Author Topic: Dew Point calculation  (Read 1532 times)

0 Members and 1 Guest are viewing this topic.

Offline sharadaprasad

  • Member
  • *
  • Posts: 9
    • Akshayanagar Weather
Dew Point calculation
« on: January 07, 2015, 11:11:21 PM »
Hello All,

I have sent the below text to Ambient support but I also wanted to post it here to get the inputs from the community

I need your help in calibrating my new Ambient Weather 1400-IP installed in Bangalore, India - http://www.wunderground.com/personal-weather-station/dashboard?ID=IKARNATA20.

Today morning the was dew deposition on all the cars parked on our streets but the temperature recorded by the station did not go below the dew point temperature. I am new to weather data. But I am assuming that the dew deposition occurs only if the outdoor temperature falls below the dew point.

The difference between the dew point and outdoor temperature was 0.5C. I tried to calibrate the temperature down by 0.5C. The dew point also reduced by 0.5C. I think that is not how it should be fixed.

Please peruse the readings at 7:47AM, 7:52AM and 7:57AM - http://www.wunderground.com/personal-weather-station/dashboard?ID=IKARNATA20#history

At 7:47, I reduced the temperature by 0.5C through the calibration page. The dew point also dropped simultaneously (@7:52). I reset the calibration to 0C @ 7:57AM

My indoor sensor is not working yet. Is that the reason? Or is my understanding of dew point wrong?

Also, what should be the relative pressure I should be offsetting in the calibration page? My station is located here - https://www.google.co.in/maps/place/2nd+A+Cross+Rd,+Akshayanagara+East,+Akshayanagar,+Bengaluru,+Karnataka+560076/@12.8719809,77.619573,17z/data=!3m1!4b1!4m2!3m1!1s0x3bae6b31fe002ddb:0xb15eff264b4ecaf6?hl=en

Please help me get the best out of the station and provide accurate information to the community.

Best,
Sharada

Offline DanS

  • Chiang Mai weather
  • Forecaster
  • *****
  • Posts: 5433
    • ThaiWx
Re: Dew Point calculation
« Reply #1 on: January 08, 2015, 03:08:13 AM »
You could probably use India's Met. Dept. to get the barometric pressure for VOBL. Just take the Qxxx number from the METAR.

Offline sharadaprasad

  • Member
  • *
  • Posts: 9
    • Akshayanagar Weather
Re: Dew Point calculation
« Reply #2 on: January 08, 2015, 03:20:08 AM »
Thank you.

Offline DanS

  • Chiang Mai weather
  • Forecaster
  • *****
  • Posts: 5433
    • ThaiWx
Re: Dew Point calculation
« Reply #3 on: January 08, 2015, 05:29:48 AM »
If it were my weather station I believe I would let it run a couple days or so to let it settle/adjust. With it being new and seeing your humidity readings already go up to 97% I get the feeling the station may be working fine. Give it a little more time and it may start showing results closer to what you're looking for with the dew point readings. :-)
« Last Edit: January 08, 2015, 05:33:19 AM by DanS »

Offline sharadaprasad

  • Member
  • *
  • Posts: 9
    • Akshayanagar Weather
Re: Dew Point calculation
« Reply #4 on: January 08, 2015, 06:16:55 AM »
Sure.

Offline ericfynne

  • Contributor
  • ***
  • Posts: 139
Re: Dew Point calculation
« Reply #5 on: January 08, 2015, 06:52:40 AM »
Cars tend to cool faster than the surrounding air, so dew can form on them even though your temperature reading is not at the dew point.

Eric

Offline sharadaprasad

  • Member
  • *
  • Posts: 9
    • Akshayanagar Weather
Re: Dew Point calculation
« Reply #6 on: January 08, 2015, 07:02:26 AM »
Aha! Good point. Thank you!

Offline Cutty Sark Sailor

  • WxElement panel
  • Forecaster
  • *****
  • Posts: 3004
    • Frankfort Weather - TwinHollies WeatherCenter
Re: Dew Point calculation
« Reply #7 on: January 08, 2015, 08:52:34 PM »
Barometric pressure, Temperature, and humidity have to be correct. Or as near as possible. Start with getting station altitude accurate, and pressure.
 

Offline dport

  • Senior Contributor
  • ****
  • Posts: 170
Re: Dew Point calculation
« Reply #8 on: January 09, 2015, 08:29:01 AM »
FWIW I installed the ip-1400 last night and the humidity and dew point levels are still way off this morning. I have a ws-2080 in my yard for comparison. The temps are spot on. Let's hope this shakes out in a day or two. My station ID is KPABRYNM2 if you want to have a look for yourself.

Offline dport

  • Senior Contributor
  • ****
  • Posts: 170
Re: Dew Point calculation
« Reply #9 on: January 09, 2015, 09:52:58 AM »
well, it appears i have calibrated my pressure incorrectly.  I calibrated the absolute pressure on the station and not the relative.  I will fix this tonight when i go home, but guessing this is the issue.  Absolute pressure should have a calibration value of zero, correct?

Offline ValentineWeather

  • Forecaster
  • *****
  • Posts: 5657
Re: Dew Point calculation
« Reply #10 on: January 09, 2015, 10:31:06 AM »
Hello All,

I have sent the below text to Ambient support but I also wanted to post it here to get the inputs from the community

I need your help in calibrating my new Ambient Weather 1400-IP installed in Bangalore, India - http://www.wunderground.com/personal-weather-station/dashboard?ID=IKARNATA20.

Today morning the was dew deposition on all the cars parked on our streets but the temperature recorded by the station did not go below the dew point temperature. I am new to weather data. But I am assuming that the dew deposition occurs only if the outdoor temperature falls below the dew point.

The difference between the dew point and outdoor temperature was 0.5C. I tried to calibrate the temperature down by 0.5C. The dew point also reduced by 0.5C. I think that is not how it should be fixed.

Please peruse the readings at 7:47AM, 7:52AM and 7:57AM - http://www.wunderground.com/personal-weather-station/dashboard?ID=IKARNATA20#history

At 7:47, I reduced the temperature by 0.5C through the calibration page. The dew point also dropped simultaneously (@7:52). I reset the calibration to 0C @ 7:57AM

My indoor sensor is not working yet. Is that the reason? Or is my understanding of dew point wrong?

Also, what should be the relative pressure I should be offsetting in the calibration page? My station is located here - https://www.google.co.in/maps/place/2nd+A+Cross+Rd,+Akshayanagara+East,+Akshayanagar,+Bengaluru,+Karnataka+560076/@12.8719809,77.619573,17z/data=!3m1!4b1!4m2!3m1!1s0x3bae6b31fe002ddb:0xb15eff264b4ecaf6?hl=en

Please help me get the best out of the station and provide accurate information to the community.

Best,
Sharada

Your instruments are measuring ambient conditions not the surfaces of objects. Just because you see dew or ice buildup on the surface doesn't mean the humidity is 100% or even close. Lots of things can affect surface temperature forming dew or ice, one biggy is called radiational cooling.
Also your ambient temperature will never be lower than dewpoint temperature, when they are the same is when you would be fully saturated at 100% humidity.
Randy