U

Updraft - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

Updraft Base - Alternate term for a rain-free base.

Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

Upslope - 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 "down-sloping" wind there as the air blows down from the Divide).

Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream - Toward the source of the flow, or located in the area from which the flow is coming.

Upwelling - Cold water that rises from the depths of a water body.