Relative humidity is a simple concept as weather phenomena go – but it has significant, far-reaching consequences for how we must take care of ourselves on a hot or wet day.

Letting the wind flow freely is the secret to cooling our cities

Humidity is the amount of moisture in the air around us, and there are three ways to track it. The most common of them is absolute humidity: the mass of water vapour in a given volume of the air and water vapour mixture, expressed as kg/m3. The second is specific humidity, equal to the mass of the moisture divided by the mass of air. It is expressed as a dimensionless number (but sometimes also as grams per kilogram, among other similar units.)

The third way is relative humidity: it is important because it factors in the amount of vapour that air can hold at different temperatures. Determining its value is a bit more complicated: it is the vapour density of the air divided by the saturation vapour density at the dry-bulb temperature.

Why does relative humidity matter?

On a hot day, our body uses sweat to cool itself. Sweat is released via our skin to the surface. There, the liquid evaporates. When water changes phase from liquid to vapour, it absorbs heat from the surface on which it lies. So when sweat evaporates, it absorbs heat from the skin, cooling it.

The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, it won’t easily accept more. This means that the sweat on your skin can’t evaporate. At the same time, the body keeps sweating because it is still expecting to cool itself.

As a result, if the relative humidity is high, you can sweat on a hot day even when you are sitting still – while your body also keeps accumulating heat. This can quickly become dangerous.

A relative humidity of 30-60% is generally considered to be comfortable. Environments that have lower levels than this typically use humidifiers to increase the humidity. When the level is higher, a fan will help move the air around you and help sweat evaporate better. 

In both cases, drinking water is important.

What does relative humidity imply physically?

Warmer air can hold more moisture than cooler air. So at the same absolute humidity, the relative humidity of warmer air will be lower than that of cooler air. It is like saying two vessels can hold the same amount of water – just that the bigger vessel will be less full than the smaller vessel.

This is why, for example, while Chennai had an absolute humidity of 70% at 2.30 pm on April 23, its relative humidity was lower, around 60%, because it had an ambient temperature of 32 C.

The change in the capacity for moisture as the air warms is also non-linear, meaning it doesn’t increase by the same amount as the air warms. For example, the difference as it warms over 20º C from -20º C to 0º C is small, but there is an enormous improvement in its vapour-carrying capacity as it warms over 20º C from 30º C to 50º C.

How do you measure relative humidity?

There are a few ways to measure relative humidity. One is to conduct a simple experiment with household items; another is to use maths; a third is to use an app or device.

Experiment

For the experiment, you need two mercury thermometers, some cotton, rubber bands, and access to cold water.

First, make sure both thermometers show the same reading, say, under a fan. Next, soak some cotton in cold water and wrap it around the bulb of one of the thermometers with a rubber band. Hold this thermometer under the fan, such that water from the cotton evaporates, and record the thermometer reading after five minutes. Hold up the other thermometer in the air and record its reading after five minutes. You will thus have the wet-bulb and dry-bulb temperatures, respectively.

Next, subtract the wet-bulb temperature from the dry-bulb temperature to get the temperature difference. Finally, use the relative humidity chart (see below) to get the relative humidity value. For example, if the difference was 6 C and the dry-bulb temperature was 28 C, the relative humidity would have been 59%.

A psychrometer is a device that has two such thermometers plus a chart to get the final reading. Modern, electronic psychrometers can calculate the relative humidity directly.

For a rough sense, the lower the wet-bulb temperature, the drier the air is, and the less relatively humid it is.

Mathematics

There are two mathematical ways (among others) to estimate the relative humidity, one simpler and one a bit more involved.

The first way is to divide the actual vapour pressure by the saturated vapour pressure. The vapour pressure is the amount of moisture that air contains. The ‘saturated’ value is the maximum possible amount. The formula (below) requires the dew point (Td), which you can obtain from a weather website.

R_H = (e/e_s)*100

e = 6.11 x 10^{((7.5 x Td)/(237.3 + Td))}

For e_s, use the same formula but replace Td with T, the actual temperature.

The second way is to calculate relative humidity starting from the specific humidity, pressure and temperature, using the Clausius-Clapeyron equation.

App

If you prefer software: Search for “hygrometer” in your phone’s app store to locate some options. (A hygrometer is an instrument that measures humidity.) Note that the app will only work if your phone has a sensor that can sense humidity.

If you like hardware, psychrometers and devices called hygrometers are available in physical as well as online stores.

Why does the wet-bulb temperature matter?

This said, a more direct way to understand the implications of the relative humidity for your wellbeing is in the form of the wet-bulb temperature (a.k.a. the adiabatic saturation temperature). It is the lowest temperature a surface – like your skin – can reach when water evaporates from it. The wet-bulb temperature is equal to the dry-bulb temperature when the relative humidity is 100%.

A wet-bulb temperature in an environment of 32-35º C or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade. (At least one study has shown that even a wet-bulb temperature of more than 29º C can be dangerous.)

The climate crisis is rendering heatwaves more common, more frequent, more spread out, and more potent over the Indian subcontinent. As a study published in May 2022 put it:

“Whether today’s most impactful heatwaves could have occurred in a pre-industrial climate, traditionally a central focus of attribution research, is fast becoming an obsolete question. The next frontier for attribution science is to inform adaptation decision-making in the face of unprecedented future heat.”

One way to adapt is to keep an eye on the relative humidity, drink lots of water, and cool yourself.

• Humidity is the amount of moisture in the air around us, and there are three ways to track it. The most common of them is absolute humidity: the mass of water vapour in a given volume of the air and water vapour mixture, expressed as kg/m3.

• The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, it won’t easily accept more.

• A wet-bulb temperature in an environment of 32-35º C or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade. (At least one study has shown that even a wet-bulb temperature of more than 29º C can be dangerous.)

Published – April 24, 2023 03:30 pm IST

Relative humidity is a simple concept as weather phenomena go – but it has significant, far-reaching consequences for how we must take care of ourselves on a hot or wet day.

Letting the wind flow freely is the secret to cooling our cities

Humidity is the amount of moisture in the air around us, and there are three ways to track it. The most common of them is absolute humidity: the mass of water vapour in a given volume of the air and water vapour mixture, expressed as kg/m3. The second is specific humidity, equal to the mass of the moisture divided by the mass of air. It is expressed as a dimensionless number (but sometimes also as grams per kilogram, among other similar units.)

The third way is relative humidity: it is important because it factors in the amount of vapour that air can hold at different temperatures. Determining its value is a bit more complicated: it is the vapour density of the air divided by the saturation vapour density at the dry-bulb temperature.

Why does relative humidity matter?

On a hot day, our body uses sweat to cool itself. Sweat is released via our skin to the surface. There, the liquid evaporates. When water changes phase from liquid to vapour, it absorbs heat from the surface on which it lies. So when sweat evaporates, it absorbs heat from the skin, cooling it.

The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, it won’t easily accept more. This means that the sweat on your skin can’t evaporate. At the same time, the body keeps sweating because it is still expecting to cool itself.

As a result, if the relative humidity is high, you can sweat on a hot day even when you are sitting still – while your body also keeps accumulating heat. This can quickly become dangerous.

A relative humidity of 30-60% is generally considered to be comfortable. Environments that have lower levels than this typically use humidifiers to increase the humidity. When the level is higher, a fan will help move the air around you and help sweat evaporate better. 

In both cases, drinking water is important.

What does relative humidity imply physically?

Warmer air can hold more moisture than cooler air. So at the same absolute humidity, the relative humidity of warmer air will be lower than that of cooler air. It is like saying two vessels can hold the same amount of water – just that the bigger vessel will be less full than the smaller vessel.

This is why, for example, while Chennai had an absolute humidity of 70% at 2.30 pm on April 23, its relative humidity was lower, around 60%, because it had an ambient temperature of 32 C.

The change in the capacity for moisture as the air warms is also non-linear, meaning it doesn’t increase by the same amount as the air warms. For example, the difference as it warms over 20º C from -20º C to 0º C is small, but there is an enormous improvement in its vapour-carrying capacity as it warms over 20º C from 30º C to 50º C.

How do you measure relative humidity?

There are a few ways to measure relative humidity. One is to conduct a simple experiment with household items; another is to use maths; a third is to use an app or device.

Experiment

For the experiment, you need two mercury thermometers, some cotton, rubber bands, and access to cold water.

First, make sure both thermometers show the same reading, say, under a fan. Next, soak some cotton in cold water and wrap it around the bulb of one of the thermometers with a rubber band. Hold this thermometer under the fan, such that water from the cotton evaporates, and record the thermometer reading after five minutes. Hold up the other thermometer in the air and record its reading after five minutes. You will thus have the wet-bulb and dry-bulb temperatures, respectively.

Next, subtract the wet-bulb temperature from the dry-bulb temperature to get the temperature difference. Finally, use the relative humidity chart (see below) to get the relative humidity value. For example, if the difference was 6 C and the dry-bulb temperature was 28 C, the relative humidity would have been 59%.

A psychrometer is a device that has two such thermometers plus a chart to get the final reading. Modern, electronic psychrometers can calculate the relative humidity directly.

For a rough sense, the lower the wet-bulb temperature, the drier the air is, and the less relatively humid it is.

Mathematics

There are two mathematical ways (among others) to estimate the relative humidity, one simpler and one a bit more involved.

The first way is to divide the actual vapour pressure by the saturated vapour pressure. The vapour pressure is the amount of moisture that air contains. The ‘saturated’ value is the maximum possible amount. The formula (below) requires the dew point (Td), which you can obtain from a weather website.

R_H = (e/e_s)*100

e = 6.11 x 10^{((7.5 x Td)/(237.3 + Td))}

For e_s, use the same formula but replace Td with T, the actual temperature.

The second way is to calculate relative humidity starting from the specific humidity, pressure and temperature, using the Clausius-Clapeyron equation.

App

If you prefer software: Search for “hygrometer” in your phone’s app store to locate some options. (A hygrometer is an instrument that measures humidity.) Note that the app will only work if your phone has a sensor that can sense humidity.

If you like hardware, psychrometers and devices called hygrometers are available in physical as well as online stores.

Why does the wet-bulb temperature matter?

This said, a more direct way to understand the implications of the relative humidity for your wellbeing is in the form of the wet-bulb temperature (a.k.a. the adiabatic saturation temperature). It is the lowest temperature a surface – like your skin – can reach when water evaporates from it. The wet-bulb temperature is equal to the dry-bulb temperature when the relative humidity is 100%.

A wet-bulb temperature in an environment of 32-35º C or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade. (At least one study has shown that even a wet-bulb temperature of more than 29º C can be dangerous.)

The climate crisis is rendering heatwaves more common, more frequent, more spread out, and more potent over the Indian subcontinent. As a study published in May 2022 put it:

“Whether today’s most impactful heatwaves could have occurred in a pre-industrial climate, traditionally a central focus of attribution research, is fast becoming an obsolete question. The next frontier for attribution science is to inform adaptation decision-making in the face of unprecedented future heat.”

One way to adapt is to keep an eye on the relative humidity, drink lots of water, and cool yourself.

• Humidity is the amount of moisture in the air around us, and there are three ways to track it. The most common of them is absolute humidity: the mass of water vapour in a given volume of the air and water vapour mixture, expressed as kg/m3.

• The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, it won’t easily accept more.

• A wet-bulb temperature in an environment of 32-35º C or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade. (At least one study has shown that even a wet-bulb temperature of more than 29º C can be dangerous.)

Published – April 24, 2023 03:30 pm IST

For a few years now, each time Sunita Uike takes a loan, she promises herself it will be the last one. Each time, the weather interferes with her plan. 

When she borrowed ₹30,000 from a private microfinance company last year, just before the sowing season in June, she was confident she would get enough work as a farm labourer to repay the monthly instalment of ₹1,840. But the rains got delayed and for days on end, Uike didn’t get any work.

As the sole breadwinner of her family in Umri village in Maharashtra’s Wardha district, with two children and a husband to feed, Uike, unable to pay up, borrowed ₹30,000 again. As the farming season progressed, her hopes of being able to eke out a living, enough to pay off her instalments, were bolstered. But a month later, the region was lashed by rains so heavy it washed away all the farms in and around the village. 

Nearly 800,000 hectares across the State were damaged. With farm owners reeling under severe losses, Uike found herself out of work once more and in heavy debt. Yet again, she borrowed ₹40,000 from another microfinance company, in part to pay off old loans and to simply survive. 

Across the village, nearly every woman has a number — many have taken two loans, others three. Some, like her friend, Neelima Kusre, have as many as six. The debt, often, comes at interest rates as high as 30%, and with coercive repayment tactics. 

Men mostly debt-free

Uike, who still has a debt of ₹1,00,000, says microfinance has caused a tectonic shift in society — men are now debt free and women are saddled in heavy debts. “Our men don’t have any loans anymore, maybe just a crop loan from a co-operative bank, nothing else. In contrast, the women are carrying massive loans around their necks,” she says. 

Microfinance, defined by the Reserve Bank of India as collateral-free loans given to low-income households earning up to ₹3,00,000 annually, was meant to create “microentrepreneurs”, according to the World Bank, by helping them access credit for their businesses to grow.

Nobel Peace Prize awardee and social entrepreneur Muhammad Yunus is widely credited to have invented the modern-day version of microfinance after his experiments in rural Bangladesh in the 1970s. In 1983, he founded the Grameen Bank and the practice of microfinance was institutionalised. Yunus is said to have offered a $27 (approx. ₹240) loan to artisans and farmers in rural Bangladesh who were unable to access credit from banks. Instead, Yunus created a model of distributing small loans to a group of borrowers who could act as co-guarantors.

In Vidarbha, the epicentre of India’s agrarian crisis with thousands of farmers dying by suicide over the last two decades, changing weather patterns and easy access to microfinance loans are creating a new crisis: pushing women farmers into an unending spiral of debt and despair. Figures with the Microfinance Institutions Network (MFIN), an industry association, show that the number of microfinance loans in Maharashtra rose from 82.89 lakh on March 31, 2021 to 91.66 lakh on March 31, 2022.

While thousands of mostly male farmers in the region have been reported to commit suicide annually, local activists suspect that unsustainable debts might even be pushing some women over the edge. But government data is unlikely to reflect this as farmer deaths by suicide are not segregated by gender, says the Maharashtra government’s Revenue and Forest department in response to an RTI plea.

The crisis is not restricted to Maharashtra. The agrarian State of Punjab has also reported its women being burdened with unending cycles of microfinance loans. As of September 30, 2020, the number of women borrowers in Punjab stood at 12.88 lakh with an outstanding principal amount of ₹4,387 crore, according to a statement from MFIN. 

In Tamil Nadu, a survey by the All India Democratic Women’s Association in December 2021 revealed women were increasingly turning to microfinance loans and enduring abuse and ill-treatment at the hands of loan agents when unable to repay.

Loans for weddings, household expenses, school fees…

When she borrowed money, Uike told the lending company that she was going to buy goats. In reality, “women take loans to buy things for the household, for farming, for daily expenses, for weddings at home, for their children’s education”, says Madhuri Khadse, head of Prerna Gram Vikas Sanstha, a not-for-profit based in neighbouring Yavatmal district, working on issues of women farmers. 

In Yavatmal, 35-year-old Savita Bhiwalkar, an Anganwadi worker, earns ₹2,200 a month. But her instalments for the two loans she has taken for ₹80,000 add up to ₹4,700 each month. To make up for the deficit, Bhiwalkar labours on the farms of villagers for ₹150 a day. But most of last year was a washout, with weather disruptions. “It has been a very difficult year,” Bhiwalkar says. “In a season, there is only a limited window of work for farm labourers like me. But when there is excess rainfall and flooding, even that window reduces.” 

However, Alok Misra, CEO and Director of MFIN, says that multiple loans are not a sign of rising indebtedness. “The RBI’s indebtedness cap also ensures that the total annual debt repayment does not exceed 50% of their annual income. Hence, even if they have multiple loans, the total repayment cannot exceed 50% and this ensures they are not indebted,” he says. 

Misra’s words don’t ring true on ground. Thanks to the way the loans are designed and the tactics employed to recover them, skipping repayment is not an option. 

Loans are given to individuals within a self-constituted group of 5-10 women — friends, family members, neighbours. The instalments can be weekly, fortnightly, or monthly, and collection takes place with loan agents holding meetings with the group on the due date. 

“When one member does not pay up, the loan agent pressurises all the others in the group so that they tell her to pay up and if that member cannot, then they shell out her contribution,” says Uike. For many women, not being able to repay an instalment results in a double-whammy of facing possible societal censure and anger as well as abusive microfinance officials.

“Agents come to your house, they insult you, call you names, even if you are late in repaying the instalment by a couple of hours,” she says. 

No fixed wages

Her closest friend, 37-year-old Maya Siram, has suffered this fate numerous times. One time, Siram, who has over four pending loans worth ₹1,20,000, missed her instalment because she had been sick. When the agent came home to demand the instalment and answers, Siram explained her state. As a daily-wage worker with no fixed wage, sick days mean no income. Her husband, too, earns daily wages. 

“I told him, we had no money to eat, leave alone pay back the loan. But he refused to leave my house,” she says. “Finally, I asked him, ‘Should I kill myself? Will that satisfy you?’ He heard that and left.” 

The next time, though, Siram was not so lucky. Farm work had dried up in the village. On a Saturday morning, when her instalment of ₹970 was due, Siram set out for the nearby town of Arvi to look for work, as a desperate, last-minute dash. When she reached the town, her phone started ringing incessantly — the loan agent was in her village, looking for her. She tried to buy time, still wandering through the market, asking for a job. But the calls continued. “The agent went to my house and called me to say, he wouldn’t leave unless I paid my instalment,” she says. 

Siram grew desperate. The phone didn’t stop ringing. She realised there was only one way she could end her misery — she pawned her phone for ₹3,000 and asked the shop-owner to transfer the ₹970 straight to the agent at her house, so that he would leave. After repaying her loans, when she went back to get her phone a month later, the shop owner charged her ₹300, or 10%, as interest. 

Mounting crisis

In States like Punjab, rising agrarian distress and indebtedness have pushed women borrowers to take their own lives as per news reports. In neighbouring Sri Lanka, microfinance loans have caused widespread turmoil with many debtors committing suicide, and political parties promising to ban this form of credit. In 2010, over 200 people in Andhra Pradesh, heavily in debt thanks to microfinance loans, died by suicide after they were hounded by debt collectors.

Calamity insurance

In India, the Reserve Bank of India last year issued regulatory directions for the microfinance industry and lifted the cap on interest rates, but asked microfinance companies not to charge “usurious” interest rates from borrowers. The RBI’s chief general manager in the department of communication, Yogesh Dayal, did not respond to a message and an email sent to him. 

MFIN’s Misra says the industry is aware of the disruptions that weather upheavals are causing in repayment of loans. “To deal with this, MFIN has already piloted a national calamity insurance, to be followed by full rollout after regulatory approval. Such an insurance will mean that if there is a flood or excess rainfall breaching the pre-decided threshold certified by remote data sensing measurement, then the insurance will take care of at least three to four repayments for the borrowers,” Misra says. 

Till then, Uike, eager to cultivate her one acre, has promised herself, yet again, not to borrow any more. 

The writer is a Mumbai-based independent journalist, and author of an upcoming book on Hindu nationalism.