Past IEM Features tagged: ceilings
Only one day left of October as we continue to march closer to the winter season for Iowa. Besides the obvious difference with colder temperatures during the season, there is also a marked difference to the number of days with low overcast clouds. The featured chart depicts a histogram of overcast cloud conditions by week of the year and level they are reported at by the automated sensor at the Ames Airport. The coldest part of the year sees the highest frequency of having low overcast clouds. The decreasing solar incidence angle and shortening daylight time conspire to allow less depth of mixing of the lower atmosphere. The colder temperatures are also quicker to saturate and create clouds. You may notice the rapid increase at 5 thousand feet AGL, this is an artifact of how the sensor and observation reporting works. The sensor is also capped with how high up it is able to sense clouds.
Monday was a rather dreary day with low clouds, freezing rain, and then rain. Winter is the season for such low clouds as shown by the featured chart. The chart depicts a histogram of cloud ceilings as estimated by the Ames Airport weather station. The darker reds represent the more conditions. There is a clear signal in the lowest levels during the coldest half of the year. Low clouds are more difficult to persist during the warmer season as solar inputs generally are able to warm the ground to mix the lower atmosphere.
Some snow visited the state again on Tuesday, but nothing to get excited about. It was a cold and dreary day with highs only in the low 40s and low clouds. The featured chart presents the frequency of the height of the overcast cloud layer as reported by the Des Moines Airport. This is referred to as the ceiling. There are caveats galore with this plot including changes in instrumentation and reporting algorithms. These caveats explain some of the bright bands in the plot at certain levels. Putting those issues aside, you can see a clear annual signal with low ceilings much less common during the warm half of the season and the very low ceilings confined to winter. The reason being that lower atmosphere temperatures are warmer in the summer with strong surface heating, this makes it more difficult for the air to remain saturated as heating lowers relative humidities. In the winter time, fog and snow are more persistent leading to lowered reported ceilings.