Last Gasp Summer
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For some parts of the country, there's an especially good reason to consider Labor Day the close of the summer season. Hi this is Dave Thurlow for the weather Notebook.

True, this is when most school systems get back to work, and it's the last warm-weather holiday on the calendar. But Labor Day also follows a distinctive hot spell that tends to strike the eastern U.S. during late August and early September. Call it the grand finale of summer.

The late-August spike in temperatures is so clear in the long-term climate record that meteorologists call it a "singularity." They believe it's tied to a recurrent pattern in how the jet stream adjusts to the Northern Hemisphere's seasonal reduction in sunlight. However, the exact process remains a mystery. You're not likely to find these singularities in the average highs and lows you see on your daily weathercast. Those readings typically fit the waxing and waning of the seasons, so that each day's average in late summer runs a tad cooler than the day before. However, long term climate records for many spots east of the Mississippi show a tendency for temperatures to hold steady or even climb for a few days before Labor Day, usually during the last week of August or the first week of September.

It's been found in Pennsylvania that this last gasp of summer comes earlier in there than it did a hundred years ago. But anywhere in the east, it's best to wait for the equinox before things really start to chill out.

The Weather Notebook, which is produced by the Mount Washington Observatory is funded by The National Science Foundation, and underwritten by Subaru -- the beauty of All-Wheel Drive.