Spring and summer are just around the corner. These are times of the year when people are outside more and engaging in activities that can be quite sensitive to the weather. It’s a time when we are checking the weather forecast more closely to find out what to expect for our next camping trip or day at the beach.
Unexpected summer storms can be more than an inconvenience, they can be a downright hazard; so an ability to anticipate this kind of weather before it pops up out of nowhere or blows in from a neighbouring region can reduce inconvenience and danger.
The official forecasts we get through the mass media can provide some good guidance for planning weather sensitive activities, but they are mostly regional in scope and not tailored for specific locations at specific times. Also, as we all know, the official forecast, even when general in nature, sometimes goes bust.
Many of us have come up with ways to try and anticipate unexpected weather changes by watching the sky ourselves, but not everyone has what I would call ‘insider knowledge’ about how to read the sky and evaluate the short term accuracy of the official forecast that covers the time for a particular planned activity. So I’m now going to try and share some of the signs in the sky that can warn the forecast may be about to go bust and send everybody running for cover.
How Cloud Types Are Identified:
The best way to tell if any unexpected storms are on the way is to start watching the sky around late morning and see what types of clouds are present. Most dry weather clouds come in four basic forms. Cumulus heaps form mostly at relatively low altitudes above ground but can grow upwards so the cloud tops may be at middle or high altitudes. Stratus sheets and stratocumulus rolls or ripples can form at low, middle, or high altitudes. Middle altitude stratus is called altostratus and high cloud of this type has the name cirrostratus. Clouds in the form of rolls and ripples (sometimes informally called ‘mackerel’ clouds) are officially named altocumulus at middle altitudes and cirrocumulus at high levels. Cirrus wisps (also called ‘mare’s tails’) are only seen at relatively high altitudes above ground.
(Since two of the four basic forms are divided into additional types according to altitude, the actual number of dry weather cloud types increases to eight. When the two main wet weather clouds identified below are included, the final total of major types adds up to 10. It is usual to see only two or three cloud types at a time when an organized weather system is approaching, but when the wet weather clouds break up after the disturbance passes, it’s not too unusual to see six to eight types simultaneously at least for a short period of time. On much more rare occasions, it is possible to see nine or even all 10 major types. I have officially recorded and reported the 10 types all at once on just two occasions in my entire life: in March of 1994 and October 2016, both times at the weather station I operate for Environment Canada in my backyard.
Anticipating Rainy Days:
Most of these cloud types can indicate a change from sunny to wet weather if they are increasingly covering the sky as the day progresses. This is especially likely if they occur in a sequence of high altitude followed by mid level then low clouds, at which time precipitation may be imminent. If the official forecast is calling for wet weather and this sequence of high to low cloud types is seen, then the wet weather is almost a certainty as stratus sheet clouds at various altitude levels merge into a single very thick and extensive layer of nimbostratus cloud that brings the rain in summer and rain or snow in winter. This scenario is typical of large scale but stable weather systems that will give you a day or two of wet weather but rarely anything severe during the spring and summer seasons.
However, as Vancouver Islanders know very well, large scale disturbances can create dangerous winds and heavy rain during the cooler months of the year, but the characteristic forerunner clouds that appear ahead of the nimbostratus at this time of year usually give plenty of warning.
Anticipating Local Severe Weather:
It is much more difficult to pinpoint when and where severe weather will strike because the disturbances that cause dangerous conditions tend to be relatively small scale and local in extent. The sky can look quite clear and non-threatening until just before a storm strikes. When scanning the sky from late morning on, it’s best to keep an eye out for any cumulus heap clouds, even if there’s only one or two to begin with and even if they look fairly small at first.
The first sign of possible trouble is the start of upward vertical growth even if it’s only a single cloud on a sunny day. The faster and taller the cloud grows, the greater the risk of severe weather in the form of lightning, heavy showers, wind squalls, and even tornadoes before the end of the day. If additional towering cumulus clouds form, the risk of violent weather goes up even higher, especially if the clouds form into a distinct line and grow into giant cumulonimbus thunder clouds with their characteristic tops made of dense cirrus clouds in the shape of anvils. Fortunately, severe weather is rare on Vancouver Island during the spring and summer seasons. The surrounding ocean and other bodies of water tend to inhibit the development of these conditions.
Unfortunately, I’m unable to provide photographs of these various cloud types in the limited space available for this column. However, those who want to explore this subject further can use a good computer search engine to look up the World Meteorological Organization (WMO) ‘International Cloud Atlas Online’, or the Wikipedia articles ‘Cloud’ and ‘List of Cloud Types’, all of which have good quality photographs.
I can vouch for the two Wikipedia articles because I have been the main contributor to both for the past few years. The International Cloud Atlas has the most and best pictures and goes into the most detail about how clouds are identified, but it requires more navigating than Wikipedia, so take your pick or check out both. With some practice, most people who are interested can become good cloud spotters, whether just for the fun of it like bird watching, or for the purpose of local short term weather forecasting or evaluation of the official forecast at any given time.
Chris Carss is a Chemainus resident and a long-time weather observer/recorder.