Metr 201 (502) Quiz # 4 Name: Solution

October 20, 2000

1) Why would you expect there to be many more cloud droplets than ice crystals in a cloud where the air temperature is –10 ^oC?

Although 0 ^oC is the maximum temperature at which ice is stable, sub-zero temperatures do not ensure that freezing will occur. Instead, freezing is initiated when a few liquid molecules combine to form a rigid “ice embryo”. At first this group of molecules is susceptible to break-up due to collisions with other liquid molecules. However, if it can grow sufficiently it becomes stable and the freezing process essentially propagates in all directions. When this occurs, the entire volume of water (droplet, glass of water, …) quickly freezes. The probability that this initiation will occur is dependent upon the number of ice embryos present in the volume at any time. Since the volume of water present in a cloud droplet is very small, there aren’t many ice embryos present. Therefore, the probability that a cloud droplet will freeze is also very low. The result is that most cloud droplets will remain as supercooled liquid, and only a few that formed on suitable ice nuclei (which are scarce) will form ice crystals.

2) How do the atmospheric conditions that produce sleet differ from those that produce hail?

Hail forms in clouds when ice crystals collide with supercooled water droplets as they fall through the cloud (called accretion). The supercooled droplets freeze on the surface of the ice crystal, each time making the crystal a little larger, and eventually colliding with so many that it is considered graupel. If this growing ice structure remains in the cloud long enough to collide with a large number of supercooled droplets, it can grow large enough to become a hailstone. For the purpose of this question, the important point is that the hailstone never melts. Sleet forms when a falling raindrop freezes as it encounters a sub-freezing layer in the atmosphere. It makes no difference whether the initial raindrop formed through the collision-coalescence process or through the ice-crystal process.