vapor pressure unit

[SATO YUKI]

May i ask a simple question about the unit of the vapor pressure. I outputed the ASCII result of Vapor Pressure, but I don't know the unit. I check the air pressure of ASCII file is hpa, and how about the vapor pressure?
Thanks very much for your replies.

[Thomas]

Hi Yuki,

the vapor pressure is in hectopascals, too.

Regards,
Thomas

[SATO YUKI]

Hi Yuki,

the vapor pressure is in hectopascals, too.

Regards,
Thomas

Thanks very much for your kind reply.
I am sorry i want to ask more detail...Now i got the ASCII result for the same time step: e.g., air pressure:1.030E+0003, vapor pressure(exterior surface):5.493970E+0000. If they have the same unit, it looks too large difference between them, and the air pressure is higher than the vapor pressure of the surface. In actually, in my case, the exterior material has built-in water.
Thanks again. I need your help.

[Thomas]

You can check the numbers if you also export the temperature and the relative humidity at the same location. Compute the saturation vapor pressure psat for this temperature and multiply the result with the relative humidity. You should get the same number that WUFI gives for the vapor pressure.

For example, if the surface temperature is 10°C and the surface humidity for the same time step is 45%, then psat(10°C) = 12.28 hPa, and 0.45*12.28 hPa = 5.53 hPa. The ASCII output for the surface vapor pressure should be the same number.

If they have the same unit, it looks too large difference between them, and the air pressure is higher than the vapor pressure of the surface.

But there should be a large difference. Water vapor is usually only a small percentage of the air. The water vapor pressure is the same as the air pressure at the temperature where the water starts boiling; for a normal atmospheric pressure of 1013 hPa this happens at a temperature of 100°C. For the lower temperatures you find in building physics, the vapor pressure is much smaller. For example, at 0°C the saturation vapor pressure is only 6.11 hPa, and the actual vapor pressure at that temperature is even less if the air is not saturated.

Regards,
Thomas