+30°C in Volgograd and +30°C in Sochi: the numbers are identical, but the experience is completely different. In Volgograd, it's hot but manageable. In Sochi, it can feel like a sauna with the door stuck shut. The thermometer isn't lying in either city. It's simply showing you only half the picture.

Everyone has heard of humidity, but few people have a clear idea of what actually lies behind the number. Yet humidity can determine whether the air feels comfortable, whether your laundry will dry on the balcony, whether a glass straight from the refrigerator will fog up, and whether 25°C feels like pleasant warmth or a steamy changing room. Understanding it is easier than it might seem.

Air Like a Sponge

The air around us always contains water vapor—an invisible gas released as water evaporates from rivers, oceans, soil, puddles, wet clothes, and even our skin. This vapor isn't a cloud or mist, and you can't see it. It's simply there, dissolved in the air much like sugar dissolves in tea.

But air has a limit. At any given temperature, it can hold only a certain amount of water vapor. Think of it as a sponge: it absorbs water until it becomes saturated. Once it reaches that point, it starts to "squeeze out" the excess. The surplus moisture condenses into droplets, creating clouds, fog, dew on grass, or the condensation that forms on a bathroom mirror.

The limit depends on temperature. Warm air can hold more water vapor; cold air can hold less.

At 30°C (86°F), a cubic meter of air can contain up to roughly 30 grams of water vapor. At 10°C (50°F), the limit is about 9 grams. At –10°C (14°F), it is less than 2 grams. That's a fifteen-fold difference, and it explains much of what can seem counterintuitive about humidity.

Absolute vs. Relative Humidity: What's the Difference?

Meteorologists use two different measures, and confusing them is the source of many common misconceptions.

  • Absolute humidity is the actual amount of water vapor in a cubic meter of air at a given moment, expressed in grams. It might be 5 grams, 15 grams, or 25 grams. The more vapor there is, the more moisture the air contains in absolute terms.

  • Relative humidity is the amount of water vapor currently in the air compared with the maximum amount it could hold at that temperature, expressed as a percentage. This is the figure shown by most weather apps, and it is one of the key factors affecting how the air feels to us.

This is where things get interesting. In winter, Moscow can have outdoor relative humidity of 80–85%, while the humidity inside a heated apartment may be just 20–25%. Outside, the air is described as "humid"; indoors, it feels "dry." Yet the absolute amount of water vapor can remain essentially unchanged. Cold outdoor air might contain, for example, just 2 grams of water vapor per cubic meter.

Bring that air indoors and heat it to 22°C (72°F), and its capacity for moisture rises to around 19 grams per cubic meter. The amount of vapor hasn't changed—it is still about 2 grams—but the air can now hold far more. Relative humidity therefore plunges from around 85% to 10–15%. The air hasn't lost its moisture; it has simply become capable of holding much more of it.

That's why heated homes in winter can leave your skin dry, lips cracked, and throat irritated. It's not because radiators literally "dry out" the air, but because warm air can hold far more moisture than it actually contains.

What Determines Humidity?

Four main factors determine how much moisture is in the air and how humid it feels.

1. Temperature

Temperature is the main regulator. Roughly speaking, every 10°C increase significantly increases the amount of water vapor air can hold. That's why tropical climates can be so humid: warm air is capable of carrying large amounts of moisture. Cold polar air, by contrast, simply cannot contain much water vapor. For the same reason, summer air generally contains far more moisture in absolute terms than winter air, even when relative humidity happens to be identical.

2. Sources of Moisture

Seas, rivers, lakes, wetlands, and forests all release water into the atmosphere through evaporation and transpiration. Coastal cities such as Sochi, Batumi, and Murmansk are therefore generally more humid than inland cities such as Volgograd, Orenburg, and Novosibirsk. Even within a single city, humidity can vary: areas near a river may be noticeably more humid than a residential neighborhood several kilometers away.

3. Wind

Wind transports moist and dry air masses from one place to another. Air arriving from the sea tends to bring moisture, clouds, and rain, while winds coming across dry steppe regions can bring much drier conditions and clear skies. A change in wind direction can alter humidity by dozens of percentage points within just a few hours.

4. Elevation

As air rises, it cools. Because cooler air reaches saturation more easily, water vapor begins to condense and fall as precipitation. That's why mountains can effectively "intercept" rain: moist air is forced upward along a slope, cools, and releases its moisture on one side of the range. The air descending on the other side is much drier.

This phenomenon, known as a rain shadow, helps explain why Sochi receives around 1,700 millimeters of precipitation a year, while Kosh-Agach, on the other side of the mountains, gets only around 100 millimeters.

Why Does the Same Temperature Feel So Different?

It all comes down to one basic mechanism: sweating.

Your body cools itself by producing sweat. As that sweat evaporates from the surface of your skin, it carries heat away—roughly 2,400 kilojoules for every liter of water that evaporates. The system works remarkably well, but only if the surrounding air is capable of taking on more moisture.

When humidity is low—around 20–40%—sweat evaporates quickly, your skin stays relatively dry, and your body can cool itself efficiently. At 35°C (95°F) in the dry air of Volgograd, it can be hot but still manageable. You may be sweating, but you barely notice it because the moisture evaporates almost as quickly as it appears.

High humidity changes the equation. At 70–90% humidity, the surrounding air is already loaded with water vapor and has much less capacity to absorb more. Sweat still forms, but it doesn't evaporate efficiently. Instead, it runs down your skin without removing much heat.

Your body keeps sweating, losing water and salts, but receives little cooling in return. That's why 30°C (86°F) in Sochi at 80% humidity can feel more like 38°C (100°F). The thermometer isn't wrong; your body simply can't cool itself as effectively.

Meteorologists account for this using concepts such as "feels-like temperature" and the heat index. These measures estimate how hot conditions feel to the human body by combining temperature with humidity.

For example, 30°C at 40% humidity feels roughly like 30°C, while the same temperature at 70% humidity can feel closer to 36°C. At 90% humidity, it may feel closer to 41°C. Meanwhile, 35°C at 30% humidity can feel slightly cooler, around 34°C, whereas 35°C at 70% humidity can feel close to 48°C (118°F).

That means the difference between dry and humid conditions at the same 35°C can be around 14 degrees in perceived temperature—the difference between "hot but bearable" and potentially dangerous heat.

Winter Works the Same Way—But in Reverse

Humidity can also affect how cold the air feels, although the mechanism is somewhat different from the summer heat problem.

In the dry, bitter cold of a place like Yakutsk, at –30°C (–22°F) and 20% humidity, clothing generally stays dry and insulation works as designed. It's brutally cold, but the conditions are relatively predictable: put on a good down jacket and you can retain your body heat.

In damp winter weather, however, moisture can make the cold feel far more penetrating. In a city such as St. Petersburg, where temperatures might hover around –5°C (23°F) with humidity at 85%, moisture can accumulate in clothing and insulation, reducing its effectiveness. Wet fabric also conducts heat much more efficiently than dry fabric, which contributes to that familiar sensation of damp, penetrating cold.

That's why a St. Petersburg –10°C (14°F) can sometimes feel worse than a Yakutsk –30°C (–22°F). It's not because people in St. Petersburg are less tolerant of cold. Moisture changes how effectively heat is transferred away from the body, making damp cold feel particularly unpleasant.

What's the Comfortable Humidity Range?

For most people, a relative humidity of around 40–60% is considered comfortable. In this range, the mucous membranes remain adequately hydrated, skin is less likely to dry out, breathing generally feels comfortable, and the body's cooling system can function effectively.

  • Below 30%, the air is generally too dry

The nose and throat can become irritated, lips may crack, hair can become static-prone, and respiratory discomfort may increase. A typical example is a heated apartment in the middle of winter.

  • Above 70%, the air can become uncomfortably humid

It may feel stuffy and heavy, sweat evaporates less efficiently, and the risk of overheating increases. High humidity can also encourage mold growth, condensation on windows, and food spoilage. Think of a subtropical coastline in summer—or a poorly ventilated basement.

How to Measure Humidity—and What to Look At

Humidity indoors can be measured with a hygrometer, a small device that sits on a shelf or hangs on a wall. It displays the current relative humidity and temperature and can be particularly useful in winter, when indoor humidity may fall to 15–25%, helping you decide whether additional humidification is needed.

Outdoor humidity is available in virtually every weather app, but the percentage itself doesn't tell the whole story. It's more useful to look at temperature and humidity together.

A pleasant 28°C (82°F) with 40% humidity can feel like an ideal summer day; 28°C with 80% humidity can feel more like a steam room. The thermometer is showing the same number in both cases, but the difference is something your entire body can feel.

Humidity is an invisible part of the weather that shapes our experience almost as much as temperature does. We've been measuring and understanding temperature for centuries, while humidity is still something many people interpret largely by instinct.

But there is one rule worth remembering: it's not just about the temperature—it's about whether your body can cool itself. If it can, heat is usually manageable. If it can't, it can become dangerous. And the deciding factor isn't the thermometer alone, but the humidity in the air around you.