Why So Hot, So Soon?

It happens every spring. Early in the season, before the trees have grown leaves and the flowers in bloom, there is always a hot dry day. Temperatures push 90 in the New York Metro region, perhaps a week or two after a cool, damp day where the temperatures don’t get out of the 40’s. For us this year, it was yesterday Wednesday April 7. The map below shows temperatures at 4pm yesterday:



Newark and Hartford have exceeded 90 and unofficial reports from Orange County suggested temperatures of 94! But why and how? Well it has to do with trees, or in this case a lack there of.

Trees do two things that tend to reduce surface temperatures, first they reflect radiation and shade the surface and secondly they are a driver in evapo-transpiration. Let me explain.

Trees, when they have leaves and are photosynthesizing, draw moisture out of the soil and release it to the atmosphere when their stomata are open and they are pulling in CO2 (go-go introduction to biology!). In effect they act to moisten atmosphere when the sun is up. In much of the solar radiation (sunlight) that would go into heating the surface instead heats the water in the atmosphere. This is the same reason that the deserts in northern Africa are hotter than the rainforests at the equator, even though the rainforests receive more sunlight. Overall trees keep temperatures down, and increase the humidity.

So if we had had Tuesday’s weather, sometime in May when the trees were fully deployed – temperatures would have been much milder, perhaps with highs in the lower 80’s - even the sun being higher and stronger in the sky!

Another interesting effect to note on the above temperature map is the influence of the cold ocean on air temperatures. With water temperatures still below 50 in many places, when the wind blows off the ocean it acts to cool costal locations. That is why Long Island showed temperatures in the 60’s and 70’s while Newark burned up in the 90’s.

Interesting weather day in the New York Metro Region.