Atmospheric Humidity, Adiabatic Lapse Rate, Types

of Cloud

Specific humidity

mass of water vapor per mass of air (g/kg)higher temperature, higher maximum specific humidity

Warm air can “hold more water” than cold air

Line is maximum (capacity) specific humidity

Relative Humidity (RH) :

actual amount of moisture in air RELATIVE to the maximum capacity amount that the air can hold

RH (%) = actual amount moisture at a temp x 100

max. capacity at the same temp

(“saturated air” means that RH = 100%)

• Maximum carrying capacity directly proprtional to temperature

• relative humidity varies with temperature

• Warm air has a higher capacity than cold air to hold moisture

• During day, relative humidity and temp. have inverse relationship

4 am : T = 5oC

actual = 7g/kg

capacity = 7 g/kg

RH = 7 / 7 x 100 = 100%

10 am : T = 16oC

actual= 7 g/kg

capacity = 14 g/kg

7 / 14 x 100 = 50%

3 pm : T = 32oC

actual = 7 g/kg

capacity = 35 g/kg

7 / 35 x 100 = 20%

Calculating RH from specific humidity:

a) If temperature = 20 °C and

actual sp. hum. is 7.5 g/kg, what is RH?

Solution:

1. Use graph to find capacity amount.

2. RH = (7.5 / 15) x 100 = 50%

b) If temperature is 25 °C and actual sp. hum. is 5 g/kg, what is RH?

RH = (5/20) x 100 = 25%

If air temperature is 35°C and wet bulb temperature is 31°C, what is relative humidity?

Dry bulb temperature is 35 & Wet bulb temperature is 31

Wet bulb depression = Dry bulb temperature - Wet bulb temperature

Wet bulb depression = 35-31= 4

RH = 74%

Dew Point

• Temperature at which air becomes saturated during cooling.

• When air temperature equals dew point, air is saturated and• Condensation (or deposition) will occur

• Examples: glass of ice water on a hot day; window in shower room

Dew formation: condensation in place

Frost formation: deposition in place

Rising parcel will cool due to expansionDescending parcel will warm due to compression

RATE depends on whether parcel is dry or wet (saturated)* differs from environmental temperature lapse rate

Cloud Development and Precipitation

Equlibrium

Atmospheric Stability

Air is in stable equilibrium when after being lifted or lowered, it tends to return to its original position – resists upward and downward air motions.

As air rises its pressure decreases and it expands and cools

As air sinks pressure increases and it is compressed and warms

Adiabatic Process

If an air parcel expands and cools, or compresses and warms, with no interchange of heat with its outside surroundings the situation is called an adiabatic process.

Dry Adiabatic lapse rate – 10˚C per 1km . (applies to unsaturated air)

Moist or Wet or Saturated Adiabatic lapse rate - 5˚C per 1km (applies to saturated air). Not a constant. Varies greatly. This number is used to keep things simple.

Determining Stability

Determine stability by comparing the temperature of a rising parcel to that of its surrounding environment.

If it is colder than its environment it will be more dense (heavier) and tend to sink back to its original level. This is called stable air because the parcel resists moving away from its original position.

If the parcel is warmer (less dense) than its environment, it will continue to rise until it reaches the same temperature of its environment. This is called unstable air because the parcel continues to move away from its original position.

Stable Air

Environmental Lapse Rate – rate at which the air temperature of the environment would be changing if we were to climb upward into the atmosphere.

Absolutely stable – the lifted parcel of air is colder and heavier than air surrounding it (its environment).

Stable air strongly resists upward vertical motion, it will, if forced to rise, tend to spread out horizontally and forms Stratus cloud

Atmosphere is stable when the environmental lapse rate is small – when there is relatively small difference in temperature between the surface air and the air aloft.

The atmosphere stabilizes as the air aloft warms or as the air near the surface cools.

Stable Air

Dry air example

Stable Air

Saturated air example

Cold surface air, on this morning, produces a stable atmospherethat inhibits vertical air motions and allows fog and haze to linger close to the ground.

Unstable Air

Absolutely unstable atmosphere – when considering both moist and dry air – the rising air is warmer than the environmental air around them.

Atmosphere becomes unstable when:Daytime solar heating of the surfaceAn influx of warm air brought in by the wind near the

surfaceAir moving over a warm surface

Unstable Air

-

Dry air example

Unstable air. The forest fire heats the air, causing instability near the surface. Warm, less-dense air (and smoke) bubbles upward, expanding and cooling as it rises.

Conditionally unstable air.

The atmosphere is stable if the rising air is unsaturated...

Conditionally Unstable Air

Suppose an unsaturated, but humid air parcel is forced to rise from the surface.

As it rises, it expands and cools at the dry adiabatic rate until it cools to its dew point.

The elevation above the surface where the air is saturated and clouds form is called the condensation level.

Above the condensation level rising air cools at the moist adiabatic rate.

Conditionally unstable atmosphere – the condition for stability being where (if anywhere) the rising air becomes saturated. If unsaturated stable air is lifted to a level where it becomes saturated, instability may result.

When the environmental lapse rate is greater than the dry adiabatic rate, the atmosphere is absolutely unstable. When the environmental lapse rate is less than the moist adiabatic rate, the atmosphere is absolutely stable. And when the environmental lapse rate lies between the dry adiabatic rate and the moist adiabatic rate (shaded green area), the atmosphere is conditionally unstable

Cumulus clouds developing into thunderstorms in a conditionally unstable atmosphere

Level of free convection

The level of the atmosphere where an air parcel, after being lifted, becomes warmer than the environment surrounding it. This air can then rise on its own and the atmosphere is unstable.

Convection and Clouds

Some areas of the earth surface absorb more sunlight than others, and thus heat up more quickly. (Discuss examples)

Thermal – a hot bubble of air that breaks away from the surface and rises, expanding and cooling as it ascends.

As a thermal rises, it mixes with cooler, drier air aloft and gradually looses its identity. But, if it cools to its saturation point, the moisture inside will condense and the thermal becomes a cumulus cloud.

Four primary means of convection(ways to form clouds)

• Surface heating (thermals)

• Topographic (forced) lifting

• Convergence at the surface

• Frontal (forced) lifting

Topography and Clouds

Orographic lift – forced lifting along a topographic barrier (mountains)

Rain Shadow – the region on the leeward side of a mountain, where precipitation is noticeably low and the air if often drier

Lenticular clouds – (mountain wave clouds) form on the lee side of mountains. Resemble waves that form in a river downstream from a large boulder.

Orographic lift, cloud development, and the formation of a rain shadow

Collision and Coalescence Process

In clouds with tops warmer than 15oC collisions between droplets can play a significant role in producing precipitation.

Large drops form on large condensation nuclei or through random collisions of droplets.

As the droplets fall (larger drops fall faster than smaller drops) the larger droplets overtake and collide with smaller drops in their path.

The merging of cloud droplets by collision is called coalescence. (Note: collision does not always guarantee coalescence)

Collision and CoalescenceIn a warm cloud composed

only of small cloud droplets of uniform size, the droplets are less likely to collide as they all fall very slowly at about the same speed. Those droplets that do collide, frequently do not coalesce because of the strong surface tension that holds together each tiny droplet.

Collision and CoalescenceIn a cloud composed of

different size droplets, larger droplets fall faster than smaller droplets. Although some tiny droplets are swept aside, some collect on the larger droplet's forward edge, while others (captured in the wake of the larger droplet) coalesce on the droplet's backside.

Warm Clouds

A cloud droplet rising then falling through a warm cumulus cloud can grow by collision and coalescence, and emerge from the cloud as a large raindrop.

Factors in cloud formation and raindrop production

The cloud’s liquid water content

The range of droplets sizes

The cloud thickness

(heaviest precipitation occurs in those clouds with most vertical development)

The updrafts of the cloud

The electric charge of the droplets and the electric field in the cloud

Ice Crystal (Bergeron) Process

Process of rain formation proposes that both ice crystals and liquid cloud droplets must co-exist in clouds at temperatures below freezing.

This process is extremely important to rain formation in the middle and high latitudes where cloud tops extend above the freezing level (cold clouds)

The distribution of ice and water in a cumulonimbus cloud.

Supercooled water

Collison coalescence occurs here

Ice Nuclei

Ice-forming particles that exist in subfreezing air

Small amount of these available in atmosphere

Clay materials, bacteria in decaying plant leaf material and other ice crystals

Saturation Vapor PressureIce vs Water

In a saturated environment, the water droplet and the ice crystal are in equilibrium, as the number of molecules leaving the surface of each droplet and ice crystal equals the number returning. The greater number of vapor molecules above the liquid indicates, however, that the saturation vapor pressure over water is greater than it is over ice.

Ice Crystal (Bergeron) ProcessThe 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.

It takes more vapor molecules to saturate the air directly above the water droplet than it does to saturate the air directly above the crystal.

Ice crystals grow at the expense of the surrounding water droplets.

AccretionIn some clouds ice crystals

might collide with supercooled liquid droplets. Upon contact, the liquid droplets freeze into ice and stick to the ice crystal – accretion or riming.

The icy matter that forms is called graupel or snow pellets.

Secondary Ice particles

In colder clouds the ice crystals may collide with other ice crystals and fracture into smaller ice particles or tiny seeds which freeze hundreds of supercooled droplets on contact.

Aggregation

As the crystals fall, they may collide and stick to one another forming an aggregate of crystals called a snowflake.fall on the ground as Rainfall or Snowfall

Types of Clouds

Cirrus, Cirrocumulus and Cirrostratus(high 6000-16,000 m)

thin and often wispy composed of ice crystals that originate from the

freezing of supercooled water droplets. Generally occur in fair weather and point in the

direction of air movement at their elevation.

Cirrus

They are made of ice crystals and have long, thin, wispy streamers.

Cirrus clouds are usually white and predict fair weather.

Cirrocumulus

They are small rounded puffs that usually appear in long rows.

Cirrocumulus are usually white, but sometimes appear gray.

Cirrocumulus are usually seen in the winter time and mean that there will be fair, but cold weather.

Cirrostratus

Sheetlike thin clouds that usually cover the entire sky.

Cirrostratus clouds usually come 12-24 hours before a rain or snow storm.

Altocumulus and Altostratus(middle 2,000 to 6, 000 m)

Middle clouds are made of ice crystals and water droplets.

The base of a middle cloud above the surface can be anywhere from 2000-8000m in the tropics to 2000-4000m in the polar regions. An

Altocumulus

They are grayish-white with one part of the cloud darker than the other.

Usually form in groups. If you see altocumulus clouds on

a warm sticky morning, then expect thunderstorms by late afternoon.

Altostratus

An altostratus cloud usually covers the whole sky.

The cloud looks gray or blue-gray.

Usually forms ahead of storms that have a lot of rain or snow. Sometimes, rain will fall from an altostratus cloud.

If the rain hits the ground, then the cloud is called a nimbostratus cloud.

Stratus, Nimbostratus and Stratocumulus(low surface to 2000 m)

Low clouds are made up of water droplets. The base of a low cloud is from the ground to 2000m.

Stratus

They are gray and can cover most or all of the sky (like a big blanket).

Stratus clouds sometimes produce light mist or drizzle.

Stratocumulus

Low, lumpy, and gray. Only light precipitation, usually

drizzle, occurs with stratocumulus clouds.

Nimbostratus

They are dark gray with a ragged base.

Produce rain or snow.

Sometimes they cover the whole sky and you can't see the edges of the cloud.

Clouds with Vertical DevelopmentCumulus and Cumulonimbus

(surface to 13,000 m)

The clouds develop by warm unstale air rising from the surface. Cumulus and Cumulonimbus clouds provide the most interesting and severe weather to our planet.

Cumulus

Puffy white or light gray clouds that look like floating cotton balls.

Cumulus clouds have sharp outlines and a flat base.

Seeing cumulus clouds in the sky can mean the weather will be good or bad.

Cumulonimbus

Known as thunderstorm clouds.

Can grow up to 10km high.

High winds make the top of the cloud flat.

Cumulonimbus clouds can produce heavy rain, hail, lightning, and tornadoes.