“It’s not the heat, it’s the humidity.” You have undoubtedly heard this cliche. There is validity to it though; the presence of large amounts of water vapor in the air decreases the rate at which water or sweat can evaporate from our bodies thereby reducing the body’s ability to cool itself down. So on humid days it feels hotter to us than if the air were dry. That is why, in addition to relaying temperatures, meteorologists will also use the heat index, or “feels like" temperature on warm and humid days. The heat index is an apparent temperature that takes into account both heat and humidity to relay how it actually feels to humans. Its winter counterpart is the wind chill which takes into account the combination of wind and cold in a similar manner.
The variable most commonly used to communicate the amount of water vapor in the air is the dewpoint temperature. Dewpoint temperature is derived mathematically from wet-bulb and dry-bulb temperature readings obtained using a psychrometer. While it is not a direct measurement of the water vapor content of the atmosphere the value of the dewpoint temperature is directly correlated with the amount of water vapor, and as such can be used as its surrogate for communication purposes.
A quick Google search for comfort-level thresholds based on dewpoint will yield many charts with slightly different interpretations. For the purposes of this discussion we can say that a dewpoint temperature of 70°F or higher is extremely uncomfortable, and becomes downright oppressive once it reaches the mid and upper 70s.
With this in mind, the past week in the NYC area was one of the most uncomfortable since record-keeping began. JFK Airport measured a dewpoint temperature of 70°F or higher for a record 183 hours (7.6 days) surpassing the previous record of 178 hours set back in August of 2003. Hourly dewpoint readings for the entire length of the streak can be seen in the chart below.
The peak of discomfort occurred on Saturday, August 13 when the dewpoint briefly soared to 84°F - the highest on record at the airport. The temperature that hour was merely 88°F, but the feels like temperature (heat index) was 112°F thanks to the outrageous humidity. A weak cold front brought dewpoints down to more reasonable levels on the morning of August 17 marking an end to the record-long streak of uncomfortable humidity.