Wind and Coriolis

Wind and Coriolis

Wind and Coriolis are the two velocity-coupled corrections to the point-mass trajectory. Both are built once as vectors/scalars in LobContext and applied every integration step in DsDx. They are grouped because neither changes the drag curve or atmosphere — they only add v-dependent accelerations in DsDx (source/solve_step.cpp drag + Coriolis).

Wind vector

Wind enters the equations only as v − w in the drag term and as w_z in the jump models. BuildWind (source/lob_builder.cpp) produces ctx.wind = {x,z} in fps:

w_x = |w|·sin(heading)
w_z = |w|·cos(heading)

heading is the user angle (0° = tailwind) mapped to the internal math angle used in BuildWind:

• WindHeading(ClockAngleT) — 12 o'clock = tailwind, 6 = headwind. Stored as (3 − clock)·30° wrapped to [0,360) so internal 90° = tailwind.
• WindHeadingDeg(value) — degrees with same 0°=tailwind convention. Stored as −value + 90° wrapped to [0,360) (source/lob_builder.cpp), so internal 90° = tailwind, 270° = headwind.

Both satisfy "12 or 0 is tailwind, 6 or 180 is headwind" (include/lob/lob.h). Speed is WindSpeedFps or WindSpeedMph (MphT → FpsT). Default 0 fps. Validation: internal radian value checked against ±360° → kLobErrorWindHeadingOOR.

Usage in the solver

DsDx forms v − w as CartesianT<FpsT>(ctx.wind.x, 0, ctx.wind.z) and uses |v−w| to scale drag (source/solve_step.cpp). No vertical wind; uniform and constant. Jump consumes only w_z via CalculateCrosswindAngleGamma(MphT(w_z), v) and the Litz/Boatright formulas (Spin Drift and Aerodynamic Jump).

Limitations

• No gradient with altitude/range, no vertical wind.
• Headwind/tailwind scales |v−w|; crosswind affects drag only via Boatright's CDa (source/lob_builder.cpp).

Coriolis

When the shooter supplies both azimuth and latitude, lob adds McCoy's rotating-Earth correction. BuildCoriolis (source/lob_builder.cpp) requires both; otherwise terms are zeroed.

Validation:

|azimuth| ≤ 360° else kLobErrorAzimuthOOR
|latitude| ≤ 90° else kLobErrorLatitudeOOR

With Ω = 7.292115e-5 rad/s (source/constants.hpp):

cos_l_sin_a = 2·Ω·cos(lat)·sin(az)
sin_l       = 2·Ω·sin(lat)
cos_l_cos_a = 2·Ω·cos(lat)·cos(az)

stored as ctx.coriolis.{cos_l_sin_a, sin_l, cos_l_cos_a}.

DsDx (source/solve_step.cpp) adds:

dvx -= vy·cos_l_sin_a + vz·sin_l
dvy += vx·cos_l_sin_a + vz·cos_l_cos_a
dvz += vx·sin_l       − vy·cos_l_cos_a

after drag, before gravity. No Eötvös variation; effect folded into deflection/elevation. Tests in test/source/lob_coriolis_test.cpp cover N/S/E/W azimuths.