Download HOW IT GOT SO HOT … HOW IT GOT SO HUMID … HOW

HOW IT GOT SO HOT … HOW IT
GOT SO HUMID … HOW COOLING
MAY HAVE PLAYED A ROLE
by Joseph D’Aleo, CCM, AMS Fellow
SPPI ORIGINAL PAPER
♦
July 25, 2011
HOW IT GOT SO HOT … HOW IT GOT SO HUMID …
HOW COOLING MAY HAVE PLAYED A ROLE
by Joseph D’Aleo, CCM, AMS Fellow | July 25, 2011
The southern plains drought, followed the second strongest La Niña (behind only 1917/18)
according to the Southern Oscillation Index (SOI), an atmospheric pressure based measure
(standardized Darwin, Australia versus Tahiti surface pressure). Positive values are La Niña,
negative El Niños. The April to April SOI showed the 1917/18 peak and 2010/11 close behind.
1917/18 was a very dry (and very cold) winter and spring in the south and east. Like this year
late in the winter it warmed in the southeast. Summer heat developed in 1918 when Baltimore
reached 105°F on successive days. This past winter started out very cold even down to Florida
(coldest ever December and January) and ended warm southeast February and March.
It was unlike 1917/18, very wet in the Ohio and Mid-Mississippi Valley and northwestern plains
and Rockies where it left a deep snowpack. It might have been wetter because unlike 1917/18
the US was cooling, not warming. The US has been cooling the last decade in winters.
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This cooling has enhanced the snowfall. 20010/11 ranked 3rd snowiest for the Northern
Hemisphere behind 1977/78 and 2009/10 and just ahead of 2007/08. The last decade was the
snowiest decade on record for the hemisphere as temperatures cooled.
The deep snow in the north and dry warmth in the south enhanced the storm track and
produced tornadoes in April and May (with April to big month) and a ton of rain along the Ohio
and Mid-Mississippi Rivers and also in the north on the Red and later the Missouri River. Many
locations in the wet areas had surpluses of over 20 inches while in the dry areas of the south
they had deficits exceed 20 inches for the 6 month period.
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The Missouri is still in flood.
The heat wave that made the news this week was actually a result of both the dry and moist
conditions.
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Heat tends to build over drought areas like we saw this year in Texas and Oklahoma. Drought
begets drought. Dry air precludes clouds, which allows for more sun which warms even more, a
positive feedback.
This process often leads to what forecasters call a heat ridge, or mound of hot air extending far
up into the atmosphere. Like a rock in a stream, and like a blocking high in high latitude as we
have seen in recent winters, it forces systems around it. The heat can get picked up by
disturbances around the edges and feed instability and strong thunderstorm complexes.
Forecasters refer to this as the ring-of-fire. It has operated for many weeks. Here is a picture of
the ring-of-fire on July 8, 2011 as forecast by the GFS model. I could have picked most any day.
When dynamic factors like cool air diving south over the west coast (where they have a year
without a summer) drive the warm air north, we get a compensating push north of warm air to
the east. The hot air is forced over saturated or even flooded fields and lush vegetation. The
plants, much like humans, transpire moisture from their stomata in the leaves (like pores in our
skin). This is an attempt to cool the plant (as sweating does for our body) as the moisture to
enter the vapor state must extract heat energy from the leaf (or our skin). That process can
dehydrate the soil (or our body) but raises the dewpoint in the air. This makes hot
temperatures even more uncomfortable.
The NWS has been stressing the heat indices justifiably, just as they do the wind-chill in winter,
because that affects how a person feels. When the air is dry and body or plant is able to easily
evaporate moisture, it can stay cooler. When it is very humid, this mechanism is less effective.
Likewise on cold days in winter when the winds blow, they strip away air molecules warmed by
contact with your skin and make you feel colder.
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In Morehead, MN when the temperature last week rose to 93°F, the dewpoint rose to an
incredible 87.7°F. That made the heat Index 134°F, equivalent to the US temperature record set
in Death Valley in 1913. But it wasn’t 134°F, it just felt that way according to at least this one
index.
This led to elevated nighttime temperatures. In fact 72.5% of the record highs (and 65% of all
record temperatures) set the last 8 days were not highest maximum temperature but highest
nighttime low temperatures, a reflection of an anomalously moistened air mass.
How does this translate into the record heat in the east Friday?
Well Friday that air mass came east. Not only was it warm to begin with, but the convection to
the west added latent heat to the air as the rising air condensed into clouds and precipitation.
When that super warm air aloft got carried east on strong west to northwest winds and
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subsided downslope from the mountains, it warmed even more by compression – about 10°F. It
cleared out any clouds and haze and enhanced the solar factor. The eastern big cities all get the
100°F days when wind are blowing downslope. Even in winter the cold air that comes in from
the west and northwest loses some of its chill by this sinking process.
In a more typical summer, when the Bermuda high is dominant with its south southwesterly
winds, the cities suffer the dog days of summer – triple H weather – hot, hazy and humid upper
80s to low 90s. That is hot and muggy, but not record setting. Often on those days the sea
breeze kicks in along the coast as southwest winds turn more southeast from the cooler water.
In the strong west to northwest wind case, that sea breeze is shut off.
That same heat ridge in the south central has suppressed the Bermuda high by causing
downstream trough off the east coast. That explains the many fine days in the east in between
the taste of heat in early June and the three day heat wave in late July.
Heavy thunderstorms have continued in the ring-of-fire area this week as the heat eased.
Chicago set a record with 6.86 inches of rain at O’Hare on Saturday and with some more
showers today, it is already the second wettest July on record with a week to go. Since they
need only 0.54 inches more to break the record and more showers are forecast this week, this
is likely to become Chicago’s wettest month.
Not because of global warming, but global cooling.
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