TS vs. CB

[plattsburgher]

I was wondering if any of you had any thoughts on this:

What is the practical difference, if any, between "CB" and "TS" in the METAR/TAF codes? I know that TS is a weather descriptor and is entered as a function of an obstruction to vis (+TSRA). CB is a cloud type and is entered with the sky cover (BKN 020CB). Are these terms necessarily linked? Since the definition of a CB is a thunderstorm, shouldn't the observation always include rain and thunderstorm (TSRA)?

Assume your company rules do not allow you to take off or land if there is a thunderstorm in the observation. If the ATIS advises CB but says nothing about TS, or any associated hazards, would you be legal?

Thanks

 

[Hugh Jorgan]

Cloud type vs. phenomenon

A CB isn't a thunderstorm, it's a cloud type as you stated. A thunderstorm is a CB that is accompanied by thunder and lightning and thus is a weather phenomenon, ie. activity in the vicinity of the report. Here's a link I've found helpful to answer the ever-confusing metar decoding...

http://www.met.tamu.edu/class/METAR/quick-metar.html

 

[Hugh Jorgan]

Assume your company rules do not allow you to take off or land if there is a thunderstorm in the observation. If the ATIS advises CB but says nothing about TS, or any associated hazards, would you be legal?

Legal? Yes.
Smart? That all depends on the situation...

 

[Noam]

METAR "service standards"

I wouldn't worry about legality in respect to METARs, unless you are at one of the largest airports in the country.

Each airport has a "service level" defined by the FAA. ASOS can only report certain things (it can't report TCU, CB, tornado's, etc...and sometimes can't even report lightning or TS).

Most airports (even most in CLASS "C" airspace) don't have METAR's at the highest service level...that means that certain data is left out of the report.

Even airports where the control tower folks are supposed to augment ASOS and add things like TCUs, CBs and TS, they frequently do not. I've heard NATCA is pissed at ASOS, just like everyone else.

What I'm trying to say is...unless you are flying into JFK, LAX, etc...your METAR most likely is missing some data or not representative of actual weather conditions.

http://www.faa.gov/ats/ars/Directorates/Arw/ASOS%20Service%20Standards.htm

P.S. -- A cumulonimbus cloud is a cumulus cloud that is producing precipitation. A thunderstorm is when an ASOS detects lightning or an observer hears thunder. Many times the ASOS will detect a TS and report it, but remember ASOS can't tell you a CB exists so don't expect to see such a remark unless you are at a big airport with an experienced observer.

http://www.faa.gov/atpubs/SWO/

ASOS SERVICE LEVELS
​

Service Level D.

This level of service consists of an ASOS continually measuring the atmosphere at a point near the runway. The ASOS senses and measures the following weather parameters: Wind, Visibility, Precipitation/Obstructions to Vision, Cloud Height, Sky Cover, Temperature, Dew Point and Altimeter. A site ranked as service level D has a stand-alone ASOS.

Service Level C.

This level consists of all the elements of Service Level D, plus augmentation and backup by a human observer on location nearby. Backup consists of inserting the correct value if the system malfunctions or is unrepresentative. Augmentation consists of adding the following elements if they are observed: Thunderstorms, Tornadoes, Hail, Virga, Volcanic Ash, Tower Visibility and any operationally significant remarks as deemed appropriate by the observer. During the hours that the observing facility is closed, the site reverts to Service Level D, stand-alone status. Generally speaking, air traffic control tower specialists provide Service Level C.

Service Level B.

This level consists of all of the elements of service levels C and D plus the augmentation of the following elements: Longline RVR at precedented sites (may be an instantaneous readout), Freezing Drizzle versus Freezing Rain, Ice Pellets, Snow Depth and Snow Increasing Rapidly remarks, Thunderstorm and Lightning Location remarks, Observed Significant Weather Not at the Station remarks. Ruing the hours that the air traffic control tower is closed, the site may revert to Service Level D. Generally speaking, contract weather observers provide Service Level B.

Service Level A.

This level consists of all of the elements of service levels B, C and D plus the augmentation of the following elements: 10 minute longline RVR at precedented sites or additional visibility increments of 1/8, 1/16 and 0; Sector Visibility; Variable Sky Condition; Cloud layers above 12,000 feet; and Cloud Types, Widespread Dust, Sand and other Obscurations and Volcanic Eruptions.

 

[plattsburgher]

Good posts:


The reason why I asked was that I was up at BIKF for a couple of weeks and they kept carrying that CB code in the METAR without any rain, the TS code, or any other hazards associated with a TS. After I had done the Blue Lagoon a couple of times and got tired of repeats on AFN or Beavis and Butthead on Euro MTV I got bored enough to break out the WX manuals.

The FAA book (AC 00-6A Aviation WX) defines "cumulonimbus" as a "cumuloform cloud type; it is heavy and dense, with considerable vertical extent in the form of massive towers; often with the tops in the shape of an anvil or massive plume; under the base of cumulonimbus, which is often very dark, there frequently exists virga, precipitation and low ragged clouds (scud), either merged with it or not; frequently accompanied by lightning, thunder, and sometimes hail; occaisionally produces a tornado or a waterspout; the ultimate manifestation of the growth of a cumulus cloud, occasionally extending well into the stratosphere." (204)

The same book defines a "thunderstorm" : "In general, a local storm invariably produced by a cumulonimbus cloud, and always accompanied by lightning and thunder." (212)

The AF Wx manual says pretty much the same things. The CB then is a cloud type that could generate the hazards associated with thunderstorm, but has not been observed to do so.

That takes me back to my original question: If the observer says it is a CB, then by definition it is a CU or TCU cloud type that has begun to rain. The METAR should then have something in the observation that reflects that (i.e., rain, showers, showers in the vicinity, distant showers, etc). If the guy doesn't see the rain, then it isn't a CB. Am I correct about that or did I get off the path somewhere?

The question in the first post about being legal was purely hypothetical; based on the assumption that a CB is the same thing as a thunderstorm. I think the commander would have some questions for me if I screwed up the schedule by holding or diverting because the ATIS called for some CBs.

Yes, I know that I need to find a hobby or something else to do.

Thanks

 

[Noam]

Good posts:

That takes me back to my original question: If the observer says it is a CB, then by definition it is a CU or TCU cloud type that has begun to rain. The METAR should then have something in the observation that reflects that (i.e., rain, showers, showers in the vicinity, distant showers, etc). If the guy doesn't see the rain, then it isn't a CB. Am I correct about that or did I get off the path somewhere?

Thanks

Yes and no. In most cases, a CB is producing some sort of precip, whether it be virga, rain, snow, whatever...sometimes do to haze or another obstruction, the observer may be able to see the top of a CB cloud, but not the base...if the CB is far away or it is hazy maybe the observer can't make out any precip at the base of the cloud.

Also, in desert areas it is possibe to have "dry" CB's and TS's...thunderstorms that don't produce precip because it is so dry out.

European METAR's/ICAO do not have the same remarks system that we do in the states...you said you were in BIKF (Keflavik, right?)...you may just have to go to the weather people there and ask them.

 

[plattsburgher]

The METAR is a common format everywhere. Member states that deviate have to publish their own exceptions. The only differences in the US are our visibility units, inches mercury, and one or two other things.

As far as the observer goes, I would think he needs to see something produced by the cloud or he can't change its status to a CB.