Fog and Cloud formation:

How does air become saturated?

North Sea Fog – a coloured visible satellite image

Reading

• Ahrens 9th ed., Chapter 5 Condensation: Dew, Fog, and Clouds, p110-121

(8th Ed: Chapter 5, p106-116)• Ahrens, 9th ed., Chapter 6 Stability and Cloud

Development, p151-157(8th Ed: Chapter 6: p149-155)

• My last lecture: condensation of cloud droplets (micro-scale)

• This lecture: specific conditions and processes that lead to condensation (macro-scale)

Dew, Frost, Haze and Fog (i) Dew: Common on vegetation early morning after a clear

night.

(b) Happens on clear nights because surface radiates heat and this is not trapped by a cloud cover.

(c) Dew forms on vegetation because it cools more quickly than (say) a road surface; vegetation has a lower heat capacity per unit area.

Dew formation releases latent heat, and this reduces the rate at which the surface cools. Therefore the dew point temperature is often the minimum night-time temperature.

(a) How? Air is cooled to Td so becomes saturated.

(ii) Frost: Forms if dew point is below 0°C. This is now called the frost point.

(a) White frost (or hoar frost) forms by ice deposition directly from the vapour, in the form of 6-sided crystals.

(iii) Haze: Dry haze is caused by particulate nuclei, which scatter light like the blue of the sky, and when there is some water deposition, the haze thickens and becomes wet haze.

(b) Black frost (or black ice) forms when the dew point is just above 0°C so the dew forms first and subsequently freezes as temperature drops below 0°C. A layer of ice is formed without the creation of small crystallites.

(iv) FOG – defined when visibility < 1 km

Fog is equivalent to a cloud at the surface.

Fog forms in one of three ways:

1. by cooling the air

2. by evaporation of water into the air

3. by mixing of two air masses such that when combined, the mixture is saturated

The types of fog are named with reference to the method by which the air becomes saturated.

ADVECTION FOG - air is cooled to saturation by having warm moist air moving over a cold surface..

San Francisco has a lot of advection fog since the surface water near the coast is much colder than the water farther off shore. Warm moist air from the Pacific Ocean is advected over the cold coastal waters, chilling the advected air from below.

UPSLOPE FOG

- forms as moist air flows up an elevated plain, hill or mountain. As the air ascends, it expands, cools adiabatically (same as any ascending air), and saturates.

Temperature °C

mix

ing

ratio

g k

g-1

0 10 20-10-20 300

5

10

15

20

25

30

AT = 0 °Cr = 3 g kg-1

BT = 30 °Cr = 20 g kg-1

But at 15 ° Crs = 10.8 g kg-1

CT = 15 °Cr = 11.5 g kg-1

EVAPORATION (or MIXING) FOG - formed by mixing two unsaturated air masses.

STEAM FOG - forms when cold air moves over warm water (e.g. heated outside swimming pool) water evaporates from the pool into the air, increasing the dew point and if mixing is sufficient the air above becomes saturated. The colder air directly above the water is heated from below and rises forming what appears to be steam.

RADIATION (GROUND) FOG - produced by the radiative cooling at the surface - common over land in late autumn and winter. - also form in low-lying areas.

Favourable conditions: 1. clear (cloud-free) nights (cold), 2. shallow layer of moist air near the ground (e.g.

recent rainfall, or near water body), 3. long nights, 4. light winds

Radiation cooling takes place from the top of the fog layer; mixing continues in the fog layer, further cooling the ground surface.

VALLEY FOG. Cold air and high moisture content in river valleys make them susceptible to radiation fog. Radiation fogs form at the ground and are deepest around sunrise - sometimes an increase in thickness at sunrise due to the evaporation of dew supplying moisture to the fog

3 methods of cloud formation:

1. Cool air directly

2. Mix with colder air3. Reduce pressure (adiabatic expansion and cooling)

The rapid drop in pressure abovean aircraft wing can result in sufficientadiabatic cooling to cause saturation

Orographic uplift

Lee Wave clouds

rotoror Lenticular clouds

As species: lenticularis

-will form ‘heap’ clouds e.g. cumulus and cumulonimbus.(may spread out if they reach an inversion, however to form St, Sc)

Ascendingthermals

Slantwise convection – forms most of the layered clouds such as Stratus. Gradient of the slope is about 1 in 100 or 200; thus air may move horizontally 200 km to ascend 1 km.

Geostrophic Wind: vg

Friction near the surface causes cross-isobar flow towards low pressure

Sub-geostrophicwind, due to theeffects of friction

Cross-isobarflow towardslow pressure

Cross-isobarflow towardslow pressure

Clicker question

Air tends to __X__ above an area of __Y__pressure, causing clouds to __Z__.

1. X=ascend; Y=low; Z=form2. X=descend; Y=low; Z=disperse3. X=ascend; Y=high; Z=form4. X=descend; Y=high; Z=disperse5. Both 2 and 3 are correct6. Both 1 and 4 are correct

Midnight (last night)

Convergence occurs in areas of low surface pressure – this is why cloud is usually associated with depressions and troughs.

Conversely, fine weather tends to occur when pressure is high, asit is associated with subsidence and surface divergence.

Rising air cools at the DALR but the dew point drops (at a rate dependent on moisture content) but typically 2 °C/1000 m. Thus Ta and Td approach each other at about 8 ° C per km.

Ahrens Figure 7.16

Calculating cloud base height

general rule for convective clouds is: cloud base 125 x (Tsurface-Td) + 200 m

Hand-in assignment

• It’s important – worth 10% of overall mark, and 1/3 of coursework mark.

• Hand in by noon 9th November to Helen McKeating, Crew 211.

• Include a signed ‘own work’ declaration.

• Penalties for late submission

Midnight (last night)

1200 Monday

0000 Tuesday

1200 Tuesday

0000 Thursday