METEOROLOGIST JEFF HABY
This series will focus on the general classification of clouds and how they form. The categories will include stratus, cumulus,
cirrus, clear skies, and composite/naming/unique clouds. This first section 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.
Cumulus clouds range from small puffy cotton ball clouds (fair weather cumulus) to huge thunderstorm complexes
of deep convection. The distinguishing feature
between cumulus and stratus clouds is that a cumulus cloud is vertically developed. For a cloud to be vertically developed requires
the ingredient of instability. Instability is released when a parcel of air is placed in an environment in which it is less dense
than the surrounding air. The cumulus cloud is a parcel of less dense air (warmer and moister) than the surrounding air. Like a helium
balloon or a bubble in a pot of boiling water, the parcel will rise. As the cloud rises it will continue to condense out more
moisture since rising air cools. As air cools, moisture in the saturated air is condensed out. This builds the condensed moisture
of the cloud as the cloud climbs in the troposphere.
The depth of the cumulus cloud will depend on how deep the layer of instability is. Shallow instability will lead to non-rain producing
fair weather cumulus. These can commonly be seen in the summer time. As the layer of instability builds, cumulus clouds can become
bigger and thus more vertically developed. When the layer of instability becomes deep enough and parcels can penetrate into this
layer, then cumulus congestus that develop into thunderstorm clouds can occur. Cumulus clouds have a photogenic appearance given
their towering cauliflower appearance in the sky.
Seriously, it is time for cirrus clouds! Cirrus clouds are the high wispy clouds. They are made of ice crystals and the
interaction with relatively strong winds aloft give cirrus the wispy feathery appearance. They can appear bright white
due to the sun shining on the ice crystals. Typically, cirrus do not produce precipitation that reaches the ground. This
is because cirrus tend to be thin and any precipitation that does fall will vaporize before reaching the ground. Lots of
cirrus can indicate that a storm system is on the way since when a storm system approaches (mid-latitude cyclone) it is
high clouds that are seen first. Even during nice weather patterns it is common to see a few cirrus in the sky. They are
thin enough that often the sun will shine right through. Cirrus can develop when lifting occurs high aloft and the air
is moist enough to develop ice clouds. In some cases the moisture can originate from thunderstorms or lifting that
transports moisture into the upper levels. As this moisture is transported any additional lifting will help develop
new high level cirrus clouds.
For this section we look at the situation in which no clouds occur that is called clear skies. Clouds require just two ingredients which
are lifting and saturated air. If one of these is lacking, then clouds will not develop. The air can be very cold or hot and it can be
very moist or very dry and any combination of these and clouds may not form. When forecasting for no clouds, the primary ingredient
to look for is sinking air throughout the troposphere. Pressure systems such as a subtropical high or polar high can produce
sinking throughout the troposphere. The late morning hours is sometimes a good time to have clear conditions. The early
morning may have fog or low clouds which can mix out to clear skies later in the day. Scattered afternoon clouds are very
common from warming and convective mixing during the day. The late morning is a good time frame between these events. It of
course can be the case that the entire day is cloud free if there is strong sinking in place and the air is fairly dry at
all levels. Getting stuck under a strong ridge can produce day after day of sunny cloud free weather. It can be surprising
difficult in some places to get a completely cloud free day (no morning fog/low stratus, no afternoon fair weather cumulus,
and no cirrus sneaking overhead). A completely cloud free day is a good indication the air is dry at all levels and there
is sinking throughout the troposphere.
COMPOSITE AND UNIQUE CLOUDS
This section looks at combinations of the basic cloud types (stratus, cumulus, cirrus), naming for the elevation the cloud occurs,
precipitation producing clouds and unique clouds. Below are composite clouds and a brief description of each:
Stratocumulus- A low level stratus cloud that has distinct clumps. They look like a combination of small fair weather cumulus
and a low level stratus deck.
Cirrocumulus- Patches within cirrus clouds that give the appearance of the mackerel sky. The patches
are often arranged in a regular pattern.
Cirrostratus- High clouds composed of ice crystal stratus. The sun will often shine through them. Halos and sun dogs can
be produced as the sun shines through cirrostratus.
Clouds can also be named for the elevation they occur. If the cloud is a low level cloud relative near the surface it will be labeled
with the basic stratus or cumulus depending on which type it is. Upper level clouds will be labeled as cirrus or one
of the two upper level combinations of composite clouds (cirrocumulus and cirrostratus). The additional cloud type is
middle level clouds which can have the “alto” prefix:
Altocumulus- Middle level cumulus clouds. They are composed of liquid drops and have patches of clouds arranged in regular patterns.
Altostratus- Middle level stratus clouds. They tend to be too high and thin to produce significant precipitation but they
can overcast the sky and produce light precipitation.
Precipitation producing deep clouds can be stratiform or cumulus. A description of these clouds follows:
Nimbostratus- A precipitation producing stratus cloud. They develop due to forced broad scale lifting of saturated air and can
cover great distances. Most non-thunderstorm precipitation is produced by nimbostratus.
Cumulonimbus- A thunderstorm cloud that encompasses the lower, middle and upper levels of the troposphere. They form by a deep
layer of rising positively buoyant air in the troposphere.
There are many unique clouds. Below are several unique clouds that can be seen:
Fractus- Fractus are also known as scud and cloud tags. They are isolated low level stratus clouds that can often be seen around
the base of thunderstorms. They often look unorganized and flow with the low level wind.
Funnel cloud- The cloud produced from condensation from a rotating column of air. The funnel cloud is the visible portion of
a tornado produced from clouds. A funnel cloud is not a tornado if the circulation is not in contact with the ground.
Lenticular- Lenticular clouds are clouds that form above mountains that have a lens and flying saucer shape. They form as
air is lifted to saturation over the top of mountains.
Mammatus- Mammatus are pouched shaped clouds that protrude downward from the thunderstorm's anvil. They form as negatively
buoyant moisture laden air sinks. The cloud remains visible until the air sinks enough that the relative humidity falls below
100%. The portion that has a relative humidity of 100% remains visible. Mammatus tend to be most prominent in extremely
severe storms but can occur when storms are not severe also.
Shelf cloud- A low ominous cloud reaching across the sky in associated with thunderstorm outflow.
Wall cloud- A lowered cloud base within the updraft region of a supercell thunderstorm. The rapid lifting of humid air causes
the cloud base to form at a lower elevation. Wall clouds will often rotate cyclonically.