Letter to the editor (Karen Larson)
Basics - Correction re: Continuity, Persistence, & Trends
I’ve just reviewed my article on ‘Reading Weather Maps’ (GOB Issue 45 - Nov/Dec ‘05), and note that I misused & failed to explain the term the “principle of continuity”* (following ‘Isobars’ page 36), wherein I said:
“... If previous surface charts are available for the last day or two, you will be able to predict the movement of weather systems over time, based upon the principle of continuity ...”
One of the most important techniques for making a forecast
is to use weather maps to estimate the speed of movement of air masses, fronts, and high and low pressure systems. There are a variety of simple forecasting techniques, the easiest ones being “Persistence” and “Trend”.
suggests that tomorrow’s weather will be same as today’s weather
"What you see, is what you get."
Persistence forecasts are generally good only for short periods of a few hours, and become less accurate as the time period lengthens.
In the tropics, especially near islands, where day after day the weather is basically the same, (because the location is affected by the same air mass, with no passages of fronts), a persistence forecast
that tomorrow is going to be the same as today ,is usually fairly accurate.
describes a phenomenon in a steady state, or one that’s moving at a constant or predictable speed. So, if the distance traversed by front is known, over a given time period, it's position can be extrapolated in time, by the formula:
Rate x Time = Distance
The Trend forecast is based on the assumption that changes will continue at the same rate they have been occurring. Thus, if a cold front, located 450 - 500 nm to your West, is approaching your location at 20 kts, then it will continue to move at 20kts in the same direction - so, you might extrapolate it’s arrival in about 24 hours.
Similarly, if a cold air mass is moving toward the station and temperatures at stations within the air mass are dropping at one Fahrenheit degree per hour, then temperatures at your location may drop at one Fahrenheit degree per hour.
Warm fronts usually move at an average speed of about 10 knots (or 240 nm/day).
Cold fronts usually move at an average speed of between 15 - 20 knots (or 360-480 nm/day).
Analyzing previous weather maps, you can adjust the averages to more
closely reflect current
(& likely future) frontal speeds - and determine a likely time of arrival at your location.
Because weather patterns (in the Northern Mid-Latitudes**) generally move from West to East, storm systems generally approach from the west. Tomorrow’s weather is possibly between 200 - 500 nm West of you right now.
** Winds in the tropics & semi-tropics generally blow from East to West.
Stationary fronts will either dissipate after several days, or change into a cold or warm front.
* The “principle of continuity” (law of the mass continuity - empty spaces are not tolerated in fluids) is actually a restatement of the principle of Conservation of Mass, as applied to fluid dynamics in the atmosphere.
Remember to adjust your forecast for differences in latitude, possible acceleration/deceleration, or intensification/deintensification of storm systems, and local effects such as topography, bodies of water
, and the land-mass heat-sink effect.
Northern sailors - look to the West for tomorrow's weather, and finally , don’t forget to look outside.
I hope this helps clear up any confusion I may have inadvertently created by my misused terminology, and failure to more fully explain my intent.