Why does Austin have two official temperature readings?



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Posted on October 24, 2010 at 8:56 PM

Updated Monday, Oct 25 at 10:25 AM

When you watch the forecast on KVUE, you'll notice two sets of high and low temperatures - one from Camp Mabry, the other from Bergstrom International Airport.

The National Weather Service records official temperatures at both of these locations, and the low temperatures between the two locations often tend to differ.

The NWS weather instrument cluster at Bergstrom International Airport is situated in a valley. This means that when the air cools at night, it will sink to the lowest part of the valley.
Because of this, it's often the low temperature at Bergstrom that's lower than Camp Mabry's, but the question is by how much.
Camp Mabry also has an NWS weather instrument cluster. Compared to Bergstrom, Camp Mabry's elevation is higher at 696 feet, and in addition, there are trees surrounding this instrument cluster. So these two factors will also have an effect on nighttime temperatures.
Wooded areas keep nighttime temperatures warmer compared to open spaces. That's because trees can trap some of the heat that escapes from the ground.
Master Sergeant Joshua Stowers is a meteorologist with the Texas Air National Guard, and monitors temperatures at Camp Mabry daily.
The weather tools he uses are standard National Weather Service instruments. Those same tools are used at both recording stations.
"This station does temperature and dewpoint. The temperature in a field environment - what you want to do is you always want to take it in the shade, with the wind blowing towards you," explains Stowers.
A white dome shades the thermometer, and this is important, because exposing the thermometer directly to the sun's rays means inaccurately higher temperatures.
Between Mabry and Bergstrom, the daytime highs normally don't differ that much.  Both sites are away from downtown. So the heat island effect--- in which concrete from buildings absorbs heat, thereby raising temperatures---is not a factor.
"Generally, the high temperatures aren't that big of a difference - it's just the low temperatures at night. We'll see an average from anywhere from 3 to 5 degrees Fahrenheit difference," says Stowers.
So why does the National Weather Service take temperatures from two different locations? One set of readings does not always represent the entire metro area, and the two stations are not close to each other.
"There's a 150 to 200 foot difference in elevation - it's just different topography. So that's why there's a need for both stations - one to continue the records for downtown; one to continue the records that were set and began at the air force base," explains National Weather Service meteorologist Steve Smart.
According to Smart, Austin's weather history began downtown in 1855. That weather station then moved to the old Robert Mueller Airport, which closed in 1999.
"There was an agreement to move that automated surface observing system from Robert Mueller to Camp Mabry to continue the weather records," says Smart, adding that Bergstrom is continuing the record keeping that began in 1942 when the airport was an air force base.
Here’s another question: Why are Bergstrom's instruments set in a valley, near Onion Creek?
Smart says choosing the site was not a simple matter, explaining the instrument cluster had to be put in “ the best location that's going to serve the interest of aviation operations and aviation safety."
Officials also had to make sure the instruments wouldn't interfere with any future airport expansion.
And as the world vigorously debates the issue of climate change, questions arise as to whether temperature readings worldwide give an accurate and reliable measure of the earth's temperature.