Atmospheric Stability & Condensation

Atmospheric Stability and Condensation

Outline of the Lecture

• The lecture addresses Adiabatic Temperature Changes.
• The lecture will cover processes that govern air movement.
• The lecture will address the different types of atmospheric stability.
• The lecture will discuss cloud formation and other forms of condensation.

Adiabatic Temperature Changes

• The key to understanding adiabatic temperature changes is the decrease in pressure with height.
• As air rises, it experiences less pressure causing it to expand.
• Air cools as it expands due to molecules spreading out, while compressed air warms as the molecules are confined to a smaller volume.
• As air rises, it will expand and cool; as air sinks, it will compress and warm.
• Adiabatic temperature changes are temperature changes that occur without heat being added or taken away.
• Air cools at a constant rate of 10°C for every 1 kilometer of ascent.
• Descending air is compressed and warmed 10°C for every 1 kilometer.
• This rate of cooling or warming applies only to unsaturated air, known as the dry adiabatic rate.
• An air parcel rises high enough, it will cool to its dew point, triggering condensation.
• The lifting condensation level is the altitude where a parcel reaches saturation and cloud formation begins.
• Latent heat gets absorbed by water vapor when it evaporates and is released as sensible heat at the lifting condensation level.
• Latent heat slows the rate of cooling as the parcel will continue to cool adiabatically.
• Wet adiabatic rate describes the slower rate of cooling ("wet" because the air is saturated).

Processes that Lift Air

• There are four mechanisms that causes air to rise: orographic lifting, frontal wedging, convergence, localized convective lifting.
• Orographic lifting occurs when elevated terrains act as barriers to the flow of air.
• Frontal wedging occurs when masses of warm and cold air collide, producing fronts.
• The cooler, denser air acts as a barrier over which the warmer, less dense air rises.
• Convergence is an atmospheric condition that exists when there is a horizontal net inflow of air into a region, forcing it to rise because it cannot go downward.
• Localized convective lifting produces pockets of rising thermals due to unequal heating of the Earth's surface.

Atmospheric Stability

• Cloud size and resulting precipitation varies a lot and depends on the stability of air.
• Air cools because of expansion when a parcel of air rises (adiabatic cooling).
• Stability can be determined by comparing the parcel's temperature to that of the surrounding air.
• When the parcel is cooler than the surrounding environment, it will be denser and sink to its original position is called stable air, resists vertical movement.
• If a rising air parcel is warmer and less dense than the surrounding air, it will continue to rise until reaching where the temperature equals that of its surroundings.
• Unstable air is like a hot-air balloon-it will rise if the air in the balloon is warmer/less dense than the surrounding air.
• The stability of the atmosphere is determined by measuring the air temperature at various heights called the environmental lapse rate.
• The environmental lapse rate is the actual temperature of the atmosphere determined via radiosondes and aircraft.

Types of Atmospheric Stability

• There are three types of atmospheric stability: absolute stability, absolute instability, and conditional stability.
• Absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate.
• Absolute instability prevails when the environmental lapse rate is greater than the dry adiabatic rate.
• Conditional instability is the most common type of atmospheric instability.
• Conditional instability prevails when moist air is located within an environmental lapse rate between the dry and wet adiabatic rates (between about 5º and 10°C per 1000 meters).

Condensation

Cloud Formation

• A cloud is a visible aggregate of minute water droplets and/or ice crystals suspended in the atmosphere above Earth's surface.
• Clouds are of continual interest to meteorologists because they provide a visual indication of atmospheric conditions.
• Clouds form when water vapor condenses in the atmosphere due to adiabatic cooling.
• Condensation requires two conditions: the air must be saturated; there must be a surface on which the water vapor can condense.
• During dew formation, objects near the ground, like blades of grass, serve as the surface for water vapor to condense.
• Cloud condensation nuclei are tiny particles that serve this purpose when condensation occurs aloft.
• Microscopic dust, smoke, and salt particles, all of which are concentrated in the lower atmosphere and are considered condensation nuclei.
• Hygroscopic nuclei (water-seeking) are the most effective sites for condensation.
• Hydrophobic nuclei are not efficient condensation nuclei because they repel water droplets.

Cloud Classification

• Luke Howard, an English naturalist, published the cloud classification that serves as our present-day system in 1803.
• Howard's system classifies clouds based on their form and height.
• Three basic cloud forms are recognized: cirrus, cumulus, and stratus.
• Cirrus clouds are high, white, and thin.
• Cirrus clouds form delicate veil-like patches or wisplike strands and often have a feathery appearance.
• The term "cirrus" translates to "curl" or "filament" in Latin.
• Cumulus clouds consist of globular cloud masses described as cottonlike in appearance.
• Normally, cumulus clouds exhibit a flat base and appear as rising domes or towers.
• “Cumulus” means “heap” or “pile” in Latin.
• Stratus clouds are best described as sheets or layers (strata) that cover much or all of the sky.
• There are no distinct individual cloud units, although there may be minor breaks.
• High clouds can be found at an altitude of 6 Kilometers or higher.
• Thin, white clouds made up of primarily ice crystals result from low temperatures and small water vapor quantities present at considerable altitudes.
• Middle clouds form in the middle altitude range (2km–6km), described with the prefix alto (meaning "middle"), and include two types: altocumulus and altostratus.
• Low clouds are formed below 2 km altitude, and there are three members of the family: stratus, stratocumulus, and nimbostratus.
• Clouds of vertical development do not fit into any of the three height categories but have their bases in the low height range, extending upward into the middle or high altitudes.

Cloud Observations

• Daily weather forecasts mention cloudy conditions and are defined by the fraction of the sky covered by clouds.
• Cloud Cover refers to the fraction of the sky obscured by clouds when observed from a particular location.
• Meteorologists use clear, few, scattered, broken, and covered words to indicate cloud cover, which this table summarizes.

Fog

• Fog defined as a cloud with its base at or very near the ground.
• Fog and a cloud have same appearances and structures.
• Essential component is the method and formation place.
• Clouds result when air rises and cools adiabatically, but fog results from cooling or when air becomes saturated through the addition of water vapor.

Types of Fog

• Radiation fog forms in the evening and when heat absorbed by the Earth's surface during the day, is radiated into the air that transferred from the ground to the air, water droplets form.
• Advection fog forms when warm, moist air passes over a cool surface, and when the moist/warm air contacts the cooler surface air, water vapor condenses to create fog.
• Upslope Fog, created when relatively humid air, moves up a gradually sloping landform.
• Air expands and cools adiabatically, the dew point is reached, and an extensive fog layer will form because of the upward movement.
• Steam fog occurs when cool air moves over warm water, so enough moisture evaporates from the water surface to saturate the air immediately above.
• The rising water vapor meets the cold air, condenses, and rises, forming steam fog.

Upcoming Schedule

• March 5, 2024: Laboratory Exercises (Lesson 3 and Lesson 4)
• March 6, 2024: Quiz No. 2 (Lesson 3 and Lesson 4)