Clear Cold Evening in Iowa

We are definitely headed for winter. I went out to the instrument shelter to take a late evening observation at 10:20 p.m. Overhead were a blanket of stars. It was a crystal clear night with no wind. Upon opening the door on the shelter I was greeted with a reading on the mercury-in-glass thermometer of 32 degrees F. This is the coldest so far this season at this time of the day. The coldest 24 hour temperature was 29 on the mornings of the 12th and 14th of October. It looks like we are about to break those readings.

Chart plotted by Digital Atmosphere available at www. weathergraphics.com.

Looking at the weather map as of the same time we see high pressure from the intermountain west over the Central Rockies of Colorado extending to Nebraska and Kansas to Iowa. Precipitation is located over Texas and Oklahoma. The station model plot indicates it is rain. We could use dry air here as harvest is well underway and farmers need dry weather with low relative humidity.

The high pressure area is a region of descending air. As the air sinks it is warmed by compression - the air is squeezed by increasing pressure at lower altitudes. This raises its temperature at a rate of 5.5 degrees F for each 1,000 feet it descends. The boxes on the map are the location of select weather stations. Skies are overcast in Texas and Oklahoma as indicated by the filled in boxes. Skies are clear under the high pressure area is indicated by the clear boxes. Note that Omaha, Nebraska is reporting a ceiling of 25,000 feet. The code for 25,000 is 250 because the code leaves off the final 2 trailing zeros.

The plots on this map are special versions of the station model plot. It is called a weather depiction plot because is shows sky cover, precipitation, and ceilings. I have added the isobars (lines of constant pressure) to show the main weather systems affecting surface weather.

Be sure to check out the station model plot section of this website to learn more about how weather observations are plotted on maps. I will share more in the future.