Weather Forecasting

When the weather turns nasty, it can trap you right where you stand, and it can kill you unless you're aware of what's coming and able to take the proper steps to save yourself. So it’s important to keep a weather eye on what’s happening. You might detect significant local atmospheric activity that has escaped the attention of weather forecasters.  

As you watch for signs of bad weather, what you are looking for are cloud patterns and movement. The atmosphere is made up of gigantic air masses that differ from one another in temperature, pressure and humidity. Interaction between these air masses results in changing weather conditions such as cloud formation, precipitation and wind. Clouds are the biggest clue to the type of weather that is coming. The three primary types of clouds that we will discuss here are cumulus, stratus and cirrus. Watching the progression of cloud evolution gives clues about what’s coming. 

Cumulus clouds are the puffy ones. They are the most unstable 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 cumulous clouds scattered in the sky like so many sheep on a pasture don’t pose a threat. But when cumulus clouds bunch together into a huge mass, or grow into towering monsters, a thunderstorm (or worse) is possible. Cumulus giants can spawn sudden downpours, lightning and thunder, violent wind, flashfloods, hail, and tornadoes. This is especially true when warm/moist air collides with cooler/drier air along a frontal boundary.

Stratus clouds form shapeless solid layers of overcast, leaving a gray, dreary sky. If there is a lot of light penetrating the stratus layer, it probably isn’t dense enough to produce much precipitation. You might get the odd shower, but it 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 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, especially if the ground is rocky or already sodden.

But there is the potential for a hidden danger with stratus clouds, because you can’t see what is happening above them, when you’re standing on the ground and looking up. It is possible that a giant cumulus formation is above the stratus layer, so be alert to the possibility of violent weather, even if things look pretty benign from below the cloud deck.

Cirrus clouds form so high in the atmosphere that they are made of ice crystals instead of water vapor. These wispy clouds (sometimes called Mares Tails because of their shape) don’t cause rain, but they can foretell the coming of a warm front that brings precipitation. 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.

Clear blue sky doesn’t necessarily mean everything is hunky-dory. If a high-pressure system moves in and pushes a low-pressure system out of the way, it brings clearing skies — but it might also bring strong and gusty wind as the pressure between the two systems attempts to equalize. Trees can be knocked down and tents blown away under clear, blue sky. 

By tapping into the available weather information and keeping an eye on the sky, you can make better judgments about what kind of weather to expect.