MOLINE, Ill. — It was a warm start to October with temperatures in the low 80s. As we continue into the second half of October usually, we are gearing up to see our potential first frost. The second week of October will finish with highs in the 60s and lows in the 50s. In the graphic below titled first frost and freeze shows the trend of the first frost happening during the second week of October, except for the year 2017. Are we on trend to follow 2017 and have a late frost? I would say so.
The conditions needed to have frost are:
- Clear skies. Clear skies allow for heat to exit into the atmosphere therefore allowing temperatures to drop rapidly.
- Temperatures need to be at least between 33 and 37 degrees for frost to form. Patchy frost can occur if temperatures are between 38-42 and frost/freeze at 32 degrees and below.
- There needs to be calm to light winds. This prevents stirring of the atmosphere which allows for there to be thin layer of super-cooled temperatures to develop at the surface. These temperatures allow for the surface to be much cooler, up to 10 degrees, then the air right about the surface, which helps frost form at the surface. For an example it could be 39 in air but at the surface it could be 32 due to calm winds.
- Topography. The shape of the land can help with frost. For example, in a valley they experience more frost since cold air settles in the bottom of the valley since it is heavier than warm air. Valleys can also shelter an area from strong wind breezes. This doesn’t mean that flat land can’t get frost it just means that frost will not occur as often.
Looking at what is needed to make frost and looking at our weather conditions when can we expect to have our first frost of the season?
Using the Global Forecast System, GFS, I can predict the weather until October 29, 2021 therefore I can check that the temperatures, cloud coverage, and winds requirements are met to produce frost. After checking for these conditions, the best dates for a frost would be Saturday October 16th and Sunday October 17th in the morning right before the sun rises, Saturday October 23rd in the morning before the sun comes up, Monday October 25th in the morning right before the sun comes up, and Friday October the 29th in the morning right before the sun comes up. The temperature for Saturday October 16th and Sunday October 17th is 40 and 43 degrees. Although this Sunday’s temperatures are just outside of the temperatures needed to produce frost, if we get enough cold air pump into our area then the temperature can drop which will then put it in the temperature ranged needed to form patchy frost. The temperature will be 38 degrees for the 23rd, 29 degrees for the 25th, and 37 degrees for the 29th. All these days have calms winds of 5 to 6 knots which is approximately 6 to 7 mph. There will be clear skies for the dates above which helps with frost formation.
The best date for us to experience a complete frost would be October 25th, the same date as the first frost in 2017. For all the other dates if frost does form it will be patchy frost at best, due to temperatures. If frost does not happen on one of these days, especially on the 25th, then it is very possible that we will not have our first frost until November surpassing the 2017 first frost.