Why is hot and humid weather so dangerous?

Air masses with excessive humidity, whether hot or cold, do not readily take in additional moisture. As is the case for sweltering weather conditions (hot and humid), you keep sweating but evaporative cooling is reduced significantly since air’s demand for water vapor shrinks as relative humidity increases. At the same time, you continue taking in more energy from the hotter environment making for a potentially lethal scenario if overexposed.

This threat to health prompted the creation of a “heat index” calculation, which translates to an “apparent temperature” after considering the actual temperature and humidity combination. Dry heat tends to reduce the apparent temperature value you feel, while hot and muggy conditions increase apparent temps that stress your body. For example, at 100°F (38°C) with a relative humidity value of 10% yields a temp that feels like 94°F (34°C). Moistening up the humidity to near 60%; however, exacerbates the effective temp to a dangerous 129°F (54°C)! According to NOAA, a heat index value of at least 130 degrees with continued exposure makes heat cramps, heat exhaustion, and heat/sunstroke “highly likely”. Even in the 90°F (32°C) to 104°F (40°C) range the same heat related illnesses are all “possible” with prolonged exposure or physical activity.

Very important context to this discussion is heat index values are based on the meteorological standard for temperatures being measured in the shade. Therefore, heat index values can be up to 15°F higher under full sun exposure, where physical activity often happens! Bottom line, pay attention to heat forecasts, stay hydrated regardless of season or time of day, listen to how your body feels, and check on heat sensitive individuals in your community. Be intelligent about your heat exposure. It’s okay to postpone outdoor events or exercise routines when heat may jeopardize our health.

Another important consideration extended heat waves that result from locations caught under a strong high pressure "heat dome". Remember, excessive sweating is dehydrating your body at the expense of trying to stay cool. Not only is your body losing water, but the act of sweating depletes precious electrolytes, as well, such as potassium, etc. So, during longer heat events it becomes vital to continuously monitor water and electrolyte intake. In other words, taking part in vigorous physical activity or having long heat exposure one day may affect your ability to resist the ongoing heat the following day without proper rehydration.

Additionally, there is concern for metropolitan areas having the urban heat island effect combining with passing summer heat domes and moisture surges that create an ideal environment for elevating overnight temperatures. Using Phoenix, Arizona, as an example during the monsoon, there are instances when the daily minimum temperatures don’t cool below 90°F (32°C) between June and September! Therefore, day-to-day accumulation of heat exposure without proper periods of cooling relief and rehydration, even when the sun is not out, is a health aspect to be mindful of affecting these communities.

Finally, heat is heat, whether weather conditions support a “dry heat” or “muggy heat” there exists the potential for heat related illness symptoms. Try to be proactive to beat the heat for you and others in your company!

-JWM

Photo Credit: Jonny William Malloy