The Mixing Ratio

[Bob Hartig]

I've been on this forum for around five years now. You'd think I'd be one of the guys with all the answers. Not so--there's still plenty of stuff I don't get, and one of them is mixing ratios.

I understand mixing ratios vaguely as a means of determining the amount of water vapor present in a mass of air, expressed in grams per kilogram (g/kg). But how do I use this information? So far I've gone about getting a sense of moisture by looking at dewpoints and Td depressions, and I've done fine that way. But moisture being as critical as it is to the convective process, I'd like to get a better understanding of some of the other ways of looking at and measuring it--so that instead of being dumb but happy, I can be happy and a little bit smarter.

So, to those of you who use mixing ratios in your forecasting: what do I need to know in order to use them? Please keep your answer as simple and application-oriented as possible. What I'm looking for is some essential, hands-on pointers for using the mixing ratio:
* How do you use it?
* What do you look for?
* Any thresholds I need to keep in mind?

Thanks in advance for your helpfulness! I much appreciate the knowledge that many of you so willingly share.

 

[Steve Miller TX]

The number one thing I use it for are Skew-T soundings to lift a parcel to moist adiabatic and therefore the LCL, CAPE, LI, etc. From what I understand of it, it takes into account air pressure which is important for higher elevated areas. In other words, a 60 degree dewpoint at Limon, Colorado is different than Oklahoma City.

I hope I got that right. If not, I'm sure I will be soon corrected.

 

[Andy Jackson]

Here's a cool app that helps to see the relationship between mixing ratios and temp/dew/pressure:
Mixing ratio calculator

I would say at looking at the different models it seems UCAR's RUC includes a model with mixing ratio's at the 925/850hpa levels. I'd say this is to view where the deep moisture is. It seems like you can just get the same thing from looking at the dewpoints at the different levels.

 

[Jeff Snyder]

An important point to remember is that mixing ratio is conserved when unsaturated air is lifted or sunk (dry adiabatically). This is in contrast to the dewpoint temperature, which decreases when a parcel is lifted adiabatically; the dewpoint "lapse rate" (or the rate at which the Td decreases when lifted or increases when "sunk") is approximately 2 C / km. So, let's say we have surface observations near Galveston, TX, (assume it's at sea level) of Td = 70 F. As that air advects northwestward, it is forced to rise with the terrain, such that by the time the air reaches Limon, CO (at an elevation of ~1600 m ASL), the dewpoint will have dropped to approximately 64 F (1.6 km * 2C/km = 3.2 C = 5.8 F). The parcel still has the same 15.7 g/kg mixing ratio that is had in Galveston, however.

So, why don't we use mixing ratio more? Well, I think it's harder to "visualize" for most weather weenies. For example, chasers like relatively low LCLs, and it's relatively easy to look at a map of surface observations (which, per the standard model, include temperature and dewpoint), and at least get a handle on LCL height. If all we have, however, is a map of temperatures and mixing ratios, most folks won't really know what to do with it. For example, in March in Oklahoma, is an 11 g/kg mixing ratio "high"? Is 14 g/kg in Iowa in July "a lot"? Granted, if we used mixing ratio more often, people would become more familiar at interpreting mixing ratio values, but you probably won't hear the local TV weatherperson say "Tomorrow will be more humid, with mixing ratios of 16-17 g/kg, so drink lots of water and stay in A/Ced buildings!"...

 

[Zach Young]

Steve is right on track. Dewpoint (and also boiling point) is pressure dependent. Mixing ratio is a much more concrete way to measure moisture content, since it's [grams of water]/[kg of dry air]. The only difference at higher elevations is that the kg of dry are takes up more volume. Using Steve's example and Andy's calculator (modified a little to make it easy on me): At sea level (1013mb) a 60F dewpoint = ~11g/kg. In Denver (~850mb) 60F = ~13.2g/kg. This is part of the reason (along with orographic forcing) why dewpoints don't need to be as high in the High Plains or in the mountains to get storms to happen.

I use the SPC mesoanalysis page fairly often. It can be a little off if the RUC isn't on it's game, but it only uses the 0hour output anyway. The "moisture convergence" map is the one I use most often, as areas of high moisture convergence are often favored for thunderstorm initiation. You can find it under basic surface.

 

[Bob Hartig]

If all we have, however, is a map of temperatures and mixing ratios, most folks won't really know what to do with it. For example, in March in Oklahoma, is an 11 g/kg mixing ratio "high"? Is 14 g/kg in Iowa in July "a lot"? Granted, if we used mixing ratio more often, people would become more familiar at interpreting mixing ratio values, but you probably won't hear the local TV weatherperson say "Tomorrow will be more humid, with mixing ratios of 16-17 g/kg, so drink lots of water and stay in A/Ced buildings!"...

Jeff, thanks for another of your typically great responses! But you left off with the very kinds of questions I'm curious about. Could you take it to the next step and answer them? What makes for a high, or maybe I should say, a significant, mixing ratio? Is there some kind of threshold? If temperature is a factor in determining whether a mixing ratio is high or low, then what would I look for at, say, 75 degrees, 80 degrees, etc.? What I need isn't so much the theory of mixing ratios as some pointers on how to use them.

Sorry to be so nit-picky. I'm just trying to figure this stuff out, with the sense that the questions I'm asking today could seem elementary or just plain goofy a year from now!

 

[Jeff Snyder]

What makes for a high, or maybe I should say, a significant, mixing ratio? Is there some kind of threshold? If temperature is a factor in determining whether a mixing ratio is high or low, then what would I look for at, say, 75 degrees, 80 degrees, etc.? What I need isn't so much the theory of mixing ratios as some pointers on how to use them.

Bob -- mixing ratio isn't really a function of the temperature (assuming we're looking at constant pressure, since T and P are related by the ideal gas law), just like dewpoint isn't a function of temperature per se (e.g. if the dewpoint is 65 F, it'll stay 65 F whether the temp is 80 F or 100 F assuming the amount of water vapor in the air remains the same). Since you are familiar with dewpoints, let's look at some common values of Td and their respective mixing ratio values.

Oklahoma City, OK (elevation: 400 m, and let's use 1000 mb MSLP, which yields 962 mb station pressure):
Td = 70 F --> W = 16.8 g/kg
Td = 65 F --> W = 14.0 g/kg
Td = 60 F --> W = 11.7 g/kg

Amarillo, TX (elevation: 1100 m, station pressure: 876 mb)
Td = 70 F --> W = 18.3 g/kg ------> Td @ OKC = 72.8 F
Td = 65 F --> W = 15.3 g/kg ------> Td @ OKC = 67.7 F
Td = 60 F --> W = 12.8 g/kg ------> Td @ OKC = 62.6 F

Limon, CO (elevation: 1640 m, station pressure: 820 mb)
Td = 70 F --> W = 19.6 g/kg ------> Td @ OKC = 74.8 F
Td = 65 F --> W = 16.4 g/kg ------> Td @ OKC = 69.7 F
Td = 60 F --> W = 13.7 g/kg ------> Td @ OKC = 64.6 F

So, as I'm sure you know, at higher elevations, you don't "need" as high of a dewpoint to have the same mixing ratio (i.e. the have the same amount of moisture in the air relative to the mass of dry air). Therefore, you don't often see very high dewpoints (>68F-70F) at higher elevations like those in eastern Wyoming. However, you'll also notice that the dewpoint "equivalents" in Limon relative to Oklahoma City aren't THAT different. In other words, you'll often hear chasers talk about how mid-50 dewpoints in eastern Colorado is often just fine for a tornadic supercell event. While this may be true, it is NOT entirely because such dewpoints aren't similar to 65-70F dewpoints in central Oklahoma. I suspect there are other reasons why you often see tornado events at higher elevations with relatively low dewpoints, but I haven't seen any comparison of mixing ratios associated with significant tornado events in different parts of the country.

So, what is "high"? Well, just like with dewpoints, "high" is relative. In March in Oklahoma, a 11-13 g/kg mixing ratio indicates pretty good moisture (relative to climatological norms). By mid-late May, we shouldn't have much of a problem getting 16-17 g/kg mixing ratios in central Oklahoma. Most Skew-T Log-P graphics you'll see online include lines of constant mixing ratio (usually just in the low-levels).

Notice, too, that the increase in mixing ratio as dewpoints increase from 60 to 65 F in OKC is 2.3 g/kg. However, over the same 5 F increase from 65 F to 70 F, the mixing ratio increases by 2.8 g/kg. Obviously, the "amount" of moisture in the air does NOT scale linearly with dewpoint (in F). The increase in mixing ratio from 70 F to 75 F in OKC is even greater. Interestingly, remember that water loading can significantly affect (detrimentally) updraft intensity. As you can see, there's almost 44% more water vapor when OKC has a 70 F dewpoint versus a 60 F dewpoint. A surface parcel at the ground in Oklahoma City that has a 75 F dewpoint has more than twice the amount of water vapor than a parcel (at ground in OKC) that has a 55 F dewpoint. So, while CAPE tends to be higher as the dewpoint increases (assuming temps aloft don't change), there will also be much more water vapor condensing to precipitation (and much more precipitation loading that can weaken updrafts).

For what it's worth, the El Paso, TX, NWSFO has some very useful weather calculators / converters on their website at http://www.srh.noaa.gov/epz/?n=wxcalc .

Sorry to be so nit-picky. I'm just trying to figure this stuff out, with the sense that the questions I'm asking today could seem elementary or just plain goofy a year from now!

Never! If nobody asked questions, few would learn! Always feel welcome to ask any question you have -- if I can't answer them (there are many that qualify for this one), we have a lot of great chasers and meteorologists on Stormtrack that can.

 

[Robert Edmonds]

Just doing a quick google search for skew-t I found this plot which might be helpful in illustrating the importance of mixing ratios.

Image can be found in link http://ccrc.unh.edu/~stm/AS/Weather_Toolbox/Skew_T.html

As a air parcel is rises it will cool dry adiabatically (i.e. follow dry adiabats) until it becomes saturated. It is saturated where the dry adiabatic line intersects the mixing ratio lines. After this the air parcel will cool moist adiabatically (i.e. follow the moist adiabats).

From a modeling stand point, most if not all models probably directly use the mixing ratios when advecting moisture. This is because you can use the mass conservation equation. To determine the dew point the models then calculate what it should be given the present mixing ratio, temp. and pressure.

 

[Zach Young]

Another useful thing about mixing ratios is that you can easily find the relative humidity using them. If you know Td you can find W (like in Jeff's latest post), but if you also know T, then you can find Ws (saturation mixing ratio). W/Ws x 100% = RH.

 

[Steven Williams]

I find the m/r good for assessing the moisture at higher levels in the atmosphere such as 700mb.

 

[Patrick Marsh]

An important thing to note is that in the absence of diabatic effects, advection occurs on isentropic surfaces. On an isentropic surface, there is no such thing as a dewpoint temperature - because temperature is the vertical coordinate. Thus, you must find a definition of moisture that does not rely on temperature -- mixing ratio, which is dependent on mass, is a perfect quantity. This is the moisture quantity that is advected in isentropic space.

An for completeness, there is an equivalent quantity to dewpoint in isentropic space. It's called the "condensation pressure" (the pressure at which condensation occurs).

 

[Bob Hartig]

These are some great responses. In fact, this thread is so helpful overall that I'm going to print it out once it has run its course. Thanks to all of you! I now see the correlation between the mixing ratio, pressure, and dewpoints. The charts and conversion apps are particularly helpful in that respect, the former because they offer comparisons and the latter because they let me learn by tinkering. At last it makes sense to me why a 55 degree Td will fuel supercells in Sterling, CO, but not in Catoosa, OK. Conceptually, the mixing ratio is finally making sense to me. Now I think I need to play with a few skew-Ts so I can interact with some real world applications.

 

[Mike Kovalchick]

Any particular reason not to just use precipitable water values instead?

Looking at the SPC Mesoanalysis maps...mixing ratios look to be more or less equiavlent to precipitable water values.

When I look at moisture, I look at surface dewpoints, 850 mb dewpoints, 700 dewpoints(for dry slots), Skew-T(to make sure the moisture thickness is adequate in case it exists just below 850) and PW values. (I look at 850 mb transport vectors and some other parameters for flooding/storm training situations.) I also consider advection, gradients, and moisture convergence. (I have a mental fudge factor in place for the plains of Colorado etc.). Finally, I consider upstream convection and whether that will block moisture transport into a target area or not.

PW values are very easy to understand/intuitive and therefore more useful in my opinion than mixing ratios. Their display on the SPC map's are look a lot less noisy..shows patterns much clearer than mixing ratio's.

As a rule of thumb, I get concerned when PW values are less than 1" or more than 2" are present. (I prefer towards the lower end-i.e. HP storms a little less likely..or rather supercells quickly becoming MCS's are more likely with the higher PW values present.)

 

[Patrick Marsh]

The reason I don't like to use PW values is that you are looking at the entire troposphere in the calculation. (Not to mention, that it is highly sensitive to *how* you do the calculation, much like CAPE, CINH, etc.) For severe weather, what we're really after is ample low-level moisture, which cannot be determined from a simple PW value.

 

[Mike Kovalchick]

The reason I don't like to use PW values is that you are looking at the entire troposphere in the calculation. (Not to mention, that it is highly sensitive to *how* you do the calculation, much like CAPE, CINH, etc.) For severe weather, what we're really after is ample low-level moisture, which cannot be determined from a simple PW value.

Thanks. I don't disagree. However, again looking at the SPC maps-they look about the same(areas of high PW are in the same areas as high mixing ratio's etc.-I assume this is because so much of the moisture column is generally contained near the surface in the warmer temperatures.) minus all the noise. Wow-PW values are so much easier to read..of course maybe this more a function of the graphics used than anything?

I will point out that moisture is important at all levels of the atmosphere for severe weather-not just the lower levels. (I point this out since it is a Education forum.) Mid-level moisture-bad(effects lapse rates), upper-level moisture-bad if it forms clouds that block insolution. Higher PW values can quickly suggest there may be TOO much moisture present.

In any case, neither parameter should be used in a vaccum..and the previous listed parameters that I mentioned should be used together to create a mental picture of what the moisture is doing on storm day. It is being able to create a mental picture of the atmosphere which helps us accurately forecast where the storms are going to be, their character and what they are going to do.. isn't that what we are trying for as chasers?
If Mixing ratios helps..fine..if it doesn't..try something else?

Thanks for pointing out that methods differ on how PW is calculated-I will have too look into that.