high and dry on a long hot day

I am a regular walker. This morning, I got out a little later than usual, that’s all. That’s why the sunshine was also a bit more and brighter than normal.

And 20 minutes of brisk walking below that was enough to induce some of those symptoms of mine. If I didn’t have access to water, I would shorten my walk and find some shade. Because if I kept going, I would be “overheated”. Maybe it’s not a medical term, but I know exactly what it means and it sounds like it. It is not a pleasant feeling.

We sweat when we work hard, which I was doing that morning. When it’s hot out, I usually think of sweating as a good thing, because it’s like my body’s natural mechanism to beat the heat. We’ve all felt it: As sweat evaporates, the body cools. Makes the heat a little more bearable.

But the heat can also make it uncomfortable. This usually happens when it is also damp. This means that there is a certain amount of moisture in the air and we can feel it. Is there any way to put a number to that feeling?

it turns out.

The number is called “relative humidity” (RH). It measures the amount of water vapor in the air.

Not as an absolute figure, but as a fraction of the maximum amount of water vapor the air can hold – described as the maximum “saturation”.

To understand this, think of a sponge. You can squeeze it, or keep it in the sun, until it dries completely. You can say that the relative humidity of the sponge is 0%. Now start using it to soak up water, let’s say from a two-liter pot dropped on the kitchen floor by an errant husband. You will reach a point when the sponge will not absorb any more water. At that point, the RH of the sponge is effectively 100%. We say it is saturated.

To make it absorb water again, you must first squeeze out what is already in it, thus lowering its RH again to near 0%.

The air around us behaves like that sponge. Over some extremely dry areas – the Sahara or the Thar Desert, or the Rann of Kutch – the relative humidity would likely be lower.

For example, in the metropolis of Timbuktu in Mali, in the southern reaches of the Sahara, as I write these words, the RH is 7%. In fact, Timbuktu experiences its lowest average RH in the year during this month of April. In August, the average RH is close to 60%. But if you travel from Timbuktu to Cherrapunji in Meghalaya, you’ll probably be in for a shock once you get off the flight (assuming there is a flight). As I write these words, the RH in Cherrapunji is 81%.

The monthly mean RH varies from 73% in February to 94% in August. Put it this way: Cherrapunji is much more humid than Timbuktu. But then, it probably doesn’t surprise you, given Cherrapunji’s fame as the rainiest place on the planet.

What if and when the RH reaches 100%? You can expect rain – think about squeezing water out of that sponge. But it is not necessary to be so. It just means that the air is saturated with water vapor.

More correctly, it means that the air is saturated with water vapor at the prevailing temperature.

That is, the capacity of air to hold water vapor varies with temperature, and quite significantly. At 0°C—the freezing point—one kilogram of otherwise dry air can hold about 3.5 grams of water vapor. Let the temperature rise by 15 °C, and the same kilogram of dry air can contain as much as 10 grams of water vapor. At 40°C, not uncommon across India these days, the saturation point is about 50 grams of water vapor per kilogram of dry air.

And if at 40°C, we only have about 25 g/kg of water vapor in the air—there are ways to measure it—that’s an RH of 50%.

Somewhat confusing, you think? No problem. The reason we monitor RH is because it has an effect on how we feel, especially when we start to sweat. On some hot summer days, you may find that you can cool off under a fan, or even by waving a magazine in your face. Conversely, we’ve all experienced days when we feel hot and sticky all the time, and nothing seems to be able to cool us down.

The difference is Rh.

In the first case, the RH is relatively low, and there is not much moisture in the air. We sweat, but it evaporates easily and this process itself works to cool us down. In another, the RH is higher, which means it’s more humid.

With that much water vapor already clogging the air, the harder it is for sweat to evaporate. That’s why we keep feeling the heat. The combination of high temperature and high RH can cause heatstroke.

By now, you know where this is going.

A few days ago, the afternoon temperature at one particular place in Navi Mumbai touched 42°C: severely hot. It is not clear what RH was at that time. One report states that an observatory in Rabale, about 20 km away, recorded 47% RH that day. But at this time of the year, Navi Mumbai averages around 74% RH.

Anyhow, more than a million people gathered in that open ground to see our Home Minister presenting the Maharashtra Bhushan award to social worker Appasaheb Dharmadhikari. They sat in the open sun for the ceremony, which began at 11 am and lasted for three hours.

When it was over, 11 people had died of heat stroke. Three more have died. The dignitaries sitting in the shadows were unaffected.

I, I wish that Appasaheb Dharmadhikari would have refused the award under these circumstances.

Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.