All about humidity, the unseen danger

[Archibald]

Humidity is a mysterious property of air that can't be seen or felt or smelled, but can mess with our photographic equipment.

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What is humidity and how can we deal with it? It is just moisture (water vapor) in the air. It is always present in the air around us. We can deal with it by understanding a bit about it.

It is important to know that humidity does not mean warmth or heat. It means having water vapor. When there is an ice fog, it is very humid and very cold at the same time. On the other hand, in a hot desert the humidity can be very low.

Air can hold only a certain amount of humidity at a given temperature. When the air is full, we say it is saturated and the relative humidity is 100%. This is a pretty common condition. It happens outdoors on cold mornings when there is dew on cars and on the grass, or when it is foggy or raining. It can happen indoors too, for instance in a bathroom where you are taking a shower, and in a butterfly house where water is sprayed into the air.

Most of the time the air isn't saturated with humidity. Typical values for relative humidity are 40-60%. That is the fraction of the possible maximum water vapor content at the current temperature. In this range, we feel comfortable personally, and our gear is safe from the perils we associate with humidity.

 

[Archibald]

Temperature has a big effect on how much water vapor the air can hold. Cold air can't hold much humidity in absolute terms. In the winter when we have an ice fog or it is snowing, the water vapor is near or at capacity. The relative humidity is about 100%, but it is 100% of a small amount.

Warm air, on the other hand, holds lots of water vapor. Heating cold air up increases its capacity for moisture and therefore reduces its relative humidity. This agrees with our experience - warmth dries things.

High levels of humidity are undesireable because water can condense out of the air and form liquid droplets on cool gear, blurring our pictures and potentially causing damage. At the right conditions, high humidity can foster the growth of fungus.

Any parcel of air will condense water if it is cooled sufficiently. The temperature at which that starts to happen is called the dew point. Early in the morning when dew is present, we are at the dew point or below. Later in the day it might be a lovely summer afternoon of 25°C (77°F) with a relative humidity of 60%. Then we are well above the dew point. From the known properties of air and water, we can determine the dew point at those conditions to be 16.7°C (62.0°F). If your camera is below that dew point temperature, it will condense water on its surface. If it is above that temperature, even slightly, it won't.

 

[Archibald]

So the critical factor is the dew point. How can we know what the dew point is?

The dew point is always at or below the temperature of the air. If the air is very moist (think tropical mornings, or foggy days in London) then the dew point equals the air temperature. Every object at that temperature or below will be wet. The other extreme is dry air in the desert, or inside a home with the heating on. The dew point will be well below the ambient temperature, possibly around 5°C (41°F). Gear will stay dry unless it is cooled below that temperature.

On a cold winter day with an outdoor temperature of -10°C (14°F), the dew point will be -10°C or less. The composition of the air doesn't change by your furnace warming it up. The dew point stays the same. OK, it will creep up after mixing with sources of water vapor in the home, but this is usually not a lot. Accordingly, there is normally little risk of getting condensation on your camera indoors in the winter.

However, this can be different in humidity hotspots in the home such as the kitchen or (especially) the bathroom, where there are sources of water vapor - or anywhere if your humidifier has run amok. If you have condensation on the windows, that is a sign of a problem. Of course there is high risk in a place like a swimming pool or greenhouse. If your gear happens to be super cold from being left in the trunk overnight, you run additional risk. Remember, the temperature of the gear should be kept above the dew point.

 

[Archibald]

There can be a significant chance of condensation if you are vacationing in an air-conditioned room in the tropics and then take the camera outdoors. If it's a balmy 35°C (95°F) out with a relative humidity of 70%, then the dew point is 29°C (84°F). Your air-conditioned hotel room is likely cooler than that. The camera will become wet when you take it out, until it warms up in the tropical air.

There are data tables and formulas available for getting the dew point at all imaginable conditions, but in most cases I don't think we need to bother with such technical stuff. It is enough to know that cool camera gear can attract moisture when taken into humid environments. It's pretty obvious when that happens - the gear gets wet. Warm it up and the problem is solved.

The best way to protect cold gear in humid conditions is to put it into a plastic bag until it warms up. However, there are a couple of problems with this. While it is in the bag, you can't take pictures! And it will take way longer to warm up - unless it is in the sun, in which case it can quickly heat up to possibly damaging temperatures. Because of these issues, I don't bother with plastic bags but try to hasten warming with my body heat. There is a risk here, so YMMV.

 

[Archibald]

Now a note about fungus. Fungus spores are microscopic and are everywhere, but don't normally grow until conditions are right. And we want to avoid those conditions, because fungus can destroy lenses. From hearsay, fungus may grow at higher than 70% relative humidity in the dark after a few days. We can encounter conditions like that in tropical areas that don't have air conditioning.

To keep the fungus from growing, you can increase the temperature or reduce the humidity. Roughly speaking, 100% relative humidity can be reduced to a safe 70% by increasing the temperature by 6°C (11°F). A lightbulb in a box can achieve that (but be careful about a fire hazard). Silica gel is very effective at reducing humidity. It needs to be fresh and kept with the gear in a tightly sealed container.

Humidity is all around us. With a bit of knowledge on how it behaves, we can keep our camera gear free of condensation and fungus.

... and if anyone asks, yes, I'm a chemist.

 

[Nextlife1]

Thank you.

P.S. So am I.

 

[Archibald]

There is NO WAY I could do a YouTube like Steve, but sometimes I can write a good story.