6. Rain and Snow 6.1 Drop-size distribution and drop breakup 6.2 Size distribution of snowflakes and breakup 6.3 Rainfall rate (Precipitation rate)

Best way to characterize rain reaching the ground is through drop-size distribution. Drop-Size distribution –Number of drops per unit size interval per unit volume. –Size interval is usually diameter (or radius) How do we mathematically represent this drop- size distribution? –Experimental studies 6.1 Drop-size distribution and drop breakup

Observations Exponential Fit

Drop Breakup When drops reach a diameter of  3mm, the surface tension is barely able to hold drop together for larger sizes, drop breaks up. Drops much larger than 3mm are unstable, exist only briefly before breaking up. Collisions between drops produce small satellite drops Why do drop-size distribution tend to be a negative-exponential form?

Young (1975) Modeled drop-size spectrum Assumed activity of condensation nuclei Coalescence, condensation, and collision-induced breakup Result: Marshall-Palmer type distribution.

Irregular aggregates of crystals. Difficult to measure. Usually expressed in terms of particle mass or diameter of the water drop formed when the snowflake is melted. Distribution can be described reasonably well by an exponential function. Breakup of snowflakes is likely due to collision. 6.2 Size distribution of snowflakes and breakup

Volume of water passing through a horizontal surface per unit time. 6.3 Rainfall rate (Precipitation rate)