Updated: Nov 1, 2021
If you were flying over the clouds below, what do they tell you about the (in)stability above and below these clouds as well as the wind direction?
Seasoned glider pilots learn how to read the clouds. Certain clouds imply the potential for lift and they can also tell the glider pilot about the direction of flow of the air aloft. Take a look at the visible satellite image below (the Florida peninsula is on the lower right with Texas on the left). What do these clouds tell you about the environment within the planetary boundary layer (PBL)?
They actually tell you quite a bit. Cloud streets pictured above (yes, you read that correct - streets, not streaks) are lines or rows of cumulus clouds (shallow, moist convection) that hang out near the top of the mixed layer. The mixed layer shown in the Skew-T log (p) diagram below is the turbulent unstable layer influenced by the Earth's surface. These cloud streets tell you that the lapse rate near the surface will have a dry adiabatic lapse rate (3 degrees Celsius for every 1,000 feet gain in altitude). Bases of the cumuliform clouds will typically form just above the lifted condensation level or LCL which is normally right near the top of the mixed layer.
Cloud streets that form in the early afternoon also tell you that there is a layer just above the mixed layer that is very stable - also referred to as a cap or lid. This is best seen on a Skew-T diagram such as the one on the right. A cap is normally marked by a very small positive lapse rate or negative lapse rate (inversion) just above the top of the mixed layer. This generally limits the vertical development of the clouds keeping them to only a few thousand feet thick - although this depends on the strength of the cap. Weaker caps can be broken allowing for deep, moist convection including thunderstorms. In the example above, the cap is strong, but can be broken if clouds start to build through about 10,000 feet or at the level of free convection (LFC). That will require some significant outside energy contribution (e.g., front) that does not generally exist when you see these cloud streets.
Cloud streets also identify the wind direction within the planetary boundary layer. The wind from the surface through the top of the cumulus clouds is typically oriented along the rows of clouds. That is, if the rows are oriented south to north (as they are in the visible satellite image above), the wind in the boundary layer will also be south to north. The winds above the clouds often shift to a flow more perpendicular to the cloud streets. This can be seen in the Skew-T above as the winds near the surface are from the south and shift to the west around 7,500 feet MSL.
Looking at the 850 mb constant pressure chart analysis shown above, the winds are indeed oriented in a south to north flow. The 850 mb constant pressure chart depicted the conditions at roughly 5,000 feet MSL or right near the top of the mixed layer. Also notice how the winds shift to a more southeasterly flow over Georgia and South Carolina. This causes the cloud streets in this area to shift to a more southeast to northwest orientation as also pictured in the satellite image above.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist