showstack

The square law

Everything with an area is a sail

Wind force is proportional to the square of wind speed. That single fact is why outdoor production has speeds written into a plan rather than judgement exercised on the day: a wind that feels twice as strong as this morning is putting four times the load into your structure, and a gust 40% above the average carries twice the force of the average.

The arithmetic is short. Dynamic pressure is q = ½ ρ v², about 0.625 v² pascals for air at sea level. Force is that pressure times the area times a shape factor. A 3 × 6 m banner in an ordinary 12 m/s breeze — Beaufort 6, the kind of day nobody cancels — is carrying more than 200 kg of push, and about twice that in the gusts.

What the plan actually needs is not one number but three: the speed at which you stop adding to the structure, the speed at which you drop or furl anything with an area, and the speed at which people leave. Those are decided in advance by whoever designed the structure, against a site-specific wind assessment, and written down where the person on site at 3pm can read them.

Water

The air holds water, and gives it back on a schedule

Warm air holds more water than cold air. Cool a parcel of air far enough and it reaches the temperature where it can no longer carry what it has — the dew point — and the surplus comes out as liquid on the nearest cold surface. Nothing has to fall from the sky for your rig to end up soaked.

This runs in both directions and both directions bite. Outdoors on a clear night, the air cools toward its dew point around dawn, and every horizontal surface on site gets wet. Indoors, gear that has been on a cold truck arrives well below the room’s dew point, and water condenses inside the amplifier before anybody has plugged anything in. Same physics, opposite geometry.

The fix is the same in both cases and it is patience: do not energise it until the surface is above the dew point, with a margin, because exactly at the dew point is already wet. On a cold case in a humid venue that is usually a couple of hours, and the mistake is opening the lid, which lets humid air straight onto the cold metal.

The rating

What IP65 actually promises, and what it does not

An ingress protection rating is two digits and each one is a specific laboratory test. The first is solids, the second is water, and a higher second digit is not automatically better — IPX7 is an immersion test and does not include the pressurised jets of IPX5, which is why some equipment carries two ratings.

What the rating covers is the assembly as tested: closed, sealed, mated, in the orientation the laboratory used. What it does not cover is nearly everything that actually happens on a site. A connector is rated when mated; the same connector hanging open in the rain is rated IP-nothing. Condensation forms inside the enclosure from air that was already in there. Gaskets age, get pinched by a hurried lid, and lose their rating without looking any different. And no IP digit says anything about UV, salt, or a truss clamp resting on the housing.

Heat and sun

A black case in direct sun is a different machine

Equipment specifications are quoted at an ambient temperature, usually somewhere between 20 and 35 °C, in still air. Direct sun on a dark enclosure adds a solar gain that has nothing to do with air temperature: surface temperatures of 60 °C and above on black flight cases in summer sun are ordinary, not exceptional.

Three things follow. Amplifiers and servers throttle or shut down, and the shutdown is usually thermal protection working correctly rather than a fault. Cable ampacity falls with ambient temperature — the derating is a published factor and the cable derating calculator applies it. And LED walls lose brightness and shift colour as they heat, which is why a wall calibrated at 8am does not match itself at 3pm.

The counter-measures are dull and effective: shade the thing rather than cooling it, keep dark surfaces out of direct sun, leave the airflow paths the manufacturer designed actually open, and plan the day so the heaviest thermal load is not at the hottest hour.

Measuring it

What an anemometer is telling you, and what it is not

A wind action plan with trigger speeds in it needs somebody measuring wind, and the measurement is less straightforward than the instrument suggests.

A cup or vane anemometer is mechanical and averages by nature — it has inertia, so it under-reads a short gust by design. An ultrasonic unit measures the time of flight between transducers, has no moving parts to seize or wear, responds fast enough to actually catch gusts, and reports direction as well. For anything the plan depends on, ultrasonic is worth the money precisely because gusts are what take structures over and a cup will smooth them away.

Where it sits matters more than what it is. Wind speed rises with height and is disturbed by everything nearby, so a reading taken at head height beside a truck is not the wind hitting the top of your structure — it can easily be half of it. The meteorological convention is 10 m in open ground, and a site anemometer should be as high and as clear as it can practically be, with its actual position written into the plan so a reading means something.

And the averaging window is the number people forget to ask about. An instrument reporting a 10-minute mean and one reporting 3-second gusts will disagree by 40% or more on the same afternoon, and a trigger value copied from a structure’s documentation refers to a specific one of those. A plan that says “stop at 18 m/s” without saying which 18 m/s is not a plan, and the two readings can be a whole Beaufort force apart.

Rain, and the thing rain actually does

Which is rarely getting into the box

Rain gets measured in millimetres per hour, and outdoor events usually care about three thresholds that have nothing to do with the number itself.

The first is standing water, which is about drainage rather than rainfall. Any flat surface on a stage — a deck, a dimmer rack lid, a case lying down — becomes a pond, and an IP rating that covers falling rain says nothing about immersion. Most water ingress on shows arrives from underneath or from a surface the equipment was standing in, not from the sky.

The second is wind-driven rain, which arrives horizontally and reaches things a vertical test never contemplated. The IP water tests spray from defined angles; a 20 m/s crosswind is not one of them, and a fixture rated for rain falling on it can be getting rain fired into its underside.

The third is weight. Rain collecting in a banner, a roof pocket or a sagging cover is a load nobody put in the calculation, it accumulates quietly, and it is heaviest exactly when the wind is worst. A pocket holding 100 litres is 100 kg hanging somewhere the design did not expect, and it is the classic cause of a structure failing on a night that was not especially windy.

Which is why the practical answers are about geometry rather than about ratings: get things off the deck, give every surface a fall, make sure water has somewhere to go that is not into a connector, and check for pockets that can fill before the weather rather than during it.

Control in the weather

What outdoor control gear actually needs

Putting a node, a splitter or a control panel outside is not just a matter of buying the rated version, and four things decide whether it survives the run.

The enclosure has to breathe without leaking. A sealed box cannot convect, so it cooks in the sun and then, as it cools at night, draws in humid air through whatever gap it has — and that air condenses inside. A breather vent, a membrane gland that passes air and not water, is the standard answer, and a sealed box without one accumulates water over a week of ordinary weather with no leak anywhere.

Cable entries are where ratings die. A gland done up on the wrong cable diameter, an unused entry with no blank, or a cable entering from above so water runs down it into the box, each throws away the rating of everything behind it. Enter from below where you can, and where you cannot, leave a drip loop so the water leaves the cable before the cable reaches the gland.

Connectors are rated mated. An unmated weatherproof connector is a funnel, and the caps that come with them exist for exactly that. A run that will sit unmated overnight needs capping as part of the get-out, not as an afterthought.

And heat and IP fight each other, which is the trade sitting under all of it. The higher the ingress rating the less the box can ventilate, so an IP66 enclosure in direct sun may need shade or active cooling that an IP54 one in the same place does not. Shading the enclosure is nearly always cheaper than cooling it, and it does not compromise the seal.

Stopping

The decision is a document, not a judgement

The last thing on this page is the only one that is not physics. Every outdoor event needs an adverse weather plan that names the trigger values, names the person who calls it, and names what happens at each stage — and it needs to exist before the day, because the day is exactly when nobody wants to be the one who stops a show.

The reason to write it down is not bureaucratic. On site, at the moment the decision matters, the people best placed to see the weather are the people with the strongest reasons not to act on it: a crew that has been building for two days, a promoter with a gate, and an audience already in. A trigger agreed in a quiet room a month earlier removes that conflict from the moment it would otherwise be resolved badly.

Lightning has a similar shape, and a similar answer. There is no way to make an open field safe; there is only a distance and a clock, and a plan that says where people go. The common rule of thumb — if the gap between flash and thunder is under 30 seconds, take shelter, and wait 30 minutes after the last thunder — exists because it is short enough to be remembered under pressure.

Where this goes next

The calculators on this page

Wind load gives the force and the overturning check. Dew point gives the condensation margin on a surface. Cable derating applies the temperature and bundling factors. All three are screening numbers: they tell you whether to have the conversation, not whether the thing stands up. Temporary demountable structures are designed to the governing standards by somebody competent to do it, and that is not a web form.