Cloud Freezing Threshold Temperature Explained

The spontaneous freezing of cloud droplets into ice crystals, known as homogeneous nucleation, occurs at a specific low temperature without the need for ice nuclei.

Spontaneous Freezing of Cloud Droplets

Cloud droplets are supercooled water droplets that remain liquid below their freezing point ($0^\circ \text{C}$). For these droplets to transition directly into ice crystals without external help (like dust particles), a significant reduction in temperature is required.

• At temperatures slightly below $0^\circ \text{C}$, droplets typically remain liquid if pure.
• As the temperature drops further, the kinetic energy of water molecules decreases, making the formation of an ordered ice structure more probable.
• The critical temperature threshold at which a significant fraction of pure water droplets will freeze spontaneously is widely recognized in atmospheric science.

Identifying the Threshold Temperature

Research and observations in atmospheric physics establish the temperature at which homogeneous nucleation becomes highly likely:

• $0^\circ \text{C}$: Freezing point of water, but droplets are supercooled below this.
• $-33^\circ \text{C}$: A temperature below $0^\circ \text{C}$, but not the universal spontaneous freezing threshold.
• $-40^\circ \text{C}$: This is the widely accepted threshold temperature where nearly all supercooled cloud droplets will freeze spontaneously. Below this temperature, liquid water droplets are extremely rare.
• $-50^\circ \text{C}$: Occurs well below the spontaneous freezing point.

Therefore, the threshold temperature for all cloud droplets to freeze spontaneously into ice crystals is $-40^\circ \text{C}$.