As appeared in the
December 2015 Edition of Up Portland

Welcome to UpPortland's weather column! Before I dig into one of the many fascinating aspects of Portland's weather, I'd like to introduce myself. My name is Jack Sillin and I'm a sophomore at North Yarmouth Academy. 

My main hobby outside of school is weather, however I also like to ski, hike, run and debate in my spare time. In addition to my monthly column on Portland's weather, you can find my forecasts daily through the UpPortland website (, on my blog (, as well as on Twitter @JackSillin.

Now for the weather... 

To begin my column for UpPortland, I'll take the first couple months to describe some of the factors that influence the weather here, such as the mountains and the ocean. Because this is a monthly publication, I will stay away from looking ahead to future weather (though I will do that on my blog and on the newspaper’s website link so check my daily forecasts there) but instead here I will focus on education by explaining some of the factors behind the weather in Portland. 

I’ll also try and explain recent prominent weather events that have occurred here. This month's column will focus on one of the not often thought about factors behind the weather in Portland which is the mountains.

We all know that they're there, but they are far away, beyond the horizon most of the time, perhaps faintly visible on a clear day from the Western Prom or the Observatory, but they seem distant, far off places. They actually have a huge influence on the weather here in Portland as with all of Maine.

The first and most common way the mountains influence the weather in Portland is through a process known as downsloping which is what I will talk about this month. While many people know that when air rises, it cools and the water vapor contained in the air condenses (Remember your 2nd grade science class water cycle?), few know that the process can work backwards and actually produce drastic results.

Downsloping is when winds blow air down a mountain range or higher terrain onto an area of lower terrain. Here in Maine, this means that when winds are westerly or north-westerly, air travels across the Great Lakes / Southern Canada and then rises up the west side of the Appalachians and then descends onto the coastal plain of Maine.

Downsloping can have two main impacts on the air over Portland. Air after downsloping is both warmer and drier than it was before. This has to do with how air in the atmosphere is (usually) warmest at the surface and cools as you rise in elevation. As the air downslopes off the mountains, it loses elevation and gains proximity to the warm ground (heated by the sun) thus warming.

The most striking example of this recently was on the 6th of November. A cold front passed through the area late in the evening and as winds shifted to the west, temps spiked between 4 and 6 degrees in a single hour in the middle of the night.

This was because the winds were downsloping off the mountains and it didn't take long for that extra warm air to arrive at the coast following the cold front. 

Factors such as the sharpness of the front, the qualities of the air just above the surface, and the post-frontal airmass must align perfectly for this to happen quite so dramatically, so not all cold fronts/downslope situations see this type of spike but it was a perfect example of the power of downsloping to warm the air.

The other thing that downsloping does is that it dries out the air in two ways. First, most downsloping situations have accompanying upslope situations as the air must first rise in order for it to fall. Most upslope situations involve some sort of precipitation or precipitation enhancement which removes some moisture from the air. However, this is not the only way air dries from downsloping.

Saying the air dries just plain and simple implies that water vapor is actually being removed from it. However, that doesn't happen in downslope cases. Even after the air finishes upsloping and causes whatever precipitation it wishes that way, it is still fairly moist. In downsloping, the air dries in terms of it becoming more and more removed from its saturation point. That's kind of confusing so lets use a metaphor.

Think of a gallon jug filled with water. The jug represents the air's capacity to hold water. The full gallon is saturated or nearly saturated air before downsloping. Because warmer air can hold more water and the air warms as it descends, the one gallon jug slowly grows into a 5 gallon jug then a 10 gallon jug and so on until the air stops descending. 

At the beginning, we would've said the full one gallon jug was “wet” or “saturated”, being 100% full of water. However, if we ended up with a 10 gallon jug filled with only 1 gallon of water, the 10 gallon jug as a whole would be termed 'mainly dry' being only 10% full. 

Now returning to the atmosphere, air at the top of a downslope setup is often saturated (think of the clouds and showers associated with upslope). As it descends, no water vapor is removed from it (it doesn't rain/snow on its way down) but the capacity of the air to hold water increases just like the size of the water jug increased. This is how air “dries” even though no moisture is actually removed from it.

Downsloping is one important way that the mountains impact Portland's weather by warming and drying the air. Next time we have a warm and pleasant afternoon following cold morning downpours, thank downsloping!

Look for more on how the mountains influence our weather next month right back here in UpPortland.

—Jack Sillin