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July 13, 2009 July 13, 2009 July 13, 2009 July 13, 2009
Heat is a four letter word, and if you?ve lived in Oklahoma this summer, it has become
one that typically invokes a very negative reaction. For an interesting people
experiment, try dropping it into a normal conversation and you?ll probably get a
reaction similar to one if you used one of those other four letter words. But today and
for a couple more posts, we?re going to foray a bit deeper into some of the impacts of
heat and how where you live may just have an impact on how much heat you may
experience.
If you?ve been following Gary on the Twitter version of the OCS ticker
(http://twitter.com/ocsticker), you may have noticed the following Tweet:
What helps u get 2 115F? This!
http://okfire.mesonet.org/data/public/mesonet/models/realtime/nfd/images/greenness/200907061200.vg.fire.gif
So, just what is being displayed in the image and how does it impact temperature?
Well, the image displays a composite image of ?greenness? at the surface determined
by satellite. Or, in other words, those locations with very active, healthy vegetation are
shown as more green than those areas that are not. The most recent Visual Greenness
image shows that those areas in eastern Oklahoma are very green while locations in
western Oklahoma are not.
Now an interesting physical principle of thermodynamics is that the evaporation of
water is a more efficient ?heat? removal mechanism than conduction (e.g., warm soil to
relatively cooler air). Thus, during the summer of 2009, for locations in eastern
Oklahoma, the vegetation can tap into surface and deep-layer soil moisture and
transpire water to the atmosphere during photosynthesis. This results in enhanced
evaporation and ?cooler? air temperature values. That is not the whole story of course,
but we?ll get back to that.
In western Oklahoma, the vegetation is not in as good of shape due to reasons including
an overall lack of rainfall and the recent harvest of wheat from north central into
southwest Oklahoma. Thus, the exposed bare soil and dormant vegetation warms up
quickly and the ?heat? is transferred to the atmosphere via conduction (which is not
very efficient) which further means that the surface continues to warm and the
atmosphere has a hard time keeping up with the heat transfer. The end result is that
the near-surface atmosphere warms as well and the temperature values climb much
faster than over the vegetated areas where evaporation occurs. You can think of it in
this way ? your own body sweats to evaporate water and remove heat from your core
to the atmosphere. Once you becomes dehydrated, you?ve evaporated the available
water in your body and your core temperature rises (which is not good).
So, back to Gary?s point. How can you reach 115F during the summer in Oklahoma?
Well, start with limited rain, dry out the soil, and remove the vegetation. Then add in
one nasty ridge in the mid-troposphere (i.e., a dominant high-pressure system) and
everything is in place to get the cooker cooking. Unfortunately, theses types of
scenarios tend to feed back upon themself. In this case the higher temperatures only
add to the dryness of the land surface, and until a significant, larger atmospheric impact
brings rain and an infusion of air from farther north those with the hot temperatures
will likely see more of the same.
Now, back to eastern Oklahoma which has cooler temperature values. So, why then
does it ?feel? so bad/hot? Well, back to sweating. Those trees, lakes, and weeds in
eastern Oklahoma have added more moisture to the atmosphere as they sweat. Now,
the ?rate? of evaporation is related to how much moisture is in the atmosphere which
means that, the more moisture in the atmosphere, the ?slower? the rate of evaporation.
Thus, when it is humid, the body also struggles to remove the heat to the atmosphere
and it feels hotter than what it really is ? the body simply can?t evaporate enough
water/sweat. The ?Heat Index? is a formulation which takes into account the combined
impacts of temperature and humidity and is often used this time of year to quantify just
how hot it feels outside. Now, evaporation is also modulated by wind speed (more wind
equals more evaporation). That explains why a breeze feels ?cool?.
Finally, take one more look at that visual Greenness image and you may be able to make
out the larger metropolitan areas of the state. They will be the subjects of a soon-to-be-
written Ticker as related to the four letter word mentioned earlier. Until then, follow
the heat in real-time on the Oklahoma Mesonet website (http://www.mesonet.org/)
and don?t forget our other links of interest:
http://twitter.com/ocsticker
http://www.facebook.com/group.php?gid=80762543949
Most importantly, please avoid the heat as much as possible and take the necessary
precautions to avoid heat stress, heat stoke, and something worse.
Ticker Author: Jeff Basara
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