Temperatures had reached the lower 90s already by lunchtime Tuesday, but relatively low humidity was keeping the heat index in check in the Baltimore area.
A temperature of 95 degrees was recorded at BWI Marshall Airport about noon, but the dew point, a measure of humidity in the air, was 58 degrees. That is on the low side this time of year, and it is keeping the humidity from adding to any perception of heat.
During the heat wave earlier this month, dew points topped 70 degrees, bringing the heat index upward of 110 degrees.
The low humidity is preventing the National Weather Service from issuing a heat advisory Tuesday. Bryan Jackson, a meteorologist with the weather service's Sterling, Va., office, said the threshhold for a heat advisory is a heat index of 105 degrees; for an excessive heat warning, the threshhold is a heat index of 110 degrees.
What meteorologists call "mixing out," where drier air has made its way lower in the atmosphere, is occurring across the Baltimore area Tuesday, Jackson said.
But humidity could return Wednesday. A heat advisory is likely, Jackson said, with heat index values of 105 degrees to 108 degrees.
Cooling centers are open around the region, with Baltimore health officials declaring a code red heat day Monday, and Anne Arundel County opening cooling centers Tuesday.
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