Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy.
To feel confident and safe at sea, we all want to know what’s likely to happen weather-wise over the next few hours and the days ahead.
To give structure to what is a pretty complex topic, I thought we could look at the order of how to determine a local area forecast, and work our way through understanding each bit over two or three newsletters. Let’s give this a go!
Here are nine steps for how to determine a weather forecast for your local area. We’ll cover #1 this week and #2 (clouds) next week. I’ll also share a great set of online sites and apps to help with overall forecasting.
- Study the synoptic charts to get the general conditions and information about impending cold/warm fronts.
- Verify the forecast. Compare the barometer, wind speed and direction, and clouds. Do they match the forecast?
- Use the general synopsis from the shipping forecast to understand how features will move, and the area forecast to give the average wind for your sea area.
- Use an inshore water forecast to see how local forecasters think the general land mass and local land features will affect the wind (sea breeze, thermal effects etc).
- Refine the forecast for your position within the sea area for the movement of the synoptic systems.
- Continue to watch out for signs of looming meteorological dangers; squalls, thunderstorms, gusts, gails, fog etc.
- Watch clouds for local circulation.
- Check tides to avoid wind against tide,especially around headlands.
- Continually monitor and log the wind, barometer and clouds and compare them with the forecast.
I’m using British terminology here, the namings of some of these forecasts may be a bit different depending on where you are in the world.
Let’s start with Synoptic Charts – but first a bit of context
Weather can be split up into seven variables, all of which are linked and together affect the conditions we sail in. The most obvious conditions for sailors are wind and visibility. These are linked to precipitation, clouds, temperature and humidity, which in turn are linked to atmospheric pressure and the general synoptic situation. Even though we’re usually only interested in a small geographic area, because everything is interconnected we do need to be aware of what’s happening at each ‘layer’.
These ‘layers’ are called weather scales, there are four of them…
Global: Elements spanning thousands of kilometres: trade winds, jet streams, the tropics, mid-latitudes, polar regions and ozone layer.
Synoptic: Large-scale systems of high and low pressure.
Mesoscale: Between high/low pressure systems and individual clouds. It’s roughly the size of one sea area in a shipping forecast.
Microscale/local/boundary: The small area where coastal features are significant (e.g. hills) and sea breezes are likely. This is where we sail.
The Synoptic Chart and Weather Fronts
Where to find synoptic charts?
These charts can be found on your local governmental websites. Here are some links for Canada, USA, Germany, Australia and the UK.
How do you read a synoptic chart?
- This video from the BBC gives a nice, simple explanation.
- If you’re after something more detailed to read, this document contains everything you need to know (and more!).
What are weather fronts?