Snow

1. Snow And more rain

2. The Precipitation Process • The Bergeron Process • Also called the ice crystal process • Need: • Water Vapor • Ice crystals • Supercooled water drops in equilibrium with the vapor

3. Bergeron Process Saturation Vapor Pressure with respect to Liquid Water Vapor Pressure Saturation Vapor Pressure with respect to Ice 0C Temperature

4. Bergeron Process Below 0C, the saturation vapor pressure with respect to ice is less than that with respect to a plane surface of liquid water at the same temperature. So, for equilibrium there is a lower vapor pressure for ice than for liquid water. Consider a supercooled liquid water drop next to an ice crystal: Key Point: If the liquid drop is in equilibrium with the environment, then the ice crystal will be in a supersaturated environment.

5. Bergeron Process Since the ice is now in a supersaturated environment, vapor molecules will go to the ice crystal and the ice crystal will grow.

6. Bergeron Process Since the vapor molecules are going toward the ice crystal, the drop now finds itself in an unsaturated environment. The drop will begin to evaporate, thereby supplying more water vapor molecules for the ice crystal to acquire.

7. Molecules from Water to Ice Figure 8.8 Ice crystals have lower saturation vapor pressures than liquid droplets, creating a gradient of high to low water molecules from liquid to ice that encourages ice growth. This growth is critical to the ice-crystal precipitation process. Figure 8.9

8. Bergeron Process The ice crystals grow by deposition at the expense of the liquid drops shrinking by evaporation. Rainfall Ice crystals may group together to form larger ice crystals. This process is called aggregation and may lead to the snowflakes we see on the ground. As the ice crystals fall, they may fall into warmer air. If so, then the ice crystals will melt into rain drops. If the crystals never melt, they will fall as snow.

9. Rain Drops Keep Falling On My Head • The drops can make it to the ground if: • The drop is large enough • The cloud base is low enough • The air between the cloud base and the ground is not too dry.

10. Virga • Hydrometeors falling from a cloud that, to a distant observer, do not appear to reach the ground. • Evaporation --The material falling out of the cloud evaporates as it falls. • Melting -- Snow is highly reflective. If the snow melts to a less reflective rain, a distant observer will see the snow “disappear” as it falls to the ground.

11. Falling Rain • How fast does it take a rain drop to reach the ground from 2 km? • Assume the average fall speed is 10 m s-1. • So an average drop takes about 3⅓ minutes to fall to the earth from a height of 2 km.

12. Summary • Warm rain formation • Cold rain formation • Bergeron process

13. Snow And other cold precipitation

14. Cold Precipitation • Snow • Snow formation • Different types of snow • Freezing rain

15. Ice Crystal Growth The ice crystals grow by deposition at the expense of the liquid drops shrinking by evaporation. • Methods of Ice Crystal Growth • Diffusional growth • Water vapor molecules diffuse toward and deposit on the ice crystal. • Riming • A crystal falling through supercooled droplets will collect many of these drops. The drops will freeze on contact.

16. Ice Crystal Growth • Methods of Ice Crystal Growth • Aggregation • Formation of snowflakes • Ice crystals collide and stick together.

17. Snow Ice crystals may group together to form larger ice crystals. This process is called aggregation and may lead to the snowflakes we see on the ground. As the ice crystals fall, they may fall into warmer air. If so, then the ice crystals will melt into rain drops. If the crystals never melt, they will fall as snow.

18. Snow • Precipitation composed of white or translucent ice crystals, chiefly in complex branched hexagonal form and often agglomerated into snowflakes. • Dendrites -- Fernlike, branched • Columns -- Taller than thin • Plates -- Flat • Other types

19. Snow • Snow shapes, or habits, are temperature dependent. Temperature (C)Crystal Habit 0 to -4 Thin Plates -4 to -10 Columns -10 to -12 Plates -12 to -16 Dendrites -16 to -22 Plates Much of the snow we see are aggregates of these crystal habits.

20. Falling Flakes • How fast does it take a snow flake to reach the ground from 2 km? • Assume the average fall speed is 1 m s-1. • So an average flake takes about 33⅓ minutes to fall to the earth from a height of 2 km.

21. Snow Aggregates • Dendrites • Easiest to hook to each other • Classic “snowflake” • Plates/Columns • Don’t stick to each other well • Temperature plays a significant role in the formation of ice crystal aggregates • Warm (near 0ºC) aggregation is more likely • Cold (T < -10ºC) aggregation very unlikely (except for dendrites at -12 to -16ºC)

22. What Do Snow Crystals Look Like? Simple Plate Crystal with Broad Branches Dendrite

23. What Do Snow Crystals Look Like? Solid Column Sheath Hollow Column

24. What Do Snow Crystals Look Like? Combination of Bullets Bullet Combination of Needles

25. A Potpourri of Crystal ShapesPart 1

26. A Potpourri of Crystal ShapesPart 2

27. Freezing Rain • Precipitation that falls to the ground as a liquid then freezes upon impact with the ground or exposed objects. Forms a glaze of ice on the objects and ground.

28. Freezing Rain Cold: T < 0ºC Warm: T > 0ºC Rain freezes upon contact with below freezing surface. Shallow freezing layer

29. Sleet -- Ice Pellets Warm: T > 0ºC Cold: T < 0ºC Rain freezes before reaching the surface. Deep freezing layer

30. Summary • No two snowflakes are identical • Then again, no two of anything are absolutely identical • Snow can become rain can become ice pellets can become rain can become freezing rain