What the humidity metric is hiding?

Question

So I have been spending time in Moscow Russia which essentially records very similar climatic humidity to my home city of London. However in Moscow my throat is permanently dry, I need to consume more water, many people have humidifiers in their bedrooms and the government spray the roads with water to prevent citizens choking on the dry dusty air.

Clearly the climate is 'drier' (or another more accurate technical adjective). What metric will show this difference between Moscow and London ?

And why is the humidity metric the same ?

Answer 1

Weather services rarely state the absolute humidity because it is not easy to determine. Instead, they state the relative humidity.

Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture per cubic meter of air ($\mathsf{g/m^3}$).

Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.

A better measure of how atmospheric moisture will affect people and the environment is the dew point.

The dew point is, in short, the point at which dew droplets form on objects like grass – in other words, when a relative humidity of 100 percent is achieved.

If you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

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Edit 31 January 2020

Taken from Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass

Dew point is the indicator of the amount of moisture in the air ... it's the temperature that the air needs to cool to in order to form cloud.

When temperatures are very high, relative humidity can under represent how much moisture there is in the air, making dew point the better measure.