We are now moving through that
transition period between the end of our spring and
beginning of our long four-month summer season down here
on the gulf coast. This is characterized by a gradual
decrease in the number of cool frontal systems able to
make it down this far south before either stalling out
or dissipating.
There are two primary reasons for
this. The first is the jet stream, that river of air
aloft responsible for steering storm systems and their
fronts on their way, as well as amplifying or
diminishing their strength, and determining what kind of
fronts we get (Pacific, Canadian, Arctic). Then there is
the development of this big bubble of summertime high
pressure off the Atlantic seaboard, known as the Bermuda
High.
As our days get longer, the jet
stream tends to dip southward less frequently, driving
cool frontal systems our way less and less often, while
at the same time, The Bermuda High begins to build up
off the Atlantic seaboard, ridging into the gulf with an
increasingly prevalent southerly flow of steamy tropical
air off the gulf. This ridge tends to also act as a
block against what few fronts do make it this far south,
and by the end of May, cool spells down on the gulf
coast become rare events indeed until mid September at
the earliest, and in some years, we don't see our first
significant cool down until October.
In some respects this is good,
because it takes the tornado-severe spring storm season
and moves it northward up into the Great Plains, where
these clashes between the cool dry and warm humid
airmasses still take place. Right at about the time our
tornado-severe thunderstorm season is winding down on
the gulf coast, it is reaching a peak in the "tornado
alley" area of the Great Plains in May and June.
Where then does our summer rain come
from, if we no longer get frontal systems down here? The
answer lies in that sultry southerly flow of tropical
air, and the sun. Instead of organized storm systems as
such, we rely on thermally induced
thunderstorms for the lion's share of our rain around
here during the summer months.
As the sun comes up and begins to
heat the air near the Earth's surface, it warms
up....and warm air being lighter than cool air becomes
buoyant and begins to rise, taking the steamy water
vapor up in the air along with it. These thermal
updrafts begin to become visible as white puffy cumulus
clouds as they get up into the cooler air aloft and
start to condense out the water vapor, which in turn
releases more heat to the air, since condensation of
moisture is a warming process, keeping the air inside
the cloud warmer than that outside of it. In other
words, this cumulus cloud is working exactly like a hot
air balloon. How do you get a hot air balloon to rise?
You pump hot air into it with a propane heater, making
the air inside the balloon lighter than that outside of
it.....so it rises.
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Now, of course we don't get thermal
thunderstorms every day in the summer. Sometimes you see
these white puffy cumulus towers build up only to
evaporate before doing anything. This is where the
variable comes in. The upper atmosphere holds the key as
to what kind of day we will have, where the storms will
develop, how heavy they will become, and which direction
they will move once they do.
Sometimes we have high pressure aloft
build up out to our west, and this tends to produce a
lid of warm dry air above the juicy tropical air at the
surface. When this happens, our cumulus cloud crop runs
into trouble once it builds up into this warm dry layer
of air. First of all, the warm updraft supporting the
building cumulus cloud will only continue rising as long
as the air inside the cloud is warmer (thus lighter)
than the air outside the cloud. Once an equilibrium is
reached, that's it. It stops rising. However, if the air
aloft is also very dry, you add insult to injury by
vaporizing the water droplets near the top of the cloud,
and evaporation being a cooling process (like when
perspiration evaporates off of your skin on a low
humidity day), the cloud begins to collapse in on
itself, since cool air is heavier than warm air.....thus
it sinks. If on the other hand, the cumulus cloud is
able to break through the cap into cooler air aloft, it
will go on to blow up into a mature thunderhead,
releasing a torrent of heavy rain and dragging down a
shaft of cool air from aloft with it. This is why you
will oftentimes hear me refer to these things as "mother
natures natural air conditioners".......because quite
frankly, this is about our only source for any genuine
cooling around here during these hot humid summer
afternoons.
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I will have more to say soon in this
space about the summer thunderstorm, as we get into the
heart of the season. I will discuss some of the
variables, such as upper level disturbances that can
enhance the activity, the nocturnal thunderstorm that
builds up in the warm coastal waters offshore during
nighttime hours, its counterpart along the so-called
"sea breeze front" during the day, and we will have a
highly detailed review of lightning safety tips for you.
While the vanilla thermal
thunderstorm makes up the lions share of our rainfall
around here during the summer months, the balance of it
is produced by tropical weather systems, being born in
water temperatures above 80 degrees, and yielding
anything from a benign tropical wave to a full blown
hurricane. Again, this is another subject I will go into
in greater detail as we get to the prime time part of
the season. Already we have seen a goodly amount of
advance publicity for the season by the issuance of
these largely worthless forecasts predicting the exact
number of storms that will develop. After being off the
mark by over 100% in 2005 (original prediction of about
13….and we ended up with 28) you would think that they
would have given it up. Then the very next year in 2006,
they predicted 17 and we ended up with only 9!! However,
the main reason I say "largely worthless" is because it
is not the NUMBER storms that develop that
is important, rather it is WHERE these things eventually
end up going that counts. When you hear these dubious
forecasts, remember this: It takes
only ONE storm scoring a direct hit on this area
to make for a very bad season. If Rita had been the one
and only hurricane to hit the U.S. coast in 2005,
crashing ashore where it did, we would have still
considered it to be a bad season. Prepare for each and
every hurricane season as if this were the year
our number was coming up, because you will not get a
clue from these full season forecasts.
I know of no better way to prepare
yourself for hurricane season than to obtain one of our
KYKZ full-color hurricane tracking charts. You will find
a highly detailed rundown on everything you need to know
about these dangerous storms, and how to prepare for
them. My suggestion is that you save the energy you
would have spent worrying about all of the "hurricane
hype" being circulated about on the airways, and
concentrate it instead on studying our write-up on these
charts.......and then execute it. You can
find a copy of our 2008 hurricane tracking chart free
for the asking at any one of the many branches of
Cameron State Bank, or in the lobby of KYKZ on Broad
Street.
There is one other hidden danger in
our summer weather around here that many people never
even consider until they fall victim to it. This is the
combination of high heat and humidity that can send the
"heat index" soaring into the danger zone. The heat
index is a "feels-like" comfort index based upon the
combination of the relative humidity and temperature as
it relates to the cooling caused by the evaporation of
perspiration off of your skin. The lower the humidity,
the more readily sweat can evaporate and cool your skin,
thus the lower the heat index. In a desert airmass,
where the temperature is 104 and the humidity is only
10%, you would have a heat index of only 90 degrees,
because the evaporative cooling of sweat off of your
skin would be so rapid that it would make it feel even
cooler than the actual air temperature.
This rarely happens around here.
The real danger kicks in when the
combination of heat and humidity are so high that the
perspiration cannot evaporate off of your skin rapidly
enough to compensate for the heat being released by your
body onto your skin. This mark is generally considered
to be around a heat index of 105 degrees. A combination
of 96 degrees and 60% humidity would produce a heat
index of 110, enough to induce heat exhaustion, and if
not attended to, heat stroke in many people. On most
summer afternoons around here we normally see a heat
index of somewhere between 95 and 105 with the actual
air temperature between 88 and 92. Whenever the heat
index gets into the 105 plus danger zone, I will
automatically begin to give the heat index and warn you
about it on the air.
Well, that should serve as an
adequate introduction to our summer season. Next time I
am going to present a highly detailed report on
lightning safety tips, along with some striking (pun
intended) lightning photography I have captured on my
cameras over the years. Thanks again for sticking with
your "weather station", KYKZ-96!
Rob Robin
