How this instrument works
Moving air carries kinetic energy. Bring it to rest against any flat surface and that energy reappears as pressure, and ½ρv² says exactly how much: 15 Pa at 5 m/s, 245 Pa at 20 m/s, 980 Pa at 40 m/s, close to 3 kPa inside category-5 hurricane winds. Those figures look trivial beside atmospheric pressure of 101,325 Pa — barely 3% of it even at hurricane strength. What makes them structurally serious is one-sidedness across area. Three kilopascals acting on 200 m² of warehouse wall, with nothing pushing back from inside, comes to 600 kN.
Henri Pitot built his measuring instrument for this quantity in 1732 — bent tubing turned upstream, lowered into flowing river water, whose column rose by an amount set by flow speed alone. Henry Darcy refined that idea in 1858 into today's total-plus-static pair, and every airspeed indicator flying still reads ½ρv² by exactly that trick. Structural engineering learned more painfully. Thomas Bouch designed his Tay Bridge to wind allowances of 10 lb/ft², roughly 480 Pa — what 28 m/s of wind delivers. On 28 December 1879 storm winds took down thirteen spans and a train. Britain's subsequent Board of Trade inquiry fixed 56 lb/ft², about 2,680 Pa, as design practice, and Benjamin Baker built Forth Bridge to that harder number.
Two assumptions hold this sheet together. Dynamic pressure ½ρv² treats air as incompressible, honest below roughly Mach 0.3 — near 100 m/s — so no weather on Earth strains it. Drag coefficient is where candour gets harder: C_d is empirical, drifts with Reynolds number, aspect ratio and surface roughness, and 1.2 suits square flat panels held face-on. Long ribbons and free-standing hoardings climb toward 2.0; smooth spheres sit near 0.47 until Re passes about 3×10⁵, then fall abruptly toward 0.1. Nothing here models gusting, vortex shedding or flutter, either. Tacoma Narrows tore itself apart in 1940 at only 19 m/s, killed by resonance rather than raw push.
- Set Air density: 1.225 kg/m³ is sea level at 15 °C. Thinner at altitude, denser in hard frost.
- Enter Wind speed in m/s, km/h, mph or knots — use peak gust figures, never daily averages.
- Choose Drag coefficient: near 1.2 for flat panels face-on, up to 2.0 for long hoardings, 0.47 for spheres.
- Give Exposed area as projected area facing into flow, not total wetted surface.
- Read Dynamic pressure in pascals and Wind force in newtons; both recompute as you type.
Worked example — a 10 m² hoarding in a gale
A roadside hoarding 5 m wide by 2 m tall gives Exposed area 10 m². It meets 20 m/s of gale — about 72 km/h — in standard sea-level air, so Air density stays at 1.225 kg/m³. Dynamic pressure comes first: q = ½ × 1.225 × 20² = 0.5 × 1.225 × 400 = 245 Pa. With Drag coefficient 1.2, right for flat signage held square to flow, Wind force follows: F = 245 × 1.2 × 10 = 2,940 N.
Two thousand nine hundred and forty newtons is very nearly 300 kg of dead weight — three adults standing on one signboard that masses perhaps 60 kg itself — arriving sideways, at head height, on posts sunk into soil. Push Wind speed up to 40 m/s and force does not double; it quadruples, to 11,760 N. That square law explains why storm damage arrives so abruptly, and why brackets sized by eye survive ten winters then fail inside one afternoon.
Questions
What is dynamic pressure, physically?
It is kinetic energy per unit volume of moving air, ½ρv², measured in pascals. Pitot-static probes read it directly as that gap between total pressure facing into flow and static pressure sampled at right angles. It is not what barometers see: one sheltered at sea level still reads 101,325 Pa whether or not gales howl past outside.
Why does doubling wind speed quadruple the force?
Because v enters squared. Air arriving twice as fast delivers twice as much mass every second, and each parcel carries twice as much momentum, so force rises fourfold. 20 m/s on 10 m² of panel gives 2,940 N; 40 m/s on that same panel gives 11,760 N. Storm design is dominated by this behaviour, which is why codes ask for peak gust speed — a gust 40% above mean speed carries roughly double its pressure.
Should I enter average wind speed or gust speed?
Gust speed. Codes such as ASCE 7 and Eurocode EN 1991-1-4 specify a short-duration peak — conventionally a 3-second gust measured 10 m above open ground — precisely because squaring punishes averaging. Speed also grows with height above terrain, so values quoted from 10 m weather masts understate conditions at 30 m by something like 20%.
What drag coefficient should I use?
For flat panels held square to flow, 1.2 is a sound starting point; long narrow hoardings and free-standing walls run higher, toward 1.8 or 2.0. Circular cylinders sit near 0.7 at ordinary wind Reynolds numbers, spheres near 0.47, streamlined aerofoil sections under 0.1. These values are measured rather than derived, and published tables assume particular shapes, aspect ratios and mountings — panels sitting close to ground behave unlike ones held clear of it.
Is dynamic pressure the same as pressure on a wall?
No. q is a reference value; real surface pressure varies point by point over any structure. Windward faces typically see positive pressure around 0.8q, leeward faces see suction near 0.5q, and roof corners in separated flow can spike well past 2q. Multiplying q by a single Drag coefficient yields net force, which is what this sheet reports — it says nothing about where across a facade that force concentrates.
Which units does this instrument work in?
Everything is held internally in SI: kilograms per cubic metre, metres per second, square metres, yielding pascals and newtons. Wind speed also accepts km/h, mph and knots, Exposed area accepts cm² and ft², and outputs convert to kPa, psi, kN or lbf. Drag coefficient is dimensionless and carries no unit at all — attaching one to it signals a mistake further upstream.