Lawson LDDC (City variant, UK)
Back to Pedestrian Wind Comfort and Safety
Lawson LDDC (City variant, UK) distinguishes frequent sitting from occasional sitting. ArchiWind uses the City Lawson variant documented below; the LDDC name also appears on other variants with different thresholds.
Pedestrian Wind Comfort
| Category | Threshold wind speed | Exceedance probability | Activity | Color |
|---|---|---|---|---|
| A | 2.5 m/s | ≤ 5% | Frequent Sitting | |
| B | 4 m/s | ≤ 5% | Occasional Sitting | |
| C | 6 m/s | ≤ 5% | Standing | |
| D | 8 m/s | ≤ 5% | Walking | |
| E | 8 m/s | > 5% | Uncomfortable |
Exceedance probability is the percentage of time the wind speed exceeds the threshold wind speed, after combining all wind directions. Read the table from the top: each location takes the first category whose condition it meets. Comfort is classified on the mean wind speed; the gust-equivalent mean (GEM) is used for safety only.
Pedestrian Wind Safety
This variant checks how often wind exceeds 15 m/s. Mean wind and gust-equivalent mean (GEM) are assessed separately, and the worse safety class is used.
| Category | Threshold wind speed (mean / GEM) | Exceedance probability | Meaning | Color |
|---|---|---|---|---|
| A | 15 m/s | ≤ 0.022% | Safe | |
| B | 15 m/s | > 0.022% | Unsafe |
This scheme has two safety classes, without Lawson 2001’s separate Dangerous class at 20 m/s. For an annual result, 0.022% is approximately two hours per year.
Safety Wind Speed: Mean and Gust-Equivalent Mean
Safety uses the same mean-wind and gust-equivalent mean calculation as Lawson 2001; only the thresholds differ, as shown in the tables above. Comfort is classified on the mean wind speed. Note that the City of London guidelines classify comfort on both mean and GEM; ArchiWind currently uses the mean wind speed for comfort.
Sources
City of London, Wind Microclimate Guidelines (2019): technical requirements, comfort and safety tables, and Annex A.