The 850 mb Forecasting Technique
During the day, convective mixing dominates. The ground heats the air from below; as the air warms, it bubbles upward and is replaced by cooler air from above. The cycle continues through most of the day, and the process is so efficient that the lowest several hundred meters of the atmosphere is essentially well-mixed.
Among other things, how hot the surface air gets depends on the initial temperature of the lowest several hundred meters, not just the initial surface temperature. Cold air a few hundred meters up will hold back the temperature at the surface. Warm air aloft will allow the surface temperature to warm too.
You can't tell from surface observations how warm the atmosphere is above the surface. Usually it gets colder as you go up, but there is often an inversion present, which means that the temperature increases aloft. So you need more than surface observations to make a surface temperature forecast.
FORECASTING TECHNIQUE: How hot could it possibly get?
Sometimes you can't rely on persistence to forecast the maximum temperature, because of changing clouds or some other reason. You therefore need a way of estimating the maximum temperature just from the weather patterns and forecasts.
When weather conditions are changing rapidly, it is often difficult to figure out how "warm" an air mass is. For example, if it's cloudy one day and clear the next, you can figure that the next day will probably be the warmer one. But how warm will it get?
To obtain an estimate of the warmest possible temperature, use an 850 mb map. You can use forecasted 850 mb maps, but often the coldest areas are not forecasted to be cold enough so you may be better off using the current 850 mb map and moving the pattern forward an appropriate distance. By whatever method, obtain an estimate of the forecasted 850 mb temperature during the day in question.
The method assumes the daytime boundary layer will extend to at least 850 mb (about 1.5 km above sea level). If that happens, then the temperature (and the rest of the atmospheric variables) would be well mixed between the ground and 850 mb, and you can use 850 mb temperatures to estimate surface temperatures.
It turns out that the temperature will not be constant when the atmosphere is well mixed. Instead, it will decrease at the rate of 10 degrees Celsius per kilometer. (You'll see why in a later module.) So if 850 mb is 1.5 km above sea level, the sea level temperature would be 15 degrees warmer than the 850 mb temperature. Add less than 15 degrees if the ground is significantly above sea level, or if you expect the day to be humid.
