Precipitation

Why do most clouds not produce precipitation?

It evaporates before it gets large enough

What is the size of a typical cloud drop?

0.02 mm diameter

What is the typical size of a rain drop?

2 mm diameter

Why do cloud drops mainly stay small?

- Due to curvature

- High surface area relative to volume/mass

- Higher rate of evaporation (evaporation on the surface)

- To remain saturated, attract more than evaporate

- Less energy required to evaporate

How do aerosols help droplets?

- Impure solution

- Slows rate of evaporation, energy required to evaporate solid nuclei

What factors affect rate of fall or water droplets?

- Max speed increases with rain drop size

- Gravity (heavier, more affected by gravity)

- Up/downdraft speed (stronger updraft, longer it has to get bigger)

- Droplet size and mass

What is bergeron?

Ice crystals attract water vapour and grow (fall, shatter and create more ice crystals)

Where does bergeron occur?

Above and below FZL

Why do ice crystals attract more water vapour than SCWD?

They are better freezing nuclei

Where does coalescence occur?

Warm clouds below FZL

What is coalescence?

Droplets attract other drops and get larger, this continues as it rises and falls until it can't be held and falls out of the cloud

How does freezing rain form?

- Downstream of mountainous continental environments

- Cold air trapped, stagnates

- Warm front passes over, few degrees above zero

- Snow at mid-high levels, falling through warm layer

- Large drops remain at 0°C then fall into sub-zero layer

- Doesn't refreeze until on nuclei

Where is freezing rain most likely to occur?

- Continental climates with large mountain ranges

- Canada

- Eastern United States

- Scandinavia

- Russia