Weather Blog

What, exactly, is a 'heat dome'?

What, exactly, is a 'heat dome'?
An unidentified bicyclist waits for a stop light next to a time and temperature sign in Lawrence, Kan., Thursday, July 21, 2011. (AP Photo/Orlin Wagner)

Over the winter, we had "Snomageddon" for the big snow storms back east. Not to be outdone, national meteorologists have come up with a new buzz word to capture the essence of the major heat wave back east: The "Heat Dome".

But this term isn't made up; it's a real weather term, just one not needed to be used very often.

Here is the explanation from a National Weather Service forecaster that was in the AP a few days ago:

"The heat dome forms when a high pressure system develops in the upper atmosphere, causing the air below it to sink and compress because there's more weight on top. That raises temperatures in the lower atmosphere, said Eli Jacks, a meteorologist with the National Weather Service in Silver Spring, Md.

The dome of high pressure also pushes the jet stream and its drier, cooler air, farther north - it's now well into Canada - while hot, humid air from the Gulf of Mexico circulates clockwise around the dome, traveling farther inland than normal. Combined with generally clear skies and the sun's higher summertime angle, "it gets really hot," Jacks said.

The trick with this heat dome is that it is so massive, it's tough to budge, and it's one reason why our own weather in the Northwest has been tough to budge too.

Think of the dome like trying to moving a dresser filled with clothes across the bedroom without those neat little plastic feet you slide on the carpet. You can push and lean against it with all your might, but it's so dense and heavy, it takes a lot to make it budge.

In the meantime, this does have an effect on the atmosphere nearby. As the ridge of high pressure reaches north, it pushes some of the colder air in the northern latitudes down around it -- sort of like when you have water trapped in a tarp on your campground and you poke it with a stick -- the water comes down around the edges, only in this case, it's cold air. So strong ridges are typically punctuated by colder weather on either side.

(In our winter, our cold outbreaks are mainly caused when a big ridge of high pressure builds into central Alaska and thus pushes that arctic air down the ridge's east side through B.C. and into the Northwest. And incidentally, we have also had somewhat of a ridge in that general area of late, keeping lower pressure here)

And with that ridge not really moving, the pattern gets stuck. And we're stuck in the cool phase.

Here is one model image I could find that sort of illustrates this:

Note how higher pressure dominates much of the southeast and mid-Atlantic States.

Some incredible heat readings

And for those inside the dome, conditions are flat out miserable, as you might have heard. Here are some heat statistics from Friday:

  • Newark, NJ reached 108 degrees, smashing their all-time record high of 105. Their heat index (what it felt like when you combine heat and humidity) was 117 at the time.
  • Baltimore's weather station at their inner harbor reported a temperature of 107. Their peak heat index has been 130 -- on par with the Middle Eastern deserts.
  • New York City's Central Park hit 104 with dew points in the low 70s. Highest heat index: 115. JFK reached 102 and LaGuardia hit 103.
  • Dulles Airport in Washington, D.C. reached 105 -- I think that is their all-time record high, eclipsing their previous record of 104. Heat index at the time is 116.
  • Boston hit over 100 for just the 25th time in their very long weather history, topping out at 102, but their humidity was dropping as the heat dome was easing in New England.
  • Philadelphia hit 102 with a heat index of 119
  • Norfolk, VA hit 100 with a heat index of 111
  • Richmond, VA hit 101 with a heat index of 112
  • Annapolis, MD hit 100 with a heat index of 116

And here in Seattle? It was 64 with a heat index of 63.

'Ring of Fire' brings storms, relief

Now, on the edges of the heat dome, you can get some pretty intense storms as the heat begins to react with the cooler weather on the reaches -- with plenty of moisture around from the high humidity. (I've heard this feature referred also as a "ring of fire" as a colloquial term, although not connected to the volcanic "ring of fire.")

Chicago found out Friday morning what can happen when the heat dome finally begins to move away.

A huge thunderstorm carrying incredible rain blew over O'Hare Airport Friday morning. The airport measured 0.28" of rain in 2 minutes and 0.55" of rain in 4 minutes! To put that in perspective, Seattle has had 0.58" of rain for all of July -- and that's spread out over 7 days of rainfall!

Yet the rain kept falling in Chicago. By the time the storm abated 64 minutes after it began, the airport received 1.23" of rain. That kind of rain is unheard of around Seattle, where our record 5 minute downpour is 0.18" and the record 1-hour downpour is 0.54."

But most importantly for Chicagoans, it put a big damper on the temperatures. After sweltering at 97 on Thursday, the temperature was knocked down to 70 during the storm and was "only" 81 at 2 p.m. on Friday -- 15 degrees cooler than the day before.

I think a lot of other cities would gladly take a thunderstorm if it meant the same relief!

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