# Psychrometric Question re: Weather

OK, I never had an HVAC class in Engineering school, but I know how to use a psych chart. Or so I think…

Anyhow, Channel 4 Weather came on a bit ago, and claimed the following statistics on their weather screen (as the weatherman was talking about what he ate for dinner or some such drivel):

Temperature - 64 F
Dew Point - 43 F
Relative Humidity - 46%

Now - this doesn’t seem right to me. So I grabbed my handy Carrier Psychrometric Chart, and looked it up. If I take “Temperature” to mean “Dry Bulb”, and Dew Point to mean “Wet Bulb”, then I cannot resolve 64 F Dry Bulb, 43 F Wet Bulb, and 46% RH.

In fact, at 64 F Dry, and 43 F Wet, I get a roughly a 9% RH.

Now, I know “Dew Point” is not equal to “Wet Bulb” by definition, but on my Carrier Psych chart it lists “Wet Bulb, Dew Point, or Saturation Temperature” as being the curved line on the top border of the chart.

Am I missing something here?

I actually sat down and did the math for this.

Using the formulas for determining relative humidity from this page, for an air temperature of 18°C and a dewpoint of 6°C I get:

Es = 6.1110[sup][7.518/(237.7+18)][/sup] = 20.7

E = 6.1110[sup][7.56/(237.7+6)][/sup] = 9.2

9.2/20.7= 0.45

Looks like the TV people got it right for once.

You actually pay attention to the weatherpeople? Wow… you got balls…

The problem is the assumption that the dew point and wet-bulb temperatures must be nearly the same. I did the following estimates for a dry-bulb temperature of 60ºF and various levels of relative humidity.

``````

Dry Bulb Temp  RH   Dew Pt  Wet Bulb Temp   WBT minus DP
-------------  --   ------  -------------   ------------
60        100    60         60              0
60         80    50         56              6
60         60    42         52             10
60         40    32         48             16

``````

Thanks guys, you’ve pointed me in the right direction. But I am still confused - why does my psych chart list “Wet Bulb, Dew Point, or Saturation Temperature” as all being on the same line (the curved line on the top border of the chart)? Am I missing something here still?

Does it have different scales for the different values? E.g. are there 3 y-axes, one labeled “WB”, one labeled “DP”, etc.?

Screw the charts…get your sling psychrometer and go check it yourself!