I recently completed a Mountain Training Association Weather Workshop run by the Meteorological Office and I wanted to write about something that was completely new to me: Thickness Lines and in particular, the Snow Line.
Thickness lines can be found on Surface Pressure Charts (aka synoptic charts). These are the charts that have isobars on them. Isobars are lines that join up points of equal pressure at sea level. Also displayed on these charts are weather fronts. These will usually lead to precipitation of some kind.
The atmosphere is three dimensional but the charts are two dimensional. Thickness lines try to address this. They appear as dashed lines. They don't appear on the current chart (https://www.metoffice.gov.uk/weather/maps-and-charts/surface-pressure) but if you look at the chart for 36 hours into the future you will see them. Go to the current chart and click on the timeline below it three times to show the forecast for the weather in 36 hours. Here you can see the thickness lines represented by dashed lines.
At the time of writing this is what the chart looks like:
If you look closely at two of the thickness lines (dashed red lines) you can see that they have numbers on them. In the picture above, there is the number 546 over the Bay of Biscay (just to the North of Spain). There is also the number 528 over Austria. (North of that is another line without a number on. This will be the 510 line. In the South East corner there is another thickness line and this will be the 564 line.)
As you gain altitude, the pressure decreases because there is less atmosphere above you. If you started at sea level and kept gaining height, you would eventually reach a point where the pressure was half that at sea level.
Let's take the 546 line as an example: everywhere along that line, if you went to a height of 5460m, the pressure would be half of what it was at sea level. Everywhere along the 528 line, the pressure at 5280m is half that at sea level.
So if you only count the bit of atmosphere between 'sea level' and 'half the sea level pressure' these lines represent the 'thickness' of the atmosphere. They join up points of equal thickness just as isobars join up points of equal pressure.
These different regions of warmer or colder air will be blown about by the wind. When isobars are closely packed together it indicates stronger winds so this can lead to rapid changes in temperature. The winds tend to go in the direction of the isobars anticlockwise around areas of low pressure and clockwise around areas of high pressure. (In the northern hemisphere).
If the thickness line crosses closely packed isobars there will be rapid change. If it crosses more widely spaced isobars there will be a slow change. If it runs parallel to the isobars there will be no change as the location of the thickness line will hardly move. This is similar to the way weather fronts move. The movement of the thickness lines helps us understand whether the atmosphere is getting warmer or colder.
The 528 line is also known as the Snow Line. If you are on the warmer side of the Snow Line any precipitation is most likely going to fall as rain. If you are on the colder side, it will most likely fall as snow.
You might argue that a greater than 50% chance of snow isn't much of a forecast but this applies at sea level. If you are higher up in the hills then the likelihood is much greater.
At the time of writing (Mon 15th Jan 2024) look at where the Snow Line (528) will be by midday on Thursday - crossing the middle of France and moving slowly south!
