Weather and Climate 天氣及氣候

What is the difference between weather and climate?

Weather refers to the conditions of the atmosphere over a short period of time at any one place.

Climate refers to the average conditions of the atmosphere over a long period of time (usually more than 30 years) at any one place.

Elements of Climate

Temperature Pressure Wind Precipitation

Air Temperature

Heat balance of the Earth

The sun is the main source of heat for the Earth

After insolation reaches the Earth, heat is released and transferred from the Earth’s surface through radiation, conduction, convection and advection

Direction of prevailing wind

Latitude

Distance from the sea

Ocean Current

Altitude

Aspect

Cloud cover

Nature of the ground

Length of the day

Tempearture is affected by

Latitude

High

Low

- small angle of incidence of insolation- insolation has to pass through a thicker layer of clouds- lower intensity of sunlight

- larger of incidence of i angle nsolation - insolation has to pass through a thinner layer of clouds- higher intensity of sunlight

Lower temperature

Higher temperature

Latitude

Altitude

High

Low

The lower density of air and dust particles

traps less heat

The higher density of air and dust particles

traps more heat

Lower temperatur

e

Higher temperatur

e

Altitude

Temperature inversion

- temperature increases with increasing altitude

- it occurs when warm wind blows over cold wind

- it also occurs when there is a rapid loss of heat from the Earth’s surface due to radiation at night

Aspect

Refers to the direction of a slope faces

Sun-facing slopes receive more insolation than shaded slopes

Effect of aspect on temperature is obvious in temperature areas

Nature of surface

Nature of surface

Land

Sea

bare

forested

Greatest diurnal and annual range of

temperature

lower diurnal and annual range of

temperature than bare ground

Lowest diurnal and annual range of

temperature as sea heats up and cools

down slower than land

Cloud cover

Cloud

cover

Thick

Thin

-less sunlight can reach the ground surface in the day time-more long-wave radiation can be trapped at night

-More sunlight can reach the ground surface in the day time-Less long-wave radiation can be trapped at night

Small diurnal range of

temperature

Greater diurnal range of

temperature

Distance from the sea

Coastal

Inland

Sea heats up and cools down slower than land

Land heats up and cools down faster than sea

On-shore wind lowers the temperature in summer and raises its temperature in winter

Temperature cannot be regulated by on-shore wind

Small annual range of temperature

Extreme range of temperature

Distance from the sea

Ocean current

Warm

Cold

Temperature is raised in the presence of on-shore wind

Temperature is lowered in the presence of on-shore

wind

Ocean currents

Ocean CurrentsThey move along the coast. Warm ocean currents: move towards the polesCold ocean currents: move towards the equator

Onshore winds blowing over a warm current, bring a warming effect in the coastal areas in winter.e.g. North Atlantic Drift

Onshore winds blowing over a cold current, bring a cooling effect to the coastal areas in summer.

e.g. the coast of Norway (North Atlantic Drift)the west coast of Canada (North Pacific Drift)

W Europe (Oporto): E Northern American (New York):

winter T higher winter T lower

Effects of warm ocean currents:

When warm ocean currents move from lower latitudes to higher latitudes, warmth is carried from equatorial/tropical regions.

This maritime moderating effect is particularly significant if the winds blow onshore.

This results in a distinct difference between the winter temperatures on the western and eastern margins of continents on similar latitudes.

Effects of cold ocean currents

When the winds blow onshore, cold currents tend to lower the temperatures of those coastal areas.

The cold Labrador Current brings a colder winter temperature:-same latitude (38 degree N)

- Washington D.C. (0.5 degree C)Lisbon (9.5 degree C)

Effects of cool ocean currents

Cool ocean currents exert cooling influence on summer temperatures in coastal areas of continents.

The winds blow onshore during summer, and coastal temperatures are lowered.

e.g. The cool Californian Current tends to lower the summer temperatures

San Diego 19.4 degree C 35.5 deg N

e.g. The warm Gulf Stream higher the summer temperatures

Charleston 27.2 degree C38.4 deg N

Direction of

prevailing wind

Onshore wind向岸風

Offshore wind離岸風

Moderating effect: warm in summer and

cool in winter in coastal areas

Extreme climate: hot in summer and cold in winter in the affected

areas

Direction of prevailing wind

Length of the day

Length of the day

Long e.g. along the Equator

Short e.g. in higher latitudes

-receive more insolation-Hotter

-receive less insolation-Colder

World distribution of temperature

• Isotherms do not form stationary straight lines parallel to lines of latitude

Causes of variations in isotherms

1. Different air temperature over land and sea surfaces

2. Different height of the land surface

3. The apparent “movement” of the overhead sun between the summer and winter seasons, which causes a slight northward or southward

“movement” of isotherms.

World distribution of temperature

Temperature pattern in January

Characteristics Reasons

Temperature decreases polewardin the two hemispheres

effect of latitude

In the northern hemisphere: temperature is generally lower the sun is overhead at the

southern hemisphere daytime is shorter in the

northern hemisphere

isotherms bend southward overcontinents as they are colderthan the sea

land cools down faster thanthe sea

less moderating effect fromthe sea towards inland

isotherms bend northward alongthe west coast of Europe as thisarea is warmer

effect of warm ocean current

In the southern hemisphere: isotherms bend northward along

the west coasts of SouthAmerica and South Africa asthese areas are cooler than thesea

effect of cold ocean current

Temperature pattern in July

Characteristics Reasons

Temperature decreases poleward inthe two hemispheres

effect of latitude

In the northern hemisphere: temperature is generally higher the sun is overhead (直射) at the

northern hemisphere daytime is longer in the northern

hemisphere isotherms (等溫線) bend

northward over continents as theyare hotter than the sea

land heats up faster than the sea less moderating (緩和) effect from the

sea towards inland

isotherms bend southward alongNE Asia and the west coast ofNorth America as these areas arecooler

effect of cold ocean current

In the southern hemisphere: isotherms have a uniform pattern greater sea surface reduces the effect

of differentialheating and cooling ofthe land and sea

Pattern of annual range of temperature

Characteristics Reasons

Annual range of temperature generally

increases poleward from the Equator

effect of latitude

Lower annual range of temperature

for coastal areas than f or inland

effect of differential heating

and cooling of the land and sea

less moderating effect from

the sea towards inland

Lower annual range of temperature for the

west coasts of Eurasia and North America

that for the east coasts in the same latitude

effect of ocean currents