CLIMATOLOGY
Weather is the atmospheric condition of an area recorded within a short period of time, which can be day after day, week after week, or after a certain period of time.
The weather condition of the area is determined by recording the behavior of weather elements such as temperature, humidity, sunshine, cloud cover, rainfall, wind, etc.
Climate is the description of the atmospheric condition recorded over a long period of time, which can be above 30 years.
It is determined by measuring or recording the behaviors of weather elements within a long period of time.
Climatology is the science which involves analysis of spatial distribution of atmospheric phenomena. It is a scientific study of different processes and conditions taking place in the atmosphere.
Micro-climate refers to the atmospheric condition of a small area which is different from the surrounding area.
Insolation is the solar radiation from the sun received in the atmosphere and on the surface of the earth, or it is the total energy generated from the sun and received in the atmosphere and on the surface of the earth.
Weather station is the place where all elements of weather are observed, measured, and recorded.
Weather forecasting is the practice of predicting future weather conditions of a given area. It can be done in two ways:
i. Traditional ways
Temperature is the degree of hotness or coldness of an area or a body.
Temperature is measured or expressed in terms of centigrade.
Temperature of the earth’s surface originates from the sun and is called solar energy. Solar energy, when it reaches the earth’s surface, is transformed into heat by the land and the atmosphere before it reaches the surface of the earth.
The amount of incoming solar radiation or insolation received in the area depends on the intensity and duration of radiation from the sun. These are determined by both the angles at which sunrays strike the earth and the number of daylight hours. These two fundamental factors plus the following five factors determine the temperature of any given area on the surface of the earth.
Other factors which influence temperature are:
i. Earth’s inclination (angle of the sun towards the earth)
The axis of the earth, that is the imaginary line connecting the North Pole through the South Pole, always remains in the same position.
It is tilted at about 23.5
away from the perpendicular. Every 24 hours the earth rotates once at that axis, while rotating the earth slowly revolves around the sun in a nearly circular, annual orbit. If the earth was not tilted from the perpendicular, the solar energy received at a given latitude would not vary during the course of the year. The rays of the sun would strike the equator and all other parts of the earth equally. When the Northern Hemisphere is directly tilted towards the sun, the vertical rays of the sun are felt as far north as 23.5° North latitude, the Tropic of Cancer, and this occurs on June 21st. It is called the summer solstice, and also the winter solstice in the Southern Hemisphere.
ii. Cloud cover
Clouds which are denser and concentrated normally tend to reflect a great deal of energy. Light-colored surfaces, especially snow cover, also serve to reflect a large amount of solar energy. Therefore, the amount of cloud cover affects the amount of sunrays that reach the surface of the earth. More cloud cover reduces the amount of heat on the surface of the earth. Energy is lost through re-radiation as well as reflection. In the re-radiation process, the earth’s surface acts as a communicator of energy; the energy that is absorbed into the land and water is returned to the atmosphere in the form of terrestrial radiation.
iii. Nature of earth’s surface
Some kinds of earth’s surface materials, especially water, store solar energy more effectively than others. Because water is transparent, solar rays can penetrate a great distance below its surface. If water currents are present, heat is distributed even more effectively.
The land surface is opaque, so all the energy received from the sun is concentrated at the surface. Therefore, the temperature of the earth between water bodies and the land is different, whereby the land experiences higher temperatures than the water bodies. Because the earth heats and cools faster than water, hot and cold temperatures are recorded on the land and not in the sea or water bodies.
iv. The distance of elevation above sea level
The temperature on the surface of the earth varies with the variation in altitude, but for every 100m rise above sea level, the temperature decreases at the rate of 0.65, and this process is called temperature lapse rate.
v. The distance from the sea
Note that coastal areas have lower summer temperatures and higher winter temperatures than those places at the same distance from the equator excluding sea coasts. This difference in temperature during different seasons is influenced by water bodies.
vi. Ocean currents
Ocean currents are divided into two types:
a. Warm ocean currents: warm ocean currents raise the temperature over the areas they flow.
b. Cold ocean currents: cold ocean currents lower the temperature of the areas they flow.
Temperature lapse rate
A temperature lapse rate is the atmospheric condition where temperature decreases with an increase in altitude. It is mostly found in the troposphere and mesosphere. There are two types of temperature lapse rate.
i. Normal lapse rate
It is also called environmental or static lapse rate. It refers to the decrease in temperature due to increase in altitude at the rate of 0.65
for every 100m above sea level.
ii. Adiabatic lapse rate
Is the rate of decrease in temperature with the rising air mass. It is divided into two types.
a. Dry adiabatic lapse rate (DALR) / unsaturated
It is the rate of cooling of a rising dry air at the rate of 1 per 100m or 10 per 1km.
b. Wet adiabatic lapse rate (WALR)/saturated
It is the rate of cooling of a rising saturated air at the rate of 6 for every 100m.
Temperature inversion
Under normal environmental conditions, atmospheric temperature decreases with increase in altitude, but on some occasions, atmospheric temperature tends to increase with increase in altitude.
Temperature inversion is the atmospheric condition where temperature increases with height or altitude. It is mostly observed in the stratosphere and thermosphere.
In the stratosphere, temperature increases due to concentration of ozone layer or ozone gas which absorbs incoming solar radiation from the sun and converts it into heat energy, making the upper part of the atmosphere more heated than the lower parts.
In the thermosphere, temperature increases with increase in height due to concentration of atomic oxygen and presence of electrically charged particles which absorb incoming solar radiation.
i. Presence of ozone gas
In the atmosphere, the gas absorbs solar radiation and changes it into heat which eventually increases the temperature on the upper part of the stratosphere and other layers of the atmosphere.
ii. Presence of atomic oxygen gas and electrically charged particles
In the thermosphere, these gases and particles increase heat in the atmosphere as the altitude increases.
iii. Terrestrial radiation
During night, there is no sunshine; thus the earth’s surface, especially the land, loses heat more rapidly and this makes warm air to be uplifted or rise up from the land. The process warms up the upper part of the atmosphere, increasing the temperature as the altitude increases.
Long wave radiation is responsible for this process.
iv. Convergence of air masses (formation of air fronts)
Convergence of air masses of different temperature conditions makes warm air to be forced up by cold air which descends from the atmosphere to the surface of the earth.
v. Presence of water vapour (atmospheric components)
Clouds, dust, and other materials in the atmosphere reflect and absorb incoming radiation from the sun, changing them into heat energy which later warms up the atmosphere.
Weather results from the interaction of solar radiation on the earth’s atmosphere and the earth’s surface. The two movements (rotation and revolution) explain the changing elevation of the sun as well as latitudinal and seasonal variations in length of the day, receipt and escape of radiation, and weather.
The sun is the source of all energy on the earth’s surface. Only a portion of sun radiation reaches the earth’s surface as direct radiation. The remainder is reflected, absorbed, or scattered by the atmosphere or atmospheric components. Therefore, the sun emits almost 100% of the heat but not all of it reaches the surface of the earth, as shown below:
The total amount of solar radiation received on a horizontal surface is about 43%. Out of this, 27% penetrates directly to the earth’s surface and 16% arrives as diffuse sky radiation. So 43% reaches the ground together. The atmosphere including clouds absorbs 15%, and the remaining 42% is reflected back into space, which represents the albedo of the earth. The ground accounts for 33% and the diffuse reflection makes up the remaining 9%.
It is noticed that diffuse radiation towards the ground (16%) is considerably greater than the returned to space (9%). This difference is due to the fact that larger dust particles scatter more radiation in the direction away from the sun than towards the sun.
In the figure, the radiation received by the earth and atmosphere is counted positive (+ve) and the radiation emitted or reflected and scattered to space is denoted by negative (-ve).
42% of incoming solar radiation is returned directly back to space and the remaining 58% is absorbed by the ground and the atmosphere.
This 58% must be radiated back to space since the yearly mean temperature of the earth as a whole remains the same. The radiation from the ground is called effective radiation.
The 24% represents the difference between the actual radiation from the ground and radiation from the atmosphere to the ground. Out of this 24%, 16% is absorbed in the atmosphere while 8% returns directly to space. The other 50% is radiated back to space by the atmosphere.
Terrestrial radiation is the radiation from the earth (from the land masses and water bodies).
The radiation emitted from water and land is long wave radiation, and the radiation from the sun to the surface of the earth and to the atmosphere is short wave radiation.
Atmospheric pressure
Atmospheric pressure is the downward force exerted by the weight of air per unit area of the earth’s surface.
The distribution of it is not the same in all regions; hence it differs from time to time and from one place to another.
Factors affecting atmospheric pressure
i. The vertical height above the earth’s surface (altitude)
Atmospheric pressure varies with altitude; on the surface of the earth, it decreases due to increase in altitude.
Therefore, near sea level, pressure is higher than in mountainous areas.
ii. Temperature radiation from one place to another causes variation in atmospheric pressure. When air is heated, it becomes less dense and rises up causing a low pressure zone; when it cools, it contracts and exerts high pressure.
iii. Overhead sun (apparent movement of overhead sun)
When the sun is overhead in the tropics, it creates considerable seasonal changes of atmospheric pressure over the earth’s surface in the respective area. During that time, the rate of insolation is high resulting in low pressure as the air is heated and expands. Other areas not experiencing overhead sun have low insolation resulting in cooling of air which creates a high pressure zone.
iv. Latitude
Atmospheric pressure varies from one place to another depending on the latitudinal position of an area. For example, in equatorial regions there is low pressure due to high solar radiation caused by high temperature at the equator.
In polar areas, there is high atmospheric pressure because of low temperature caused by little insolation from the sun.
v. The earth’s rotation
Rotation of the earth causes day and night which results in variation in atmospheric pressure as the earth is inclined at different angles towards the sun.
Wind system
Wind is air in motion from the region of high pressure to the region of low pressure.
Wind can be classified into two types:
i. Local winds
ii. Interplanetary winds
i. Local winds
These are wind systems operating only in a small area within a short period of time. These winds include the following:
a. Land breeze
This is the movement of air from the land to the sea or ocean.
Land breeze occurs during night when the land is colder than the sea; hence the land develops high pressure and the sea develops low pressure.
b. Sea breeze
This is the wind that moves from the sea to the land. It occurs during the daytime when the ocean is cooler than the earth or the land. During that time, the water bodies, especially the oceans and seas, develop high pressure and the land develops low pressure.
c. Anabatic wind
The word anabatic is derived from the Greek word “anabatos” meaning moving upward. Anabatic is a warm wind which blows up steep slope maintain side, driven by heating of the slope through insolation. It is also known as upslope wind as it blows from the valley towards the mountain slope or valley slope.
This wind occurs during daytime when hillside is heated by calm sunny weather. It is common in mountainous areas.
d. Katabatic winds
Katabatic, the word is derived from the Greek word “katabasis” meaning descending. It is the technical name for drainage wind, a wind that carries high-density air from higher elevation down the slope under the force of gravity.
These winds are sometimes called fall winds. They occur during the night in mountainous areas when the highland areas are losing more heat due to high rate of terrestrial radiation.
The word monsoon is derived from the Arabic word “mausin” meaning season. Monsoon winds are winds whose directions are reversed from one season to another due to the change of pressure belts caused by apparent movement of overhead sun. They develop due to difference in season when the sun is overhead in tropics. During summer when the sun is overhead in the Tropic of Cancer, wind blows from high pressure belt in the southern hemisphere to the Middle East especially in Asia. During winter when the sun is overhead in the Tropic of Capricorn, central Asia develops high pressure hence wind blows from Asia to Australia. Monsoon wind is common in India, Japan, Australia, Indonesia, and other parts of the Middle East.
ii. Interplanetary winds are as follows:
a. Trade winds
These are prevailing patterns of earthly surface winds found in tropics within the lower portion of the earth’s atmosphere, in a lower section of the troposphere near the earth’s equator. They blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during winter and when the Arctic oscillation is in its warm phase. The wind blows from the sub-tropical high pressure belts towards the intertropical convergence zone. They are divided into two types:
i. North East trade wind
These winds blow from the northeastern direction in the Northern Hemisphere because of rotation which affects wind direction.
ii. South East trade wind
They blow from the southeastern direction in the Southern Hemisphere.
b. Mid latitude westerlies
The westerlies comprise the air flowing from the sub-tropical highs to the sub-polar lows from about 30 to 60 N and S of the equator.
These belts move north and south with the seasonal change of pressure belts.
These winds originate from high pressure areas in the horse latitudes (30 N and S of the equator) towards the 60 N and S of the equator (sub-polar low pressure belts). The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
Westerlies are strongest in the western hemisphere and at times, when the pressure is lower over poles while they are weakest in the Southern Hemisphere and wind pressure is higher over the polar.
c. Polar winds
They blow from the polar ice towards the equator. The movement is pronounced in both hemispheres. Polar winds are dry and cold winds that blow from high pressure areas of the polar at the north and south towards low pressure areas (sub-polar low pressure 60 N and S).
Air masses are formed when air remains stationary over a place for several days. During this time, the air is likely to assume the temperature and humidity properties of that area.
Air masses are a body of air covering a relatively wide area, exhibiting approximately uniform properties through any horizontal section.
Air masses are a large volume of air in the atmosphere that is mostly uniform in temperature and moisture.
For a place to develop an air mass it should have the following conditions:
i. There should be a large uniform surface, for example ocean or a desert or any other surface which is uniform.
ii. There should be stagnation in atmospheric circulation or no change of weather condition.
iii. There should be relatively constant temperature.
Air masses can be divided into groups according to their source regions and characteristics, both of temperature and humidity.
On the basis of temperature, they are known as polar or tropical, and on the basis of humidity they are known as marine (having crossed the oceans and so moist) or continental (originating over the continent and so dry). Their combination allows four main categories of air masses to be distinguished.
i. Polar continental air masses (pc)
This air originates over the continental interiors, the northern tundra land of northern America, Asia, and Greenland forming masses of cold dry air. Their air mass yields little or no rainfall.
ii. Polar maritime air masses
These are air masses which originate and travel over high latitude oceans (60 N and S) such as the northern Atlantic Ocean. They are cooler in winter but warm in summer. They have low temperature and low moisture content.
iii. Tropical continental air masses (TC)
These are air masses which originate and develop from tropical deserts such as Sahara and Australian desert. They are warm and generally tropical continental air masses originate from tropical deserts.
iv. Tropical maritime air masses
These originate in tropical latitudes but having crossed the water bodies towards the north and south pole or originate from tropical oceans such as Pacific and Atlantic. They are warm and moist, yielding heavy rainfall with lightning and thunders.
A frontal zone is the point where two air masses of different characteristics in terms of temperature and humidity meet each other after they have moved outward from the various major high pressure areas.
The situation results in the condition of large scale atmospheric instability in the frontal zones.
Conditions for the occurrence of frontal zones:
i. There must be two contrasting air masses moving towards one another.
ii. There must be a convergence of two air masses.
i. Warm frontal zones
This is formed when a strong warm air is moving or advancing and is forced to override a weak cold air mass. It is formed when a strong warm air mass meets with a weak cold air mass and slides up over it. The whole surrounding is covered by warm air.
Weather conditions associated with warm frontal zones:
i. Formation of clouds
ii. There will be a formation of warm weather condition
iii. Formation of cyclonic rainfall characterized by lightning and thunderstorms
It occurs when a strong advancing cold air mass overcomes a body of weak warm air mass. During air meeting, weak warm air is forced to rise above causing the surrounding to be covered by cold air mass.
Weather conditions associated with cold frontal zones:
i. Formation of little or sparse clouds
ii. Formation of cold weather condition because the surface will be characterized with low or cool temperature
Air is said to be stable since dew point may not be reached and therefore the atmosphere produces few clouds with little or no rainfall at all.
The situation occurs when dry adiabatic lapse rate is greater than normal lapse rate.
The atmosphere stability is associated with low degree of dampness in the atmosphere with no cloud, high day temperature.
Air instability occurs when the rising saturated air cools less rapidly than the surrounding air. The rising air remains warmer and lighter than the surrounding air.
If the air gets sufficient moisture and the dew point is reached, this condition may result in heavy clouds, thunderstorms, heavy rainfall, and high degree of atmospheric dampness and very small daily range temperature.
It occurs when environmental lapse rate is greater than dry adiabatic lapse rate.
Humidity is the amount of water vapour in the atmosphere. It can be absolute or relative humidity.
Is the actual amount of water vapour present in a certain volume of air at a given temperature.
Is the amount of water vapour present in a mass of air expressed as a percentage.
Factors affecting humidity
i. Altitude
The amount of humidity increases as the altitude from the surface increases. This is because of normal lapse rate.
ii. Temperature
High temperature increases humidity in the atmosphere due to increase in rate of evaporation from the earth’s surface.
iii. Rainfall
High rainfall increases the amount of water on the surface of the earth which when evaporates results in the formation of humidity or water vapour in the atmosphere. Also rainfall increases the amount of water in the atmosphere due to increased moisture on the surface.
iv. Availability and size of water bodies on the earth’s surface
Many water bodies like oceans and seas increase the amount of water vapour or humidity in the atmosphere through evaporation.
v. Vegetation cover
Areas covered by denser vegetation like equatorial region have high atmospheric water vapour due to high rate of evapotranspiration from the vegetation.
vi. Human activities such as afforestation, construction of water reservoirs also influence humidity in the atmosphere.
Condensation is the formation of water droplets when rising air has been cooled beyond its dew point.
A dew point is a temperature where air becomes saturated as a result of cooling process in the atmosphere.
It is the point where atmosphere does not contain any further water vapour and condensation begins to take place.
The cooling of air in the atmosphere occurs through the following ways:
i. When there is horizontal movement of warm air on the cold surface
ii. Through movement of air from warmer to cooler latitudes
iii. Through ascending of warm air.
iv. By direct radiation from the earth’s surface to the atmosphere.
Haze is impaired visibility of 1 to 2 km as a result of dust and other small particles in the atmosphere.
Mist is impaired visibility caused by condensation of water vapour into small droplets that form clouds at the ground level. It reduces visibility to less than 2 km.
Fog is the collection of liquid water droplets or ice crystals suspended in the air at or near the earth’s surface. Fog can be considered as a type of low-flying cloud and is heavily influenced by nearby bodies of water, topography, wind conditions, and even human activities.
Fog forms when the difference between air temperature and dew point is generally less than 2.5. They begin to form when water vapour condenses into tiny liquid water droplets suspended in the air.
1. SMOG
It is the combination of smoke and fog, it reduces visibility to about zoom; hence it is very dangerous and can cause accidents.
2. RADIATION FOG
Is formed by cooling of land after sunset by thermal radiation in calm condition with clear sky. Cooling ground produces condensation. Radiation fog occurs at night and usually does not last long after sunrise, but they can persist all day in warm months.
3. GROUND FOG
It is the fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.
4. ADVECTION FOG
It occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passing over an area with significant snow-pack. It is common at the sea when moist air encounters cooler waters including areas of cooler waters upwelling.
5. EVAPORATION FOG (STEAM FOG)
These fogs are formed over bodies of water overlying much cooler air.
6. FREEZING FOG
A freezing fog is composed of droplets of supercooled water which freezes to surfaces on contact. They are very common in temperate regions and occur during winter seasons.
7. PRECIPITATION FOG
This fog is formed as precipitation falls into drier air below the cloud; the liquid droplets evaporate into water vapour. The water vapour cools and at the dew point it condenses and forms fog.
8. UPSLOPE FOG
This fog forms when the moist air is rising up the slope of a mountain or hill which condenses into fog on account of adiabatic cooling and to a lesser extent the drop in pressure with altitude.
9. VALLEY FOG
This fog forms in the mountain valleys often during winter season.
Clouds are formed when water vapour from the earth’s surface reaching the atmosphere cools and condenses at different heights in the atmosphere.
Clouds may be classified based on two main criteria:
i. According to the height
ii. According to the general form/appearance.
According to the height, clouds are classified into the following types:
1. HIGH LEVEL CLOUDS
These clouds are formed above 20,000 feet or 6000m and since the temperature is so cold at such high elevations, these clouds are primarily composed of ice crystals. High level clouds are typically thin and white in appearance but can appear in magnificent array of colors when the sun is low on the horizon.
High level clouds include:
a. CIRRUS
These are fibers or feather-like clouds in a blue sky. They indicate fair weather condition (stable).
b. CIRROSTRATUS
These are thin sheet clouds found at high altitude but still spread out over a large area.
c. CIRROCUMULUS
Small heaped clouds with ripples or wave appearance.
2. MEDIUM LEVEL CLOUDS
The bases of medium level clouds typically appear between 6500 feet to 20,000 feet. Because of their lower altitude, they are composed primarily of water droplets; however, they can also be composed of ice crystals when temperature is cool enough.
Medium level clouds include:
a. ALTOCUMULUS
These are white-grey head clouds appearing like waves and are separated by patches of blue sky.
b. ALTOSTRATUS
These are greyish sheet clouds which are much denser than cirrostratus clouds.
3. LOW LEVEL CLOUDS
These are mostly composed of water droplets since their bases generally lie below 6500 feet. However, when temperature is cold enough, these clouds may also contain ice particles and snow.
Low level clouds include:
a. NIMBOSTRATUS
These are very low sheet clouds found at a very low height. If rain falls from these clouds, it is known as nimbostratus rain.
b. STRATOCUMULUS
These are dark and heavy clouds with pronounced waves.
Depressions are large areas of low pressure due to the meeting of warm equatorial and cold polar air. They are oval or circular shaped on maps with closed isobars. Air of depression circulates in anti-clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere where they blow towards the centre.
Depressions mainly develop over the oceans, mid or temperate latitudes where humid tropical air meets with the cold polar air especially around latitude 60 North and South of the equator. It is at this point where westerly winds meet with the polar winds. The zone where these two winds meet or converge is called polar front and it is in this zone where depressions form.
Weather conditions associated with depressions:
i. Clear sky with the formation of cirrus clouds which are a bit high.
ii. Winds blow from the southeast; after a definite time, a cloud cover develops and rain occurs heavily.
iii. When rain stops, wind direction changes and it starts blowing from southwest, temperature rises and there occurs more humid air.
A cyclone is a storm or a system of winds that rotates around a centre of low atmospheric pressure.
An anticyclone is a storm or a system of wind that rotates around a centre of high pressure.
Destructive weather patterns tend to be associated with both cyclones and anticyclones as follows:
i. Cyclones (commonly called lows)
Generally are indicators of rain clouds and other forms of bad weather (lightning & thundering).
ii. Anticyclones (commonly known as highs)
They are predictors of fair weather.
iii. Wind in cyclones blows clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
iv. In cyclones, air close to the ground is forced inwards towards the centre of the cyclone where pressure is lowest; it then begins to rise up, expanding and cooling in the process. The situation increases humidity of the rising air which results in clouds and high humidity in the cyclone.
v. In anticyclones, the situation is reversed. Air at the centre of an anticyclone is forced away from high pressure that occurs there.
Tropical cyclones are areas of low pressure system which originate in temperate latitudes between 20 to 60 north and south of the equator. They occur all over the oceans except in the northern Atlantic Ocean.
Tropical cyclones develop where the air mass brought by northern and southern trade winds meet, that is along the inter-tropical front. They form over the oceans as the air masses which have travelled over oceans.
Weather conditions associated with tropical cyclones:
i. The air becomes still, temperature and humidity rise.
ii. Thick clouds appear or develop.
iii. Winds blow violently and finally dense clouds and torrential rainfall reduce visibility to a few meters.
Effects of tropical cyclones:
i. There is rapid rising of air giving rise to torrential rainfall that causes floods.
ii. Strong blowing wind which causes considerable damage to property such as electricity, buildings, etc.
iii. Formation of ocean wave storms, radial surges resulting from high winds.
iv. Landslides which can result from heavy rainfall where buildings have been erected on steep and unstable slopes, as in the case of Hong Kong where landslides were responsible for 480 deaths between 1948 to 1998.
A natural region is a large area of earth’s surface with similar characteristics of temperature, rainfall, and vegetation cover. They are classified according to climatic type and natural vegetation.
The major climatic types are classified based on temperature and rainfall pattern.
i. If a place has a temperature above 20, it is said to be hot climate.
ii. If a place has temperature between 10 and 20, it is said to be warm climate.
iii. If a place has temperature between 0 and 10, it is said to be cool climate.
iv. If a place has temperature less than 0, it is said to be cold climate.
i. If a place has rainfall more than 1500mm, it is said to be very wet climate.
ii. If a place has rainfall between 1000mm-1500mm, the climate is said to be wet climate.
iii. If a place has rainfall between 500mm and 1000mm, the climate is said to be moderate.
iv. If a place has rainfall between 250mm and 500mm, the climate is said to be dry climate.
v. If a place has rainfall below 250mm, the climate is said to be very dry climate or desert climate.
Areas which experience hot climate include the following climate regions:
i. Equatorial climate
ii. Tropical climate
iii. Monsoon climate
iv. Hot deserts
I. EQUATORIAL CLIMATE
This climate covers areas between 0 – 5 north and south of the equator. It includes places like:
– Amazon basin
– Congo basin
– Guinea
– Southeastern Asia
Characteristics of equatorial climate:
i. They experience high temperature throughout the year; sometimes a monthly temperature may be above 27.
ii. Have very small annual temperature range which is less than 4.
iii. They have double maximum rainfall, i.e., they receive heavy rainfall twice a year.
iv. They have thick forest and even green such as Amazon forest and Congo forest.
v. They have high and low vegetation, i.e., vegetation in this area are grown in layers with no undergrowth.
vi. The areas experience high humidity and high cloud cover throughout the year.
Economic activities taking place in Equatorial climate:
i. Lumbering activities, examples in Gabon, Amazon forest.
ii. Tourism activities.
iii. Agricultural activities especially cultivation of rice, banana, cocoa, coffee, etc.
It covers the areas located within the tropical belt. They are divided into two:
a. Tropical continental climate (savanna)
It is found in the interior of continents and located within 5 – 20 north and south of equator. It includes places like:
– Central Africa
– Venezuela
– South Australia
– Parts of Sudan
This climate in Africa and Australia is known as savanna because it is dominated by grassland. In Brazil, it is known as campos and in Europe is known as Steppe.
Characteristics of savanna climate:
i. It experiences moderate rainfall which is convectional in type.
ii. Temperature is high during hot season and low during cold season.
iii. They have high annual range of temperature between 8 and 11.
iv. The zone is under the influence of intertropical convergence zone (ITCZ).
v. They experience moderate and sometimes wet rainfall.
vi. Natural vegetation which dominates the region are grasses with very little scattered trees.
Economic Activities
i. Tourism activities because of natural vegetation or grasses which are good for wildlife.
ii. Cultivation of crops such as maize.
iii. Pastoralism which is done in pastoral societies. Example: Fulani in Northern Nigeria, Maasai in East Africa.
b. Tropical marine climate
They are found on the eastern side of major landmass or of the continent especially where there is a steep highland. Such areas are like:
– Philippines
– Mozambique
– Hawaii
Characteristics of tropical marine climate:
i. They experience conventional and orographic/relief rainfall.
ii. They face trade wind coast.
iii. They experience high temperature with very small fluctuation.
It is influenced by seasonal wind. It is found in:
– South East Asia
– China
– North Australia
– Burma
– Indonesia
– Somalia
Characteristics of monsoon climate:
i. They experience seasonal wind.
ii. The summer season is hot with warm air.
iii. The annual rainfalls vary from one area to another, due to the influence of water bodies, relief, and natural vegetation.
Hot deserts occur on the western side of the continent between 20
north and south of the equator. Examples of the deserts are:
– Atacama desert
– Namib desert
– Sahara desert
– Kalahari desert
– California desert
Characteristics of hot desert:
i. They experience very high daily temperature above 35, which vary from daytime to night. The daytime is higher and night temperature is lower to about 16.
ii. The area is very dry as they receive very little or no rainfall. The annual rainfall in deserts is below 250mm but it increases as you approach the transitional zone between desert and tropical climate.
iii. They are characterized by little or scarce vegetation cover.
iv. The velocity of wind in desert is high due to lack of vegetation cover and little or no water bodies.
Vegetation is the general assemblage of plant species and the ground cover they provide. It is the total plant community growing in a certain area under the influence of natural and man-made conditions.
Vegetation can be divided into the following types:
1. NATURAL VEGETATION
These are primary or original plant cover that grows in the area even when there is no human influence. They exist in the given geographical area naturally.
2. SEMI-NATURAL VEGETATION
These are wild vegetation whose existence directly or indirectly has influence from man. Such vegetation includes palm, rubber, which were wild but now grown by human beings.
3. CULTIVATED VEGETATION
Refers to the crops and cultivated trees planted to replace the destroyed vegetation.
1. ECOSYSTEM
An ecosystem is a community of living organisms in conjunction with the non-living components of their environment (things like air, water, and mineral soil) interacting as a system. These biotic and abiotic components are regarded as linked together through nutrient cycles and energy flows.
2. HABITATS
Is a surface of the earth adjacent to the atmosphere in which all organisms live, or in which all organic life exists. It is the natural home or environment for animals, plants, and other organisms.
3. PLANT SUCCESSION
Plant succession is the changing of plant communities over time. Plant communities go through all sorts of changes over time depending upon various changing conditions, disasters, human influence, animal influence, etc.
Or
Plant succession is the orderly process of one plant community gradually or rapidly replacing another.
The community begins with relatively few pioneering plants and animals and develops through increasing complexity until it becomes stable or self-perpetuating as a climax community.
a. Primary succession
Primary succession is a dynamic which begins with colonization of an area that has not been previously occupied by an ecological community such as newly exposed rocks, lava flows, newly exposed glacial tills.
Stages of primary succession include:
i. Pioneer plants (lichens and mosses)
ii. Grass stage: grasses, smaller shrubs, and trees.
Animals begin to return when there is food for them to eat.
b. Secondary succession
Secondary succession is a successional dynamic following severe disturbance or removal of a pre-existing community. Dynamics in secondary succession are influenced by pre-disturbance conditions when the former vegetation is cleared or burnt by wild harm fire caused by volcanic eruption and later the area becomes colonized by a new vegetation or plant community quite different from the former vegetation. Disturbance of the former vegetation can be by natural forces and human beings through different activities.
Secondary succession is often a shorter process by which plant communities can shift rather rapidly if given the proper opportunity. Initially diversity increases, can decrease or hit a terminal point by which little change occurs.
c. Old field succession
Old field succession is the inversion of plant communities following the abandonment of the managed plant community by humans.
Qn.1 In brief, describe the characteristics of tropical rain forest vegetation.
Qn.2 What are the characteristics of desert vegetation which have made them thrive the hard condition of desert climate?
Qn.3 Write short notes on the following:
i. Temperature lapse rate
ii. Plant community
iii. Fog
iv. Heat budget
v. Insolation
vi. Ecosystem
vii. Terrestrial radiation
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