Hot waves: Hot facts and safety tips
| By Tony Schumacher |
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Even in the coolest of summers there are at least a few days of sweltering heat that makes your life a bit more difficult. A serious heat wave of recent memory occurred in July 1995 when over 500 people died in the Chicago and Milwaukee areas. How about the majority of the drought summer of 1988 when repeat
episodes of triple-digit heat claimed over 7,000 lives in the United States? The all-time record highs for our area are generally in the 100- to 115-degree range. Hopefully you won't have a sporting event planned when such a day develops, unless it's in the water!
So what causes heat waves? A prolonged period of hot summer weather occurs when a strong ridge of high pressure in the upper atmosphere becomes nearly
stationary over a given area. The high pressure causes the air to sink, which inhibits cloud formation. Thus the summer sun bakes the ground and air for days on end. If the ground is especially dry, the temperatures rise even faster than normal. In the eastern half of the nation, the upper-level high pressure that builds is often an extension of the Bermuda high. This is a semipermanent feature over the Atlantic Ocean near Bermuda.
In the summer it intensifies and expands west over the eastern United States. Since air circulation around high pressure is clockwise, and we're on the western flank, we may experience five or six days in a row of southerly winds. This pumps very muggy air from the Gulf Coast area into the Midwest.
Finally, strong upper-level highs prevent cold fronts from making too much progress south. Often in July and
August they'll limp their way through parts of Minnesota, Upper Michigan and Wisconsin before stalling. This may bring some temporary relief to the Northwoods with high temperatures retreating to the 70s. However, cities like Chicago and Cedar Rapids are often left in the sizzling 90s about five to eight days more in summer than just 200 miles north in places like Green Bay and Minneapolis.
Speaking of cities, did you know heat waves are accentuated in large urban areas? The pavement and buildings soak up the sun's heat during the day and cool off only very slowly at night. It's not uncommon during a serious heat wave for overnight low temperatures to hover around 75 degrees in big cities, while it falls to the upper 60s only 10 miles out of town!
One variable that doesn't significantly change between city and country is the humidity. Is the saying, "It's
not the heat, it's the humidity," really true? The answer is a resounding yes. A chart was developed in 1979 by R.G. Steadman, which is used to determine the heat index or apparent temperature. It's based on the premise that the more humid the air is, the harder it is for your body to cool itself through the evaporation of sweat. For example, if the air temperature is 90 and the relative humidity is 60%, typical for an Upper
Midwest heat wave, the heat index is 100 degrees. That's how hot your body thinks it is.
Let's consider dry heat. If the air temperature is 90 and the humidity is only 20%, common for desert locations like Phoenix, the heat index is 87. That's right, your body can cool so efficiently through the evaporation of sweat that it feels cooler than the temperature you see on the thermometer.
You still need to be careful of dehydration because it's estimated that you can lose a half-gallon of water in 10 minutes if you're exercising when it's 90, the air is very dry, and you're in direct sunlight.
In any case, the following heat index levels have been linked to the following risks.
1) CAUTION – 85 to 94 degrees: Physical activity may cause general fatigue.
2) EXTREME CAUTION – 95 to 105 degrees: Possible heat cramps or heat exhaustion with prolonged exposure.
3) DANGER – Greater than 105 degrees: Possible heat stroke with prolonged exposure.
If you're participating in your favorite sporting activity in large cities, respiratory problems are more likely during heat waves because of the excessive buildup of ozone and air pollutants. Also keep in mind the heat index is not a perfect tool. For instance, you'll obviously feel cooler than the chart indicates if there's a
decent breeze or you're in the shade; however, it's a good starting point. Generally the National Weather Service will issue a Heat Advisory if the heat index is expected to be in the 100-to-115 range and an Excessive Heat Warning for plus-115-degree weather. This varies by region of the country because folks in Florida and Texas are a lot more prepared to deal with the heat than we are in the Upper Midwest.
I'd like to dispel some myths about heat and humidity. On a steamy afternoon, I often hear people saying, "Boy, it's awful; the humidity must be 100%."
Actually, that's never the case once the air temperature gets above 80 or so. Relative humidity is a relationship between air temperature and dew point temperature. The warmer the air is, the more moisture it takes to saturate it. When the temperature and dew point are the same, the relative humidity is 100%. The
moisture in the air, measured as dew point, has a fixed upper limit, however. It generally never gets above 80° F in the
Upper Midwest. So when the air temperature is in the 90s, the relative humidity is generally in the 55% to 70% range.
If you're watching the weather report on TV and notice the dew point is 60 degrees, it starts to feel sticky for a few people. If it's 65, most people feel uncomfortable, and when it reaches 70 or more, the air feels oppressive.
A second false statement I hear a lot goes like this: "The air is so thick and heavy, you can cut it with a knife."
Hot, muggy air is actually lighter than cool air. Water vapor molecules weigh less than dry air molecules.
No matter whether the air is heavy or light, there are some common-sense ways to stay safe in the heat. Obviously you need to drink plenty of fluids, especially water. Take frequent breaks from heavy activity,
especially during the hottest part of the day. Wear lightweight, light-colored, loose-fitting clothing if possible. Also, eating smaller, more frequent meals may help you feel more comfortable. Finally, avoid sunburn because it diminishes your body's ability to stay cool, not to mention the cancer risk. You're most at risk from sunburn from 10 a.m. to 3 p.m. Actually 1 p.m. is the peak time for UV radiation exposure, while it drops 50% just three hours from then.
Of course, overcast, rainy conditions reduce UV exposure considerably; however, partly to variably cloudy skies do little to ease the threat. Being at higher elevations, such as in the mountains, increases the risk of sunburn, while smog reduces it. Be especially careful if you're spending a lot of time near sand or water because the reflection of sunlight off these surfaces boosts your UV exposure.
We can feel fortunate in the Upper Midwest that our summers are generally tolerable. After all, the average high temperatures in July and August are generally in the low to mid 80s, a lot cooler than other parts of the country. Usually, with the exception of about seven to 10 days, it's fairly safe to participate in your favorite outdoor activity. If you can't afford to miss training on one of those scorchers, at least try to do it during the early morning or evening.
By the way, the forecast for July and August calls for above-normal temperatures in much of the nation, our area included. In that case, maybe 12 or 15 days will be rough for a good workout or race.
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