When Dallas Raines or the Times' weather page reports that it was 33C in Los Angeles yesterday, their data likely come from one source: the National Weather Service's downtown Los Angeles station on the University of Southern California (USC) campus.
Wedged between Vermont Avenue and USC's tennis courts on land leased from the university, this fully automated and remot ely monitored station has made LA's official weather observations since 1999. Stocked with a battery of high-tech instruments, the Automated Surface Observing System (ASOS) is too advanced for a traditional mercury thermometer. Instead, its electronic hygrothermometer uses physics to determine the ambient air temperature in five-minute intervals. Passing an electric current through a coiled platinum wire, the sensor measures the wire's electrical resistance and then runs the result through an algorithm. In simple terms, higher resistance equals hotter air.
Mercury thermometers were still state-of-the-art on 1 July 1877, when Sergeant C. E. Howgate of the US Army Signal Corps stood atop the roof of LA's Ducommon Building at Main and Commercial and recorded a high of 25C. Since then, the official weather station has moved seven times — introducing discontinuities in the historical record — but meteorologists have amassed a daily log of LA's weather that spans 136 years.
To find LA's hottest day ever, one needs only to turn back to 27 September 2010. With an offshore wind blocking the cooling effects of the ocean that day, the city baked under the summer sun, and the weather station recorded a temperature of 45C 12.15pm. The high-tech sensor then promptly overheated, denying meteorologists absolute certainty that the record wasn't actually higher.
"Unfortunately, it's kind of like when you need things the worst, things break down. That did happen on that day," said meteorologist Eric Boldt of the National Weather Service's Oxnard office, which is responsible for the downtown Los Angeles ASOS.
Still, Boldt is fairly confident that the weather station didn't miss an even higher temperature that day.
"Our thinking was that it happened at the time of day when the temperature had likely peaked."
Of course, the ASOS does more than take the city's temperature. Its sensors — ranging from a ceilometer that shoots a laser into the sky to measure the height of clouds to an 20cm cylinder automated tipping bucket that measures precipitation — track the various weather conditions that might mean life or death for aviators. In fact, most ASOSes live at airports, which makes this urban station somewhat unusual.
"Just from my background, most of the ASOSes are installed in airports for safety of flying," Boldt explained. "This is a little unusual that we have one in downtown. And they wanted to keep a record for the main city area. That's why we've kept this going."
Here's a view of the full array of weather instruments:
This is the hygrothermometer that failed during L.A.'s hottest day on record:
This forward scatter visibility sensor measures air visibility:
The anemometer measures wind speed and direction:
The heated tipping bucket measures precipitation. Within the wind shield is a cylindrical bucket that tips when it collects 0.25mm of liquid precipitation:
A Moreton Bay fig and eucalyptus trees stand near the station. Their proximity may influence weather readings, but they also approximate the conditions most suburban Angelenos experience: