Dewpoint vs mixing ratio vs relative humidity

[Andrew Clope]

I'm pretty sure I understand dewpoint temps and relative humidity regarding severe storm forecasting, but I'm confused as to what mixing ratio is? I know that it's basically just the ratio of water to air in a given parcel, but how do the 3 (dewpoint/MR/Relative Humidity) affect weather forecasting?

Thanks!

 

[JamesCaruso]

The meteorologists and other more knowledgeable chasers can correct me as needed, but my understanding of mixing ratio is that it is a better absolute measure of the amount of moisture in the atmosphere. Dewpoint is a measure of the temperature at which you will have condensation. This is also affected by air pressure, which is why you can get great storms in Colorado at lower dewpoints - I can't give you a proper equivalence, but just throwing out numbers as an example Colorado dewpoints in the 50s might represent the same amount of moisture in absolute terms as 60s in the lower terrain further east in the Plains.

Relative humidity is the relationship of the moisture in the air to what the air can hold at its temperature (and, I assume, pressure). So if the air temperature is 70 and the dewpoint is 60, the RH is higher than if the air temperature is 80 and the dewpoint is 60. Or, if the temperature is 70, you will have a higher RH at a dewpoint of 60 than you will at a dewpoint of 50. RH is quoted a lot in mainstream public forecasts but I think it is pretty meaningless in that context because people associate high humidity with uncomfortable conditions, but as shown in the example above, you can have a given dewpoint, raise the temp, and the RH actually goes down, yet clearly you would experience hotter and more uncomfortable conditions.

Another moisture related metric is precipitable water which, I learned on this forum via a question I posted a year or so ago, is a measure of the total moisture in the atmospheric column, whereas dewpoint is generally cited as a surface measure. Dewpoint is also measured at other levels of the atmosphere, but it is measured at each level individually, not for the column in totality which PW does measure.

Jim


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[Andrew Clope]

Jim,

Thank you for your response.

 

[Robert S. Thompson]

Jim essentially has it correct. If you look at the bottom portion of a Skew-T that should help you understand further.

High PWATs indicate a greater potential for flooding, especially when compared to normal. Also, higher PWAT value reduces the chance that low level moisture will "mix out."

RST

 

[Jeff Duda]

I agree that James gave an informative response. I'd like to add a few things to what he said.

Mixing ratio is indeed the ratio of the mass of water vapor to the mass of dry air (excluding water vapor). Mixing ratio remains constant for vertical motion of an unsaturated air parcel. This is important for understanding parcel theory. When you see soundings from a well-mixed/unstable PBL, the mixing ratio is what is well mixed, and it is constant through the depth of the PBL. It turns out mixing ratio is actually a derivative of another way of measuring moisture content called vapor pressure, which is the partial pressure of water vapor in the air. Relative humidity is the ratio of the vapor pressure to the saturation vapor pressure (the latter being an exponential function of temperature). As James noted, RH is generally not very useful for weather forecasting, since widely varying combinations of temperature and dewpoint can result in the same RH values. It's more useful in numerical models to determine when to create clouds/rain etc.

Finally, precipitable water is actually the vertical integral of mixing ratio. It literally is the amount of rain that would fall straight down out of a given column of the atmosphere if all the water vapor in that column were condensed into liquid and allowed to fall to the ground (without evaporating). You can think of it as a maximum potential rainfall amount at a given point if no other forces added water to the column. Typical PWAT values are in the 0.5"-2" range, and there are clearly observations of well over 2" of rain in a short period. This is a different beast related to flash flood forecasting, but it is still interesting to know.