EAS 1420 Exam #2

Stable Atmospheric Conditions

lifted parcel = colder/denser than its environment, it SINKS back down

Unstable Atmospheric Conditions

lifted parcel = warmer/less than its environment, it keeps RISING

Dry Adiabatic Lapse Rate

10* C/km

Moist Adiabatic Lapse Rate

6.5* C/km

Level of Free Convection

point where the parcel become unstable

Lifted Condensation Level

point where parcel becomes saturated (at cloud base)

Methods to lift air parcels

• Convection

• LCL and cloud base

• Topography

• Surface convergence

• Fronts

Complexities to lifting air parcels

changing moisture, latent heat release, non-uniform humidity

How to determine stability from surface parcels

LCL, parcel temperature, and comparing parcel to the environment

Absolutely unstable atmosphere

slope (ELR or environmental lapse rate) > 10* C/km

Absolutely stable atmosphere

slope (ELR) < 10* C/km

Condensation does not follow saturation conditions because

• no water surface in atmosphere

• water droplet must form first

• less neighbors, so molecules can evaporate easier

Condensation nuclei

molecules that water vapor condenses onto

Hygroscopic nuclei

aids in gas → liquid (common)

Deposition nuclei

aids in gas → solid (rare, most don’t work unless temps are less than -10* C)

Freezing nuclei

aids in liquid → solid (common but most don’t work unless temps are less than -10* C)

Ways that parcels of air become saturated

• expansional cooling (lifting air parcels)

• sensible heating (contact with cold surface)

• mixing air parcels

When does dew form?

• overnight

• humid near the surface

• clear skies

• calm winds

Fog

A cloud in contact with the surface

Types of fog

radiative, advection, steam/sea/mixing

How are clouds classified?

stability: stable vs. unstable; altitude: low, middle, high

Cloud classifications

High clouds

• Cirrus

• Cirrostratus

• Cirrocumulus

Middle clouds

• Altostratus

• Altocumulus

Low clouds

• Stratus

• Stratocumulus

• Nimbostratus

Clouds with vertical development

• Cumulus

• Cumulonimbus

Terminal velocity

speed at which falling droplets stop accelerating; droplet at rest is pulled down by gravity

How raindrops are created from cloud droplets

Collision-coalescence: large cloud droplets capture smaller droplets as they fall

Bergeron Process

process that generates snow/develops snowflakes

Difference between freezing rain and sleet

Freezing rain: falls as liquid, and freezes on contact

Sleet: raindrops refreeze into ice before they hit the ground

Temperature profile of freezing rain and sleet

Freezing rain: shallow freezing layer

Sleet: Deep freezing layer

Newton’s 2nd Law

Forces act to accelerate objects (F = ma)

Forces on air parcels

• Pressure gradient force (PGF)

• Coriolis Force (Co)

• Centrifugal force (Ce)

• Gravitational force

• Friction (Fr)

Pressure Gradient Force (PGF)

force pushing air from high to low pressure

direction: high to low pressure

strength: how fast pressure changes

Coriolis Force

direction: deflects to the right of the motion (left in southern hemisphere)

strength: greater towards poles, greater with stronger winds

Centrifugal Force

the outward push when traveling in a circle

direction: outward, does not depend on the direction of the rotation

strength: radius of curvature (how sharp of turn), speed

Gravitational Force

attractive force between any two objects with mass

direction: toward each object

strength: mass and distance

Gravity

combination of the gravitational force and centrifugal force

Forces in balance with hydrostatic equilibrium

Vertical pressure gradient force and gravity

Forces acting in the geostrophic and gradient balances

Pressure gradient force (PGF) & Coriolis force