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Meteorological Inquiries |
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| How
is dewpoint determined?
|
A lot of
local airports measure the dewpoint by a group of weather instruments
called the Automated Surface Observing System (ASOS). This system
has a hygrothermometer as one of the many weather instruments.
The ASOS' hygrothermometers
calculate dewpoint by using a chilled mirror method. By definition,
the dewpoint of a sample of air is the
temperature at which the water vapor in the air condenses. In the
chilled mirror method, a mirror is cooled to the point where a fine
film condensate is present on the mirror's surface.
The presence of condensation is detected by the reflection of an
infrared light off the surface of the mirror. Internal circuits
refrigerate a small mirror, and by using an optical feedback loop,
maintain the mirror at exactly the temperature at which the mirror
surface is slightly clouded with condensed water vapor from the
sampled
air. (ASOS Site Technical Manual)
If ASOS (Automated Surface Observing System) fails, we can use a
sling
psychrometer to determine the temperature and wet-bulb temperature.
Using these numbers, we have a psychrometric calculator to determine
dewpoint and relative humidity.
Bill Murrell
Meteorologist
|
| What
year did the Red River in Shreveport freeze?
|
|
The Red River froze over in December 1983. That was the first
time it
froze over since February 1895.
Bill Murrell
Meteorologist
|
|
What is
the worst hailstorm and when did it happen?
UPDATED |
In the
United States, the largest hailstone ever recorded fell in
Coffeyville, KS on September 3, 1970. The circumference was 44 cm
(about 17 inches) and weighed 776 grams (1.7 pounds).
** On June 22, 2003, a Severe Thunderstorm
over Aurora, NE, produced a hailstone measuring 7 inches in diameter
and a circumference of 18 3/4 inches. No weight was determined **
One of the worst hail storms ever, occurred over Fort Worth, Texas,
on
May 5, 1995. This storm caused damage close to a billion dollars.
Bill Murrell
Meteorologist
|
|
Why
is the air temperature colder if there is snow on the ground.
|
|
There
are two reasons why it would be colder if snow was on the ground.
Snow is a good reflector (high albedo) of the sun. So when the
ground
is covered in snow, the sun has a hard time heating the land because
the solar radiation is being reflected back into the atmosphere.
On a clear night with
calm winds, but with no snow cover; the
atmosphere will radiate heat away from the surface, but will gain
some
heat from the ground radiating some heat. However, snow acts as
an
insulator and does not allow the warmth from the ground to radiate
into
the air above.
Therefore, temperatures
will be colder during both day and night with
snow cover. In addition, an arctic outbreak of cold air will be
colder
if areas north of the region have snow cover even if the your
location
is snow free because the air mass moving southward will have less
time
to modify.
Bill Murrell
Meteorologist
|
| What
are the conditions that cause a blizzard?
|
| The definition of
a blizzard is sustained winds or frequent gusts to 35
mph or more for at least 3 hours and considerable falling and/or
blowing snow reducing visibility frequently to less than 1/4 mile.
These conditions occur
most frequently across the Northern Plains,
Upper Midwest and the New England area. Once it is cold enough
for
snow, all that is needed is a storm system that has enough moisture
to
produce some snow. There is no set amount of snow for a blizzard;
just
that it reduces the visibility to less than a 1/4 of a mile for
at
least 3 hours with sustained winds or frequent gusts to 35 mph.
So the
main criteria is that it be windy, which is caused by a tight
pressure
gradient between the surface low or storm system and a Canadian
surface high.
Sometimes in the Northern
Plains and Upper Midwest, only a little
amount of snow actually falls, but because of the very windy conditions
it causes considerable blowing and drifting of snow causing white
out
conditions.
On the other hand,
across New England, more snow usually falls during a
blizzard because the storm system is pumping in Atlantic moisture.
Bill Murrell
Meteorologist
|
What is the deformation zone?
|
A deformation zone is
an area where the wind becomes stretched.
Streamlines converge asymptotically (axis of dilatation) from high
and
low pressure systems. Deformation is a primary factor in the process
of frontogenesis and frontolysis. (Glossary of Meteorology)
Here is a link that explains a little about deformation zones:
http://205.156.54.206/pub/im/wrta8615.pdf
Bill Murrell
Meteorologist
|
|
www.weather.gov
