Reading the weather is not as mysterious as you might suppose. At its
most basic, what you’re looking for are changes in cloud
patterns and density, and shifts in the direction of cloud
movement. To make the whole process more reliable, if you have
simple instruments, you’ll be able to detect important clues
about what the weather is going to do, even if there are no
clouds (at the moment) to observe, and there are a variety of
pocket “weather stations” available that you can take camping
with you.
Most of the time, the trends are gradual and bear watching
over a period of several hours before you can draw any
conclusions. But there are times when sudden violent outbursts
occur quickly — a wall cloud appears, and out of it drops a
funnel cloud that becomes a tornado. Or a cumulus cloud goes
into hyperdrive and grows into a cumulonimbus monster with an
anvil-flat top and black ugly base that suddenly disgorges,
creating a flash flood that sweeps across miles of terrain,
destroying everything in its path. These are events that can
seriously disrupt your sailing, kayaking, camping trip or
anything outdoors, so it’s good to know what’s going on in the
atmosphere around you.
Cloud Spotting
Clouds are the most obvious clue to the type of weather that
is coming. Although there are many types of clouds, the most
significant we’ll discuss are cirrus, stratus/nimbostratus,
cumulus and cumulonimbus. Watching the progression of cloud
evolution can tell the story of what’s coming.
Cirrus
Cirrus
clouds form so high in the atmosphere that they are made of ice
crystals instead of water vapor. These wispy clouds (sometimes
called Mare’s Tails because of their shape) don’t cause rain,
but they can foretell the coming of a warm front that brings
precipitation. A rainbow ring around the sun is formed by these
ice crystals and is a forecasting clue. If stratus follows
cirrus, and if that stratus evolves into a thicker and darker
layer, expect rain. How quickly the rain comes depends on the
speed that the front is moving.
Stratus
Stratus clouds form shapeless solid layers of overcast low in
the sky, making for a gray, dreary day. If there is a lot of
light penetrating the stratus layer, it probably isn’t dense
enough to produce much precipitation. It normally takes a cloud
thickness of 4,000 feet or more to produce steady rain. But if
the clouds become dark and low, expect showers or drizzle.
Stratus clouds don’t usually result in sudden and violent
downpours the way cumulus clouds do, but the rain can continue
steadily for hours or even a couple of days, so there is still a
danger of flooding. Stratus clouds can present the potential for
hidden danger, because you can’t see what is happening above
them. It is possible that a giant cumulus formation is above the
stratus layer, so be alert to the possibility of violent
weather.
Cumulus
Cumulus clouds are the puffy ones. They are the least stable
type of clouds and are often associated with cold fronts or air
rising over mountains. The puffiness indicates that there is
some degree of upward movement (a rising air mass), causing air
to climb to a colder altitude where the water vapor in the air
condenses and “grows” the cloud at the top. A bunch of little
cumulus clouds scattered in the sky like so many sheep on a
pasture don’t pose a threat, but bigger cumulus clouds bear
watching.
Cumulonimbus clouds are like cumulus clouds on steroids. They
form when cumulus clouds bunch together into a huge mass, or
grow into towering monsters with an anvil-shaped flat top.
That’s when a thunderstorm (or worse) is possible. These giants
can spawn sudden downpours, lightning and thunder, violent wind,
flash floods, hail, microbursts, and tornadoes. This is
especially true when warm/moist air collides with cooler/drier
air along a frontal boundary.
Weather Fronts
When
a warm air mass follows cold air, it is called an approaching
warm front; a cold front is arranged just the opposite. It is
along frontal boundaries that most of the exciting weather
develops as warm and cold air mixes, often creating violent
storms with high winds and precipitation. If you listen to a
weather report on the radio or TV, pay close attention to what
they’re saying about frontal boundaries. If your sailing trip
lies in the path of one of these systems, you can count on the
weather being unsettled.
Warm air has the ability to hold more moisture than cold air.
When warm air that holds moisture cools, the moisture in it
condenses and forms clouds — this happens as warm air rises into
a colder upper atmosphere. It also happens when warm air moves
against colder air at or near ground level, resulting in fog.
It is not only the formation of clouds that is important to your
ability to predict the weather, but the sequence in which the
clouds approach. The sequence tells whether a cold front or a
warm front is coming, and that is important for two reasons: One
is because they approach at different speeds and the other is
because their arrival brings storms of different intensity.
Cold Fronts
When a cold front approaches, the cold air, being heavier than
the warm air ahead of it, forces its way below the warm air
mass. The result is an abrupt transition from clear skies to
mountainous thunderheads and heavy rain, as the warm, moist air
is forced into the cold upper atmosphere. Cold-front storms
approach rapidly and violently, but pass more quickly than do
warm fronts.
The sequence that indicates an advancing cold front begins with
thunderheads, lightning, thunder and violent squalls. As the
worst of it passes, the sky may lighten, the air may be colder,
and the storm can often give way to lighter rain and eventually
high cirrus clouds and finally clear skies.
Warm Fronts
Warm fronts bring warm, moist air that rides gradually up over
the top of the cooler air ahead. Because of this gradual
approach, storms associated with the actual frontal boundary may
be several hundred miles behind the wispy cirrus clouds, which
are the first indication that the front is approaching.
The cloud sequence that indicates an approaching warm front
often begins with very high cirrus clouds then proceeds to lower
and lower cloud decks that become increasingly darker, finally
bringing rain from low nimbostratus clouds (“nimbo” refers to
rain, so a nimbostratus is a stratus cloud that produces rain).
Watching clouds in the sky is not just a game for children; it
can be an important way to keep an eye on the weather and help
you know when to stay and when to go. Keep an eye to the sky,
is good advice.
