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 SKEW-T: A LOOK AT MAXT

METEOROLOGIST JEFF HABY

1. What is MAXT?

MAXT gives a forecaster some insight to the expected high temperature that day and is found through the thermodynamic process of the PBL mixing out during the day.

2. How is MAXT determined?

Surface pressure is noted. Second, the temperature is found on the sounding 150 mb above the surface. Third, a parcel of air on the temperature sounding that is 150 mb above the surface is brought down dry adiabatically to the surface. This new parcel temperature at the surface is the MAXT.

See sounding below (zoomed in portion is also given). The surface pressure is 1000 mb on this sounding. Thus, 150 mb above the surface will be the 850 mb level. Note the 850 mb temperature... which is 20 C. Now, bring this 20 C parcel at 850 mb back down to the 1000 mb surface pressure at the dry adiabatic lapse rate. Once the parcel is back down to the surface it has warmed dry adiabatically to a temperature of about 34 C. Sounding parameter shows a value of 33.9 C.

3. Operational significance of MAXT:

It approximates how much warming the PBL and surface will be able to take during the day. Once the PBL mixes out to the dry adiabatic lapse rate, further warming is slow and difficult. This is why temperatures tends to increase most rapidly in the first half of the day and more slowly in the second half of the day.

4. Pitfalls:

a. Method does not work on cloudy days or days with significant precipitation.

b. There is a tendency that the temperature will be higher than predicted on days the wind is light and will be lower than predicted on days the wind is strong. This is because the low level wind effects the depth of mixing (in which method always assumes it is 150 mb). When the wind is light solar heating will remain near the surface. When the wind is strong, this warming air will be mixed with cooler air aloft thus keeping the temperature cooler.

c. Method assumes air is mixed only between the surface and 150 mb above the surface. If the air mixes to a height significantly above or below 150 mb above the surface the technique will not work accurately. Convection and strong wind will significantly alter and contaminate the mixing depth.

d. Daylight hours effect accuracy. There is a significant difference in daylight hours between the warm and cool season.

e. Method only works in a barotropic atmosphere. Fronts or differential advection will contaminate technique.

f. Method does not work well in regions with complex topography or near mesoscale temperature gradients such as coastal areas, very hilly areas, and areas near large lakes.

g. Method is most relevant for morning soundings and not afternoon sounding.