MET 10 Lecture 6

Atmospheric Stability and Cloud DevelopmentChapter 5

Dr. Craig ClementsSan Jose State University

Atmospheric Stability

Hydrologic Cycle

Atmospheric stabilityAtmospheric stabilityWe determine the stability of air by comparing the temperature of a

rising parcel to that of its surroundings or ‘environment’. If the parcel is colder than its environment, it will be more dense

(heavier) and tend to sink back to its original level. This condition is called stable.

Stable: Air is stable when it resists upward displacement.

If the parcel is warmer than its environment, it will be less dense (lighter) and will continue to rise until reaching the same temperature as the environment. This is an example of unstable air.

Unstable: A lifted parcel of air will be warmer than the surrounding air and thus will continue to rise upward, away from its original position.

Atmospheric Lapse RatesAtmospheric Lapse RatesAdiabatic process: when a parcel of air expands and cools, or compresses and warms, with no interchange of heat with the outside surroundings.

- As long as the air in the parcel is unsaturated (RH<100%) the rate of adiabatic cooling or warming is constant Dry Adiabatic Lapse Rate = 10°C /1000 m (5.5°F /1000 ft.)

- If the rising air cools to its dew-point temperature, the RH becomes 100%, condensation occurs, a cloud forms, and latent heat is released. This added heat during condensation offsets some of the cooling. The air now cools at a lesser rate

Moist Adiabatic Lapse Rate = 6°C /1000 m (3.3°F /1000 ft.)

Dry Adiabatic Lapse Dry Adiabatic Lapse RateRate

The atmosphere is stable when the environmental lapse rate is small, that is, when there is relatively small difference in temperature between surface air and the air aloft.

The atmosphere becomes more stable ( it stabilizes) as the air aloft warms or surface air cools.

Stable Atmosphere

What can cause the surface air to cool?

Becoming more stable

Unstable Atmosphere

The atmosphere is unstable when the environmental lapse rate steepens, that is, when the temperature of the air drops rapidly with increasing height.

The atmosphere becomes more unstable (destabilize) as the air aloft becomes colder or the surface air warms.

What can cause the air to cool aloft?

The surface air to warm?

Becoming more unstable: destabilizes

Convection: Stable or Unstable?

Two lapse rates at once!

Cloud Development and Cloud Development and StabilityStability

Most clouds form as air rises, expands, and cools.

- A majority of the clouds we observe are due to the following mechanisms:

1. Surface heating and free convection2. Uplift along topography3. Widespread ascent due to the flowing together (convergence) of surface air.4. Uplift along weather fronts (different air masses).

Air that is saturated: number of molecules escaping the water surface = amount returning.

At higher air temperatures, it takes more water vapor to saturate the air.

Convection

Lifted by topography

Convergence

Lifting along weather fronts

Cumulus clouds building on a warm summer afternoon. Each cloud represents a region where thermals are rising from the surface. The clear areas between the clouds are regions where the air is sinking.

Orographic uplift, cloud development, and the formation of a rain shadow.

Clouds that form in the wave directly over the mountain are called mountain wave clouds, whereas those that form downwind of the mountain are called lee wave clouds

Dew point represents the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.

Dew point is a good indicator of the air’s actual water vapor content.

High dew points indicate high water vapor content, low dew points = low water vapor content.

Cloud dropletsRaindrops

Condensation nuclei

Which of the three drops drawn here represents

the real shape of a falling raindrop?

Fig. 4-8a, p. 86

Precipitation Processes

Collision-Coalescence process:

• Larger cloud droplets fall faster than smaller droplets.

• The larger droplets collide with smaller drops in their path.

• This merging of cloud droplets by collision is called coalescence and occurs in clouds with tops warmer than -15°C.

• Called warm clouds.

• Coalescence is enhanced if the colliding droplets have opposite electrical charges.

Dew and Frost

On calm, clear nights, the surface cools rapidly by what process?

Air near the ground cools to the dew point quickly, reaching saturation.

Water vapor condenses on blades of grass at the ground, forming tiny specks of water called dew.

Precipitation Processes in Warm Clouds

• Most important factor in the production of raindrops is the liquid water content of the cloud.

• Other important factors:1. The range of droplet sizes2. The could thickness3. The updrafts of the could4. The electric charge of the droplets and electric field in the

cloud.

Precipitation Processes in Cold Clouds

Ice-crystal process (Bergeron):

• This process proposes that both ice crystals and liquid cloud droplets must co-exist in clouds at temperatures below freezing.

• Important in area where clouds can extend upward into regions where the air temperatures are below freezing.

• These clouds are called cold clouds.

• Water droplets existing at temperatures below freezing are referred to as supercooled.

The distribution of ice and water in a cumulonimbus cloud.

Frost

When the dew point is below freezing (now called the frost point), frost forms which is composed of tiny ice crystals.

Water vapor changes directly into ice without becoming liquid first– called deposition.

The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller.

Accretion Splintering forming graupel

AggregationForming snowflakes

Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain

Cirrus cloudsThe streaks of falling precipitation that evaporate before reaching the ground are called virga.

The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks.

dendrite snowflakes

Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis

Average Annual Snowfall over US

An aircraft undergoing de-icing

during inclement winter weather

HW #4: Due Thursday October 9

1. Identify cloud types for 3 days: Friday, Saturday, and Sunday would be a good choice.

2. List whether they are high, middle or low clouds.3. List the time that you observed them.4. If you can photograph them, put the photos into your report or find similar photos on the web and include

these into your report.

Components of a standard rain gauge.

Tipping bucket rain gauge

A microwave pulse sent out from the radar transmitter. The pulse strikes raindrops and a fraction of its energy is reflected back to the radar unit, where it is detected and displayed

Doppler RadarRadar- radio detection and ranging

A radar is a remote sensing instrument: - measuring the atmosphere from a distance

Doppler radar display showing precipitation intensity

The numbers under the letters DBZ represent the logarithmic scale for measuring the size and volume of precipitation particles

Doppler radar display showing 1-hour rainfall amounts

Reading and HW assignment:Due: Monday, Oct. 11

Reading: Precipitation Types. pages: 126-134.

Homework: Chapter 5 problems page 138.

Answer: 4, 20,23, 24, 25