What is Humidity & How Does it Affect Weather?

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You probably hear humidity referenced a lot when talking about HVAC. The cycle of water may not be something you have thought much about since Earth Science class, but it is vital to our global climate and to providing humidity, which affects many different aspects of temperature. It is something that happens all around us but is not visible to our eyes. It is incredibly important to everyone because it affects us in multiple different ways. As the cooling season approaches, humidity is an important topic that not many people know a lot about. What is it exactly? What does it have to do with AC? In this article, we will talk about what humidity is, how we can measure it in cold and warm air, what a high dew point or relative humidity means, why it is important to take it into account, and how you can control high humidity levels indoors.

What is Humidity?

In the air around us, there is always a certain volume of water that is evaporating and condensing in a never-ending cycle. When water evaporates, it rises into the surrounding air as water vapor, and humidity is the measure of this amount of water vapor in the air around us. Most water vapor is contained only within the first 10,000 feet above the Earth’s surface. Places with warmer temperatures will have a higher amount of water vapor and higher humidity since water evaporates faster in warm air. The higher the temperature, the more water vapor the air is able to hold. Humidity plays only one part of the water cycle that is essential for all life on Earth. Through this cycle, water is able to be distributed everywhere it needs to go by traveling through the surrounding air molecules. It is the reason why we have clouds and rain.

The liquid water evaporates from the oceans, lakes, and other bodies of water on the Earth’s surface and goes up into the sky to eventually be condensed into clouds. Clouds are essentially made of tons of tiny water droplets suspended together in the air. After the water vapor is condensed in the clouds, and if there is less air pressure in the Earth’s atmosphere, it will create colder air and go back to liquid form as it comes down as liquid water droplets or ice crystals. The fallen rain eventually sinks back into the ground and bodies of water, causing the process to start all over again.

Measuring Humidity

Absolute Humidity

One method of measuring the amount of water vapor, or humidity, in the outdoor air is the absolute humidity measurement. The absolute humidity measurement would be the total amount of water vapor contained within the air volume. The absolute humidity is considered the most technically accurate way to measure humidity, but because of the more complicated way it is expressed, absolute humidity measurements do not often give one a good idea of how humid it really feels outside. Absolute humidity is measured in grams of moisture per cubic meter of air, which most likely does not give a good picture of how humid it is outside. Instead of grams per cubic meter of air, a more accurate measurement for that would be the dew point.

Dew Points

A dew point temperature is the air temperature that has to be reached in order for the water to condense and evaporate at the same amount of time. When the temperature in the air matches the dew point, the air is at the point of saturation where condensation won’t form. Since water evaporates faster when temperatures increase, condensation won’t form when it begins to climb above the dew point. But when the temperature begins to drop below the dew point, that is when condensation forms. It is essentially a measure of the moisture content of the outdoor air.

Relative Humidity

The next point of measurement is the relative humidity. The dew point is used to calculate this measurement. This is a percentage measurement of the water vapor saturation in the air relative to the maximum saturation point, or how close together the air temperature and dew point are. This means that the closer the relative humidity level is to 100%, the more humid the air will actually feel. 100% relative humidity means the maximum amount of moisture for that particular temperature is currently in the air. It means the air is completely saturated with water vapor. Relative humidity is a measurement of how much water vapor is in the air relative to how much the volume of air is able to hold. The combined effect of both the air temperature and humidity is referred to as the heat index. The higher the air temperature and relative humidity, the higher the heat index.

Warm air can handle a lot more water vapor than cold air can. Therefore, colder air temperatures will have lower humidity levels than when air temperature increases. As far as indoor air temperature goes, tasks such as cooking, showering, washing clothes or dishes, and even breathing will increase the moisture in the air and therefore the relative humidity levels. While these are obviously natural everyday tasks that can’t just stop, it is important to control the relative humidity levels indoors for healthy air quality.

Humidity Levels: Why It’s Important in Cold Air and Warm Air

Increased humidity levels can have a serious effect on your health as well as your house in general. You need a certain amount of water vapor in the air; you do not want too much, but you do not want too little either. As far as relative humidity goes, you want the level inside to be between 30% and 60%. Anything higher than 70% or lower than 25% is an unhealthy level that should be regulated.

Humidity levels that are too high can lead to any of the following:

Mold Growth

Since increased humidity will increase dampness in your home, it will leave ample opportunity for mold and mildew to fester in certain areas of the house. It is also an easy way for bacteria to grow around the house as well.

Respiratory Illnesses

Asthma and allergies can be greatly irritated by an excess amount of water vapor in the air. Because the air is more damp and dense from the water vapor present, it can create difficulty breathing for anyone, but people susceptible to allergies and asthma can be particularly affected.

Dust Mites

Dust mites survive off of the moisture in the air, so they can absorb the water from the high humidity in the air. These creatures can also worsen respiratory diseases.

Heat Stroke

Humidity decreases the efficiency of our body’s natural cooling system, making high humidity extremely dangerous in some cases. If there is already an excess amount of water vapor in the air, your sweat, which is how your body cools itself down, will take a lot longer to evaporate. When that happens, you can become quickly dehydrated or fatigued and even suffer heat stroke or heat exhaustion because your body temperature is unable to be brought down.

Overworked AC

Air conditioners do more than give your home cold air. Your air conditioning system is designed to remove both heat and humidity from your house. But when the outside air is already filled with moisture, the AC will be forced to work a lot harder to effectively cool your home, which can increase energy usage. If you are noticing high humidity in your home, such as air feeling uncomfortably moist, fogged windows, or a strong musty smell, it could mean your AC is not working as efficiently as it should be.

On the other hand, you do not want humidity levels to be too low. Low humidity at an unhealthy level can lead to:

Dry Skin

When the humidity in the air is too low, you will have dry air that can dry out your eyes and skin. This is because there is not enough moisture in the air to protect the tear film in your eyes, and dry air will extract moisture from your skin. This can worsen conditions such as eczema. Using eye drops and lotion is important to prevent these harmful effects of low humidity.


The reason the winter season has been labeled flu season is because the low humidity levels have been linked to the higher spreading rate of influenza. The transmission of the virus increases when the air is more dry.

Maintaining Humidity Levels and Air Temperature Indoors

While there are many ways humidity levels can negatively affect indoor air quality, there are also several solutions to implement. When air temperature increases and relative humidity rises, keep these tips in mind when trying to contain high humidity levels in your house:


One of the most important aspects of maintaining indoor relative humidity is to make sure you have good ventilation throughout the house. Regularly opening windows to allow fresh air when the outside air is at a healthy temperature and utilizing bathroom and kitchen fans are great ways to reduce the humidity levels inside.


Adding a dehumidifier to your air conditioner can help reduce how much energy your system has to exert to cool your home and remove water vapor. It removes the moisture from the air before it recirculates back through your home. This decreases condensation forming inside your home and within your ducts, where mold and mildew can begin to grow.

If you are finding that your house is experiencing low humidity levels, try following any of these steps:

Dry Clothes Indoors

Ensure you have proper ventilation before setting your wet clothes out on a drying rack inside the house, rather than using a tumble dryer. This can help get more moisture into the air.

Open Door When Showering

Having the bathroom door open while showering is also a great way to increase the moisture content in the air, since the moisture from the shower can spread throughout the house.

Humidity is healthy and a natural part of the water cycle. Liquid water evaporates into the air around us, turning into water vapor at a rate that depends on the air’s temperature. Warmer air causes the water vapor to be created more rapidly because more water vapor can be held in the air. Therefore, there will be more humid climates wherever the air’s temperature is hotter. We can measure humidity through the absolute humidity, which will use a value of grams of moisture per cubic meter of air, or we can measure water vapor in the air through the dew point and relative humidity. The relative humidity measures water vapor percentages in the air compared to the maximum amount of water vapor at the current temperature. The air temperature will have a higher relative humidity level if the air is cooler and a lower relative humidity in hot temperatures.

Humid conditions can cause a lot of problems for indoor air quality, including mold growth, respiratory problems, dust mites, body temperature problems, and possibly an overworking AC. However, humidity levels that are too low can lead to very dry air, which dries out your skin and eyes and even increases the chance of influenza spreading in cases of low absolute humidity.

You can control high humidity levels and excess moisture inside your home by making sure you have proper ventilation. Opening windows when you can, making sure air conditioners have clear ducts, and using kitchen and bathroom fans to decrease moisture content in the air are all great ways to control the relative humidity indoors. A dehumidifier is also a method of controlling the relative humidity levels so that your AC does not need to be overworked. For humidity levels that are too low, you can increase the moisture in the air by drying your clothes inside instead of in the tumble dryer or showering with the door open to spread the moisture through the house.

As the summer months approach with warmer air that will increase relative humidity as more water vapor fills the air, it is vital to know all about relative humidity levels, what their effects are, how the levels are measured, and what you can do to keep the air inside your home healthy. Though we can’t see it, humidity occurs all around us all the time and can affect indoor air temperature as much as outdoor air temperature. As long as you follow the advice given in this article, you will be able to control the relative humidity levels in your home so that you are able to breathe healthier air in your house.

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(707) 452-3464



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San Rafael
(415) 942-6565
Santa Rosa
(707) 452-3464