To many meteorologists, summer is June, July, and August, the three warmest months of the year. So you could say that today is the last day of meteorological summer. In the month to come, the inevitable slide back into the cold season really starts to become noticeable.

You can see it in the length of daylight. On September 1st, the sun's above the horizon for more than 13 hours, but that falls off to 11 hours and 45 minutes by month's end. In response, the average high temperature drops from the low 80s on the 1st to the low 70s by the 30th. Ninety-degree days are still not that unusual in September, especially early in the month. At the other extreme, the temperature has not officially dropped below freezing during the month at either Wilmington or Philadelphia, but there have been a few late-month readings in the low 30s.

As summer's heat and humidity fades during the course of the month, so too does the risk of thunderstorms. But with ocean temperatures peaking in the Atlantic in September, hurricane season enters its peak time. So all eyes turn south and east, not so much for the potential of wind as for flooding rains.