As Far As Heat Goes, July Was The Cruelest Month

A year ago, New Jersey sweated through its fifth-hottest July ever. This July was even hotter, claiming the fifth slot and bumping last July into sixth place.

Detect a pattern? The past four Julys all rank among New Jersey's six hottest since record keeping began in 1895. And seven of the 10 hottest Julys have occurred since 1993.

It reached the 90s somewhere in New Jersey on 18 days in July. But most striking about the month were the minimum temperatures. On 26 July days, the mercury failed to dip below 70 degrees at weather stations around the state.

“There's no question the nighttime average lows in July far surpassed any other month on record,” David Robinson, a Rutgers University professor and the state climatologist, told The Record. “We just shattered the record in terms of the lack of nighttime cooling.”

Intense humidity helped keep temperatures elevated during the nights - but it also kept things from getting even hotter than they were during the day, Robinson said. Because of the humidity, heat at the surface and in the lower atmosphere could not escape through radiation at night. At the same time, the humidity and moist surface conditions tempered daytime highs because some of the sun's energy went toward evaporation, not heating, and produced afternoon cloud cover.

The excessive humidity was produced by a Bermuda high sitting off the East Coast and an unusually large dip, or trough, in the jet stream over the Midwest, which combined to pump warm, moist air from the Gulf of Mexico up the East Coast.

But that pattern changed in mid-July as the Bermuda high moved west and stalled over Ohio and Pennsylvania. That pushed the jet stream trough farther west, keeping things hot and humid but reducing actual rainfall.

“The Bermuda high became a bully for a week, shoving that jet stream trough west,” Robinson said.

In late July, the pattern changed again, as the Bermuda high retreated back over the Atlantic and the jet stream trough moved east, centering over the Great Lakes. That enabled cooler air from Canada to spread farther south than usual, which has accounted for the relatively low temperatures we have experienced into early August. If not for that change, July could have matched the record for heat set in 1955, Robinson said.

As went the state, so went North Jersey. The Hawthorne weather station recorded 21 days with minimum temperatures of 70 degrees or higher, and 13 days with highs in the 90s, according to Bob Ziff of the North Jersey Weather Observers. It was the first July in several years without a 100-degree day in Hawthorne, he said.

The pattern that has allowed cooler Canadian air down into our region is expected to continue for another two weeks, Robinson said.

“If you don't like hot weather in August, this is your year,” he said.