This time of year, with the longer nights and the pseudo-chilly nights, the overnight low temperature ranges are all over the place. Although we have to commit to one low temperature on the 7 day forecast, it’s hardly as simple as that.
These days, the low temperature can range from the upper 30s to the upper 50s from the coldest suburban valleys to Logan Airport. Radiational cooling – the process by which warmth from the afternoon radiates out to space during clear, calm nights – is the main culprit at play here. Since the cool air is relatively dense, and likes to "puddle" in low spots across the countryside, creating those cold pockets in isolated areas. However, in the cities, the concrete and pavement are an almost inexhaustible source of warmth, and keep the temperatures up at night. It’s a process called the Urban Heat Island effect and it can mean the difference between being in the 30s/40s in the suburbs and near 60 in downtown Boston.
So where’s the balance? Well, we typically take an average of the suburbs and the city and come up with a low for our 7 day forecast. It’s hardly representative of what you may experience, but we can’t litter the 7 day with the "ifs ands and buts" now can we?
Onward and…downward? Yes, the cooler air is coming. A subtle change in the wind direction and the arrival of a brand spanking new high pressure on Thursday and Friday will mean our temperatures slide a good 7-10 degrees by late week. Certainly more indicative of the season, and definitely a setup for ideal radiational cooling.
Pete