Brown Dwarf Weather: Patchy Clouds & Rotational Variability

Substellar Weather

L/T transition

Rotation period 4.5 h

Cloud coverage 45%

Viewing inclination 60°

Cloud/clear contrast 18%

Photometry

relative flux vs. time (last ~3 rotations)

Rel. flux now

100.0%

Rotation phase

0°

Measured peak-to-trough amplitude

— %

How it works

Brown dwarfs (roughly 13–80 Jupiter masses) are too light to sustain stable hydrogen fusion, so their photospheres cool over billions of years — passing through spectral classes M → L → T → Y. Around the L/T transition (~1200–1400 K) the atmosphere is thought to break up into patchy condensate clouds (silicates, iron) separated by clearer, hotter gaps, rather than a uniform deck. As the object rotates, cloudy and clear patches sweep in and out of view, modulating the disk-integrated brightness:

F(t)/F₀ ≈ 1 − A · Σᵢ max(0, n̂ᵢ(t)·ô) / N_eff
n̂ᵢ(t)  = surface normal of cloud patch i (rotates with the body)
ô      = line-of-sight unit vector to the observer
A       = cloud/clear brightness contrast
N_eff   = ¼·N_total  (average visible-hemisphere projection for N_total patches spread over the sphere)

This is the exact mechanism behind observed brown-dwarf photometric variability — e.g. the binary Luhman 16B shows ~1–2% flux variations with a ~5 hour period, interpreted as patchy clouds rotating in and out of the visible hemisphere.

Rotation period — sets how fast the patch pattern sweeps past the observer; this is the period you would recover from a real light curve.
Cloud coverage — fraction of the surface carrying cloud patches; more coverage means more mass modulating brightness as it rotates.
Viewing inclination — tilt of the spin axis relative to the line of sight. Near 0° (pole-on) the same patches stay in view and variability nearly vanishes; near 90° (equator-on) the full modulation is visible.
Cloud/clear contrast — how much dimmer a cloudy patch is than a clear one, setting the light-curve amplitude directly.

Clear photosphere (hot) Silicate cloud patch (cool)

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