CB vs TCU: Cumulonimbus or Towering Cumulus in a METAR
CB vs TCU is the only cloud type question a METAR ever asks you. Every other layer is reported as cover and height and nothing more. These two get named, and they get named for a reason.
A cloud layer normally tells you where the sky is. These two tell you where the air is moving hard enough to hurt you.
What CB vs TCU means in the report
Both are suffixes. They attach to the end of a cloud group, after the height.
BKN040CB is a broken layer with its base at 4000 feet, and that layer is
cumulonimbus. SCT025TCU is a scattered layer at 2500 feet made of towering
cumulus. The cover and height are read exactly as normal. The suffix adds the
type.
CB is cumulonimbus. That is a fully formed convective cloud, tall enough that
its top has frozen into ice crystals and spread into an anvil. It produces
lightning, hail, severe turbulence and strong downdraughts.
TCU is towering cumulus. Same violent updraught, same rapid growth, but the
top has not glaciated yet. No anvil, no lightning.
How an observer tells them apart
The test is the top of the cloud, not the bottom. From the ground both look like a tall lumpy tower.
A towering cumulus has a hard, sharp, cauliflower edge all the way up. Every part of it is still water droplets, so the outline stays crisp.
A cumulonimbus has gone soft and fibrous at the top, and usually flattened out sideways into an anvil. That fuzziness is ice. Once the top freezes, the cloud has the full charge separation that makes lightning, and it earns the CB code.
The change from one to the other can take twenty minutes. A field reporting TCU in one hour and CB in the next has not seen two different clouds.
Why both are worth avoiding
TCU is often read as the safe one. That is the mistake this comparison exists to stop.
The updraught inside a towering cumulus is already strong, often 3000 feet per minute or more. It carries the same airframe icing above the freezing level and the same severe turbulence. What it lacks is lightning and hail, which are the two hazards a passenger notices.
Neither is a cloud to fly through. The standard avoidance advice, twenty miles from a thunderstorm, applies to the mature CB. For TCU, treat it as a CB that has not announced itself yet, because in half an hour it may well be one.
Where automation gets it wrong
This is the part that catches people reading a report from an unstaffed field.
A ceilometer fires a laser straight up and times the return. That gives an honest height for whatever passes overhead. It cannot see form, it cannot see sideways, and it has no way to judge whether a top has glaciated.
So a fully automated station reports BKN040 where a human would have written
BKN040CB. The storm is there. The code for it is not.
Stations with lightning detection can add TS to the body when strikes are
detected nearby, which partly covers the gap. A storm that has not yet produced
a detected strike still goes unreported.
If a report carries AUTO and the area forecast talks about convection, believe
the forecast over the silence.
Reading the suffix in a TAF
A TAF uses the same two suffixes, and there it is a forecast rather than a sighting.
TEMPO 1418/1422 4SM TSRA BKN020CB says the forecaster expects a cumulonimbus
base near 2000 feet in that window. Nobody has seen it. It is expected.
That matters for planning. A CB in a METAR is a fact you route around now. A CB in a TAF is a window you may want to be on the ground before, or airborne well ahead of.
Watch which change group it sits in. Under TEMPO the storm comes and goes.
Under FM it is the new steady state. Under PROB30 it is a three in ten
chance, which is not small when the outcome is hail.
Verdict on CB vs TCU
Read CB vs TCU as one hazard at two stages rather than as two different clouds. The suffix is the observer telling you that this particular layer is dangerous.
CB means it is finished and firing. TCU means it is still loading. Both mean
route around it, and both mean the air near that layer is not smooth. If the
report says AUTO and neither suffix appears, that is not evidence of a quiet
sky.
Frequently asked questions
What is the difference between CB and TCU in a METAR?
CB is cumulonimbus, a mature thunderstorm cloud with an anvil top. TCU is towering cumulus, the same cloud still growing and not yet glaciated. TCU is what CB looks like twenty minutes earlier.
Why are CB and TCU the only cloud types in a METAR?
Because they are the only two that change what an aircraft can safely do. Every other cloud type is reported by cover and height alone. CB and TCU carry turbulence, shear and hail, so they get named.
Does TCU mean there is no thunderstorm?
It means no thunderstorm has been observed yet. TCU has the updraught but has not produced lightning. Treat it as a storm that has not finished forming rather than as a benign cloud.
Can an automated station report CB or TCU?
Usually not. A ceilometer looks straight up and measures height, not shape. Most CB and TCU reports come from a human observer or from augmentation, which is why an AUTO report can miss a storm beside the field.
What does BKN040CB mean?
A broken layer, five to seven eighths cover, with a base at 4000 feet above the field, and that layer is cumulonimbus. The suffix attaches to the layer it describes, not to the report as a whole.