Blocking Patterns and Highs Intensity

[Cyclenall]

I have three questions related to the topic title and they are:

1. What does "dm ***" mean when talking about "heights" when referring to a High Pressure that is present over an area? I also don't know why they call it heights but I think it is in relation to isobars of the High.

2. What is the longest time a blocking pattern was present with a High Pressure area in place over eastern North America before breaking down? Is it possible for it to exceed three weeks? The type of weather would be prolonged Heat-Waves so the longest Heat-Waves is the best place to start looking.

3. Is there a way to measure how strong a cold front is aside from the intensity of the low associated with it? I don't know if its as simple as seeing how cold the temperatures are behind it or what.

[senorpepr]

For your first question:

"dm" is a common, but incorrect usage of the SI abbreviation for dekameter. dm is really for decimeter (1/10 of a meter), but in this case dm (should be dam) refers to dekameter (10s of meters).

For the surface, pressure is contoured using isobars to denote lines of equal pressure. However, when analyzing above the surface, we tend to measure at a standard pressure level. For instance, 500 hPa (or mb). There really is no "high pressure" or "low pressure" when analyzing on a standard pressure level chart since the entire map is the same pressure.

Below is a 500 hPa chart. The pressure at every location on this chart is exactly 500 hPa. The contours are not isobars, but rather isohypses--lines of equal height. The 576 line running through northern Florida is the 576 dam isohypse, indicating that the height of 500 hPa along this line is 576 dam... or 5760 meters.

The lows and highs are using the same concept. They're showing areas of low and high heights. The low off the coast of Oregon? The 500 hPa height is recorded at 523 dam there. Same goes for the station plots. Omaha, NE is reporting 569 dam for their 500 hPa height.



No idea on question two.

For question three... as with many stats, it depends on what you're trying to demonstrate. In my opinion, you're not demonstrating the strength of a cold front by the intensity of the low. That's showing how strong the low is. I've seen weak cold fronts attached to deep lows and vice versa. I would say temperature gradient would be a better measure. What is the change in temperature over a change in distance? Look at your station plots and see what the change in temperature is over... say... 100 mi. Another tool to use is a cross section using Skew-Ts to measure the steepness of a cold front. It really depends on what you're trying to measure.

[Cyclenall]

Excellent answer(s), question 1 was well done and gave me a better understanding of what people are talking about when referring to 500 mb maps and such. There are still fundamental questions that I have about it and they are:

- Now that I know "heights" actually really, literally means heights and is not some meteorological jargon that I thought it was, I don't know what that really tells about the weather. Since its different from surface in the way its dealt with, everything is foreign. Maybe I'm not right about it being totally different from the surface but so far it seems like it.

- Lets use the "death ridge" that was over North America last July during a historic heat-wave. Why is it that it is common to see mets and others use the 500 mb level to depict the extent of the heat-wave, over the surface maps for the strength of the high? What does the 500 mb level show that the surface does not?

I don't expect very many to know the answer to question 2. Heat-Wave junkies and climatologists maybe .

For question 3, I agree with the intensity of the low not indicating how strong the cold front is but I think the answer is temperature gradient. That is just an extension of what I thought only with some simple math involved.

cross section using Skew-Ts to measure the steepness of a cold front.

I have no idea what this is.

[RL3AO]

- Lets use the "death ridge" that was over North America last July during a historic heat-wave. Why is it that it is common to see mets and others use the 500 mb level to depict the extent of the heat-wave, over the surface maps for the strength of the high? What does the 500 mb level show that the surface does not?

*Non-pro met answer*

It has something to do with a concept called thickness which is basically the difference of height between two levels (eg 1000mb-500mb thickness level). The 500mb height relates very well to temp. The higher 500mb level the warmer the temp. The lower the 500mb level the cooler the temp. The 540 line is a quick way to estimate the separation of rain and snow line. Last summer there was a 594 dam ridge which even approached 600 dam at times. The higher the height of the 500mb line the hotter it usually is.

Hopefully that made sense. If not maybe senor can expand.

[Cyclenall]

*Non-pro met answer*

It has something to do with a concept called thickness which is basically the difference of height between two levels (eg 1000mb-500mb thickness level). The 500mb height relates very well to temp. The higher 500mb level the warmer the temp. The lower the 500mb level the cooler the temp. The 540 line is a quick way to estimate the separation of rain and snow line. Last summer there was a 594 dam ridge which even approached 600 dam at times. The higher the height of the 500mb line the hotter it usually is.

Hopefully that made sense. If not maybe senor can expand.

Not well, I thought the 850 mb level related best with surface temperatures since its the closest to ground. So the higher the pressure at higher levels of the atmosphere dictates how warm it can get...I don't know why that would be. My guess would be naturally at higher levels it gets warmer and warmer (up to a certain hight though before cooling again). When the pressure rises in that sector height, it moves it whatever direction better and increases the temperature. Is this close to right?

[Ptarmigan]

*Non-pro met answer*

It has something to do with a concept called thickness which is basically the difference of height between two levels (eg 1000mb-500mb thickness level). The 500mb height relates very well to temp. The higher 500mb level the warmer the temp. The lower the 500mb level the cooler the temp. The 540 line is a quick way to estimate the separation of rain and snow line. Last summer there was a 594 dam ridge which even approached 600 dam at times. The higher the height of the 500mb line the hotter it usually is.

Hopefully that made sense. If not maybe senor can expand.

Not well, I thought the 850 mb level related best with surface temperatures since its the closest to ground. So the higher the pressure at higher levels of the atmosphere dictates how warm it can get...I don't know why that would be. My guess would be naturally at higher levels it gets warmer and warmer (up to a certain hight though before cooling again). When the pressure rises in that sector height, it moves it whatever direction better and increases the temperature. Is this close to right?

What causes high pressure?
http://www.weatherquestions.com/What_ca ... essure.htm

Maybe this could help explain.