How does Humidity affect Astrophotography | Midnight Photographer

In astrophotography, the environment plays a huge factor in the results of your photos. For instance, light pollution and clouds can completely ruin a photo. But there are other important factors which not many people talk about. Recently I wondered, how does humidity affect astrophotography? I did some research and found the answer.

In general, humidity has a negative effect on astrophotography. Water particles in the air amplify light pollution and skyglow while blocking the light from the stars. In addition, condensation from these particles can damage your camera equipment.

What is humidity?

Humidity is the amount of water vapor in the air around us. Humidity is usually measured in a percentage out of 100. 0% humidity is no water vapor in the air, 100% humidity does not mean the air is completely made of water, but the air has absorbed the maximum amount of water that it can. Humidity cannot go above 100% because the excess water forms droplets or condenses onto cold surfaces, this occurs when the temperature goes below the dew point. The dew point can vary based on environmental factors, see this Wikipedia page.

How do water particles block light?

When we look into the night sky, we see light created 2.5 million years ago. This light needs to travel through space for millions of years and through our atmosphere. Each layer of the atmosphere the light travels through, it gets weaker. If the air around us is humid, the light now has another layer to travel through – a layer of water. As the light travels through the humidity, it gets scattered by water particles making it more difficult for us and our camera to see.

What is light pollution and skyglow?

Light pollution is when man-made lights shine into the atmosphere effectively outshining the night sky. You may have noticed that in a big city, you can hardly see any stars. While in a rural area you can see thousands. This is because in rural areas there is less light, and in cities, there is lots of light. You can visualize light pollution using the light pollution map. Light pollution comes in four forms, glare, skyglow, light trespass, and clutter.

Skyglow is an increase in light across the entire night sky generally over cities and areas with lots of light. This is light being reflected back at us in the atmosphere. You may notice the sky is visibly brighter in cities as opposed to suburbs or more rural places. You can also observe skyglow from a distance coming from big cities like in this photo. Here is an example of skyglow coming from a distance:

suburban night sky
Twinsday at English Wikipedia, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Glare is the overuse of light which can get pretty annoying and obstruct your view of the sky. It’s defined by the International Dark-Sky Association as “excessive brightness that causes visual discomfort”

Clutter is a clutter of multiple light sources which can cause light trespass, skyglow, as well as glare. This phenomenon usually occurs in cities. Here is an example of clutter as well as glare.

Photo from https://www.pikrepo.com/nfcns/red-street-light-during-night-time

Light trespass is a general term for light that illuminates places that it wasn’t intended to. Like a street light illuminated the interior of your house through a window.

How does humidity increase light pollution?

Humidity increases skyglow – a type of light pollution. This is because skyglow is amplified by haze. Haze is made of particles in the sky like water droplets. Humidity can create water droplets when it comes into contact with another particle. So in summary, humidity creates water droplets, water droplets cause haze, and haze increases skyglow.

What can combat light pollution?

If humidity increases haze and skyglow, what decreases it?

Based on observations, there is visibly less skyglow and more stars in the sky after a rainstorm. This is because rain clears out the pollutants which contribute to skyglow.

What is condensation and how does it hurt my camera?

Condensation is when the gas form of water, turns into the liquid form. You’ve probably noticed this occur on a cold water bottle. Condensation requires two things, cold, and humidity. It’s usually cold at night and because of humidity, water will condense onto your camera and lens. Camera equipment and water do not mix, water can seep in and damage the electronics. Water should not come into contact with your lens, so if you don’t have a filter this may lead to problems.

How do I know if it’s humid or not?

If you go to any reputable weather website or app, the percentage of humidity should be displayed. Condensation usually occurs when the temperature is lower than the dew point. The dew point is the temperature for the air to reach 100% humidity. Humidity cannot go above 100% so the excess moisture condenses onto other surfaces. A humidity above 60% will increase skyglow by a noticeable amount.

Is it possible to do astrophotography in a humid environment?

Yes, it is possible to do astrophotography in a humid environment as it is possible to do astrophotography even during amplified light pollution. Your photos will just not look as sharp as they would’ve during lower humidity. However, I would completely avoid attempting astrophotography if the humidity is 100% and the temperature is below the dew point to avoid condensation damaging your camera equipment.

What other factors affect astrophotography?

As I’ve mentioned earlier there are other factors besides light pollution and humidity which affect astrophotography. For instance, smoke, fog, and clouds obscure the night sky. The moon is its own source of light pollution, it shines out much of the other sky. Unless you specifically photographing the moon, you’ll want to shoot during a new moon or a crescent moon.

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