IEM Daily Feature
Tuesday, 06 August 2013

13C Forecasting Rule

Posted: 06 Aug 2013 05:35 AM

When the sun heats the ground, the ground heats the lower atmosphere which drives a mixing process with warm air rising and cold air sinking. This mixing process grows vertically creating a well mixed layer. If this mixing encounters colder air aloft, relatively more heating needs to occur to continue raising the temperature of the mixed layer. If this mixing encounters warmer air aloft, the heating process can accelerate as temperatures are already warm. Atmospheric soundings sample the atmosphere twice per day. The morning sounding provides a sampling of the air above our heads that may be mixed down during the day. A informal rule of forecasting in the plains is to add 13 Celsius to the 850 hPa (millibar) level temperature to provide an afternoon high temperature. For example, the morning sounding on Monday for Omaha had 16.2°C at 850 hPa and when you add 13°C to that reading it yields a high temperature of 85°F. The actual high was 87°F! The featured chart looks at the monthly distribution of change in afternoon high temperatures from the 850 hPa morning temperature. The green bar represents 13°C. The box plots provide detail on the temperature distributions and for the warm six months of the year, this relationship closely approximates the monthly mean. The cold months are dominated by horizontal advection processes and also have decreased solar heating, so this rule does not apply well then.

Good = 99
Bad = 7

Tags:   sounding   forecasting