P&K MET

Determination of Dispersion Categories

Extract of VDI 3782 Part 1 Appendix 1. Changes made are marked in RED.
Necessary modifications, to apply this to a different location, are possible.

Dispersion Categories are to be determined by the hour with the consideration of wind velocity, cloud cover, and type of cloud as well as indication of the month and the time of day according to the scheme given in Table A1:

Table A1. Scheme for the determination of dispersion categories [22]

Wind velocity at a height of 10 m in m/s Total cloud coverage in oktas *)
for night-time hours **) for day-time hours **)
0/8 to 6/87/8 to 8/80/8 to 2/83/8 to 5/86/8 to 8/8
1 and lessIIIIVIVIV
1,5 and 2IIIIVIVIII2
2,5 and 3IIIII1IVIVIII2
3,5 and 4III1III1IVIII2III2
4,5 and moreIII1III1III2III1III1
*)For cases with a total coverage which exclusively consists of high clouds (cirrus), one proceeds from a total coverage which is reduced by 3/8.
**)Sunrise and sunset (local time) are decisive for the division of day and night. The full hour after sunset is also included in the diffusion category for night hours.

The dispersion categories which have been determined by these criteria are to be changed for special dispersion conditions as follows:

  1. If the measurements for the month of June to August and for the hours 10.00 h to 16.00 h local time results in dispersion categories under V, the next higher dispersion category is to be used for a total coverage of not more than 6/8, or a total coverage of not more than 7/8 and wind velocities below 2.5 m/s. Furthermore, apart from this, for the hours from 12.00 h to 15.00 h local time with a coverage of not more than 5/8, the next higher dispersion category - in the case of category IV, the category V - is to be used.
  2. For the months May and September, the next higher dispersion category - in the case of category IV, the category V - is to be used for the hours 11.00 h to 15.00 h local time and a coverage of not more than 6/8.
  3. For every full hour of the time intervals from 1 hour to 3 hours after sunrise (SA + 1) to (SA + 3), and from 2 hours before until 1 hour after sunset (SU - 2 to SU + 1), the dispersion categories according to Table A2 are determined not only by the columns for night-hours (KN), but also by the columns for day-hours (KT). In Table A2, all the possible combinations of the dispersion categories KN and KT are listed, indicating which dispersion category is to be used instead for the dispersion calculation. If, e.g., the sun rises at 6.25 h local time, the value for the hours of 7.25 h to 8.25 h local time is to be used for SA + 1 to SA + 2. For an hourly time sequence with observations by the hour, the determination of the dispersion category for 8.00 local time applies.

Table A2. Dispersion Categories

KNKTSA+1 to SA+2SA+2 to SA+3SU-2 to SU-1SU-1 to SUSU to SU+1
IIVI(II)*)IIIIII(I)**)II(I)**)
IIII2IIIIIII1III1II(I)**)
IIIVIIIII1III1IIII
IIIII2III1III1III1III1II
III1IVIII1III2III2III1III1
III1III2III1III1III2III2III1
III1III1III1III1III1III1III1
 KN dispersion category, at night; SA sunrise; KT dispersion category, at day; SU sunset
*)For the month of March to November and wind velocities of more than 1 m/s the value in brackets are to be used.
**)For the months January, February, and December, wind velocities below 1/ms and a total cloud coverage below 6/8 the value in brackets is to be used.
  1. For the month of December, January, and February, the dispersion category IV is to be replaced by the dispersion category III2.

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