Paper 1 · November 2024 · Climatology
The Bergeron-Findeisen process explains rain formation in cold clouds where supercooled water droplets and ice crystals coexist. What is the key physical fact that drives ice crystals to grow at the expense of the surrounding droplets?
ASupercooled water droplets are chemically unstable and spontaneously convert to ice without any vapour exchange.
BThe saturation vapour pressure over ice and over water is always identical, so no net transfer occurs, which would leave no mechanism at all for ice crystals to grow faster than the surrounding supercooled droplets.
CThe saturation vapour pressure over ice is lower than over supercooled water at the same temperature, so vapour deposits onto the ice while droplets evaporate to compensate.
DIce crystals are heavier than water droplets, so they simply fall through and absorb the droplets by direct contact.
Explanation
Because the air is saturated with respect to water but supersaturated with respect to ice, water vapour deposits onto the ice crystals; this lowers local vapour pressure, causing the supercooled droplets to evaporate to compensate, so ice crystals grow rapidly at the droplets' expense.
Derived from ZIMSEC Advanced Level Geography 6037/1 Paper 1: Physical Component, November 2024 session, Q1