Late summer inevitably brings shifts in weather behavior that are harbingers ofmore dramatic happenings to come.Weakening of sunlight, including shortening days, allows areas in the north tocool significantly during the last few weeks of the season.

In answer, the jet stream strengthens in its course across the Northern states,as do the lows and cold fronts that attend to it.

Many areas get their first chilly nights since spring.

Morning fog becomes more common thanks to lengthening nights together withlingering summer humidity.

Still, there is plenty of warm air left at this time of the year, and there canstill be memorable heat waves.

More seriously, September is the biggest month for hurricanes by far.

This is because surface water temperatures of the tropical and temperate oceansare at their yearly highs, even as the land begins to cool significantly. Thisshifts the focus of thunderstorms from land to sea, including the very warmseas where tropical storms and hurricanes normally spin up.

Story by AccuWeather.com Senior Meteorologist Jim Andrews