Clouds and precipitation

Earth science final chapter 2

Frontal lifting

Occurs as air is displaced upward along the leading edges of contrasting air masses. (i.e., fronts)

orgographic lifting

occurs when air is forcibly lifted upslope as it is pushed against a mountian.

Convergent lifting

occurs as parcels move toward one another (i.e., surface low pressure)

convective lifting

Air rises on its own due to instability often caused by local surface heating.

Air can be forced to rise due to: (F.O.C.C)

fronts, orography, convergence, convection

A cloud (top) is..

An aggregation of tiny water droplets and/or ice crystals that are suspended in air.

Fog (bottom) is..

A low-lying cloud in contact with the ground.

The composition of clouds depends on

the surrounding temperature

low clouds and/or clouds in warmer, tropical regions tend to

contain only liquid droplets

clouds that occuer exclusively at upper level of the troposphere tend to

contain only ice

deep clouds can contain

both liquid and ice (mixed-phase)

liquid and ice can occur at

the same level in a cloud

once air rises and cools eventually it will

reach saturation

As air continues to cool, condensation will begin on

cloud condensation nuclei

cloud condensation nuclei is

microscopic particles suspended in the atmosphere

without cloud condensation nuclei

A cloud cannot form

Clouds occur in 4 primary altitude classes

low, middle, high, vertically developed

Clouds occur in 3 basic forms

flat: stratiform, puffy: cumuliform, wispy: cirroform

high clouds all have some form of “___” in the name

cirrus

high clouds are made up exclusively of

ice crystals

Generally high clouds are

too thin and high to ever produce precipitation at the surface

Cirrus

Thin and wispy

Cirrocumulus also called a “mackerel sky” because…

The individual small cloud elements look like fish scales

Cirrostratus

Thin layered clouds where the sun is visible through the cloud, often with a halo around it

if you hold your hand and an individual cloud element is about the wdith of 1 finger, that suggests the presence of

cirrocumulus

middle clouds all have some form of “___” in the name

alto

Middle clouds can be made up of

ice and/or liquid depending on environmental temperature

on rare occasions, middle clouds can produce

light precipitation at the surface, but any liquid/ice that does start to fall usually evaporates before reaching the surface (virga)

Altocumulus

Individual medium-sized cloud elements

Altostratus

layered clouds that are thicker than cirrostratus

if you hold our hand and an individual cloud element is about the width of 2 fingers, that suggests the presence of

altocumulus

altocumulus do not represent a

mackerel sky

low clouds all have some form of “___” in the name

stratus

low clouds can be made up of

ice and/or liquid depending on environmental temperature, but typically have more water

only nimostratus produce measurable

precipitation

stratus

thick, layer cloud (sun is not visible)

stratocumulus

individual relatively large cloud elements with little to no blue sky visible between elements

nimbostratus

layered clouds that are thicker than stratus

key difference between nimbostratus and stratus is that

nimbostratus produce measurable precipitation

if you hold your hand and an individual cloud element is about the width of 3 fingers or larger that suggests the presence of

stratocumulus

vertically-developed all have some form of “___” in the name

cumulus

vertically-developed can be made up of

ice and/or liquid, depending on environmental temperature and how vertically developed they are

taller versions of vertically-developed clouds produce

precipitation

cumulus look like

cotton balls with much blue sky visible between cloud elements

cumulus have

relatively little vertical development

cumulonimbus clouds

can extend over nearly the entire depth of the troposphere with a characteristic anvil shape at the top

cumulonimbus clouds can produce

very heavy rain, hail, and other severe weather

what is this

cumulonimbus cloud

anvil is caused by

the stability of the top of the troposphere in addition to strong winds stretching the top of the cloud out.

presence of fog tells us that

the air and dew point temperatures near the ground are nearly identical

nearly saturated conditions

100% relative humidity

3 types of fog

radiation, advection, evaporation

Radiation fog caused by

faster radiative cooling of near-surface air compared to the air higher up

Radiation fog requires

long, clear nights (longer time for cooling) and light winds and a dry atmosphere aloft

if winds are stong (during radiation fog)

there will be too much mixing, and cooling near the ground will be inhibited

advection fog occurs when a

moist, relatively warm layer of air is blown over a cooler surface

winds can be

relatively strong and long nights are not needed

evaporation fog occurs when

water evaporates from a warm water surface adding vapor to the cold overlying air

what requirements are there for evaporation fog for wind or long nights?

none

precipitation

liquid water or ice that falls from a cloud and reaches earth’s surface

precipitation occurs when

condensed water droplets or ice crystals in a cloud grow large enough to fall with gravity Condensed

2 ways for cloud droplets or ice to grow to precipitation size

collision-coalescence and bergeron process

collision-coalescence

liquid drops only

the smaller droplets..

collide with drops as they fall and stick to them helping them grow large enough to fall out of the cloud

bergeron process requires

a mix of liquid and ice

if both phases are present the liquid will..

tend to evaporate

the extrra vapor will..

deposit onto the ice helping it grow until it eventually becomes large enough to fall from the cloud

ice grows at the

expense of the liquid water

rain

precipitation in liquid form

rain requires

a deep layer of air at the surface above the melting point

snow

precipitation in the form of ice crystals

freezing rain

precipitation that is liquid in the air near the surface but freezes on impact with a surface

as little as 0.25 inch of ice can

being down tree branches and power lines

sleet

precipitation in the form of ice pellets

sleet can make travel

hazardous but generally won’t cause power outages

the key difference between sleet and freezing rain is

how deep the near-surface cold layer is

shallow cold layer for sleet does

allow refreezing

hail

large pieces of ice (0.2 up to 8 inches in diameter) that fall to earth

hail requires

strong updraft and much supercooled liquid water

hail conditions only present

in cumulonimbus clouds

hail starts with a

smaller piece of ice that adds layers of ice as it moves up and down in the cloud eventually becoming too heavy to stay in the cloud