PrecipitationObjectives:1. To understand how precipitation is formed.2. To describe three types of rain – frontal,

orographic and convectional.

What are the six types of precipitation?

• Rain• Drizzle• Snow• Sleet• Hail• Dew

• Condensation produces minute water droplets, less than 0.5mm in diameter (or ice crystals if temperature is below freezing).

• They are so small they are kept buoyant by the rising air that creates them.

• So, although condensation produces clouds, clouds do not always mean precipitation.

But…

• These water droplets (or ice crystals) become large enough to overcome the lifting mechanism and fall to the floor.

• There are two theories…

1. Bergeron-Findeisen Mechanism

• Through adiabatic cooling of air as it rises the upper parts of clouds are below freezing point.

• Even at the equator upper cloud may be as cold as -65°C.

• Freezing nuclei (salt and fine soil particles) attract super-cooled water droplets and increase in mass.

• At critical mass they fall towards the surface, either melting or remaining frozen.

2. Collision & Coalescence Process

• Water droplets are constantly moving within a cloud.

• They collide, fuse and increase in mass until they reach critical mass and fall.

Marketplace• Design a poster to teach others about one type

of rainfall:1. Frontal2. Orographic3. Convectional

• Sources of information:– Essential AS Geography p200-201– Advanced Geography p158– Waugh p219-220– Geog.2 p27

Fog

Objectives:1.To name two types

of fog.2.To explain how these

types of fog are formed.

Advection FogHaar fog off East coast

Chinese tourist spot hidden in early morning fog

Advection fog near a river – horizontal movement of air over colder surface

Salt provides an ideal hygroscopic nuclei to help form this haar fog off California

Golden Gate Bridge San Francisco – 105 foggy days a year

Radiation Fog

Thick fog makes travelling very difficult & dangerous

Pollution gets trapped under a temperature inversion producing the orange smog over LA

Summary