Tuesday was the third day this month Portland has reached or exceeded 90 degrees.  While June was comfortable, most days this month have averaged above normal and since the 10th of the month, the afternoons have been particularly warm.  Humidity has ebbed and flowed much of the month bringing us cool temperatures for the first couple of hours in the morning.

You might have heard the term “heat dome” several times this summer. While this term is catchy, there’s nothing meteorologically significant about it nor is it an official term we would have learned about in weather school. However, it is a great descriptor and does help partially explain why it’s been so hot the past couple of weeks.

There are generally two types of pressure systems, highs and lows.  In the low pressure system the air is rising while in the high pressure one, the air sinks.  This is the key to the dry weather and heat as sinking air warms and dries.

The image below illustrates how the high pressure system builds heat beneath it.  As the air sinks, it warms and becomes trapped under the sinking air.  Day after day the air gets warmer and warmer and becomes more and more difficult to dislodge.  While these types of pressure systems are more common in the center of the country their effects can spread into New England and the all the way to the coast of Maine.

High pressure caps the atmosphere and can act as a dome keeping temperatures at the ground quite warm

High pressure caps the atmosphere and can act as a dome keeping temperatures at the ground quite warm

High pressure has been the dominant player in our weather since early June and is why parts of the region are in drought.  While not always bringing us extreme temperatures, the sinking air is keeping a cap on the atmosphere and thwarting most showers and thunderstorms from being very productive in their rainfall.

Rainfall has been typically sporadic this month with some areas such as the midcoast seeing adequate rainfall the past week, while Greater Portland still is about 40 percent below average rainfall this month.

Friday brings the opportunity for more widespread showers than we have seen in a while.  This won’t be a severe weather day, rather an area of low pressure bringing rain.  Just as in winter, the exact track of the storm will determine where the heaviest rain does fall.

The weekend is shaping up to be a nice curtain call to July with a blend of clouds and sunshine along with pleasantly warm temperatures and moderate to low humidity.

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