Lecture 5: (In)stability and local winds 2

Question 1

LOCAL WIND SYSTEMS : THERMAL CIRCULATION

Answer 1

• Local heating and cooling result in regional changes of air density.
• Dipping of the isobars results in a horizontal pressure gradient force aloft.
• As the air begins moving at altitude the surface pressure begins to change and a PGF results near the surface with opposite wind direction.
• Local circulations brought on by changes in air temperature are called thermal circulations.

Question 2

Sea and land breeze

Answer 2

• Anothertypeofthermal circulation is the Sea and Land Breeze.
• Duringdaytimelandheatsup more quickly than the sea and a thermal circulation forms with a sea breeze.
• During night time the land surface cools down much more quickly and to lower temperatures than the sea, reversing the direction of the thermal circulation.
• The temperature difference during night is usually smaller such that the land breeze is normally weaker than the sea breeze.

Question 3

IMPACT ON CIRCULATION

Answer 3

On a clear, relatively calm night, a weak country breeze carries pollutants from the outskirts into the city, where they concentrate and rise due to the warmth of the city’s urban heat island.

This effect may produce a pollution (or dust) dome from the suburbs to the center of town

Question 4

THE URBAN HEAT ISLAND EFFECT

Answer 4

The Urban Heat Island Effect refers to a significant increase in air temperatures in a metropolitan area in comparison to the surrounding rural area

Question 5

THE URBAN HEAT ISLAND EFFECT- formation

Answer 5

The formation of a heat island is the result of the interaction of the following factors:

the release (and reflection) of heat from industrial and domestic buildings

the absorption of heat by concrete, brick and tarmac during the day, and its release into the lower atmosphere at night

enhanced cloud formation due to pollution can trap heat

the relative absence of water in urban areas means that less energy is used for evapotranspiration and more is available to heat the lower atmosphere

the absence of strong winds to both disperse the heat and bring in cooler air from rural and suburban areas

Question 6

MOUNTAIN AND VALLEY BREEZE

Answer 6

• During day time the sloped valley walls receive more insolation per surface area.
• The air above the valley walls gets heated more efficiently and a gentle upslope wind, the valley breeze, is formed.
• Duringnighttimethecool(dense)airglidesdownhill into the valley, providing a mountain breeze.
• This daily cycle of wind flow is best developed in clear summer weather when prevailing winds are light.

Question 7

NIGHT-TIME INVERSION CAUSED BY OROGRAPHY

Answer 7

At night, cold air and pollutants drain downhill and settle in low-lying valleys.

Question 8

INCREASED POLLUTION CAUSED BY SUBSIDENCE INVERSION

Answer 8

• Subsidenceinversions form when air above a deep anticyclone (high pressure) slowly sinks (subsides) and warms due to compression.
• Colderairatthesurface is prevented from mixing with the sinking warm air (it is more dense than the air above)
• A thick layer of polluted air is trapped in the valley.
• The top of the polluted air marks the base of a subsidence inversion

Question 9

ADIABATIC PROCESS

Answer 9

When an air parcel cools and warms or expands and compresses without exchange of matter and energy with the surrounding air - this is called an adiabatic process

Question 10

more on the adiabatic process

Answer 10

• Air pressure and density decrease with altitude.
• As an air parcel rises it is moved into an area of lower surrounding pressure.
• As a consequence the molecules inside the air parcel perform work to expand the boundaries of the parcel.
• The energy used for this expansion must come from the molecules inside the air parcel.
• This reduction in kinetic energy results in a lower temperature.
• When the air parcel is moved to lower altitude with higher surrounding pressure, the surrounding air molecules compress the boundaries of the air parcel, increasing the kinetic energy (average speed) of molecules inside: the air gets warmer.

Question 11

ADIABATIC PROCESSES AND ENVIRONMENTAL LAPSE RATE

Answer 11

The rate at which the temperature decreases with altitude is referred to as the Environmental Lapse Rate (ELR).
• This temperature gradient (the ELR) is crucial in determining the extent of vertical motion in the atmosphere.

Question 12

ATMOSPHERIC STABILITY: PRINCIPAL CONCEPT

Answer 12

• Atmospheric stability refers to a condition of equilibrium with respect to vertical motion.
• Air that is in a stable equilibrium will resist vertical motion.
• Air in an unstable equilibrium will, given a slight dislocation, move farther away from its original position

Question 13

DRY ADIABATIC LAPSE RATE (DALR)

Answer 13

In unsaturated air (i.e. when the relative humidity is less than 100%) the rate of warming and cooling due to vertical motion remains constant.
• This rate of temperature change is approximately 10oC for 1000 m of change in elevation.
• This rate of temperature change is called the dry adiabatic lapse rate (DALR) and applies only to unsaturated air.

Question 14

MOIST ADIABATIC LAPSE RATE (MALR)

Answer 14

• As the rising air cools to its dew point temperature the relative humidity reaches 100% and the air becomes saturated.
• Further lifting of the air parcel will result in condensation, a cloud forms, and latent heat is released inside the air parcel.
• As a result the cooling due to expansion gets partly off-set by the latent heat release.
• This rate of temperature change is
on average 6oC for 1000 m of change in elevation.
• This rate of temperature change is called the moist adiabatic lapse rate (MALR) and applies only to saturated air.

Question 15

ATMOSPHERIC STABILITY

Answer 15

Atmospheric stability is determined by comparing the rate of a rising air parcel’s temperature change with the ambient environmental lapse rate.