Is Average Relative Humidity A Meaningless Measurement?

Relative humidity is one of the most widely reported weather parameters and is familiar to most people.

Scientists sometimes misunderstand relative humidity.

Still, it is not uncommon to encounter it being misused.  Here are two examples:  

1. My sister recently stated that her son was experiencing 45℃ and 100% humidity while walking around during the day in the Philippines.
2. In scientific literature, I often find figures displaying daily average relative humidity over a period of weeks or months.  

Both of these examples show a misunderstanding of what relative humidity is and how it can be used.

What is relative humidity?

Relative humidity (h_r) is the ratio of the vapor pressure (e_a) in the air over how much vapor pressure there could be if the air were saturated at that air temperature (saturated vapor pressure, e_s(T_a)).

While vapor pressure is a reasonably conservative quantity, meaning it doesn’t change drastically with time (i.e.hours), e_s(T_a) is solely tied to temperature, shown by the empirical Tetens equation:

where T_a is air temperature, and b =17.502 and c = 240.97℃ (constants).  As the equation shows, saturated vapor pressure is only a function of temperature, so relative humidity in natural conditions will simply show a sinusoidal pattern that is inverse to air temperature.  

When humidity is higher, the vapor concentration difference is smaller so we lose less water, reducing our ability to cool.

Why do we estimate it poorly?

When temperatures are elevated above our comfort zone, we begin to feel hot. Our bodies, which are adept at keeping us cool, evaporate water from our skin to return us to a comfortable skin temperature.  When humidity is higher, the vapor concentration difference is smaller so we lose less water, thus reducing our ability to cool.  In an attempt to balance the humidity, our body moistens the skin surface with sweat, leaving us feeling damp and sticky. This makes us feel like the air is nearly saturated, but in reality, the higher humidity has simply limited our ability to cool ourselves.

It is a relatively simple thing to convince ourselves that daytime humidities are never 100% unless it’s raining. We know that daytime temperatures are almost always higher than nighttime, due to solar radiation. And, we are familiar with dew that forms on surfaces as nighttime temperatures cool to the point that they begin to condense water out of the air (dew point temperature). If we assume that the vapor pressure of the air (e_a) is the same as the saturation vapor pressure when the dew began to form (nighttime low temperature), then any air temperature throughout the day (T_a, which we assume would be higher) generates a saturation vapor pressure (e_s(T_a)) that is higher than e_a and thus, relative humidity would be less than 1.

So, what about my nephew in the Philippines? Right now, a typical low temperature is 24℃ with a high of 34℃ (when it’s not raining).  Under that scenario, the relative humidity, although it would feel quite high, would only be around 56% at midday.

Next Week: Learn what’s wrong with using average relative humidity in scientific papers and what measurement should be used instead.

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