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