METEOROLOGIST JEFF HABY
The next 5 Haby Hints will focus on the general classification of clouds and how they form. The 5 writings will include stratus, cumulus,
cirrus, clear skies, and composite/naming/unique clouds. This first writing looks at the characteristics and formation of stratus clouds.
Stratus clouds are flat clouds that look like sheets. They can occur at any tropospheric elevation including at the surface (fog), near
the ground, in the middle levels and upper levels of the troposphere. The two ingredients that form stratus clouds include dynamic
lifting and air with a high enough relative humidity that it can be saturated upon lifting.
Dynamic lifting is a slow lifting. This is why stratus clouds are flat looking instead of vertically developed like cumulus clouds
are from convective lifting. The relative humidity will vary with height and regions of lifting (if present) will vary with height. The
best combination of high relative humidity and lifting or where lifting is first able to saturate the air is where stratus
clouds will first form.
Unlike typical cumulus clouds, stratus clouds can cover the entire sky. This is because dynamic lifting tends to occur over a large
region while convective lifting tends to be more focused over a smaller area. Dynamic lifting occurs from large synoptic scale
lifting mechanisms such as fronts, low pressure convergence, low level warm air advection, positive vorticity advection
and jet stream divergence.
Low level stratus clouds can also form as a ground fog without the aid of significant lifting (although lifting helps sustain
the clouds). These clouds require saturated air and the other basic ingredients to develop fog. During the day the ground can
warm enough to mix out the fog at ground level but there can still be left a blanket of stratus clouds just above the surface. This
blanket of clouds may or may not mix out depending on a variety of factors such as thickness of clouds, wind speed/direction, time
of year and changes in lifting during the day.