Past exam of the medicine course of the University of Oxford / 1864 / First examination / Physics / 12 / Solution 2026-10-31
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A common optical halo is produced by refraction through hexagonal atmospheric ice crystals. For a prism path and ice refractive index about , the minimum deviation relation gives
(1)
Random suitable crystal orientations produce a ring around the Sun or Moon, with concentration near that minimum angle. Optical dispersion puts red near the inner edge. Other crystal paths or orientations produce other halos and arcs; the explanation should not treat every ring as the water-drop mechanism of a rainbow.[ref]
A waterspout is a rotating atmospheric column over water. Converging air and an updraft can stretch and intensify existing rotation; cooling and condensation make part of the funnel visible, while strong surface winds raise spray. The visible funnel is not simply a solid column of sea water sucked intact to the cloud. Fair-weather waterspouts and thunderstorm-associated tornadic waterspouts have different developmental settings; a single thunderstorm-only explanation does not cover both.[ref]
Hoar frost forms by deposition of atmospheric water vapour as ice on a sufficiently cold surface. The surface must be below freezing and below the relevant frost point. Radiative cooling on a clear quiet night can make a surface colder than air at ordinary thermometer height, so a positive reported air temperature does not rule out surface frost. Freezing previously formed liquid dew is frozen dew, a different formation route.[ref]
Hail grows in a thunderstorm when ice embryos collect supercooled liquid droplets that freeze on them. A strong updraft increases residence time and can support growing stones; they fall out when their trajectories and weight take them out of adequate support. Clear and opaque ice layers reflect changing growth and freezing conditions. They are not proof that every layer required a complete repeated ascent and descent: horizontal movement and variations across an updraft matter too. Nor is the very cold cloud top necessarily the best growth region, because appreciable growth requires liquid water.[ref]
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