Topic: Humidity readings

[Dave Nelson]

Hi Gang,

     ok ... Im desparately trying to understand the humidity reports off ABC radio weather reports.

 example ... today  12 july 07    for penrith region ....  temp  12C,   humidity  99%

  HOW HOW  !!!   can you have a 99% humidity without cloud/fog   or the heavens opening up with rain?

  it seems pretty regular for out west to have 85% ++ humidity on cool mornings.
 but it just seems so weird to me.   my mind is stuck on the idea that with that much moisture present
it should just about be visible ??

  cheers
Dave N

[Jimmy Deguara]

Dave,

Out here, fogs are not uncommon especially when there has been rainfall around. I can report we even had F 0.2 this morning here in Schofields. As air cools which it does out here, fogs will form given sufficient moisture. Also take into account the caculations are based on automatic weather stations and their associated dew point readings compared to the air temperature.

The high humidities can also have been attributed to the cool moist air we have had this year.

Regards,

Jimmy Deguara

[Mike]

While on the 'subject' i have a really important question to ask relating to DPs. 

Is it possible to calculate DPs rising or falling in ten minute increments reading from only the first and second skew-t soundings if that was all that was available to you?

If not, how could you calculate it?  You would need regular hourly soundings to do this no?

And if you can how so ?


Edit: What are DP's - Deputy Principals or Dew Points? I think dew points. For all concerned, be careful when abbreviating given the audience sometimes may not understand
Mike

[Mike]

Deputy Principals?  Ah yes, point taken!  Anyway....it's definitely dew points thank you.

Mike

[Mike]

So if someone only had the 00Z sounding (if you only had that to view)  and said that the dew points at 3pm had risen a certain number of degrees in 10 minutes they'd be incorrect or lying? - unless one had a fresh sounding at 3pm or used all the mathematical calculations and had a thermometer and the other instruments needed to do it without the fresh sounding?

Mike

[Mike]

Just on your humidity question, John.

 Up here in Darwin we still record humidity levels between 60-80% in the dry season on some mornings.  If we have dominant but light flowing easterlies our nights are very cool.  Is what you are referring to is relative humidity?  I thought this was a dimensionaless ratio expressed in percentage ranges of the amount of atmospheric moisture present, but relative to the amount that 'would be' in the air if it was saturated - hence not seeing any cloud.  Humidity is a function of both moisture content and temperature.  I hope i got that right...and as is i think, fog is an effect of the above and then dew points come into play then also.

( I was going to move my question of dew points, but they're both associated with each other as far as atmospheric moisture is concerned.)

Mike