This text is part of the Weather Preparedness & Resilience Toolbox developed by the YOUROPE Event Safety (YES) Group within YOUROPE’s 3F project (Future-Fit Festivals). It is aimed at everyone involved in planning, building, and operating open-air events. It helps festivals and other outdoor events become truly weather-ready by offering both practical and research-based resources as well as background information on weather and climate. Learn how to design safer and more weather-resilient outdoor events.
Doing the maths
Weather hazards and crowd dynamics are domains where assumptions fail quickly and consequences escalate fast. Intuitive judgments such as “people will move faster,” “the storm is still far away,” or “the exits should be sufficient” are unreliable once conditions deteriorate due to rain, mud, heat, wind, darkness, or stress. In these situations, time, capacity, and movement are not abstract concepts but measurable variables that directly determine whether a protective action succeeds or fails.
Doing the math transforms weather risk management from opinion-based discussion into defensible, operational decision-making. Calculations make visible how long alerting actually takes, how quickly people can realistically move under degraded conditions, and whether shelter or evacuation capacities are sufficient for the crowd present at that specific moment and site. They also reveal hidden bottlenecks and margins that are otherwise overlooked until it is too late.
Most importantly, calculations create decision certainty under pressure. When storm arrival times, lightning proximity, heat exposure, or ground conditions change rapidly, pre-calculated scenarios allow event leaders to act decisively rather than negotiate assumptions.
In weather and crowd management, calculation is not bureaucracy – it is a core safety control that links forecast information to real-world human movement and ensures that protective actions are timely, proportionate, and achievable.
Things you always should calculate
- Flow rates under normal conditions (-> how long will it take to get to the carpark?)
- Flow rates under difficult/degraded ground conditions (e.g. mud)
- Procedure times, e.g.
- Ingress procedures
- Show-stop procedures
- Evacuation procedures (incl. alarm & activation)
- Infrastructure closure procedures
Evacuation Time Calculation: Dry Ground vs. Muddy Ground
Scenario
- Open-air festival, rural site
- Audience present: 18,000 persons
- Trigger: Severe thunderstorm with expected wind gusts and lightning
- Strategy: Full evacuation to off-site areas
Step 1: Alert and Activation Time
Includes:
- Internal decision and confirmation
- Mobilisation of stewards, traffic management, and medical teams
- PA / screen / app notification
| Component Alert & Activation | Time |
| Decision & confirmation | 5 min |
| Staff activation | 3 min |
| Audience notification (redundant channels) | 5 min |
| Alert & Activation Time | 13 min |
Step 2: Movement Time (Dry Conditions – Baseline)
Assumptions (validated during planning):
- Effective exit width: 60 m total
- Flow rate (dry, stable ground): 82 persons/min/m
- Total flow capacity: 60 m × 82 = 4,920 persons/min
- Movement time: 18,000 ÷ 4,920 ≈ 3.7 min
➡ Rounded conservatively: 5 min
Step 3: Movement Time (muddy conditions realistic)
Observed and literature-supported reduction:
- Muddy ground: 30–50% flow reduction
- Applied factor: ./. 40%
Adjusted flow rate: 82 × 0.6 = 49 persons/min/m
Adjusted capacity: 60 m × 49 = 2,940 persons/min
Movement time: 18,000 ÷ 2,940 ≈ 6.1 min
➡ Rounded conservatively: 8 min
Step 4: Apply Safety Factor (25%)
Total time before safety factor: 13 min (activation & alert) + 8 min (movement) = 21 min
Safety margin: 25% of 21 min ≈ 5,3 min
Total Required Evacuation Time ≈ 26 minutes
Operational Implication: If lightning ETA is < 25 minutes, evacuation is not viable → sheltering strategy must be activated instead.
Sheltering Time Calculation with Capacity Constraint Scenario
- Open-air festival, rural site
- Audience present: 18,000 persons
- Trigger: Severe thunderstorm with expected wind gusts and lightning
- Strategy: Shelter-in-place using tents and solid structures
Shelter Capacity Check (Critical!)
- Total certified shelter capacity: 12,000 persons
- Audience present: 18,000 persons
➡ Sheltering not possible for all
➡ Requires hybrid strategy:
- 12,000 shelter
- 6,000 directed off-site
Step 1: Alert and Activation Time
| Component | Time |
| Decision & confirmation | 5 min |
| Staff activation & positioning | 5min |
| Audience information & instruction | 7 min |
| Alert & Activation Time | 17 min |
Step 2: Movement Time to Shelters (muddy ground)
Assumptions:
- Average walking distance to shelter: 180 m
- Speed on muddy ground: 0.6 m/s (≈ 2.2 km/h)
Movement time: 180 ÷ 0.6 ≈ 5 min
Congestion factor at entrances: +3 min
➡ Total shelter movement time: 8 min
Step 3: Apply Safety Factor (25%)
Base time: 17 min + 8 min = 25 min
Safety factor: 25% ≈ 6,2 min
Total Required Sheltering Time ≈ 31 minutes
Operational Implication
Decision matrix must combine:
- Weather ETA
- Ground condition
- Shelter availability
- Ability to split audience safely (not every audience will follow your instructions properly!)
- RISK of overcrowding in shelter?
Egress After Show Stop: Muddy Ground Effect
Scenario
- Show Stop already executed
- Audience leaves in a controlled egress phase
- Rain ongoing, ground heavily degraded
Assumptions
- Audience remaining: 22,000 persons
- Exit width: 70 m
- Flow rate reduced by 50% due to mud, fatigue, poor visibility
Adjusted flow: 82 × 0.5 = 41 persons/min/m
Capacity: 70 × 41 = 2,870 persons/min
Egress time: 22,000 ÷ 2,870 ≈ 7.7 min
Realistic adjustment (groups, stops, hesitation): × 1.8
➡ ≈ 14 minutes
Safety Factor: +25% ≈ 4 min
Total Controlled Egress Time ≈ 18 minutes
Operational Implication
Post-show egress in bad ground conditions may take 2–3× longer than planned.
This directly affects:
- Medical staffing endurance
- Traffic management handover
- Timing of reopening restricted areas