Dewpoints and Tornadic Storms

[Darrin Rasberry]

Dewpoints are probably my weakest area of basic understanding, as I only have a Wikipedia knowledge of it. I would like to understand more about dewpoints, particularly its effect on immediate development on tornadic supercells/squall lines and how to use them in part to find, before a projected outbreak occurs, the best place to wait for a particular chase. I am specifically interested in their interaction with other ingredients for storms.

Thank you in advance.

 

[Bill Schintler]

RE: Dewpoints and tornadogenesis

It is generally agreed that supercell tornadoes require a complex interaction between a rotating updraft of moist air and the RFD (downdraft of rain-cooled air). Dewpoint spread is the difference between the air temperature and the dewpoint temperature. A large dewpoint spread is usually associated with higher cloud base heights (LCL), and low spreads are associated with lower LCLs. When the spread is large, the descending RFD experiences more evaporational cooling, which causes it to accelerate. When this happens, the RFD is more likely to "undercut" the updraft instead of forming a rotating vortex. Tornadogenesis is more likely within supercell storms when the LCL is less then 1000m AGL. The downside of low LCLs is that storms tend to be less photogenic, as there's often more haze and low clouds that obstruct the view of storm structure.

Regarding convection in general, dewpoint is associated with instability. With other factors constant, increasing the dewpoint will increase instability (CAPE). Other factors that affect instability include surface temperature and lapse rates (rate at which temperature decreases with altitude). Taking the average of dewpoint (and temperature) throughout a layer above the surface (usually lowest 100mb, represented as MLCAPE) often gives a better representation of the actual instability. This is because the rising parcel is made up of not only the air at the surface but rather from a layer at and above the surface.

It should also be mentioned that in areas at higher altitude, such as CO, lower surface dewpoints are needed to attain a specific degree of instability. In Oklahoma City, dewpoints above 60F are generally considered more favorable for robust convection. By contrast, 50F is a healthy dewpoint in Denver.

- bill

 

[Keith Mullen]

On more basic terms, the other aspect to consider is how dewpoints also can be indicative of lift/CAPE/convergence (more of a flag). Hot humid air is more indicative of the potential for creating an ascending air parcel than comparatively dry hot air, or cool moist air.

Being able to visually see where there is enough deep layer "punch" to break up and out in relation to your other factors of convergence, drylines, and favorable conditions for backing and other necessary conditions for rotation is key.

Now if I could only figure out future timing, I could help you out more. The SPC site is great for playing around with all of the features if you are unfamiliar, then once you get the jist, progress to some of the other resources suggested here.

Learning how to interpret a skew-T chart would be my first recommendation on how to really understand the difference between surface obs and real true deep moisture/dewpoints. A surface reading may look swell, but a skew-t will either confirm or deny what you first thought.

More takers please.

 

[Gene Moore]

To get an idea of the dew point extremes involved in tornadogenesis check out this link to the LBB NWS office.

http://www.srh.noaa.gov/lub/events/2007/20071227/index.php

Numerous weak tornadoes occurred in an environment of 30's dew points, some did minor damage. I would have loved to have been on this case.

In my chase career the lowest measured dew point I've observed a full condensation tornado was a 49 dew point; the tornado lasted about 15+ minutes. At that time the Altus AFB (LTS) observation was 69/49. Just behind the tornado in the hail field from where I shot video, I could see my breath. This recent LBB case had conditions far below that.

 

[Mike Krzywonski]

It should also be mentioned that in areas at higher altitude, such as CO, lower surface dewpoints are needed to attain a specific degree of instability.

Is this because of lower surface temps. at higher altitudes?

In Oklahoma City, dewpoints above 60F are generally considered more favorable for robust convection. By contrast, 50F is a healthy dewpoint in Denver.

This is relative to surface temps. & weather seasons, correct? Doesn't it vary throughout the year?

 

[Darrin Rasberry]

To get an idea of the dew point extremes involved in tornadogenesis check out this link to the LBB NWS office.

http://www.srh.noaa.gov/lub/events/2007/20071227/index.php

Numerous weak tornadoes occurred in an environment of 30's dew points, some did minor damage. I would have loved to have been on this case.

In my chase career the lowest measured dew point I've observed a full condensation tornado was a 49 dew point; the tornado lasted about 15+ minutes. At that time the Altus AFB (LTS) observation was 69/49. Just behind the tornado in the hail field from where I shot video, I could see my breath. This recent LBB case had conditions far below that.

Excellent stuff to know (also applies to the other posters here - thanks!). One additional question to this - is there a general point where the temperature/humidity is too HIGH, as the 40's are too LOW? Which one is the better scenario, higher than a neighborhood of 60 or lower given the non-Colorado great plains region?

 

[Jeff Snyder]

Is this because of lower surface temps. at higher altitudes?

Dewpoint temperature is not the best measure of the total amount of "moisture" in a given layer or level. A better measure of "moisture" is specific humidity, absolute humidity, or mixing ratio, which are measures of the mass of water in a unit vol or unit mass parcel. When parcels change elevation or height, the mixing ratio is conserved; the dewpoint temperature is not conserved as parcels rises or sink. What this means is that as an unsaturated parcels moves from, for example, 1000mb to 900mb, the dewpoint of that parcel decreases slightly (while the mixing ratio does not). Most skew-t diagrams show the constant-mixing-ratio lines, and one can see that they do not parallel the constant temperature lines. So, as a parcel or blob of air moves from the Gulf of Mexico (0 feet above sea level) to eastern Colorado (several thousand feet above sea level), the dewpoint temperature decreases (though the total amount of water vapor, the mixing ratio, remains constant). For an unsaturated parcel, the dewpoint decreases approximately 2C per every km.

From an anecdotal standpoint, though, this does not entirely explain why tornado events in higher elevations are often associated with lower dewpoints. In other words, from my half-minded observations, typical tornado events in Colorado tend to have lower mixing ratio than typical events farther southeast. No concrete obs behind that, though.

 

[Gene Moore]

Excellent stuff to know (also applies to the other posters here - thanks!). One additional question to this - is there a general point where the temperature/humidity is too HIGH, as the 40's are too LOW? Which one is the better scenario, higher than a neighborhood of 60 or lower given the non-Colorado great plains region?

I'm going to answer this differently than Jeff who provided great input. Humidity too high: for the short answer I'll say no, the more low level moisture the better. That said, the assumption here would be that we're talking about 100 percent humidity in a reasonable (surface) temperature range...and that goes back to what Jeff was discussing with regard to proper measurement. So, let's say we have a situation with a 65 temp and a 65 dew point, that's 100 percent humidity. Of course one would want the temperature to rise from that point so they could see the tornado, right! The general rule is when the temp-dew point spread becomes 5 degrees or less fog (clouds) form. This is true in the upper atmosphere and on the ground. So any temperature spread less than 5 degrees will imply some reduction in visibility due to increase humidity. Where is this important synoptically, on or north of a warm front or near the coast to name a few.

In most cases using dewpoint for surface observations is better than RH (rel humidity). That said, what if you have 90F+ degrees and a 50 dew point. That computes to a RH of less than 30 percent. In those situations the parcel will have to rise too high to achieve condensation, thus the cloud base may be too high for tornadogenesis. This is an area where the use of RH is still relevant and I use this technique often in the early summer months on the high plains.

 

[Bobby Prentice]

Dewpoints are probably my weakest area of basic understanding, as I only have a Wikipedia knowledge of it. I would like to understand more about dewpoints, particularly its effect on immediate development on tornadic supercells/squall lines and how to use them in part to find, before a projected outbreak occurs, the best place to wait for a particular chase. I am specifically interested in their interaction with other ingredients for storms.

Dewpoint is defined as:

dewpoint—(Or dewpoint temperature.) The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur.

When this temperature is below 0°C, it is sometimes called the frost point. The dewpoint may alternatively be defined as the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Isobaric heating or cooling of an air parcel does not alter the value of that parcel's dewpoint, as long as no vapor is added or removed. Therefore, the dewpoint is a conservative property of air with respect to such processes. However, the dewpoint is nonconservative with respect to vertical adiabatic motions of air in the atmosphere. The dewpoint of ascending moist air decreases at a rate only about one-fifth as great as the dry-adiabatic lapse rate. The dewpoint can be measured directly by several kinds of dewpoint hygrometers or it can be deduced indirectly from psychrometers or devices that measure the water vapor density or mixing ratio. See dewpoint formula.

Dewpoint is related to the total quantity (or amount) of water vapor in the air near the surface. It is the "fuel" that drives thunderstorm formation via latent heat release.

Thunderstorms require three ingredients:

1. Sufficient water vapor ("dewpoints") to produce condition 2.

2. Unstable air. The airmass associated with showers and thunderstorms is almost always conditionally unstable. With conditional instability a lifting mechanism is required to lift the air ("above the cap") to release the instability.

3. A lifting mechanism to release the instability. Think of this a lifting the air parcels above the cap (temperature inversion). The lift can be from fronts, drylines, surface lows/troughs, upslople flow, sea/lake breeze fronts, outflow boundaries from previous convection, etc.

For more information about dewpoint, stability, and thunderstorm formation, please read chapters 5 and 6 from the link below:

Aviation Weather AC 00-6A
Aviation Weather AC 00-6A, chapters 4-6 - PDF format

This document is a bit dated, but does a good job of explaining the concepts to the lay person.
 

[Darrin Rasberry]

David Drummond pointed out to me the aforementioned event in Lubbock on Dec. 27th. Here in Iowa, a similar (but not as drastic) event happened when we had tornadoes track near Ames while many of my students in windbreakers were wondering if I was out of my mind when I asked them to be careful of severe weather. Temps were in the fifties with dews very close, and my chase partner had to call off spotting a tornado-warned cell near his home because of fog. That day last week reminded me of this thread, and of some lingering questions I have (divided by subject matter):

1. How big is a rising parcel of air - my foot? My house? My hometown? Related to that question, what is the distribution of these parcels like over a given unstable area on a stormy day, and what prevents all of this warm air and moisture from rising up all at once as one huge parcel?

2. What is the limit for dew points? I've seen that the world record is 95 in Saudi Arabia, but do they ever go into, say, the 80's during the summer?

3. I still am not sure why the cap does what it does. Let me give you my impression: If a capping inversion is too high up, then there's enough room for the parcels of surface-warmed, moist air to reach air cool enough to condense and form clouds before those parcels even get to the cap - forming a mess. Is this why the formation of lines of thunderstorms (or an MCS) occur as a severe day progresses and the cap weakens sufficiently, while only supercells occur first, due to the "survival of the fittest" scenario where only the most organized and strongest-lifted moist areas form over the cap?

4. Furthermore, I often hear that the southern plains are "too capped" in the later part of the storm season. Yet strong thunderstorm lines and derechos will form, especially in June over the southern Oklahoma/northern Texas areas, although supercells at that time seem limited. Since a line forms, this seems to me that the cap has to weaken significantly - so why don't supercells still form while the cap weakens? My own interpretation is that the cap begins strong and then becomes very weak in a very short amount of time, meaning that supercells just don't have time to develop, or to live long before the line overtakes them.

As a line trucks along, supercells can still "develop ahead of it." I am assuming this is because the mechanism that is pushing the squall along is also acting as a mechanism for thunderstorm potential ahead of the line. Is this the right idea?

The squall line last Tuesday (April 8) did not have very many supercells form, as was predicted. The line wasn't moving especially fast at once point, so what prevented numerous supercells from forming in front of it? Are the mechanisms on days such as that just not strong enough to initiate development past the cap? And if so, why does an already-existing squall line continue to live on when it hits an inversion layer?

I know that's tons of questions, and anyone replying certainly doesn't have to answer all of them - I just had to vent those longstanding curiosities.

 

[Mikey Gribble]

I can't answer most of your questions, but I'll do what I can.

1. What prevents all the surface air (boundary layer) from rising up (turning over) is a temperature inversion. If there is ever absolute instability ( I think that's what it's called, but it's been a long time since I read something with that in it), then the unstable layer turns itself over and releases that instability.

2. I've seen 80's quite a bit along the equator in the summer.

3. The cap is a temperature inversion. If you lifted a surface parcel to that height it would be colder than the surrounding air and hence negatively bouyant. The strength (temperature difference of the lifted parcel and the inversion) and depth of this capping inversion is important. People are often talking about convergence along boundaries and lift with upper level disturbances because this can lift the surface parcel through the cap (breaking the cap). As surface temperatures rise throughout the day you can reach the convective temperature, which is the temperature needed for a surface parcel to be warmer than the capping layer (after being lifted to that height). Dewpoints play into this a lot because a saturated parcel will cool more slowly than an unsaturated parcel as it rises through the atmosphere.

Darrin said...
"If a capping inversion is too high up, then there's enough room for the parcels of surface-warmed, moist air to reach air cool enough to condense and form clouds before those parcels even get to the cap - forming a mess"

I'm not sure exactly what you mean by this, but I've never heard of that before. Too high of relative humidity can cause problems with cloud cover, but that isn't associated with the height of the cap. Surface parcels condensing (forming clouds) is actually going to help it break through the cap since it will continue to rise at the moist adiabatic lapse rate (remember saturated air cools more slowly as it rises then unsaturated air).

4. Convective mode (squall lines vs. discrete supercells) is determined by a number of things, but I don't think a cap weakening quickly is one of them. The shear vector being normal to the boundary, initiating boundary and the amount of forcing are big ones.

The free warm sector storms (like what was supposed to form the other day) is largely accomplished through reaching the convective temperature since there is no focusing boundary. Upper level support may help to lift a surface parcel through the inversion layer too. Subtle differences in surface heating, old outflow boundaries, areas where wind speed or direction may change slightly, and lots of other things can help to focus convection in one area or another over the warm sector.
A squall line has a cold pool behind it from evaporational cooling (rain cooled air) and this helps to force the warm moist air ahead of the squall line up and over the gust front. Some times this can help to keep a squall line going when other storms may struggle with a weak cap.

 

[Dustin Wilcox]

2. What is the limit for dew points? I've seen that the world record is 95 in Saudi Arabia, but do they ever go into, say, the 80's during the summer?

E NE and W IA, especially SW IA, tops the 80 mark a couple times a year during the heart of summer, when the corn is really sweating.

 

[Brian Emfinger]

I actually found this site today while trying to update some of my links. It has all the high and low weather parameters from the whole world. Here are the high dewpoints from today:

http://coolwx.com/extreme/city.dewp.php

and their main site:

http://coolwx.com/extreme/