Training Tip: From CU to TCU

Aside from turbulence in the air below those clouds, they seem a rather benign feature of the weather picture, usually dissipating as the sun goes down and taking the bumps with them.

Now as you update your destination’s weather, you notice an item concerning reported cloud cover hinting that those common cumulus clouds are evolving into something more attention-getting: “1853Z 17017KT 10SM FEW050TCU SCT100 SCT250 32/18 A2980 RMK AO2 SLP086 TCU N T03170183.”

Note the two references in the METAR to TCU, or towering cumulus clouds. As you draw closer to the destination, you can see some big white clouds resembling cauliflower massing in the sky ahead.

Okay, so instead of summer cu, there’s TCU—what’s the big deal?

Page 12-16 of the Pilot’s Handbook of Aeronautical Knowledge has a succinct answer to that question: “Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent. These types of clouds often develop into cumulonimbus clouds or thunderstorms.”

How likely are they to develop as described in that passage?

There’s another clue to that probability in the metar. Have you spotted it yet?

Look at the dew point. Not the temperature-dew point spread, which is ample and suggests no immediate risk of fog. But the dew point, 18 degrees Celsius or about 65 degrees Fahrenheit, is high enough to sound an alarm bell of its own, because a dew point that high corrrelates well with the possibility of thunderstorm development, as discussed in the June 12, 2015, Training Tip.

If that happens, those TCUs will become cumulonimbus clouds, or CBs (sometimes abbreviated Cb outside the context of a METAR).

How can you tell the difference between towering cumulus and cumulonimbus by visual inspection?

According to this glossary of weather terms, a towering cumulus cloud is “a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb.”

Just over an hour later, when you are safely on the ground, this textbook weather scenario plays out, as indicated in the new metar: 2004Z 18012KT 10SM TS BKN060CB BKN100 OVC250 27/20 A2982 RMK AO2 PRESRR OCNL LTGICCG OHD AND N TS OHD AND N MOV E T02720200.