EG1204: Earth Systems: an introduction Meteorology and Climate

Lecture 4Fog and Cloud formation

Topics we will cover

•Formation of fog•Formation of clouds•Cloud types•Observing cloud

Formation of FOG #1• The process of condensation that

forms fog and clouds is not so simple. It is not simply the case that saturation (dew point) must be reached

• There must be airborne particles on which water vapour can condense

Formation of FOG #2• Although air looks clean - it never really is.

Air contains many tiny particles (impurities)• many of these particles serve as a surface

on which condensation can occur• These particles are called condensation

nuclei• Some condensation nuclei are very small

with a radius of < 0.2µm (Aitken nuclei)• Particles 0.2 - 1µm are called large nuclei• Particles > 1µm giant nuclei

Formation of FOG #3• As the relative humidity reaches 75 -

100% (saturation) water condenses onto condensation nuclei

• As the air cools and becomes more saturated the droplets of suspended condensed water get larger until visible to the naked eye

• We can see these clouds of droplets as fog

Radiation fog

Formation of FOG #4• City air (with its extra impurities)

produces a thicker fog as there are more condensation nuclei

• London often suffered from very thick fog as a result of pollution and industrial activity until legislation was introduced early in the 20th century

City fog – exacerbated by pollutant particulates

Formation of FOG #5• Fog often forms near the ground on

a natural surface (e.g. football pitch)• This is exacerbated on clear nights

when radiation leaves rapidly and cools the ground down and the moist air directly above it

• This is known as radiation fog

Formation of FOG #7• Fog may form when warm moist air

travels over a cold surface. This is known as advection fog

• Fog may form by the mixing of two unsaturated air masses - called evaporation (mixing) fog. It forms when cold unsaturated air settles over warm water from which water may be evaporating - explains why fogs form over lakes and ponds in summer

Advection Fog in a valley

Formation of Clouds #2• Cloud formation may be convectional,

orographic or frontal • Convectional clouds form when moist air is

carried upwards by the action of vertical convection (due to solar heating of the surface). Moist air cools as it ascends until it becomes saturated. At the point of saturation, the moist air condenses to form cloud (Convectional Condensation Level)

Formation of Clouds #3• Strong surface heating where

there is very moist air can lead to the development of intense storm clouds - so called "anvil shaped" structures that produce large amounts of precipitation

Formation of Clouds #4• Orographic cloud forms when moist air is forced

upwards - usually when it flows over a plateau or mountain. The air forced upslope cools until it becomes saturated forming clouds near to or above the mountainous structure (Lifting Condensation Level).

• The air (now free of moisture) flows down the lee side of the mountain at a higher temperature as energy lost during condensation is carried away by the wind. Rainfall usually occurs on the lee side, forming a rain shadow on the upslope side.

Formation of Clouds #5• With frontal cloud formation, moist

warm air is forced above cooler air (the cooler air acting like a wedge). As this moist air is forced upwards it cools until the air becomes saturated and condenses into clouds. The clouds are usually easily seen as a visible ridge along the line of an active front.

Formation of Clouds #6• When winds are forced to converge at

low level (the ITCZ near the equator for example) air will be forced upwards where it cools and condenses. This is known as convergence clouds and explains why the equatorial ITCZ is visible from space as a zone of active cloud systems along the equator.

Cloud TypesNAME ABBR. HEIGHT (km) CATEGORYCirrus Ci 6 – 10 HIGHCirrocumulus Cc 6 – 10 HIGHCirrostratus Cs 6 – 10 HIGHAltostratus As 3 – 6 MEDIUMAltocumulus Ac 3 – 6 MEDIUMStratocumulus Ac < 3 LOWStratus St 1 – 2 LOWCumulus Cu 0.6 – 6 CUMULIFORMCumulonimbus Cb To the

tropopauseCUMULIFORM

Ice crystals

No precipitation

Altocumulus CloudWater and Ice clouds – usually bring precipitationAfter 15 – 20 hours

Stratus CloudTypically overcast or drizzle conditions

Cumulus CloudAssociated with gusty winds and heavy precipitation

ITCZ (Inter-Tropical Convergence Zone)

Observing Cloud #1• Clouds are observed at regular intervals

at ground stations• The degree of cloud cover in the sky is

expressed as oktas (or eighths)• A value of 4 oktas would mean that

about half (50%) of the sky was covered with cloud

• A value of 8 oktas means that the entire sky is cloud-covered (totally overcast)

Observing Cloud #2• Sometimes it is preferable to

estimate the cloud cover for the three main layers of the troposphere (low, middle and high)