The instant expert: Humidity

This week Rick Arthur gets steamy about the water vapour in the air that makes high temperatures seem worse.

THE BASICS Let's work up a sweat, students. Humidity is dampness in the air. It plays an important role in climate and weather. The word comes from the Middle English humidite, from Old French, and from the Medieval Latin humiditas, from the Latin humidus.

HOT AND BOTHERED In the coastal regions of the UAE, the high humidity brought by sea breezes off the Gulf exacerbates the temperatures. "You feel hotter than if the air was dry," says a spokesman at the National Centre of Meteorology and Seismology. High humidity also explains why the bespectacled Instant Expert is irritatingly rendered blind as his glasses fog up when he steps outside first thing in the morning.

ARID MEGATHERMAL CLIMATES That's a fancy name for deserts, which, if they're not on the water, "are famous for their intense heat and their very low humidity," says Scott Sistek, a meteorologist at KOMO TV in Seattle (a city with humidity as high as 83 per cent) and one of the Instant Expert's vast stable of, well, instant experts on call around the globe. "Middle East and Northern Africa deserts have intense high pressure that brings sinking air, and air becomes hotter and drier as it sinks. The deserts of the American south-west frequently have relative humidity in single digits - Las Vegas, Nevada registered a humidity of 1 per cent in late June."

BELABOURING THE OBVIOUS For the dunce in the corner over there, when the outside temperature is high, the body wants to sweat; but when the humidity also is high, the air is so full of water that the rate at which perspiration evaporates on the skin is lower than it would be in dry heat. Thermoregulation, or the ability to keep the body temperature within safe boundaries, consequently suffers. Thus the health warnings when it is hot and humid. Only mad dogs and Englishmen, you know.

STRESSED OUT Humidity joins air temperature, wind speed and exposure in calculating the thermal work limit (TWL) by the Health Authority-Abu Dhabi. It's a heat stress index meant for labourers and employers. It complements the midday break law that prohibits work in the sun from 12.30pm to 3pm. If you ever see a TWL below 115 - The National now publishes the number every day - beware.

FRIZZ ALERT High humidity means frizzy hair. The Weather Channel's Daily Beauty Forecast includes a Frizz Alert "based on a mathematical equation that factors in humidity, temperature, wind and more to create an indicator of how weather will affect hair's frizziness". The alert is sponsored by Frizz-Ease Smooth Start Repairing Shampoo & Conditioner from John Frieda. What a happy coincidence, eh?

GEEK ALERT The Finland-based global company Vaisala (which has an office in Dubai) offers a wide array of environmental products and services, including 15 gadgets to measure humidity. The Vaisala Humicap Humidity and Temperature Transmitter Series HMT330 (price on request, as if from a Katie Trotter fashion shoot) appears to be a bit much for one's flat, but the free humidity calculator might provide some madcap meteorological fun (or not). It can be downloaded at www.vaisala.com.

NOT SO SPORTING Tennis players at Wimbledon this year complained about higher humidity under the Centre Court roof. The water vapour in the trapped air produced a wetter, slower ball, the physicist Andy Kidger told The Daily Telegraph, which would aid slow-court specialists such as Rafael Nadal. Alas, the final was played under the sky, and Rafa fell in four sets to Novak Djokovic.

THE DISSENTING OPINION Uh, can there be one? On humidity? OK, there's probably some scientist in nerdy love with the phenomenon, but high levels generally are bad for plants, animals, human beings and machinery. We hate it.

A glossary

We have to get technical, but think of the meteorological bon mots that will fall from your lips this sweaty UAE summer.

HUMIDITY 1) Water vapour content of the air. 2) Some measure of the water content of air.

ABSOLUTE HUMIDITY 1) The ratio of the mass of water vapour to the volume occupied by a mixture of water vapour and dry air. 2) Mass of water contained in a unit volume of moist air.

RELATIVE HUMIDITY The (dimensionless) ratio of the actual vapour pressure of the air to the saturation vapour pressure. The relative humidity is usually expressed in per cent, and can be computed from psychrometric data.

SPECIFIC HUMIDITY The mass of water vapour per unit mass of air, including the water vapour (usually expressed as grams of water vapour per kilogram of air).

DEW POINT The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapour content in order for saturation to occur. When this temperature is below 0°C, it is called the frost point.

Source: National Snow and Ice Data Center at the University of Colorado Boulder in the US