Go Back   IceInSpace > Equipment > ATM and DIY Projects

Reply
 
Thread Tools Rate Thread
  #1  
Old 05-11-2015, 07:04 AM
ribuck (Richard)
Registered User

ribuck is offline
 
Join Date: Jun 2013
Location: Newcastle, UK
Posts: 33
Question DIY Weather Station - Sky Temp Vs Ambient ??

Hi All,

I've built a DIY arduino based weather station and have all the hardware and Ascom driver working fine, but the last piece of the puzzle which i need to understand is the relationship between Sky Temp vs the Ambient Air temp to determine if it's cloudy or not.

i think i read somewhere once, that there is a direct relationship between the Air & Sky temps which determines if it's cloudy or not, and it's that bit i dont yet know.

All i know is that clear sky is supposed to create a bigger difference in temperatures, then cloudy which can get very close to ambient.

Anyone got any idea's on the approx temp differences ??

e.g. Clear Sky = (30 degree difference in temp ???)
Light / Whispy Cloud (15 degree difference in temp ??)
Full / Thick Cloud ( 5 degree difference in temp ??? )

Hope my question makes sense

Rich.
Reply With Quote
  #2  
Old 06-11-2015, 01:34 AM
ribuck (Richard)
Registered User

ribuck is offline
 
Join Date: Jun 2013
Location: Newcastle, UK
Posts: 33
Anyone, any idea's ?
Reply With Quote
  #3  
Old 06-11-2015, 07:14 AM
Ryderscope's Avatar
Ryderscope (Rodney)
Registered User

Ryderscope is offline
 
Join Date: May 2006
Location: Glanmire, NSW
Posts: 1,216
I cannot answer your question but for what it is worth, I use the Aurora cloud sensor which does exactly that - it measures the sky temperature to work out whether it is cloudy or not. It has settings which you can adjust which change whether it reports cloudy or clear. It also reports what it calls sky 'clarity' which I suspect would be what you are referring to - the difference between ambient and sky temps. I have only been playing with it for a few weeks by comparing what it is reporting against what I see in the sky and it works quite well. The unit also has a rain sensor which can operate a relay which can then be used for other actions such as closing the observatory roof etc.
R
Reply With Quote
  #4  
Old 06-11-2015, 10:53 AM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,796
In the thermal IR (used by radiant temp sensors), the zenith sky can appear to be less than -40C if the air is dry. Humidity and haze will increase this significantly.
Clouds can appear very cold at altitude (simply because it is cold up that high), but can be near ambient at low altitude.

ie, I don't think that there is a magic difference between ambient and sky that will tell you categorically if clouds are present. However, there are obviously proprietary strategies that can do a good job (eg for Boltwood). Maybe a good place to start would be to set the gear running in clear conditions and look for (say) a 10-15C increase in sky temperature (relative to ambient) to indicate cloud cover. That should account for the humidity on the night and give you a good chance of detecting cold high level clouds, as well as intermediate and low level stuff. You will probably also find that there is an absolute temperature, below which the sky will always be clear.

You will have probably have to experiment to see what works best with your sensor, since spectral response, zenith angle and acceptance angle will affect the results. Grateful if you could keep us informed of your findings - others will be interested.

ref: http://journals.ametsoc.org/doi/pdf/...2007JAMC1615.1
Reply With Quote
  #5  
Old 03-11-2019, 05:24 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,103
Quote:
Originally Posted by ribuck View Post
Hi All,

I've built a DIY arduino based weather station and have all the hardware and Ascom driver working fine, but the last piece of the puzzle which i need to understand is the relationship between Sky Temp vs the Ambient Air temp to determine if it's cloudy or not.

i think i read somewhere once, that there is a direct relationship between the Air & Sky temps which determines if it's cloudy or not, and it's that bit i dont yet know.

All i know is that clear sky is supposed to create a bigger difference in temperatures, then cloudy which can get very close to ambient.

Anyone got any idea's on the approx temp differences ??

e.g. Clear Sky = (30 degree difference in temp ???)
Light / Whispy Cloud (15 degree difference in temp ??)
Full / Thick Cloud ( 5 degree difference in temp ??? )

Hope my question makes sense

Rich.
I've been working on a similar project. I've currently got mine set as 100% cloudy=5 degC delta T and 100% clear = 13.7 degC delta T. I've been watching the predictions vs observed conditions on and off and I'm getting close to a good prediction. I'm finding it slightly overpredicts cloud at the lowcloud percentage end meaning I probably need to reduce my 13.7 number a bit - maybe to 12.7 degC. I'm using a linear model in between the two temperatures and I've written a bit of simple code to calculate the slope and intercept:

clear = 13.7; // initialise delta t for clear sky
cloudy = 5; // initialise delta t for cloudy sky
slope = (100 - 0)/(cloudy - clear);// slope of cloud model
xin = 100 - (cloudy * slope);// intercept of cloud model

Then calculate the delta T
dt = ambient - sky;

adt = (adt * 2 + dt) / 3; // calculate rolling 3 point average dt
// use the average dt to smooth out cloud detection

// cloud calc using rolling average delta T
cloud = xin + slope * adt;
cld = constrain(cloud, 0, 100);
//ensure the cloud reading is between 0 and 100%

It takes a bit of tuning however with this code I only need to adjust the constants clear and cloudy.

I'd be interested to hear how your worked out.

Peter
Reply With Quote
  #6  
Old 05-11-2019, 02:30 PM
markas (Mark)
Registered User

markas is offline
 
Join Date: May 2012
Location: Melbourne Australia
Posts: 281
Rich,

I use a hand-held IR thermometer with ~10 degree FOV. I have not relied on it, but I record its readings on imaging nights.

I have found no hard and fast rules.

However, if the zenith temperature is below -40 C, the sky is generally clear. No absolute guarrantee, as I have also recorded -40 C when there is definitely high cirrus.

On the other hand, if the measured temp is >-30 C the image is almost always noticeably compromised.

If there's low cloud in the air, the IRT will usually read between -20 and -5.

You can get a reasonably good idea of these indicators in daylight (obviously, don't aim near the Sun!) so that you can see what you're pointing at.


Sorry I can't say that the use of the IRT is anything more than a rough guide to the cloud state.....


Mark
Reply With Quote
Reply

Bookmarks

Tags
arduino, ascom, cloud, diy, sensor

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 06:32 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
Advertisement
Lunatico Astronomical
Advertisement
Star Adventurer
Advertisement
EQ8-R
Advertisement
Celestron RASA
Advertisement
Testar
Advertisement
Astromechanics
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement