Where does the water at Brighton or Alta come from? All the water within the area surrounding Alta and Brighton comes from precipitation (i.e. rain, snow, etc.). In an average year, over 50 inches of water, mostly in the form of snow, falls from the sky at Alta. This is in contrast to the Salt Lake Valley that averages between 15 and 20 inches of water a year. The disparity between these two locations in the annual precipitation might seem surprising given the proximity (less than twenty miles) of them. However, the mountains act to enhance or sometimes create precipitation when moisture moves into the region. Though these processes can be quite complex at times, a few simplified explanations exist. A name that is usually applied to this entire phenomenon is called orographic precipitation. In other words, orographic precipitation is when precipitation is either enhanced or created by air moving up and/or over the mountains.
A more detailed explanation is as follows: As air in the atmosphere approaches mountains it is typically forced up and over the mountain barrier. When this occurs, the rising motion of the air results in the air cooling. Given sufficient moisture, water vapor will condense on small dust particles, compounds, etc. in the atmosphere. This results in the formation of small water droplets or, to our eyes, clouds. If the rising motion occurs for a sufficient amount of time and moisture remains available, these cloud water droplets collide and eventually become big enough to fall out of the sky as rain. If the temperature is below freezing than these rain droplets may fall from the sky in the form of snow or graupel.
Often in the summertime or preceding a storm when the flow (wind) is coming from the southwest, we have another process that leads to vast differences in precipitation between the mountains and surrounding valleys. Sometimes rain or snow falls from well above the mountains into a dry air mass that is in place over the valley and mountains. Due to the mountains being 5000 to 7000 feet higher than the valley floor, the precipitation is able to reach the ground in the mountains before it completely evaporates. In contrast, the precipitation completely evaporates over the valley and leads to zero precipitation on the valley floor. This phenomenon is called virga and is a common sight in Utah, particularly during the summer months.
We have learned how and why the mountains receive significantly more precipitation than the valleys. But we have yet to fully answer the question of where the water comes from. We know the water we see in the streams and lakes in the mountains comes from precipitation, but where does the moisture that produces this precipitation come from originally? This is a question that even the most knowledgeable scientists still don’t know the exact answer to! During the cooler months, some of this moisture likely comes from water vapor that has evaporated off the Pacific Ocean. In the late summer months when the monsoon brings afternoon thunderstorms to our area, some of the moisture may come from as far away as the Gulf of Mexico (and even farther south). Evapotranspiration, the process of water moving from bodies of water, soil and plants (through transpiration) to the air, adds water vapor (moisture) to the atmosphere. At least some of the moisture is likely to be recycled locally through this phenomenon. How much all these processes contribute to the rain and snow we receive both in the valleys and mountains here in Utah is largely unknown! This is one of the more important meteorological questions scientists have yet to answer here in the western United States. It is likely necessary for us to understand the answer to this question in order for us to make an educated, and hopefully accurate, prediction of how global warming will affect our weather and most specifically, future droughts.
