The 850 mb Map

The last map on our tour is the 850 mb map. This map has been discussed in other modules, and we have been using it to forecast the evolution of the temperature pattern, with particular attention to maximum temperatures. But it can also be used to forecast, you guessed it, vertical motion.

Remember I mentioned the omega equation, and that one of the processes associated with upward motion is low-level warm advection? Well, I did. Warm advection occurs if warmer air is replacing colder air, as is happening, for example, out in the Pacific along the left edge of the map. And there is upward motion there too, judging from the humidity on the 700 mb map, although it's pretty wimpy. The warm advection at 850 mb is often not as strong an effect as the 500 mb vorticity advection. You can probably make it through the rest of the semester without figuring out vertical motion from any 850 mb map. But then, you'll probably get burned on a couple of forecasts by failing to do so.

As you know, the 850 mb map can be very useful in surface maximum temperature forecasting. Recall that when air rises or sinks, it cools or warms at the dry adiabatic lapse rate, 10 C per km. During the day, there tends to be much mixing within the lowest km or so of the atmosphere. If the mixing extends all the way up to 850 mb (about 1.5 km), you can take the 850 mb temperature forecast and get a surface temperature forecast by correcting for the difference in elevation between the ground and 850 mb.

Hypothetical example: Rapid City is 1 km above sea level. The 850 mb temperature for 00Z Thursday is 4 C. The maximum temperature forecast for Wednesday afternoon, based on the 850 mb forecast would be 9 C, since the 850 mb surface is about 500 m above Rapid City.

This technique works best on warm, sunny days. On any kind of day, though, you can use the temperature patterns to deduce the motions of air masses. Using the same example as before, you might see where the 850 mb temperature is 4 C today and check what the surface temperatures are like there. I use this technique a lot.

On cloudy, rainy days, the 850 mb map can be used to determine whether the precipitation will be rain or snow. Usually the rain-snow line is somewhere near the 0 C isotherm at 850 mb. In this example, the 0 C isotherm lines up pretty nicely with the Mason-Dixon line. So Pennsylvania should be getting mostly snow, and Maryland should be getting mostly rain.

Here's an observation from Baltimore, in the northern part of Maryland:

KBWI 301754Z 02009KT 1 1/2SM -RA BR BKN006 BKN010 OVC014 04/04 A2989 RMK AO2 SLP121 P0004 60006 T00390039 10039 20033 58016=

And here are some observations from State College, PA, near the center of the state:

KUNV 301547Z 04008KT 2SM -SG BR BKN004 OVC015 00/M01 A2995=

KUNV 301747Z 04007KT 2SM -PL BR OVC044 00/M01 A2993=

KUNV 301805Z 04006KT 1 1/2SM PL BR OVC044 00/M01 A2993= 

Not exactly snow. They had some snow granules, and now they're reporting moderate sleet (PL stands for ice pellets). There must be some warm air between 850 mb and the ground. We'll talk about this more in the Snow Forecasting module.

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