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Once again, Philadelphia faces an especially hot summer

It has been hot for days, for weeks, and for that matter, remarkably often in the last 23 years. In the period of record dating to 1874, officially eight of the 10 warmest summers in Philadelphia have occurred since 1988.

This latest heat surge continues an extraordinarily warm period that took hold in March, right after the historic snows disappeared. In New Jersey, the March 1-June 1 period - the meteorological spring - was the warmest statewide on record, said David Robinson, a Rutgers University professor who is the state climatologist.

The Philadelphia area is now in the midst of its sixth heat wave of the season. The National Weather Service recorded a temperature of 91 degrees before noon at the Philadelphia International Airport. A heat wave is three days in a row of temperatures of 90 and above. The highs Wednesday and Thursday were 93 and 91, respectively.

There have been 16 days with highs of 90 or higher this month, including two 100-plus days.

The eastern U.S. cooked for another day Wednesday as unrelenting heat again sent thermometers past 100 degrees in urban "heat islands," buckled roads, slowed trains and pushed utilities toward the limit of the electrical grid's capacity.

Records fall again in East as heat swelters on

Philadelphia hit 100 degrees for second straight day, breaking a record of 98 degrees set in 1999. Baltimore hit 100 for the third straight day and Newark, N.J., hit triple digits for the fourth straight day. New York's Central Park was at 99 degrees at 2 p.m.

Having just completed the warmest spring on record, Washington, DC, and other cities in the eastern United States are enduring a record-breaking heat wave that is consistent with climate change. Last week’s temperatures in Washington D.C. broke a century-old record and forecasters expect this month to be the hottest June on record for the area. These patterns fit the long-term trends of more frequent heat waves driven by climate change

Characteristics of the current heat wave in the Washington, DC area include record daytime highs, record high overnight lows, and the long string of days above 90 degrees, all of which are consistent with the trends in the U.S. driven by climate change. The back-to-back heat waves experienced in Philadelphia this June also reflect the long-term global warming trend, as do the record-breaking average temperatures witnessed this past spring in Washington D.C., Maryland, Pennsylvania, and New Jersey.

As she did for winter 2010, this summer Mother Nature has truly outdone herself, this time by brewing up a miserable combination of heat and humidity that has enveloped not only the northeastern U.S., but much of the Northern Hemisphere as well. As Ian Livingston reported on Saturday, July was the warmest single calendar month of all time in Washington (tied with July 1993). The warmest day in the city was July 7th, when the temperature soared to 102 degrees Fahrenheit at Reagan National Airport. The day before, the temperature rose to a sweltering 105 F at Baltimore-Washington International Airport. In total, Washingtonians sweated through 21 July days during which temperatures exceeded 90 degrees. Baltimore set a new record for the average daily high temperature, which was 92.5 degrees, beating July 1988's 91.9 degrees. New York City and Philadelphia experienced their second-warmest months since records began in the late 1800s. New York recorded an average monthly temperature of 81.3 degrees, which was 4.8 degrees above the July average. The warmest July on record there occurred in 1999, when the average temperature was 81.4 degrees.

Although long-term global climate change doesn't directly cause a particular heat wave, the pronounced warming trend in global average temperatures during the past century has increased the odds of more frequent and severe heat waves. For example, scientists have partially attributed the deadly 2003 European heat wave, which killed tens of thousands, to manmade climate change.