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Past Questions 3
ArkLaTex
Meteorological Inquiries
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On August
27, just as a storm system was hitting the Chireno area in Nacogdoches
County, the barometer shot up and I experienced a change of pressure
in my ears. Does the barometric pressure go up or down when a
tornado is in the area? What else might account for the sudden
change in pressure?
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On August 27th, a MCC(Mesoscale
Convective Complex) moved through East Texas. A MCC is made up
of a large area of thunderstorms persisting for several hours
in a rounded or linear shape that looks like a front on radar.
However this boundary is not a front as no strong temperature
contrast is observed ahead or behind the boundary. You are correct
with observance of the drastic pressure change...usually a meso
high is located just behind the boundary...which will cause barometer
readings to rise quickly. Yes this drastic pressure change can
also be sensitive to your ears...which is similar to riding an
airplane. Tornados are usually associated with meso lows so the
pressure would fall quickly.
Bill Parker
Public Outreach Meteorologist
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WHAT ARE CUMULONIMBUS MAMMATUS
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Mammatus Clouds indicates
sinking air and are usually found on the anvil portions of thunderstorms
(cumulonimbus clouds), but can be found on other types of clouds.
They are not dangerous, but they are sometimes more pronounced during
and after a strong/severe thunderstorm.
In a thunderstorm, precipitation is carried upward by an updraft.
Some of this precipitation (ice crystals and water droplets) spreads
out, and forms the anvil of a thunderstorm. The precipitation particles
are sometimes heavier than the surrounding air, and begin to sink.
This sinking and the associated evaporation cause the "pouch-like"
clouds to form on the underside of the clouds (anvil).
Bill Murrell
Meteorologist |
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WHAT IS A CIVIL EMERGENCY
MESSAGE?
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A civil emergency message
(CEM) is issued to relay non-weather related time critical information
to protect life and/or property. This information comes from federal,
state or local officials, and then we issue a CEM to help spread
the word about the problem.
Examples of potential problems that the NWS would issue a CEM for,
if requested by a govt. organization (i.e., Emergency Managers).
Chemical Spill
Hazardous Substance Release
Bomb Threat
Nuclear Power Plant Emergency
and other vital information for the public
Bill Murrell
Meteorologist |
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WHAT IS THE FORMULA TO CALCULATE
DEW POINT?
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Basic definition of
dew point: The temperature that the air must be cooled to, at
constant pressure and moisture content, in order to reach saturation
(i.e., fog).
Before the days of the Automated Surface Observing System (ASOS),
Meteorologists would use an instrument (sling psychrometer) to
determine the air temperature and wet-bulb temperature. With this
information, a meteorologist would use a psychrometric calculator
(a wheel) to determine the dew point and relative humidity.
The actual formula:
Set x = (1 — 0.01 RH)
where RH is the relative humidity expressed as a percent (a number
between 1 and 100).
Then calculate DPD = (14.55 + 0.114T)x + ((2.5 + 0.007T)x)to the
third power + (15.9 + 0.117T)x to the 14th power where T is the
temperature in degrees Celsius.
This calculation yields the difference between the temperature
and dew point in degrees Celsius.
Finally, compute the dew point TD = T — DPD. The answer is in
degrees Celsius.
Bill Murrell
Meteorologist
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DO HURRICANES AND FLOODS
AFFECT EL DORADO, AR?
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Hurricanes do not directly
affect El Dorado Arkansas, but their remnants can. As hurricanes
moves inland, they quickly diminish in strength because they have
left their energy source (warm ocean waters). However, as a hurricane
or tropical storm is dissipating they can still produce copious
amounts of rain far from the coast, including southern Arkansas.
This rain could result in flooding, but most flooding associated
with a decaying tropical system is usually within a couple hundred
miles of the gulf coast.
El Dorado, Arkansas, can also experience flooding during other times
of the year when atmospheric conditions come together to produce
copious amounts of rain. Sometimes a cold front will enter the region
and stall out. This front will then act as a focus to produce periods
of showers and thunderstorms. If the front remains stationary for
a few days and the Gulf of Mexico continues to feed moisture northward,
flooding could result as the ground becomes saturated with the repeated
rainfall. Flooding is more of a problem during late winter and spring
time. However, flooding can occur in any season.
Bill Murrell
Meteorologist |
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