Ice Fraction (Mid-Level)

Ice Fraction (Mid-Level) represents the proportion of cloud particles existing in solid ice phase versus liquid supercooled water phase within mid-tropospheric clouds (typically altocumulus and altostratus at 3000-6000 meters altitude), expressed as a dimensionless fraction from 0 (entirely liquid water droplets) to 1 (entirely ice crystals). This cloud microphysics parameter from GFS model output reveals the phase composition of mid-level clouds, which dramatically affects their optical properties and capacity to produce atmospheric optical phenomena. Cloud phase matters photographically because ice crystals and water droplets interact with light differently: liquid water droplets scatter light diffusely creating uniform white or grey cloud appearance, while ice crystals refract and reflect light at specific angles determined by hexagonal crystal geometry, producing spectacular atmospheric optics including 22° halos, sun dogs (parhelia at 22° left/right of the sun), circumzenithal arcs (upside-down rainbow above the sun), tangent arcs, light pillars, and other ice crystal displays.

Ice fraction interpretation for photography: 0.0-0.2 indicates mid-level clouds dominated by supercooled liquid water droplets—expect uniform white or grey cloud appearance without optical phenomena. Fraction 0.2-0.4 suggests mixed-phase clouds with some ice crystals present but not dominant—occasional weak halos possible but unreliable. Fraction 0.4-0.7 represents ice-dominated mid-level clouds with abundant hexagonal ice crystals favorably oriented by atmospheric conditions—high probability of distinct 22° halos, sun dogs, and related optical displays when the sun illuminates these clouds from appropriate angles. Fraction 0.7-1.0 indicates nearly pure ice crystal clouds (cirrostratus or ice-phase altocumulus)—maximum likelihood of vivid, photogenic atmospheric optical phenomena including multiple simultaneous displays (halo + sun dogs + upper tangent arc). The formation of ice versus liquid water in clouds depends on temperature and availability of ice nuclei: below -40°C, all cloud water freezes spontaneously; between 0°C and -40°C (the supercooled regime), cloud phase depends on ice nucleating particle concentrations and cloud dynamics. Mid-level clouds in the -10°C to -25°C range frequently exist in mixed-phase, with ice fraction increasing as temperatures decrease and as ice crystals grow at expense of evaporating liquid droplets (Bergeron process).

PhotoWeather's GFS ice fraction forecasts enable photographers to anticipate ice crystal optical phenomena by identifying periods when mid-level clouds will be ice-dominated: target days forecasting ice fraction >0.5 in mid-level clouds combined with thin-to-moderate cloud cover (allowing sun visibility) for best halo and sun dog photography opportunities. Ice fraction also affects cloud visual texture: ice-phase altocumulus exhibits distinctive fibrous or striated appearance (virga, fall streaks), while liquid-phase altocumulus appears as smooth, defined roundish puffs—enabling photographers to anticipate cloud aesthetics. For landscape photography incorporating sky elements, periods with high mid-level ice fraction offer opportunities to capture rare optical phenomena that add unexpected visual interest and elevate standard compositions into memorable images.