Sunday, September 5, 2010

Brief discussion of some weather terms used readily in my blog

500mb/upper levels— If someone asked me to describe “weather” in one word, I would say that it is a wave. In my opinion, weather is, in its simplest form, a series of atmospheric waves that encircle the Earth and create weather patterns. These connected waves are not visible at the surface because of friction, but in the mid and upper levels you can follow the wave pattern. The wave pattern that controls our weather is most detectable at 500mb. You may have noticed that in my discussions I like to post the 500mb chart, which is a graphical depiction of the upper levels of the atmosphere. This is arguably the most important layer of the entire atmosphere for determining what kind of weather will be experienced at a particular location. The coloring (yellows, browns, and reds) on the 500mb chart shows the absolute vorticity. Vorticity is basically energy, which can be used in the atmosphere to lift air. 500mb is significant because it is in an ideal position to account for the weather pattern at the highest levels of the atmosphere but low enough to show some of the key features of the mid levels.

Although what is going on at 500mb is vital to the surface weather, the atmosphere doesn’t get mixed up to that high a level. This means that even though the weather at 500mb indirectly determines the temperature pattern here at the surface, you would never be able to say that because the temperature at 500mb is x, then the high temperature at the surface will be y as a result. On a clear breezy summer afternoon, the atmosphere will often get mixed up to 850mb. When this occurs, then it can be said that the temperature at the surface is directly correlated to the temperature at 850mb. This leads me to my next term:

850mb/mid level temperature—In my entries, I very often make reference to the 850mb temperature or mid level temperature. This is because 850mb is the most important layer for determining the daytime high temperature here at the surface whenever the atmosphere is sufficiently mixed. In my opinion, even when not considering the actual high temperature, I think the 850mb temperatures are very useful just to get an idea of what kind of an air mass is in place and even the rate at which temperatures will heat up/cool down. In the summer, the general rule of thumb for calculating the daytime high off of the 850mb temps is to add about 25-30 degrees or slightly more when the atmosphere is mixed up to that level. A less mixed atmosphere will yield highs closer to 20 degrees higher than the 850s. There are times when the atmosphere only mixes to a very low level. This often occurs when there is extensive cloud cover and/or precipitation combined with light winds. In these instances, the 850mb temperature is just as useless as the 500mb temperature—there is little if any correlation at all. On the other hand, when downsloping and urban effects are taken into account, that number can be even greater than 30.

Downsloping—Whenever the wind is relatively strong and has a sufficient westerly component (i.e. W, WNW, or WSW), the compressional heating of air as it travels down the lee of the Appalachians must be taken into account. This can sometimes cause temperatures to be higher than one might expect based on the 850mb temperature algorithm discussed above.

Computer models—There are four main computer forecast models that I use. There is the European model (ECMWF), the American global model (GFS), the North American model (NAM), and the Canadian model (GGEM). The ECMWF often outperforms the GFS, and the GGEM is known to be quite bullish at times on weather features. In the short range, the NAM is often more similar to the ECMWF.

Teleconnections—I sometimes make reference to teleconnections in my posts. The idea behind a teleconnection is that the weather in one location can be predicted based on what is going on in another location, using the idea of a fixed wave pattern (i.e. if a ridge is in one place, then a trough must be in another, etc.). This is not a foolproof method, but it gives a very general idea of the weather pattern that should be in place. For example, the North Atlantic Oscillation (NAO) is important because it is a determining factor in how progressive/blocked the pattern is across the northeast. There are other teleconnections that are important as well.

Here is a small table of the North American teleconnections broken down into simple form and their important effects on the northeast:

-NAO = Blocked/slower progression of weather systems
+NAO = Progressive pattern/quick movement of weather systems
-PNA = Ridge in the east/trough in the west; warmer
+PNA = Trough in the east/ridge in the west; cooler
-AO = Polar vortex (upper low) farther south; turning colder
+AO = Polar vortex (upper low) farther north; turning warmer

These values are rough and are not always accurate, depending on the specifics. But what I think is interesting about teleconnections is that without seeing any weather maps you can get a very general sense of what the weather pattern is going to be like at a given time just by looking at an oscillating curve on a graph. For example, if you combine a +PNA, –AO, and –NAO together, this will bring a trough in the east, perhaps anchored by a polar vortex relatively far south, and a block to the east of it, which can cause a prolonged period of cold weather in the winter. It is ironic that the teleconnections are also individual waves on a graph, which further gives credit to the idea that weather is, in its simplest form, a “wave.”

This is certainly far from a complete list, but I am planning on continuing to update this page with more explanations as I use them in my blog entries. Please let me know if there is anything that I have not explained or did not discuss fully enough, and I will add it.