GC385 Weather and Climate Exam 2

Vapor Pressure

the amount of pressure exerted on the atmosphere by water vapor

Saturation vapor pressure

the maximum possible vapor pressure of the atmosphere when it is saturated.

Saturation vapor pressure is lower over ice or water?

over ice than water of the same temperature.

Relative Humidity – Comparison of how much water vapor is present to how much the air can hold for a given temperature

what does relative humidity depend on?

Air temp

relationship between saturation specific humidity and temp

Increases as air temperature increases

Specific Humidity:

Given mass of water vapor for a given mass of air. Does not change as air expands of contracts

Saturation specific humidity

the specific humidity of the atmosphere when saturated

The dewpoint temperature is

the temperature at which a given mass of air becomes saturated and net condensation begins to form water droplets.

At saturation, evaporation rate is equal to

condensation rate (equilibrium).

Air becomes saturated in three ways:

Addition of water vapor to air. Mixing cold with warm air. Cooling moist air to dew point

Diabatic processes

addition or removal of energy

Adiabatic processes

without transfer of energy

The second law of thermodynamic:

energy is always transferred from areas of high temperature toward those of lower temperatures.

Diabatic processes are typically associated with

fog development but not necessarily clouds.

Adiabatic heating and cooling occurs because

Rising air expands and cools. Sinking air is compressed and warms.

Environmental Lapse Rate

decrease in temp with increasing altitude

DAR

rate at which “dry” air: cools by expansion (if ascending) or heats by compression (if descending).

MAR

rate at which an ascending air parcel that is moist and saturated cools by expansion

Saturation can lead to

formation of liquid water or ice crystals

Stable conditions when

ELR < DAR and MAR: a lifted parcel is cooler/denser than its environment and tends to sink

during stable conditions what is weather like

skies are generally clear with limited vertical cloud growth.

Unstable conditions

when ELR > DAR: parcel stays warmer/less dense than environment, so it keeps rising

during unstable conditions what is weather like

clouds/precipitation likely.

conditionally unstable

When ELR is between the DAR and MAR, unsaturated parcels resist lifting, but a saturated parcel rises at the MAR. • Alternatively, if an unsaturated parcel is uplifted above saturation point, it will become unstable and keep rising.

Convergent Lifting

Air flows toward an area of low pressure.

Low pressure center

air converging and ascending, cooling and condensation occurring

Convectional Lifting

The air above the warmer surfaces is heated and rises.

Orographic Lifting

occurs when air is forced to ascend upslope as it is pushed against a mountain.

what do we use to classify clouds

appearance and height

Two other processes responsible for further droplet growth.

warm and cold cloud growth

Warm clouds (above freezing) produce rain by

collision coalescence.

In cold clouds (below about −10 °C), precipitation often forms by

the Bergeron process.

how does Lake effect snows develop

as the warm lake waters evaporate into cold air. • The below freezing air mass passes over the warmer Great Lakes, it absorbs heat energy and moisture from the lake surfaces and becomes humidified.

pressure

the amount of force exerted per unit of surface area

increasing density causes what on pressure

pressure becomes higher

If temperature is increased what happens to pressure

the molecules move faster, increasing pressure

Pressure gradients

what causes wind

Gravity balances strong vertical pressure gradients to create

hydrostatic equilibrium

The Coriolis force is an effect of

Earth’s rotation

what does the coriolis force do

Deflects moving objects

coriolis force direction

To the right of its moving direction in the Northern Hemisphere

friction affects air

within 1.5 km (1 mi) of the surface, the planetary boundary layer.

Anticyclones are

High pressure areas that have clockwise airflow in the Northern Hemisphere

anticyclones characterized by

descending air that warms and creates clear skies.

Cyclones are

Air converges toward low pressure centers

cyclones characterized by

ascending air which cools to form clouds and possibly precipitation.

Intertropical Convergence Zone (ITCZ)

Equatorial Low

ITCZ causes what weather

Heavy rainfall • Trade winds

upper troposphere only affected by

coriolis and pressure gradients

westerly winds in

upper troposphere

major wind systems

monsoonal, land and sea breezes, mountain valley breezes

Monsoonal Winds Driven by

unequal heating between the Asian landmass and the Indian Ocean

monsoon winds

Winter: high pressure over Asia, cold dry winds flow southward • Summer: low pressure over Asia, moist oceanic air flows northward

monsoon winds bring

Brings intense rainfall June … September; critical for agriculture but also causes floods

land and sea breezes caused by

land and water temp differences (land temp changes from day to night)

land and sea breezes wind

goes from low temp water to land during day

mountain valley breezes

flow up valley mountain during day

El Niño (warm phase)

weakened/reversed trade winds, warmer SSTs, reduced upwelling

La Niña (normal, cool phase)

stronger trade winds, cooler SSTs, enhanced upwelling

Air mass is

a body of air with uniform temperature, humidity, and stability characteristics.

A front is

the transition zone between two air masses with different temperature, humidity, and density characteristics.

Classifying air masses according to

the moisture and temperature characteristics of their source regions: ex mT

Cold Fronts

Cold air forces warm air upwards

Cold front causes

Precipitation behind and at front

Warm Fronts

Warm air moves up and over cold air

warm front causes

Precipitation ahead of the front

Occluded Fronts Occur when

two fronts meet, and the warm air mass between them is displaced aloft. • This typically occurs when a cold front meets a warm front as it circulates the low pressure center of a midlatitude cyclone.

Stationary Fronts

do not move. They do not advance. They are two unlike air masses side by side. • They may slowly migrate and warmer air is displaced above colder.

When a cold front approaches a warm front, what will develop

a midlatitude cyclone