It’s that time of year—so what exactly is the Heat Index?

Here’s the latest edition of Weather whys, a series of posts by Braniff Davis explaining the science behind weather phenomenon affecting Texas.

A reader asked us a great question on Eric’s post this morning about the meaning of a heat index. In a place as hot and humid as Houston, we see meteorologists refer to the heat index all the time, saying things like, “The air temperature is 97 degrees, but the humidity makes it feel like 107 degrees outside.” But what does this actually mean?

With heat indices, there’s a little bit of meteorology—and a little bit of perception. The heat index is an ‘apparent temperature,’ or a measure of how hot air actually feels against your skin. When our bodies sweat, that sweat evaporates off of our skin into the atmosphere. This evaporation of moisture releases latent heat away from the body and, in turn, cools you down. However, when there is already a lot of moisture in the air, the sweat doesn’t evaporate as easily. This makes it harder for our bodies to cool, and therefore, the temperature you feel on your skin is much hotter than the actual air temperature.

HOW DO WE KNOW HOW HOT IT FEELS?

NOAA Heat Index Table
NOAA Heat Index Table

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And so it well and truly now begins … summer

Did you miss the heat?

Houston has had a long, slow slide into summer this year, only reaching the 90s this week, and with wet soils helping to keep a lid on temperatures. That all changes. Now.

High pressure will begin building over the region today, and while we can’t entirely rule out a few isolated thunderstorms, I wouldn’t expect to see much rain over the metro area. Instead we’re going to see partly to mostly sunny skies and lots of heat. With highs likely climbing into the mid-90s today through Saturday, the National Weather Service has issued a special statement warning of elevated heat indices, from 100 to 107 degrees when factoring in humidity. Lows will only fall into the upper 70s to around 80 degrees. This is absolutely typical summertime weather in Houston.

Rain accumulation forecast for now through Saturday. (Weather Bell)
Rain accumulation forecast for now through Saturday. (Weather Bell)

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Houston just had its wettest 60-day period on record

It has been one hell of a spring storm season. Pretty much every part of the Houston region has flooded to one extent or another, with some rivers and bayous reaching historical flood levels. And now, courtesy of Brian Brettschneider, here’s another amazing data point about this spring’s rain—the period of April 14 through June 12 is the wettest 60 days in Houston’s recorded history. Any year, any time.

(Brian Brettschneider)
(Brian Brettschneider)

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It’s stormy this afternoon, but the heavy rains won’t last

Much of Houston has experienced some heavy rain this afternoon, with storms producing rainfall rates of 1 to 2 inches per hour. These rains have scuttled outdoor activities and led to some street flooding downtown, however they are largely proving manageable for the area’s drainage systems. The main hazard is lightning, so please do not venture outdoors unprotected during intense lightning events.

The Houston radar was quite active at 3:45pm CT on Sunday. (Intellicast)
The Houston radar was quite active at 3:45pm CT on Sunday. (Intellicast)

 

It seems most likely that these storms will wane this evening by or shortly after sunset, with the loss of daytime heating. Forecast models show another complex of storms developing over north Texas tonight, but it appears as though these will weaken as they approach the Houston metro area early Monday.

Later on Monday we may see some scattered thunderstorm development, but nothing like today. And then, by Tuesday, we’re going to be into a summer-like pattern where high pressure clamps down on rain chances, and increases highs into the mid-90s.

Posted at 3:45pm CT on Sunday by Eric Berger