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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 1 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 2 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 3 HOW STABILITY AFFECTS CLOUD TYPE Reading for Wednesday Chapter 5 p. 173-188 4