October 5, 2010
Filed under Feature Stories
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 camping trip (or worse), so it’s good to know what’s going on in the atmosphere around you.
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 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 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 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. If cumulonimbus clouds are forming, that’s when I start to break camp and head for shelter.
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 camping 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.
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 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.