Download what is a cloud? what is fog? - U of L Class Index

Lecture 9: Fog and Clouds
SUMMARISING AIR STABILITY CONDITIONS
Absolute Stability: ELR < SALR
- air parcel returns to original position
Absolute Instability: ELR > DALR
- air parcel continues to move upwards
Conditionally Instability: DALR < ELR < SALR
- air parcel continues to move upwards if condensation is occurring
• Absolute instability is uncommon (mixing reduces the ELR)
• Conditional instability is common: air will undergo condensation and rise
freely if there is a suitable trigger
Geog 1000 Introduction to Physical Geography Fall 2012 : Dr. Hester Jiskoot, University of Lethbridge
• Commonly air cools initially at the DALR, but if the Dew Point T is
reached subsequent cooling occurs at the MALR
WHAT IS A CLOUD?
A visible aggregate of moisture droplet & ice crystals
Need:
1. Saturation
2. Condensation nuclei Æ particulates or salt (= hygroscopic)
3. Cooling (lifting) mechanism
WHAT IS FOG?
Evaporation fog over the Oldman river and Radiation fog over ground
Indian Battle Park, Lethbridge
A ground level cloud layer: visibility < 1000m
Fog often capped by inversion layer
Types of fog:
1.
2.
3.
4.
Advection -moist air passes over a cool surface
Evaporation -sea-smoke/steam-fog: cold are over warmer water
Topographic -upslope/hill-fog: adiabatic cooling through lifting
Radiation -land cools faster than air after sunset: thermal radiation
Photo credit: Constance Sheriff, 3 Oct 2010
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CLOUD TYPES
Cloud characteristics depend on altitude & atmospheric conditions.
Cloud classification according to
CLOUD SHAPE CLASSIFICATION
altitude & shape
Anvil top
Altitude classification
Cirrostratus
Low clouds (0-2 km) Æ prefix strato (stratus, (strato)cumulus)
Middle clouds (2-6 km) Æ prefix alto (Altocumulus, altostratus)
6000 m
Towering
cumulus
High clouds (>6 km) Æ prefix cirro (Cirrus, cirrocumulus, cirrostratus)
Altostratus
Vertical (1-10 km extent): Cumulonimbus, Cumulus
Shape classification
Layers: Stratus
Heaps:
Cumulus
2000 m
Swelling
cumulus
Cirrocumulus Virga
Cirrus
Altocumulus
Alto
cumulus
Stratus
Fair weather
cumulus
Stratocumulus
Nimbostratus Cumulonumbus
Wisps/Feathery: Cirrus
NOW FOR REAL……
Altostratus clouds
Virga
Virga
Cirrus
Stratocumulus
Cumulus
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Contrail changed to cirrus by winds aloft
Contrail
Contrail
ATMOSPHERIC LIFTING MECHANISMS
1. Convergent
2. Convectional
3. Orographic
lifting
/Orographic lifting
4. Frontal
• cold air mass pushes underneath warm
air mass
INFRARED GOES WEATHER SATELLITE: CLOUDS 3 Oct 2010
• steeply sloped front
• warm T ahead of front, cold T behind it
• air rises quickly, cools, condenses,
producing thunderstorms & violent weather
• precipitation at and right behind front
• cirrus clouds far ahead of front
• warm air mass moves over cold air mass
• shallow sloped front
• warm fronts move ~50% slower than cold
fronts
• cold ahead of front, warm behind
• air rises slowly, expands, cools, condenses,
causing precipitation zone
• widespread precipitation, far ahead of front
• cirrus clouds far ahead of front
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HOW STABILITY AFFECTS CLOUD TYPE
Reading for Wednesday
Chapter 5 p. 173-188
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