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relative transparency
We're all familiar with the notion of relative humidity, so there's no need to go into that. But is there a meaningful metric for humidity/transparency such that a less transparent sky allows less radiative cooling and thus causes less dew ? Tonight I was out at 90% relative humidity. Ambient air temperature was 65F and the dewpoint (I am told, I have not checked these numbers against each other) was 50 degrees F. I was expecting dew like mad, yet my optics stayed dry. The night was clear, maybe 10% cloud cover or less, but not very transparent. The naked eye just didn't see many stars. Was the lack of dew caused by the same thing that caused the bad transparency ? Was the lack of dew caused by the bad transparency (not the same question) ? Has anybody quantified this ? Would it be useful to do so ? I think my next eyepiece just morphed into a weather station! |
#2
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relative transparency
On Tue, 22 Aug 2006 22:25:44 -0400, Tom Rauschenbach
wrote: We're all familiar with the notion of relative humidity, so there's no need to go into that. But is there a meaningful metric for humidity/transparency such that a less transparent sky allows less radiative cooling and thus causes less dew ? Tonight I was out at 90% relative humidity. Ambient air temperature was 65F and the dewpoint (I am told, I have not checked these numbers against each other) was 50 degrees F. I was expecting dew like mad, yet my optics stayed dry. The night was clear, maybe 10% cloud cover or less, but not very transparent. The naked eye just didn't see many stars. Was the lack of dew caused by the same thing that caused the bad transparency ? Was the lack of dew caused by the bad transparency (not the same question) ? Has anybody quantified this ? Would it be useful to do so ? I've made some limited comparisons between atmospheric water vapor (I have an IR camera) and the temperature of objects. There is a simple cloud detector that compares the temperature on the top and bottom of a plate which depends on this relationship. On very transparent nights, I've had water bowls freeze over when the air temperature never dropped below 50°F. If it weren't for the fact that the dewpoint here is usually tens of degrees below ambient, that would be a big problem. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#3
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relative transparency
Tom Rauschenbach wrote:
We're all familiar with the notion of relative humidity, so there's no need to go into that. Actually, relative humidity is widely misunderstood and misreported. I wish that weather stations *never* reported it, and *always* reported dew point instead. The problem with relative humidity is that it swings wildly over the course of a day as the actual temp varies, whereas the dew point is fairly stable. So people often report things like "humiidity of 90% and temp of 90 degrees," combining the morning's humidity with the afternoon's temp, and not realizing that this is a nonsensical result. Tonight I was out at 90% relative humidity. Ambient air temperature was 65F and the dewpoint (I am told, I have not checked these numbers against each other) was 50 degrees F. According to the relative humidity calculator at www.esa.act.gov.au/firebreak/humidity.html at temp of 65F and a dew point of 50F yields a relative humidity of 50% -- a far cry from the 90% you reported. I was expecting dew like mad, yet my optics stayed dry. Yes, that's very common. The other critical variable you didn't report, though, is the wind. Even a light breeze inhibits dew dramatically. Also, most dew is re-condensation from the ground, so when the ground is dry, the dew tends to be light even when the humidity is fairly high. - Tony Flanders |
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relative transparency
On Wed, 23 Aug 2006 03:05:40 -0700, tony_flanders wrote:
Tom Rauschenbach wrote: We're all familiar with the notion of relative humidity, so there's no need to go into that. Actually, relative humidity is widely misunderstood and misreported. I wish that weather stations *never* reported it, and *always* reported dew point instead. The problem with relative humidity is that it swings wildly over the course of a day as the actual temp varies, whereas the dew point is fairly stable. So people often report things like "humiidity of 90% and temp of 90 degrees," combining the morning's humidity with the afternoon's temp, and not realizing that this is a nonsensical result. Tonight I was out at 90% relative humidity. Ambient air temperature was 65F and the dewpoint (I am told, I have not checked these numbers against each other) was 50 degrees F. According to the relative humidity calculator at www.esa.act.gov.au/firebreak/humidity.html at temp of 65F and a dew point of 50F yields a relative humidity of 50% -- a far cry from the 90% you reported. That's why I took pains to point out that I didn't perform the check that you did ! Also, I had no means to determine either the dewpoint or the relative humidity (or ambient temp for that matter) I just took numbers off a weather web page. I was expecting dew like mad, yet my optics stayed dry. Yes, that's very common. The other critical variable you didn't report, though, is the wind. Even a light breeze inhibits dew dramatically. Also, most dew is re-condensation from the ground, so when the ground is dry, the dew tends to be light even when the humidity is fairly high. No *that's* a useful observation ! While considering how I might quantify this phenomenon (assuming it exists) I'll be sure to keep that in mind. I guess the next question might be : is there a quantified metric of transparency ? - Tony Flanders |
#5
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relative transparency
On Wed, 23 Aug 2006 10:36:59 -0400, Tom Rauschenbach
wrote: I guess the next question might be : is there a quantified metric of transparency ? Transparency is closely related to limiting magnitude. Instrumentally I think you could just look at the absolute intensity of a reference object. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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