Wind check for lifting

Colours show the gust band at your chosen height. Mean wind is the average over the hour, shown for reference. Alerts show when the Met Office forecast gives a 5% or higher chance of lightning (given in 3-hour blocks), a 50% or higher chance of rain, any snow, visibility under 1km, or a frost or ice risk (2°C or colder, or feeling like 0°C or below). Sunrise and sunset are worked out for your site with the standard NOAA method; hours in the dark are shaded in the Time row.

The forecast is the Met Office site-specific forecast for the point nearest your site, from the Met Office Weather DataHub. It is hour by hour for roughly the next two days, then in 3-hour blocks up to 7 days ahead; we show each 3-hour value across its three hours. We fetch a fresh copy at most every 30 minutes, and the time the Met Office issued it is shown under the place name. All times are UK time.

The Met Office forecasts the wind 10m above the ground. Gust is the highest gust expected around that hour: a gust is the wind speed averaged over about 3 seconds, the same measure crane wind limits use. Mean wind is the steady wind speed without the gusts, shown for reference. Gusts can be well above the mean, especially in moderate winds, which is why the limits apply to the gust. Rain mm is the rain forecast in that hour; where the Met Office gives 3-hour blocks, the block’s total is shared evenly across its three hours.

The ground slows the wind, so it gets stronger with height. A crane’s wind limit applies at the boom or jib tip, where its anemometer is, and a MEWP’s at the platform, so the 10m forecast is scaled up to the height you choose. The switch on the chosen day shows the original Met Office figures at 10m; your limit is marked on both, but it applies at the tip, so the scaled figures are the cautious ones to plan with.

Gust at height = 10m gust × (height ÷ 10)^0.105
Mean at height = 10m mean × (height ÷ 10)^0.14

Gusts grow less with height than the mean wind does, because the air higher up is less gusty. For example, a 20 mph gust at 10m becomes about 23 mph at 40m. The factors for each height:

Why the boom tip height, not the height of the load?

A crane’s wind limit is the 3-second gust at the boom or jib tip, where the anemometer measures it: BS EN 13000 applies the gust at the crane’s height, not at 10m (CPA CIG/TCIG 2507). When the manufacturer allows for wind on the load, they assume the load feels that same tip-height wind, even though it hangs lower, which is deliberately cautious. So the forecast is compared at the boom tip, and so is the limit for a load that catches the wind. The exception is local: a load passing between buildings can meet funnelled gusts the anemometer does not feel, which is for the site assessment.

Where the height factors come from

The gust factors match the height factors in Liebherr’s guidance on the influence of wind on cranes (1.18 at 50m, 1.27 at 100m, 1.33 at 150m), which the Construction Plant-hire Association refers to in its guidance on lifting operations in wind (CIG/TCIG 2507). They also match the wind profile for open country in BS EN 1991-1-4, the British and European standard for wind on structures. The mean wind factor is the standard profile for open country.

Choosing what you are lifting with fills in a cautious planning figure where published guidance or makers’ charts give one. Always use the figure for your machine and its set-up from the load chart or manual, and change the preset to match.

Cranes are designed to lift loads that catch a set amount of wind. Cladding, shutters, pods and other large, light loads catch more, so they need a lower wind limit. With a crane chosen, the Load button works this out. The load’s wind area is its largest face × a shape factor (1.2 for a box or flat panel, 2.4 if unsure, 1.0 for a round load). The limit for the load is the crane’s limit × √(wind area the crane allows ÷ the load’s wind area), and never more than the crane’s limit; the forecast is then marked against it.

Mobile and crawler cranes (BS EN 13000) allow 1.2 m² of wind area per tonne on the hook, the method in BS 7121-1 and CPA CIG/TCIG 2507. Tower cranes (BS EN 14439, designed to BS EN 13001-2) allow 2.4 × the larger of 0.8 m² or 0.5 m² per tonne, so at least 1.92 m²; CPA CIG/TCIG 2507 notes the same approach is useful for tower cranes. Mobile tower cranes use the mobile crane figure, the more cautious of the two, so check the manufacturer’s own guidance too. It is not offered for MEWPs or other machines, whose manufacturers set their own rules. It is a planning estimate: the appointed person confirms the limit in the lift plan.

The factors assume open, fairly flat country, as the Liebherr tables do. In towns and among tall buildings, on hills and on the coast, the wind can be stronger or weaker than shown, and buildings can funnel it into sudden gusts. The forecast covers an area rather than your exact site, and it becomes less certain further ahead: use the later days for planning, and check again nearer the day. On the day, the anemometer and the operator decide, within the limits in the lift plan set by the appointed person.