The Weather for Noobies

FI is Fantasy Island - used for discussing charts more than 6 or 7 days away.

ECM is European centre for medium range weather forecasting. A quick google should explain any of the terms for you.

A

I think stickying a thread with this title might be no harm, the weather business is full of abbreviations and acronyms, it must be quite off putting for casual visitors to these parts.

Maybe if we leave this thread run for a while we would build up a useful collection that could be aggregated into one post which could be stickied or added to one of the existing stickies.

Ok, i can do this. As there is many abbreviations/acronyms in weather, there are some used more in Summer than Winter eg: CAPE, LI when forecasting for thunderstorms. So to keep them all together instead of doing just season abbreviations/acronyms, a sticky can be used for these in alphabetical order.

To SC and DE, propose a sticky for this with access only for mods to edit and lock it for viewing only and keep it to just one post or leave it open for discussion so comments and questions can be made on that abbreviation keeping with one post to edit and add on abbreviations/acronyms?

Pangea wrote: »

Thanks, I did try google but no luck, it didnt explain the terms for me .

Wikipedia is a great resource and has loads of links to various sites they mention.

Pangea wrote: »

Ok so whose gona start explaing stuff ?

Start asking and the first person who can, will explain, Snowbie and the other mods will tie it all together when they have time.

great links thanks a lot

Pangea wrote: »

This was posted by the greengiant ,little help please, what is this chart about ,is that -10 , -20 etc. air tempertures?
"
Aye, its looking fairly good for a god cold spell again, lasting for most of next week. Quite possibly some snow especially along southern to north eastern coasts depending on precipitation of course. Here's what the chart looks like for Monday at 850hPa "

[/URL]

Ok, I'll take a stab at this.

This chart shows temperature and pressure. The solid black lines show pressure contours, isobars. The closer the isobars are together then the stronger the wind. The coloured regions represent the temparature and you can see at the bottom the colours that the temperature relates to. But keep in mind this is 1.5km up in the air, not on the surface.

holly1 wrote: »

This may sound really dumb,but could you explain Precipitation.

Water from the sky!

Okay, that map being discussed is a special blend, it shows surface pressure isobars and temperatures at the 850 mb level using the colour code at the bottom of the map.

Let me try to explain these levels. The average pressure near the surface is a little over 1000 mbs (about 1012 mbs). Mbs stands for millibars. Your old-school barometer would read 29.53 inches at 1000 mbs.

Weather maps called "surface" weather maps are actually for the pressures corrected to sea level. Otherwise, they would look more like topographic maps in reverse, because pressure falls with height. If you see a 1030 mb high over the mountains of Scotland, the actual pressure on top of those mountains is likely to be well under 900 mbs.

The standard levels for measuring the atmosphere at "upper levels" are 850, 700, 500, and either 300 or 250 mbs. Now it would be just as good to draw up pressure maps at standard levels, but this is easier -- they send up balloons that signal back to a weather office at what elevation they have broken through 850 mbs, 700 mbs, etc. These are called "heights" by weather folk. A map of these heights is drawn up, and it looks like a pressure map. In the upper levels, the winds tend to blow parallel to the height contours more than at the surface, so when you're looking at a height chart, you can get a pretty good idea of the flow at that level.

These weather balloons also transmit back the temperature and humidity at those levels above the ground, so these get plotted onto the maps to give some idea where warm and cold air masses are meeting at that level, and where there is moisture concentrated. All of that goes into the numerical weather prediction models, updated every 12 hours (with some 6 hour updates for one model).

On the map you're looking at above here, the 850 mb temperatures are plotted on a surface weather map. This gives you a nice idea of how warm the atmosphere is up around cloud base heights of 5,000 ft, which is about where 850 mbs is found. That in turn gives you an idea of what would be falling from those clouds if they were dropping any kind of precip. If it looks to be colder than -5 C up there, this is quite likely starting off as snow, but it could melt on the way down if the surface layers are warm enough. This is why in some cases it snows on top of hills and melts to rain or sleet lower down. If you have a very mild upper temperature over surface cold air, this could signal freezing rain, but this is rare in your climate. If the upper temps are -10 or lower, anything falling would almost certainly be snow, the only exception might be over really warm water that was capable of warming the air over top to somewhere high enough to melt the falling snow.

Hope that gets you oriented a bit. Obviously there are many details and complications, but you'll get used to how the levels interact if you follow the weather on a regular basis, and learn as you go.

Just one other detail to keep in mind -- thickness. You'll hear this term a lot, sometimes without knowing it is thickness being discussed. The 500-mb heights are one thing, but in weather forecasting, the "1000-500 mb thickness" is used more than the 500 mb height as a forecasting tool. What is it? It's the difference in height between 1000 mbs and 500 mbs in the atmosphere. The bigger the difference, the thicker the atmosphere, meaning the more space it has to store heat. You may already have run into the semi-mythical term "528 dam" in regard to snow. Some just leave out the "dam" which just stands for decameters, so you'll read something about sub-528 air coming across Ireland being good for snow, and a discussion of that.

As it happens, snow in the British Isles usually needs a thickness even a bit lower than 528 dam, but that is definitely about where it starts. If you click on this parameter on the GFS charts (1000-500) you'll see these thickness lines in colour codes that just happen to start into the blues at 528 dam. Stronger shades of blue indicate lower thicknesses, the lower the better for snow potential. In the summer, a really warm air mass will have the 564 dam thickness as a sign that it's of tropical origins. But anything much over 552 dam is usually fairly warm, especially at this time of year. Just try the model options for the GFS on any large website that offers them, and see what different parameters you can find, then if some of them don't make sense, ask. Some are pretty easy to follow, like 2m temperature, that's basically the temperature. I don't know if someone explained dew-point already, but you can think of that as the temperature that the air would reach if it suddenly became saturated (this is not quite right, but close enough). So if someone says, I have a temperature here of 4.4 and a dew point of -1.0, then the air outside at their location has enough moisture to fog up an air mass that was actually at -1.0 C. Someone else might be saying, I'm at 4.7 and my dew point is 2.8 C. Then they would have a similar temperature but a lot more moisture in their air. Quite often, when the sun goes down, the temperature will fall to near the dew point, and either a frost or dew will form, or fog will develop, depending on how dry the air above is.

Others may report a wet bulb temperature, this is somewhat similar to the dew point but it is measured on a different basis, and is usually a bit higher than the dewpoint. And relative humidity tells you what percentage of the amount of moisture is actually present for the temperature of the air, compared to what would be there at that temperature if you had a saturated air mass. If the "r.h." was 90% you would know that this air mass was just about saturated, if it was 10% it would be almost free of moisture.

1st and probably my only post in here...

google "let it snow"