Abu Dhabi, UAEFriday 18 October 2019

Why does it feel so much hotter? Humidity in the UAE explained

With the summer heat upon us, 'The National' explains the science behind humidity

It is not only the air temperature that is causing discomfort in the Emirates this summer. Jeff Topping / The National
It is not only the air temperature that is causing discomfort in the Emirates this summer. Jeff Topping / The National

It is that time of year again. The outdoor terraces are closed and the balcony is off limits.

But while air temperatures are relatively consistent, why are some days in the UAE much more uncomfortable than others?

The answer is humidity, which is likely to approach 90 per cent in coastal regions this week, according to the National Centre of Meteorology. But what is the science behind the sweaty work shirt, and is the way we commonly measure humidity wrong?

What is humidity?

Humidity is a measure of how much water vapour is in the air around us. The more moisture in the air, the higher the humidity.

Usually, the humidity figure we see or hear on weather forecasts is relative humidity, meaning it represents the moisture in the air as a percentage of the maximum amount of water vapour the air can hold. How much moisture the air can hold depends on the air temperature.

Why does it affect how how comfortable we feel?

In might be 38 degrees on both Monday and Tuesday, but Tuesday might feel far hotter. The reason is likely to be humidity, and more specifically, how our bodies react to it.

We tend to deal with high temperatures, in low humidity, relatively well. When it is hot, but there is low humidity, we sweat and this moisture is easily evaporated into the atmosphere. This process helps us cool down.

However, in very high humidity, the atmosphere is already heavy with moisture, meaning it is so clogged up there isn’t room for much more vapour. This means fluid from our skin evaporates more slowly, causing our bodies to overheat.

This is why there are often two numbers in weather forecasts – the actual temperature, and a ‘feels like’ figure, which is supposed to take into account humidity.

What causes humidity?

Humidity comes from water evaporating from large bodies of water. As temperatures heat up, so do the seas, for example, causing more moisture to evaporate into the atmosphere. This is why humidity is worse in coastal areas, such as Abu Dhabi city and Dubai.

How should we cope?

At times of high humidity, it is especially important to stay hydrated. Where possible, it is advised to avoid going outside, and those who insist on exercising outdoors should be particularly aware of the potential dangers of heat stroke. In the home, a dehumidifier can help by sucking moisture out of the air.

What about the dew point?

Meteorologists often use the dew point as a measure of how much water vapour is in the atmosphere, and many argue this gives a far more accurate prediction of how uncomfortable humans are likely to feel, compared to relative humidity.

Crucially, warmer air can hold far more moisture than cooler air.

The dew point is the temperature at which dew begins to form on things like grass or condenses into mist or fog. In other words, it is the point at which air can no longer hold all of its vapour and some must condense into liquid, meaning relative humidity is 100 per cent.

The dew point is always lower or equal to the air temperature. The closer the air temperature is to the dew point, the more humid it will feel. Equally, if there is a large distance between them, it will feel comfortable.

Critics of the relative humility measure argue that while simple, it is often misleading. This is because the amount of moisture the air can hold varies according to the temperature. So while relative humidity tells us how close the water in the air is to saturation, it does not tell us how much total moisture is actually in the air.

For example, 70 per cent relative humidity may sound high, but if the weather is cold, this would feel less stuffy than 50 per cent relative humidity in very hot weather, because a higher quantity of actual moisture would be in the air.

What other problems can humidity cause?

It is not just feeling uncomfortable. Humidity is often blamed for mold, as water from the air settles on surfaces. It can also affect electronics or contribute to causing hurricanes and cyclones.

Is it getting worse?

Research has suggested that global warming may lead to heatwaves beyond the limit of human survival, including in places such as Abu Dhabi and Dubai, with humidity a crucial factor.

A study published in 2015 said wet bulb temperature (WBT) - a combined measure of heat and humidity - is set to increase if global carbon emissions continue at current rates.

At a WBT of more than 35C, even the fittest person would find it impossible to cool their body naturally by sweating, meaning humans could potentially die within the space of just six hours.

The scientists predicted such extremes would occur every decade or two - after 2070 - along much of the Gulf coast if global warming is not curbed.

Updated: July 10, 2019 09:50 AM

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