Wednesday, July 21, 2010
Two different kinds of heat
The sauna or the oven? That is the question. Ok, so maybe Hamlet didn't mutter that during his pondering of whether he should continue his existence on Earth, but the question is a valid one when describing the types of heat in the western Corn Belt. We know that at some point it will be hot in the summer, but will it be a drier heat that is akin to an oven or will it be a humid heat that is akin to a sauna? To help answer this question, I give you a case study of similar air masses yielding two different results.
The upper air charts (compliments of the Storm Prediction Center) from July 19, 2006 and July 14, 2010 show that both days featured very warm temperatures in the lower and mid levels of the atmosphere, with 850 mb temps at Omaha of 29 degrees C and 30 degrees C on 7/19/06 and 7/14/10 respectively.
* Side note
Meteorologists often use the 850 mb temp as a guide for predicting how hot it could possibly be, assuming winds are strong enough to allow for optimal mixing.
*End Side note
Both days featured a cold front moving southeast across the northern plains and both days had strong southerly winds at the surface advecting warmer air from the south. Given all of the similarities, one would expect both days to have equally high maximum temperatures. But the maximum temperatures on 7/19/06 and 7/14/10 were quite a bit different (see maps).
* 2nd side note
Comparisons in this study are only valid in eastern and southern Nebraska. Both cases featured an approaching cold front but the cold front had advanced much further southeast on 7/14/10 than the cold front on 7/19/06, thus the maximum temperatures in the 80's on 7/14/10 in northern and western Nebraska.
* End 2nd side note
So what caused the difference in maximum temperature to be 10-15 degrees F different over the study area? I'll give you a hint. Look at the SMI maps from 7/20/06 and 7/12/10. The summer of 2006 featured moderate to severe drought conditions (see U.S. Drought Monitor maps) across much of the U.S. heartland and every single AWDN station in Nebraska had an SMI well below 0.0 (indicative of water stress) by late July 2006. That stands in stark contrast to this summer, which has been the wettest in many years over much of the western Corn Belt. So, long story short, the copious amounts of soil moisture available on 7/14/10 allowed for high, unimpeded vegetation (especially corn) transpiration rates, while the drought conditions damaged vegetation health and suppressed transpiration rates. It is this difference that is primarily responsible for the difference in temperature.
The last figure featured in this case study shows the hourly dewpoint and air temperatures from the Mead Agrofarm AWDN site on 7/14/10 and 7/19/06. Both days began with equally warm temperatures but notice how the dewpoint keeps slowly increasing to 80+ F on 7/14/10, while the dewpoint on 7/19/06 slowly drops to the mid 60's. Conversely, the slope of the morning air temperature increase on 7/14/10 is much less than on 7/19/06 when temperatures quickly climbed to the upper 90's. The air temperature continued to slowly rise throughout the day on 7/19/06, topping out in the low 100's, while air temperatures remained relatively invariant during the afternoon on 7/14/10.
As alluded to earlier, the differences in moisture led to the difference in maximum temperature. All else being equal, days with high humidity are going to have lower air temperatures than days with lower humidity, because dry air is much easier to heat up (or cool down) than humid air.
* Lesson of the Day
The specific heat of air is around 1 J/g*K while the specific heat of water is around 4.18 J/g*K, so it takes far more energy to warm water 1 K (or 1 deg C) than it does to heat totally dry air 1 K.
So to sum up, the air was far more humid on 7/14/10 than on 7/19/06 because of vast differences in soil moisture and plant health. A full profile of soil water means that plants don't have to "work hard" to obtain moisture and can undergo unimpeded photosynthesis. When plants are undergoing photosynthesis, the stomata are open, and the plant will transpire. When there is little soil water to work with, as was the case on 7/19/06, then plants close their stomata to prevent water loss, and thus are not transpiring. If you recall nothing else from this post, just remember this:
Moist soils -> high transpiration and latent heat flux -> increased dewpoints -> decreased air temperature
Dry soils -> low transpiration and latent heat flux -> reduced dewpoints -> increased air temperature (lower latent heat, higher sensible heat)