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UPPER AIR OBSERVATIONS
As
mentioned in the forecasters section
of this tour, much of the forecast is based on computer guidance. These
computer models need to be initialized with current information before
the atmospheric equations can be run to produce a forecast. Some of that
information is garnered from surface observations; thousands of sites across
the country maintain a continuous weather watch with temperature, humidity,
clouds, visibility (and any obstructions), winds, and atmospheric pressure
data. Often, these sites can be found at airports as accurate, current
weather information is necessary for aircraft to take off or land. Much
of the time, this "weather watch" is maintained by a group of computerized
instruments. However, this only handles what is occurring on the ground;
since weather is three-dimensional, we need data from all layers in the
atmosphere. |
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To
accomplish this task, instrument packages attached to helium-filled
balloons are launched at 70 sites in the continental United States,
with an additional 22 sites in Alaska, Hawaii, and Pacific Territories,
and others in Canada and Mexico. (For
a map of launch sites, click here.) To synchronize the launches
in North America and elsewhere around the world, the standard time
zone for weather information worldwide is Greenwich Mean Time-- the
time zone for Greenwich, England-- which is also know as Universal
Coordinated Time, or UTC. Balloon flights occur at midnight and noon
Greenwich time, or 00 UTC and 12 UTC. The actual launch takes place
an hour earlier, so the data can be collected and transmitted by the
observation time listed above. Locally, this translates to an observation
time of 8 am/pm in the summer, and 7 am/pm in the winter, again with
launch time an hour earlier.
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The instrument package used to take these observations
is shown to the right. This package is known as a radiosonde, or “sonde” for
short. The data which the sonde collects includes pressure (from a pressure
cell
located
inside the packaging), temperature from “relative electronic resistance”,
a way to compute temperature based on the way electricity reacts to the
air (this wire is pointing up from the Styrofoam package as shown), humidity
from moisture or ice collected on a chemically treated plastic plate (this
sensor is in the front of the package), and wind speed and direction calculated
by tracking the sonde during the flight using GPS technology to improve
accuracy in tracking both vertical and horizontal resolution of the data.
The sonde is powered by a water-activated battery. A regular battery would
work initially but it isn’t durable enough to last through the rough
weather conditions that are found higher up in the atmosphere.
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This instrument package is attached to a helium-filled
balloon, which can be seen to the left. The balloon is approximately 5 feet tall
and 4 feet wide at launch, and filled with approximately 1500 grams of helium.
However, since air pressure decreases with height, this allows the balloon
to expand to approximately 35 feet wide before it finally bursts at a height
of roughly 15 miles above the Earth’s surface.
As previously mentioned, GPS technology is utilized to precisely track the
movement of the radiosonde. After the weather balloon and instrument package
are launched, we listen for the radio signal the sonde sends back to earth
with a ground based radar dish. This device is located under a 20 feet wide
white dome at the launch site. A new set of weather observations is sent
each second through the radio signal and our radar dish records the information.
This recording is then carried by underground wire from the dome directly
into our office computers. Our computers then send this information to the
weather forecasting computers near Washington D.C. so the weather data can
be used to make the next weather model forecast. The computer and control
display by which we monitor the flight from our office is shown at the bottom
center of the page.
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The launch is illustrated in the next three
images below. The computer and control display by which we monitor the
flight can be seen in a fourth picture, found below right. |
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Once
the balloon bursts, it is free to fall back to Earth. For this reason, a
parachute
is attached between the balloon and the
sonde, so it will float back to the ground, and no damage will be done to
any personal property. The parachute is the orange object below the balloon;
it can be seen fully opened to the right.
If
found, most radiosondes can be returned (using a mailer inside) and reconditioned.
Due to the west-to-east steering
currents over the mid-latitudes, if the winds are strong enough, sondes launched
from this site can reach the Atlantic Ocean. Sondes from this
office, howevere can land just about anywhere within a couple hundred miles
of our office. Maybe you will find one some day. If you do, remember to
send them back!
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weather.gov 
