Baltimore gets a shade warmer, according to new averages

The National Weather Service's new 30-year temperature and precipitation averages, which are used by farmers, utilities and meteorologists, show a warmer, drier and snowier Baltimore.

Compared with the previous data, the updated numbers show that:

• The city's average annual temperature has increased by 0.5 degree Fahrenheit.

• Annual precipitation has dropped by 0.06 inch.

• Average annual snowfall has jumped by 2 inches, to 20 inches per year.

Climate scientists say the warmer trend and even the snow data are consistent with a warming planet.

Farmers dealing with weather on a month-to-month basis may not see the trend. But other agricultural interests are already thinking about how they will adjust in the long term to an increasingly warm future, said Dave Martin, University of Maryland extension educator for Baltimore County.

"Various parts of agriculture, from farmers to researchers to the supply industry, all have raised the question about what changes will need to be looked at … how that will affect weed control, crop maturity and maybe even some species — crops that haven't grown here that we'll be able to grow here, or vice versa, crops we have grown that will gradually be harder to grow."

The 30-year averages are used by farmers to make decisions about crop selections and planting times. Weather forecasters use them for comparisons in daily weather reports. And electric utilities use them to project long-term and short-term energy usage.

Since 1950, all daily, monthly and annual weather statistics have been compared against a moving 30-year average that's adjusted once every 10 years.

Since 2001, the 30-year average has been based on data compiled from 1971 through 2000. On Aug. 1, the 1970s were dropped. The new averages are based on weather data collected from 1981 through 2010.

The new normals are not a simple averaging of temperatures over the 30-year period. The NOAA National Climatic Data Center (NCDC) says they also involve a number of additional statistical tools; they adjust for missing or doubtful data, and issues relating to individual NWS weather stations, such as site and instrument changes.

The most striking difference between the old and the new data sets is the increase in temperatures. But that shouldn't be surprising, said NCDC Director Thomas R. Karl.

"The [continental U.S.] climate of the 2000s is about 1.5 degrees [Fahrenheit] warmer than the 1970s, so we would expect the updated 30-year normals to be warmer," he said.

And they are.

Across the lower 48 states, the annual average temperature for the 1981-2010 period increased by 0.5 degree over the 1971-2000 normals. The annual maximum and annual minimum temperature averages increased in every state, the NCDC said.

"That's why we think it's global climate change," said Kevin Trenberth, former head of climate analysis and now Distinguished Senior Scientist at the National Center for Atmospheric Research in Boulder, Colo.

"It's not spotty; it's systematic, on a very large scale. That's consistent with everything we know about increases in greenhouse gases," he said. "It's actually a very good measure of the climate change that's going on."

At BWI-Marshall Airport, the annual average temperature also increased by 0.5 degree. It was up by 0.7 degree at Washington's Reagan National Airport, and by 1.1 degrees at Dulles International Airport.

Much of the increase in the long-term temperature averages can be attributed to warmer nights.

The monthly average for January at BWI inched upward from 32.3 degrees to 32.9 degrees in the new climate normals. The average high for January increased very little — by just 0.2 degree to 41.4 degrees. But the average low was nearly a degree higher, up from 23.5 degrees to 24.4 degrees.

For July, the monthly average increased a half-degree, from 76.5 degrees to 77.0 degrees. But while the average maximum was unchanged at 87.2 degrees, the average low climbed a full degree, from 65.8 degrees to 66.8 degrees.

Precipitation averages were mixed across the region.

The BWI data show a slight decrease in precipitation averages from the old data set to the new. Average annual precipitation was down 0.06 inches. Dulles Airport, too, saw a decrease in precipitation, down 0.26 inch. But Washington National was up 0.39 inch.

Jared Klein, climate program leader at the NWS' regional forecast office in Sterling, Va., sees a seasonal variability to the new precipitation data for the region. "Monthly precipitation normals at all three stations have generally trended drier during the late winter and early spring, followed by a notable wetter trend during the late spring and early summer," he said.

Baltimore and Washington also show an increase in precipitation averages in late autumn. The biggest declines in precipitation averages occurred in January and August at all three locations.

Meanwhile, the city's annual average snowfall jumped in the new 30-year averages, up 2 inches to 20.2 inches. The snowfall norm dropped by less than an inch at Washington, while increasing by less than an inch at Dulles.

Most of the explanation for Baltimore's big increase was the huge snow total during the winter of 2009-2010. That winter saw a record 77 inches of snow at BWI, most of it in three blizzards in late December and early February.

Klein said that if the snowy 2009-2010 season were removed from the data, "then the 1981-2010 normal seasonal snowfall would decrease by 2.6 inches at BWI," Klein said. "It would also shift the normal seasonal snowfall totals slightly below the 1971-2000 normals."

In other words, if we had succeeded in wishing away those storms, the new 30-year snowfall average for Baltimore would be 15.6 inches, instead of 20.2 inches.

Maryland weather blog: Frank Roylance on meteorology

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