Clouds

Objective:To be able to recognise and name different types of cloud.

Why do clouds form?

• Clouds form when air cools to dew point and vapour condenses into water droplets or ice crystals.

• General classification was proposed by Luke Howard in 1803.

Luke Howard’s Four Cloud Types:

Cumulus

Stratus

Cirrus

Nimbus

Howard used 4 Latin words:

– Cirrus (meaning lock of curly hair)

– Cumulus (meaning pile or heap)

– Stratus (meaning layer)

– Nimbus (meaning rain bearing)

He also compiled composite names and added the prefix ‘alto’ for mid-level clouds:

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 need to define each• 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

Exam Question

• Explain how clouds are formed. Why do only some clouds produce rainfall? (10 marks)