We have lived near the town of Atenas in the Central Valley of Costa Rica for nearly four years. In all that time and until just recently, we never encountered any single day when the combination of air temperature and humidity produced a "feels like" temperature of over 100 °F (37.8 °C). "Feels like" temperatures of over 100 °F are not uncommon in the Atlantic and Pacific coastal regions, but are rare in our region of the Central Valley (elevation ~ 2760 ft. / 840 m.)
In all of last year, the hottest daytime air temperature where we live was 92.1 °F (33.4 °C). The hottest daytime "feels like" temperature (also known as the "heat index") was 94.1 °F (34.5 °C).
But, in late April of this year, there was an abnormal increase in humidity. On two successive days, the air temperatures were around 90 °F (32.2 °C), but the "feels like" temperatures climbed above 100 °F, the maximum being 101.0 °F (38.3 °C). The spike in the following chart illustrates the occurrence of these unusual conditions.
Even the Ticos (native Costa Ricans) were complaining. Nevertheless, and perhaps partly due to some acclimation, and avoiding any serious exertion, we remained quite comfortable without air conditioning. During the peak temperature part of the days, our house remained relatively cool as the result of good built in cross-breeze ventilation (a very important feature of any house where you might choose to live in Costa Rica).
In any case, a two day heat wave with high humidity is nothing like a one week, two week or more heat wave as can occur in eastern Canada and the U.S. during the summertime. Another distinction is that our heat wave was limited to daytime heat. During the nights, there was significant cooling. Sleeping was comfortable - again without any need for air conditioning.
For those interested, the "heat index" equations for calculating "feels like" temperatures based on air temperature and humidity can be found here.