‘Feels like’ heat index nothing to take lightly

Sep 15, 2019 by Jordanna Sheermohamed

Sea Science: by Jordanna Sheermohamed 

As we leave summer behind and prepare to embrace the cooler winter temps, lingering moisture is quick to remind us who really calls the shots in  those transition days of autumn. It’s a common debate: dry heat versus a swampy heat; the extra chill of a damp winter night versus the T-shirt-still-possible temps of fresh, high-altitude snow. It’s moisture that tips the scales from comfortable to unbearable.

Blistering temps are often presented with a “heat index,” which can be easily misunderstood as an actual temperature. The heat index – also referred to as the “felt air temperature,” “apparent temperature,” “real feel,” or the “feels like” temperature – is how hot things feel when humidity is factored into the actual air temperature.   

It feels hotter because the excess humidity (moisture) in the atmosphere makes it harder for the body’s perspiration to evaporate. Minimized evaporation helps sweat to remain on the body and inhibits the ability to cool down with a passing gentle breeze, as would occur with a wind chill. While the heat index can be slightly subjective, given an individual’s body mass, clothing choices, heat tolerance or activity level, the general perceived conditions are still important to understand.

Consider this: A temperature of 90 degrees Fahrenheit (32 degrees Celsius) with 50% humidity would feel like 95 F (35 C). By comparison, if the humidity increases to 70%, that same location would feel like 105 F [40.5 C]. That 10-degree difference is enough to initiate “extreme caution” or even “extreme danger” warnings of heat. The associated effects on the body range from fatigue, muscle cramps, dizziness  or nausea to loss of consciousness and an altered mental state associated with a heat stroke, which occurs when the body’s core temperature exceeds 103 F (39 C).

The moisture originates from local bodies of waters, from small lakes to large seas and neighboring oceans, ready to hitch a ride on the winds through all levels of the atmosphere. Onshore winds help to drag this local moisture inland, elevating humidity at coastal locations as well as hundreds of miles inland. In contrast, offshore winds can maintain lower and comfortable levels of humidity, in and around 40% to 50%.

The unfortunate circumstances occur when prolonged and elevated heat indices occur in an area in which existing housing lacks air-conditioning units. Furthermore, the heat index values refer to conditions in shady areas with light winds. This means that those who are exposed to the elements, whether at work or at leisure, may be  dealing with even higher “feels like” temperatures, by as much as 15 degrees.

 Recognizing the effects the weather has on our body isn’t new. We dress to protect from the cold or carry umbrellas to shield from the rain. Understanding the heat indices helps to keep us safe from the sunshine we love, in all its excessive glory.

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm (WeatherForecastSolutions.com). Comments are welcome below.


About Jordanna Sheermohamed

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a private weather-forecasting company (www.WeatherForecastSolutions.com). Comments are welcome at editor@the-triton.com.

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