Milky Way and the Dew Point Temperatures

[DanK]

Interesting. I realized later that there’s another reason clouds could be relevant— they slow radiant heat loss, so all other things equal, the air temperature should drop to the dew point more quickly if there is less cloud cover. Right?


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[Manfred M]

I was puzzling over this as well. I was thinking that the presence of wind might slow condensation slightly by disrupting the formation of droplets, but that may have been farfetched. Basically, once a drop in temperature brings relative humidity up to 100%, water has to condense as the air chills further. Would wind matter much?

Clear skies per se have nothing to do with it. However, clear skies are generally a sign of relatively dry air, and dry air cools faster. It's an interesting question: dry air cools faster but has to cool more to reach the dew point, while humid air cools more slowly but doesn't have to cool as much. Which reaches the dew point faster?

There are three types of heat transfer; conduction, convection and radiation. In this scenario, convection is the most important heat transfer mechanism and the movement is always from the warmer component to the cooler one.

The presence of wind will cool the camera gear more quickly, much like the wind chill that we notice in the winter. Stand in front of a fan on a hot day (as long as the ambient temperature is lower than your body temperature) and you will feel cooler. From a technical standpoint this is referred to as "forced convection". Heat loss from the camera gear is due to convection, so keeping a steady stream of cooler area moving over it will get the gear down to the dew point more quickly, allowing condensation to form.

Radiation will become important in the cloudy vs clear sky scenario. The clouds will be warmer than outer space (which is ultimately where heat from earth escapes to), so radiative heat loss will be slower in overcast conditions as the temperature differential is lower.

[Manfred M]

I didn't know that the dew point will vary widely for all sorts of slight changes. Where did you see that, may I ask?

The dew point does not vary at all Ted; physics drives that, but what does vary is the local relative humidity. Geography and topography will have an impact.

Get yourself into a small secluded valley with small lake or river running through it, the relative humidity will be higher (sometimes far higher) than a rocky hill top that is just a few kilometers / miles away.

Unless you travel with a sling psychrometer and a psychrometric chart, you won't know for sure.

[Manfred M]

This isn't her issue. This is the issue when walking into the outdoors. It isn't the issue with night photography, which is her concern. The main issue with dew in night photography is temperatures dropping below the dew point. I have had condensation on my camera when doing night photography when my camera was at outside temperature when I started.

Absolutely right Dan. Once your camera / lens temperature drops below the dew point, they will have condensation form. This is no different than anything else; that is how dew forms on the grass and rocks that we encounter first thing in the morning, when the ambient temperature is usually at its lowest.

This is far more of an issue in warm weather when the air has a far higher capacity to hold water (in its gaseous form). Once the air temperature drops below freezing, its capacity to hold moisture is quite low (but not zero), so condensation is much less of an issue. That's also the reason that our landscape images seem clearer in the late fall and winter; far less humidity in the air because it is cooler.

[xpatUSA]

However, clear skies are generally a sign of relatively dry air, and dry air cools faster. It's an interesting question: dry air cools faster but has to cool more to reach the dew point, while humid air cools more slowly but doesn't have to cool as much. Which reaches the dew point faster?

Dan, may I ask what the units of dryness and humidity are? I'm struggling with how "dry air cools faster" and "humid air cools more slowly" ...

... Be that as it may, I'm thinking that, at the same heat flow out of a given volume of air, the "humid air" gets there first:



Less change of enthalpy at the same heat flow = less time.

[xpatUSA]

The dew point does not vary at all Ted; physics drives that, but what does vary is the local relative humidity. Geography and topography will have an impact.

Get yourself into a small secluded valley with small lake or river running through it, the relative humidity will be higher (sometimes far higher) than a rocky hill top that is just a few kilometers / miles away.

Thanks Manfred - we were typing at the same time. So that difference in RH would be due to a combination of temperature difference (hill height, wind) and water content (evaporation from valley lake/river).

You said "The dew point does not vary at all Ted; physics drives that"

But the chart does say that a fall in both temperature and water content could result in a variation of dew-point temperature.

For a carefully chosen but reasonable example, at a dry-bulb of 25C and 0.016WC (in the valley) - RH is 80% and DPT is 22C whereas at a dry-bulb of 24C and 0.015WC (up the hill) - RH is still 80% and DPT falls to 20C.

So I must disagree about the invariant dew-point. Unless you were assuming constant water content which is quite unlikely for your wet valley/rocky hill example.

[revi]

As long as your camera body and lens are warmer than the outdoor temperature, you should not get any condensation issues.

The problem is actually that the lens/camera/telescope cools down to below the dew point during a photo or astro session.

And it will especially be a problem when you have a clear night with high humidity: while the ambient temperature will drop, the dew point will not drop: the amount of water in the air stays the same,
but colder air can hold less moisture, so relative humidity will increase until your equipment ends up at a temperature below the dew point... condensation will happen...
This is aggravated by the mechanism of cooling: radiation from the earth and all object on it, so your equipment can end up being a bit colder than the ambient air; and the difference only needs to be smal
when you are at high rel. humidity (according to the chart above: as small as 2°F). This actually happens often in certain regions, where near sunrise everything is covered in dew.

If just the air cools down to below the dew point, you get fog.

[xpatUSA]

Originally Posted by DanK ... dry air cools faster but has to cool more to reach the dew point, while humid air cools more slowly but doesn't have to cool as much.

Dan, may I ask what [your] units of dryness and humidity are? I'm struggling with how "dry air cools faster" and "humid air cools more slowly" ...<>

Dan, my question asked again.

[DanK]

Most people who have lived in humid and dry climates have experienced this. Starting at the same daytime high temperature, the air cools more slowly in a humid climate. I've never looked for a systematic explanation, but I suspect it's two things: the heat content of the water vapor, and the humid are radiating more heat back--primarily the former, I would guess.

[xpatUSA]

Most people who have lived in humid and dry climates have experienced this.

I've lived in Bahrein and also up in the Arctic Circle. Never stayed outside long enough in either place to notice the night-time cooling rate. Too bloody uncomfortable!

Starting at the same daytime high temperature, the air cools more slowly in a humid climate. I've never looked for a systematic explanation, but I suspect it's two things: the heat content of the water vapor, and the humid area radiating more heat back--primarily the former, I would guess.

Thanks Dan. All a bit moot, then.

The total (dry plus vapor) mass density holding the heat doesn't change much with water content.

Was reading while you posted. It seems far more complicated than we might think.

https://www.weather.gov/source/zhu/Z...Influences.htm

Quite good, Clouds and Dewpoints get mentioned with respect to air temperature near the ground.

https://journals.ametsoc.org/jamc/ar...Boundary-Layer

A bit more technical.

Again, posted FWIW. I'll read both links but doubt if I'll be able to bring anything concrete to this sub-discussion.