What Is Humidity and How Does Humidity Affect Temperature?

By: Mack Hayden  | 
Whether you're in a muggy region or a place with a "dry" heat, it can be tough to cool down during the summer. simarik / Getty Images

When discussing the weather, humidity is a constant topic of conversation and consternation, especially during those muggy summer months. When people say, "At least it's a dry heat," that's just another way of saying, "Phew! At least it's not humid!"

But what is humidity and why does it make our shirts so sticky and our lives so miserable?


Understanding Humidity

Humidity refers to the amount of water vapor present in the air. Water vapor is the gas form of water and it’s a crucial component of our atmosphere. Meteorologists define humidity in two primary ways:

  • Absolute humidity measures the actual amount of water vapor in the air, regardless of the air temperature. It’s the literal grams of water vapor per cubic meter of air.
  • Relative humidity, on the other hand, is a percentage comparing the current amount of water vapor in the air to the maximum amount the air can hold at a given temperature.

That's why the air can feel different depending on the weather conditions. On a hot day, high relative humidity can make it feel significantly warmer than it actually is, while on a cool day, low absolute humidity can make the air feel crisper and drier.


Understanding how to measure humidity is crucial for both meteorologists and everyday people. Humidity is a key factor in weather forecasts, indoor comfort and even health. There are various ways to measure humidity, each providing different insights into the amount of moisture in the air.


Water Vapor and Its Role

Water vapor plays a vital role in the Earth’s weather and climate systems. When liquid water evaporates, it becomes water vapor and enters the atmosphere. This process is essential to kickstart the water cycle, a continuous movement of water on, above and below the surface of the Earth.

  • Evaporation occurs when heat from the sun causes water from oceans, rivers, lakes and other sources to turn into water vapor.
  • Condensation happens when water vapor cools down and changes back into liquid water, forming clouds.
  • Precipitation occurs when these water droplets in clouds become heavy enough to fall back to Earth as rain, snow, sleet or hail.

Water vapor also acts as a greenhouse gas, trapping heat in the atmosphere and playing a significant role in regulating the Earth’s temperature. Without water vapor, the planet would be much colder and less hospitable.


Where Does Humidity Come From?

Humidity is a natural part of our atmosphere and originates from various sources. The primary source of atmospheric humidity is the evaporation of water from large bodies of water, such as oceans, seas, lakes and rivers. The sun’s heat causes this water to evaporate and become water vapor, which then enters the air.

However, many everyday activities also contribute to indoor humidity levels. Cooking, cleaning, showering and even breathing add moisture to the air. For instance, when you cook, the steam from boiling water adds water vapor to the surrounding air. Similarly, when you dry wet clothes indoors, the water evaporates and increases the humidity level inside your home.


Temperature and Humidity

Warm air holds more water vapor than cold air, which is why humidity tends to be higher in warmer air and lower in colder air. This is why the air can feel so much more oppressive and heavy in the summer when the temperature is high and the air is jam packed with moisture.

In warm climates, the air is often more humid because heat causes water to evaporate more quickly. This leads to a higher concentration of water vapor in the atmosphere. In cooler climates, the air holds less water vapor, resulting in lower humidity levels.


That's why certain places feel dry and others feel humid, even at the same temperature.

What Relative Humidity Measures

Relative humidity measures of how much water vapor is in the air compared to the maximum amount that could be present at a given temperature. For instance, if the relative humidity is 60 percent, the air is holding 60 percent of the water vapor it's capable of holding at that temperature.

When the air temperature rises, it can hold more water vapor, and thus the relative humidity can change even if the absolute humidity remains constant.


The dew point is the temperature at which air becomes saturated with water vapor and cannot hold any more, leading to condensation. If the dew point rises, it means there’s more moisture in the air. The dew point is always equal to or lower than the current air temperature. When the dew point is close to the air temperature, the relative humidity is high, leading to a muggy and uncomfortable feeling.

Why Is Some Air Dry and Other Air Moist?

Very dry air typically has low absolute and relative humidity. This can occur in places with cold air, as cold air cannot hold much water vapor. On the other hand, moist air has high humidity levels, often found in warm, humid climates where the air can hold a large amount of water vapor.

Cool air also generally holds less water vapor than warm air. This is why relative humidity can be low even in places that are relatively humid during warmer seasons. When air cools, its capacity to hold water vapor decreases, often leading to condensation if the air is saturated.


Evaporative cooling is a process that takes advantage of this principle. When water evaporates, it absorbs heat from the surrounding air, cooling it down. This is why sweating helps cool the body. Evaporative coolers, or swamp coolers, use this principle to cool indoor air by blowing warm air over water-saturated pads.

Practical Effects of Humidity

Humidity can cause a lot of headaches, both literally and figuratively. Even if it's a necessary element of the water cycle, it can be a real pain. Here are just a few of its potential effects.

Health Effects of Humidity

High humidity levels can significantly affect how we feel and function. When the air is humid, it feels warmer than it actually is because our sweat doesn’t evaporate as quickly, impairing the body’s natural cooling mechanism. This can lead to several health issues.


  • Dehydration: High humidity slows down the body’s ability to cool itself, which can lead to dehydration if you don’t drink enough fluids.
  • Fatigue and muscle cramps: The body has to work harder to cool itself, which can lead to fatigue and muscle cramps.
  • Heat stroke and heat exhaustion: In extreme cases, high humidity can cause severe conditions such as heat stroke and heat exhaustion. These conditions occur when the body overheats and cannot regulate its temperature.
Impact of Humidity on Daily Life

Humidity can also impact daily life, especially in our homes. High humidity levels can make air-conditioned homes feel less comfortable because the air conditioner has to work harder to remove the excess moisture from the air. This can lead to increased energy costs and wear and tear on the AC system.

Signs of high indoor humidity include a persistent feeling of stickiness on the skin, discomfort and reduced air circulation. High humidity can also cause problems such as mold and mildew growth, which can damage your home and affect your health.

Ideal indoor humidity levels usually range between 30 percent and 50 percent. When the relative humidity level is too high, the air feels muggy and can lead to discomfort and health issues like mold growth. When it’s too low, the air feels dry, which can cause skin irritation and respiratory problems.

Managing indoor humidity levels involves using devices like humidifiers to add moisture to the air in dry conditions and dehumidifiers to remove excess moisture in humid conditions. Air conditioners also play a role in managing humidity by cooling the air and reducing its moisture content.


Managing Humidity

Managing humidity levels is essential for maintaining comfort and health. Here are some strategies to control humidity.

  • Good ventilation: Ensuring proper ventilation in your home can significantly reduce humidity levels. Opening windows and using exhaust fans can help improve air circulation and reduce moisture.
  • Dehumidifiers: Using a dehumidifier can be an effective way to control indoor humidity. Dehumidifiers work by removing excess moisture from the air, which can help make your home more comfortable and prevent problems like mold and mildew. Of course, if your home is too dry, you may want to use a humidifier instead. See, sometimes humidity isn't bad!

By understanding and managing humidity, we can create more comfortable living environments and reduce the negative impacts of high humidity on our health and homes.