Why Does Humidity Rise at Night?


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Humidity levels are higher during the night-time. Often, this will manifest itself in the form of fog or dew on the grass. We can see this to be true, but most people do not seem to know the answer as to why this is.

Humidity levels are higher during the night because cold air cannot hold as much moisture as warm air. Cooler air has a lower saturation point, and when the air can no longer hold any more moisture, it gathers on the ground in the form of dew. Humidity levels are relative to the overall temperature.

We now know that generally speaking, cool air is incapable of holding as much moisture as warm air, but a closer look is required in order to fully understand why this is the case. In this article, we will examine the various factors that prevent cool air from retaining as much moisture as its warmer counterpart.

Why does humidity rise at night

Why Does Cold Air Hold Less Water Than Hot Air?

To answer this question, we must first understand that molecules move at different speeds according to the temperature. Faster moving particles generate more of what is known as kinetic energy. 

Evaporation vs. Condensation

  • Warmer temperatures dictate that, on average, particles will be moving faster. Faster moving particles are more likely to enter into what is known as the vapor phase.
  • Cooler temperatures dictate that particles will be moving slower on average and are thus more likely to form what is known as condensation.

Examples

Evaporation and condensation are processes of water changing between states due to alterations of the temperature. The three states in which water is able to manifest are solid (ice), liquid (water), and gas (steam).

  • An example of evaporation is boiling a pot of water. Due to the extremely high temperature involved (water boils at 100 degrees Celsius / 212 degrees Fahrenheit), the particles begin speeding up, causing the water to become steam. If left long enough, the entirety of the water contained in the pot will evaporate, leaving the pot completely empty. 
  • An example of condensation would be the formation of fog. When the air rapidly begins cooling and the saturation point is reached, the excess moisture will often manifest itself in the form of fog.

Humidity Is Relative

Previously mentioned in this article was the fact that humidity levels are relative to the overall temperature. This simply means that not only do we measure the humidity level by itself, but we must also factor in the temperature to truly put the question of Why Does Humidity Rise At Night? into context. Failure to factor in the temperature will only result in a measurement of what is known as absolute humidity (AH). 

Absolute humidity is the direct humidity level devoid of all other factors.

How This Works

Let us imagine for a moment that the humidity level on a warm day is 60%. As night begins to fall, and the temperature begins to decrease, this percentage of humidity will rise linearly as the air cools without the actual amount of moisture ever having changed. This shift in humidity percentage can easily deceive us if we are to take it at face value. 

Why does humidity rise at night

What Does All of This Mean?

This means that in regards to the original question of why it is more humid at night, the problem lies in how we define and contextualize the question. While yes, it is more humid during the night, this phenomenon can have some misleading implications if not properly understood. 

How to Measure Relative Humidity (RH)

To accurately measure the level of relative humidity in a given time and place, a hygrometer is most often used. The hygrometer was originally invented by Leonardo Da Vinci in the late 15th century. A hygrometer is a tool that works by measuring many factors such as temperature, air pressure, mass, and absolute humidity (the amount of humidity present regardless of the temperature). It then produces a read-out of the relative humidity level.

Where to Buy

Thankfully, modern hygrometers are readily available for purchase to anyone at a low cost. 

The AcuRite 00613 Digital Hygrometer is at the leading edge of indoor hygrometers. Indoor hygrometers are a great way to optimize home comfort when used in conjunction with an air humidifier and dehumidifier combination. For air humidifiers, I personally recommend the Everlasting Comfort Humidifiers for Bedroom, and for a dehumidifier, I would highly recommend the Pro Breeze Electric Mini Dehumidifier.

Health Benefits of In-Home Humidity Regulation

There are many health benefits to optimizing the humidity levels inside your home. These benefits include but are not limited to:

  • Helping to reduce symptoms of the flu or common cold
  • Preventing dry and itchy skin (especially in winter months)
  • Improving sleep comfort and quality
  • Providing relief for dry and irritated lips

Optimal Indoor Humidity Level

In general, it is best to try and maintain indoor humidity levels at around 40-60% relative humidity. Too high a humidity level can cause dampness leading to the formation of mold. Not only can this cause structural damage within your home, but it can also lead to adverse health effects such as respiratory ailments. Too low a humidity level can also negatively affect the structure of your home by drying out wooden structures and surfaces.

Safety Precautions for Outdoor Activities

For anybody who is planning to spend the day outdoors, it is crucial that you consider safety when the humidity levels are high. This is especially true when performing physical activities such as jogging, hiking, playing sports, etc. 

High humidity makes it difficult to breathe and regulate our body temperature. The body will begin to sweat profusely in order to cool our core temperature, and as a result, we are at an increased chance of dehydration, fainting, heatstroke, and nausea.

  • Drink ample amounts of water every hour when performing outdoor activities
  • Seek shade and reduce the amount of direct sunlight exposure as much as possible
  • Listen to your body: never ignore signs like dizziness, headaches, dry mouth, etc

Pets and Animals

Always be considerate of the wellbeing of your pets, especially during times of high humidity. Most animals, such as dogs and cats, cannot perspire due to a lack of sweat glands. This makes it extremely difficult for them to regulate their body temperature on hot, humid days. Make sure your pets have access to water, and never leave them in a car with the windows rolled up. 

What to Do if You See an Animal in a Hot Car

If you see an animal locked in a hot car, Peta.com recommends that you record the car’s license plate number, have the owner called or paged in the nearest stores, contact the police and/or humane society, and have somebody stay to watch the animal closely until the incident has been resolved. 

Final Thoughts

In this article, we have come to understand two main things:

  1. Why humidity levels are in fact, higher during the night
  2. Why we must also put into context the difference between relative and absolute humidity

In the case of why humidity rises at night, a simple answer cannot entirely suffice, and a little bit of deeper understanding is necessary to fully explain the way this works. Humidity is relative to the temperature, and a colder temperature has a lower saturation point. Once the saturation point is reached, the water condenses and forms on the ground as dew or fog.

Nicole Sutton

Nicole is a writer from Missouri who enjoys horseback riding, writing in her journal, and watching reruns of Friends.

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