Meteorologist Kirstie Zontini talks about the storm threat today and the high heat for Independence Day.

A massive ‘heat dome’ has opened the summer, and it’s already setting records

It is a term that we use a lot in the summertime, and we’ve certainly had to get use to lately. Since late June, a massive “heat dome” has encompassed nearly two-thirds of the continental United States.

While these weather events are a normal occurrence this time of year, the latest one over the last few days has broken quite a few records.

First, let’s talk about what exactly a “heat dome” is. This weather phenomena occurs when a certain set of conditions align, setting the stage for extreme heat to develop over a large geographical area.

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These conditions occur when the jet stream, a river of fast-moving air that typically flows across the northern United States or southern Canada this time of year, begins to ridge northward. At the same time, the air pressure builds across areas south of the jetstream in the upper-levels of the atmosphere. Since the pressure is high, well above the ground, it forces the air toward the ground in a sinking-fashion. Typically, this type of air-movement will squash any chances for large areas of rain to develop, as storm clouds need rising air.

But what really causes the heat to build is the actual sinking process of the air. As the air gets pushed toward the ground, it gets compressed due to the ‘weight’ of the sinking air. When air gets compressed, it heats up.

The stronger the high pressure is over an area, the hotter the temperatures can get. In Denver, Colorado, the temperature soared to 105 degrees over the weekend, tying an all-time record for the warmest temperature ever reported.

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But what happened in Denver wasn’t the complete story of the oppressive heat that is ongoing across much of the country. With high pressure, winds, while sinking, typically rotate clock-wise around the center of the high-pressure system. When the center of the system moves toward or just of the southeastern United States, the winds being to pull-up moisture from the Gulf of Mexico far inland.

When a heat dome develops, this moisture can get pulled much farther north into the continental United States than what is normal. When this occurs, the moisture – or humidity – combines with the extreme heat to increase what is called the heat index.

The problem for us humans is, when it is both really hot and really humid, the body’s natural ability to cool itself off gets short-circuited a bit. Our bodies cool by sweating and then having the sweat evaporate from your skin. This evaporation cools the body temperature.

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But when it is humid outside, the sweat cannot easily evaporate into the air because the air already has a lot of moisture in it. So, without the sweat evaporating as easily, your body cannot cool as easily.

Think of the heat index as a measurement of how difficult it is for your body to cool itself. Basically, the higher the heat index, the harder your body has to work to keep cool, and thus, the easier it is for you to suffer heat stress or even worse.

One thing is for sure, despite a chilly end to spring, our summer thus far has started out well above normal. While we may get a brief break from the extreme heat, don’t count on the break to last long. Almost all of our long-range models call for above normal temperatures – perhaps way above – to be around through most of July.

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