Different parts of the rules use wind speed categories slightly differently.
For example, a Fog Cloud (or some magic items modeled on it) will be dispersed with a moderate wind, which is described as at least 10 miles per hour. However an Eversmoking Bottle may have its smoke dispersed in one minute with a moderate window (11 to 20 miles per hour), or in one round with a strong wind (21 or more miles per hour).
It's clear that the categories are originally based on 10 mile-per-hour groupings, so calm winds would be 0 miles per hour, a light wind would be 1-10, a moderate wind would be 11-20, a strong wind would be 21-30, etc. However it's often more convenient to say "at least 10 miles per hour", or similar. It does feel likely to create ambiguity. A wind speed is likely to be stated as exactly 10 miles per hour; is that a light wind or a moderate wind?
An Eversmoking Bottle describes things as 11-20 miles per hour, and 21+ miles per hour. Grenade smoke and Stinking Cloud are described with "at least 10 miles per hour" and "at least 20 miles per hour" for the same dispersal results. But both attribute those speeds to moderate and strong winds, respectfully. Warding Wind creates a Strong wind, at 20 mile per hour. Control Winds provides more reference points.
The only rules I can find on the effects of winds are in the DM's Guide under Wilderness Survival, where it describes the effects of strong winds (presumably the 21-30 mph category). However it only categorizes winds into "None", "Light", and "Strong". That leaves a gap where "Moderate" is expected.
Meanwhile, Control Weather describes Calm, Moderate, Strong, Gale, and Storm winds, leaving out Light winds.
Those wind speeds seem to somewhat reflect the Beaufort scale of wind speeds, which cross (with some more specific variants) Light, Moderate, Strong, Gale, Storm, and Hurricane speeds. If you want to mostly round to the nearest 5-10 mph for convenience, and match up with Saffir-Simpson for the hurricane end of things, it's reasonable to set up the scale as:
- Calm: 0 mph
- Light: 1-10 mph
- Moderate: 11-20 mph
- Strong: 21-30 mph
- Gale: 31-45 mph
- Storm: 46-75 mph
- Hurricane: 76+ mph
The Control Winds spell says that Strong winds halve your movement speed (ie: cause difficult terrain, which is what Warding Wind also does with a Strong wind), which sorta matches the description on the Beaufort scale at 28-33 mph (high Strong to low Gale). The Beaufort Scale suggests that higher Gale winds prevent forward walking progress. Meanwhile, Moderate winds are suggested to cause disadvantage on ranged attacks through the area.
So, all put together, I'd see this as:
- Calm: 0 mph
- Light: 1-10 mph — Disperses trivial smoke effects
- Moderate: 11-20 mph — Disperses large smoke effects slowly. Disadvantage on ranged attacks.
- Strong: 21-30 mph — Disperses large smoke effects quickly. Halves movement speed, and can potentially push someone back. Non-magical flight is nearly impossible (creature must land at end of turn).
- Gale: 31-45 mph — Neutralizes most movement. Wind may possibly determine your movement.
- Storm: 46-75 mph — Can uproot trees. Objects blown in the wind are significant threats. Can blow people around fairly easily.
- Hurricane: 76+ mph — Can destroy structures.
Or, if you want to round the 10-mph end to the low end of each scale increment:
- Calm: 0 mph
- Light: 1-9 mph — Disperses trivial smoke effects
- Moderate: 10-19 mph — Disperses large smoke effects slowly. Disadvantage on ranged attacks.
- Strong: 20-29 mph — Disperses large smoke effects quickly. Halves movement speed, and can potentially push someone back. Non-magical flight is nearly impossible (creature must land at end of turn).
- Gale: 30-44 mph — Neutralizes most movement. Wind may possibly determine your movement.
- Storm: 45-74 mph — Can uproot trees. Objects blown in the wind are significant threats. Can blow people around fairly easily.
- Hurricane: 75+ mph — Can destroy structures.
Personally, I'm not sure which I'd prefer. Is 20 the top end of Moderate? Or the bottom end of Strong? Do you want to describe your winds with the strongest effect or the weakest effect? Or maybe just let the endpoints overlap, so you can describe it as 20–30 miles per hour, which feels more natural, and focus on the descriptor rather than the number?
- Calm: 0 mph
- Light: 1-10 mph — Disperses trivial smoke effects
- Moderate: 10-20 mph — Disperses large smoke effects slowly. Disadvantage on ranged attacks.
- Strong: 20-30 mph — Disperses large smoke effects quickly. Halves movement speed, and can potentially push someone back. Non-magical flight is nearly impossible (creature must land at end of turn).
- Gale: 30-45 mph — Neutralizes most movement. Wind may possibly determine your movement.
- Storm: 45-75 mph — Can uproot trees. Objects blown in the wind are significant threats. Can blow people around fairly easily.
- Hurricane: 75+ mph — Can destroy structures.
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