COOOOOOOOLER!

It is the time of the year that I try to forget about ragweed and long for frost.  The earliest the official temperature has dropped to 32° is the 28th of September but the average date is mid October.

When the longer clear nights of Autumn allow the temperature to drop into the mid and upper 40s in most areas, rural valleys can see freezing. Cold air is more dense than air that is warmer and as in contact with the ground is cooled by the ground it flows down hill. 

Any bicycle rider, motorcyclist or owner of a convertible can tell you in the dips along the road and in the deeper valleys it is colder. That is called cold air drainage and the more dense cooler drains down hill and pools in low spots and valleys. So when it makes the mid 40s in the suburbs it is not unusual for rural valleys to get close to freezing.

As you can see on the map above blue is on the way. Of course the blue is the cooler air that gets here this week.

For specifics on the forecast take a look at the video below.