ATMO 201 Chapter 5

Briefly describe how each of the following clouds forms: 1. Lenticular 2. Rotor 3. Castellanus

1. Lenticular- stationary lense-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction. 2.Rotor- A turbulent, altocumulus-type. Found in the lee of some large mountain barriers, rotates around an axis, parallel to the range. 3. Castellanus— A cloud species of which at least a fraction of its upper part presents some vertically developed cumuliform protuberances (some of which are taller than they are wide) that give the cloud a crenellated or turreted appearance.

List and explain several processes by which an unstable atmosphere can be made stable.

1. Lower a layer of air 2. The environmental lapse rate decreases 3. Cooling of surface air 4. Cold advection

List four primary ways clouds form and describe the formation of one cloud type by each method.

1. Surface heating and free convection. Thermals are a bubble of hot air that rises. As more and more rise the air cools to it saturation point causing moisture to condense forming a cloud. 2. Uplift along topography. Air moves horizontally across the surface. It cannot got through things it will encounter so the air is forced up. Air goes up cools and forms clouds. 3. Widespread ascent due to convergence of surface air. Air masses coming together forces air up where cooling happens forming clouds. 4. Uplift along weather fronts. Forced lifting along warm and cold weather fronts causes air to be forced up where cooling happens form clouds.

Absolutely Stable Atmosphere

The atmosphere is always absolutely stable when the environmental lapse rate is less than the moist adiabatic rate.

What is a stable atmosphere and how can it form?

The atmosphere is stable when the environmental lapse rate is small. Consequently, the atmosphere tends to become more stable as the air aloft warms or the surface air cools.

Condensation Level

The elevation above the surface where the cloud first forms.

Explain the difference between environmental lapse rate and dry adiabatic rate.

The environmental lapse rate is the rate at which the air temp changes with elevation. Unlike the dry rate, this is the rate of change of temp of the actual air

Environmental Lapse Rate

The rate at which the air surrounding us will change if we were to climb upward into the atmosphere

Explain why rain shadows form on the leeward side of mountains.

The sinking air is drier

Why are moist and dry adiabatic rates of cooling different?

They are different because dry adiabatic rates apply when the air is unsaturated and moist adiabatic rates apply when the air is saturated

Describe the necessary conditions for the formation of convective instability. How do you feel convective instability may play a role in the formation of intense thunderstorms?

warm humid surface with cold dry top

Why are downslope winds on the leeward side of a high mountain range usually much warmer and drier than the upslope winds on the windward side?

warms by compressional heating

Explain why an inversion represents an absolutely stable atmosphere.

With an inversion, warm air overlies cold air; rising air is becoming colder while the air around it is getting warmer. This causes the rising air to sink, creating stability.

How would one normally measure the environmental lapse rate?

a radiosonde

Why are cumulus clouds more frequently observed during the afternoon than at night?

During the afternoon, the ground is warmed by the mid-day sun. This in turn heats the air at the ground, which forms a warm air bubble that rises and eventually becomes a cumulus cloud. This doesn't happen often at night because the ground is cooler at night.

How does atmospheric stability influence the formation of lake-effect snowstorms?

During the summer months, bodies of water, including the great lakes, absorb huge amounts of energy from the sun and from the warm air that passes over them. They become huge reservoirs of heat. The surrounding land, in contrast, cannot store heat nearly as effectively. Consequently during the autumn and winter, the temperature of the land drops quickly, whereas water bodies lose their heat more gradually and cool slowly.

Orographic Uplift

Forced lifting along a topographic barrier (i.e. mountains)

On a warm, sunny day the sky is dotted with small developing cumulus clouds. How does the stability of the atmosphere determine whether these clouds will remain small or grow into cumulonimbus clouds?

If the atmosphere is stable, they will remain small. If the atmosphere is unstable they will grow into cumulonimbus.

Subsidence Inversion

Inversions that form as air slowly sinks over a large area

How and why does lifting or lowering a layer of air change its stability?

Lifting or lowering a layer of air changes causes the air to stretch out or compact as the air density aloft increases or decreases. When the air stretches out, it cools more quickly. The air at the top of the layer will be cooler than the air at the bottom, which steepens the environmental lapse rate and increases instability.

Entrainment

Mixing of cloud and cooler air

Explain why cumulus clouds are conspicuously absent over a cool water surface.

Over a cool water surface there is nothing to keep the air moving upwards, and so cumulus clouds are absent there

Convective Instability

Potentail instability brought about by the lifting of a stable layer whose surface is humid and whose top is too dry; associated with the development of severe storms

Under what conditions would the moist adiabatic rate of cooling be almost equal to the dry adiabatic rate?

When the rising air is very cold

Dry Adiabatic Rate

10 degrees C per 1000 meters; applies only to unsaturated air

Parcel of Air

A small volume of air

List and explain several processes by which a stable atmosphere can be made unstable.

A stable atmosphere can be made unstable when the environmental lapse rate steepens, which occurs whenair temperature drops rapidly with increasing height. Causes include: Cooling of the air aloft Winds bringing in colder air (cold advection) Clouds (or the air) emitting infrared radiation to space (radiational cooling) Warming of surface air Daytime solar heating of the surface An influx of warm air brought in by the wind (warm advection) Air moving over a warm surface

Absolutely Unstable Atmosphere

Absolute instability results when the environmental lapse rate is greater than the dry adiabatic rate

Altocumulus Castellanus

Altocumulus clouds that grow vertically and show tower-like extensions

Why are there usually large spaces of blue sky between cumulus clouds?

Because the thermals that form them on the ground are usually shaded/etc by the cloud that was just formed, and take time for the thermals to build up again.

Why do cumulonimbus clouds (thunderstorms) often have flat tops?

Because the tropopause is very stable, and thus resists the cloud from continuing to grow upwards, and thus it must spread out at the top.

Rotor Clouds

Cloud with extremely turbulent air formed when the air rising form an eddy is cool

Describe the general characteristics of clouds associated with stable and unstable atmospheres.

Clouds in a stable atmosphere tend to spread out horizontally, resulting in cirrostratus, altostratus, nimbostratus or stratus forming in the stable air. Clouds are more likely to grow vertically in an unstable/conditionally unstable atmosphere, as the warm air parcel from the ground tends to keep rising upwards.

Conditionally Unstable Atmosphere

This type of stability tends depends on whether or not the rising air is saturated. If the air is saturated,the atmosphere is unstable. If the air is unsaturated, the atmosphere is stable.

Is strong convection more likely in a stable or unstable atmosphere?

Unstable

If you see a sky full of altocumulus castellans in the late afternoon, what does this observation tell you about the stability of the atmosphere where the clouds have formed?

Unstable; indicates thunderstorms

Adiabatic Process

When a parcel of air expands and cools or compresses and warms without interchanging heat with its surroundings

What is an adiabatic process?

When a parcel of air expands and cools or compresses and warms without interchanging heat with its surroundings

Moist Adiabatic Rate

When air cools at lesser rate than dry adiabatic rate because the relative humidity is 100%

Rain Shadow

When precipitation is less on the leeward (downwind) side of a mountain because the sinking air is drier

Neutral Stability

When the environmental lapse rate is equal to the moist adiabatic rate

If the atmosphere is conditionally unstable, what condition is necessary to bring on instability?

When the environmental lapse rate moves above the dry adiabatic rate