the air must move over the mountains. As the winds blow against a mountain, the air is forced to rise up and over the peak. This rising air is moving "up" the slope of that mountain. When air rises, it cools at a rate of about 5.5 degrees Fahrenheit for each 1,000 feet that it goes up. This temperature change is called the "dry adiabatic lapse rate". The cooler air cannot "hold" as much water vapor as warm air, so the vapor condenses out to form clouds. If the air continues to rise up the mountain, more and more moisture will condense and the cloud will thicken and eventually produce precipitation.

To put this into an example, let’s take a situation where a westerly wind is pushing into western Colorado; the air will be forced to rise up as it travels into the mountains. This rising air will cool, the moisture will condense out, clouds will form and rain or snow will develop over the west facing slope of the mountain.

A "downslope" is just the opposite, as the air reaches the crest of the mountain and begins to move downhill, it will begin to warm up, due to the air being compressed as it descends (the molecules of air are being pushed closer together as the air moves down into the thicker atmosphere at lower elevations). This compression warms the air; warmer air can "hold" more moisture than cold air so the humidity drops and the clouds evaporate. Thus a downsloping wind will generally bring clearing skies and warmer temperatures.

The same weather pattern that can bring snow to the western side of a mountain can bring dry and mild weather to the opposite side of the mountain. That is why it is sometimes snowing on the west side of the Eisenhower Tunnel, but not on the east side. In fact, it can be sunny, breezy and warm in Denver with a strong westerly wind, while the mountains west of the Divide are getting hammered with snow.

In Denver, our "upslope" winds are from the east. An easterly wind will have to travel "up the hill" from the 4,000 foot elevations near the Kansas border, to our Mile High elevation here in Denver. Easterly winds will bring an increase in clouds and often a chance for rain or snow to the Denver area, especially since that air moves west of Denver and backs up against the mountains, gradually forming the clouds and precipitation over the foothills and then out across the eastern plains. In this instance, the upslope winds will often cause rain or snow here, but dry weather will hold west of the Continental Divide (remember, they would have an easterly "downsloping" wind there as the air blows down from the Divide).

It may sound a little confusing, but that is why Colorado weather is such a challenge to predict! Just remember, "upslopes" tend to bring clouds and precipitation - "downslopes" tend to bring windy, dry and mild weather.