Today’s column concerns precipitation and where the long-term data comes from. First let’s tackle precipitation. We aware that it comes in many forms from fog and freezing rain to hail, sleet, snow and rain. We will focus on rain and snow.
• Of all the weather factors discussed rain and snow are the most difficult to accurately measure. First, wind can affect the accuracy of measurement. Second, more so than the other factors discussed except wind, it is critical to have a clear open area away from any obstructions such as buildings and trees to approach something like accuracy. Finally, as anyone living in Kansas knows precipitation can vary greatly over a short distance. Farmers and ranchers can be working in relative sunshine and see the rain less than a mile away. When the long-term averages are discussed, there is an assumption that for a given area the precipitation “averages out” over the course of time and normally it does.
• Precipitation is measured as a depth which in the U.S. is inches. Snow is measured in two ways. First, what is the depth of the snow unaffected by wind which can be a bit tricky to say the least. Second, a determination is made as to how much liquid water is in the snow by melting it or determining its density. We are all familiar with light fluffy snows containing little water and heavy, wet snows. A rule of thumb is that an average snowfall of twelve inches has approximately one inch of liquid water in it. Extremely dry snow may be half that depth and extremely wet snow almost twice. Some areas determine how long the snow cover lasts. This is important to say wheat farmers as snow cover protects and insulates wheat from low temperatures.
• Weather extremes – When the all-time highs and lows for an area are reported or if it is reported that an event is the highest or lowest on record, it is a function of recorded observations. For factors such as temperature and precipitation, records only go back to when instruments were located in an area and records were kept and compiled in some type of official manner. Today, many official stations are located at airports such as in Hutchinson and Wichita. These stations consist of personnel and automated equipment. There are also weather observation sites normally kept by volunteers for a given location which have equipment but the data is typically recorded manually and sent in to the NWS. For very remote locations, totally automated systems may be in place and either download regularly via land line or more typically cellphone or in some cases the data is stored and retrieved onsite at a later date.
• Finally long-term averages – Typically these are thirty year rolling averages. Right now if you want to know the long-term average annual precipitation yearly total for this area that average is the average of the years from 1981 to 2010. In 2021 the average will be the years from 1991 to 2020.
This isn’t everything but it hopefully helps in understanding the weather data we hear or read every day.