| Estimation of p |
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| There is considerable uncertainty in estimating p and this is probably the greatest source of error. We recommend the following data for rough terrain or urban areas. This data is based on work done by Irwin in the late 1970s. |
| STABILITY | EXPONENT p |
| Very Unstable | 0.15 |
| Mod. Unstable | 0.15 |
| Slightly Unstable | 0.20 |
| Neutral | 0.25 |
| Slightly Stable | 0.40 |
| Stable | 0.60 |
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For smooth, rural terrain these values are multiplied by 0.6. We normally refer to stability classes according to the convention below which is based on classic work by Pasquill in the early 1960s but modified by Turner. |
| STABILITY CLASS | DESCRIPTION |
| A | Very Unstable |
| B | Mod. Unstable |
| C | Slightly Unstable |
| D | Neutral |
| E | Slightly Stable |
| F | Stable |
| To estimate a ventilation coefficient we need the average wind speed for the whole mixing depth. This can be found by integration but given the uncertainty in p it is probably not worthwhile. An estimate based on the wind speed at half mixing height is probably as accurate. |
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AN IMPORTANT THING TO REMEMBER: UNDER AN OVERCAST SKY THE STABILITY CLASS IS CLASS D
REGARDLESS OF ALL WIND SPEED AND DAY/NIGHT. |
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We will need to estimate a stability class for this purpose and also for later calculations. Stability class depends on lapse rates, which in turn depend on weather conditions. For other conditions the following tables can be used: 1) DAYTIME CONDITIONS |
| SURFACE WIND SPEED IN m/s | STRONG SUNSHINE | MODERATE SUNSHINE | SLIGHTLY SUNNY |
| <2 | A | A-B | B |
| 2-3 | A-B | B | C |
| 3-5 | B | B-C | C |
| 5-6 | C | C-D | D |
| >6 | C | D | D |
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Note that slightly sunny corresponds to a typical New England autumn afternoon. 2) NIGHTIME CONDITIONS |
| SURFACE WIND SPEED IN m/s | CLOUDY-EQUAL OR MORE THAN 50% COVER | CLEAR SKIES |
| <2 | E | F |
| 2-3 | E | F |
| 3-5 | D | E |
| 5-6 | D | D |
| >6 | D | D |