Geographical factors affecting climate
Climate of a particular place can differ when compared to another. There are various geographical factors causing these variations in climate.
Latitude
Latitude is the angular distance north and south of the Equator. Latitudes do affect the temperature of a particular area, due to the Earth’s inclination; the mid-day Sun is almost overhead within the tropics but sunrays reach the Earth at an angle outside the tropics. Areas within and near the Equator are hot because sunrays directly reach the Earth’s surface and hit the smallest area. Temperature, thus, tends to decrease from the equatorial regions towards the poles. This is because sunrays travel over longer distance before they reach the Earth’s surface at the polar areas. Moreover, the more the area is hit by sunrays the higher the temperature of the area. Figure 2.1 shows how latitudes may affect temperature:
Altitude
Altitude is the height of a point above mean sea level. Earth’s atmosphere is mainly heated through conduction from the surface. As such, places near the surface arc warmer than those far from the surface. Temperature, therefore, decreases with height above the sea level at the rate of 0.6°C per 100 metres of height. The higher you go, the cooler it becomes because temperature falls. Normally this happens in the first layer of the atmosphere known as troposphere. This rate of decrease in temperature with altitude (lapse rate) is not constant as it varies from one place to another as well as from one season to another. As such, East Africa is cooler than the Congo Basin area though they fall within the same latitudes. Figure 2.2 shows the temperature at different altitudes. One would expect higher temperature at higher altitudes, but this is not what happens. Water vapour and dust in the atmosphere at lower altitudes prevent the heat from escaping back into space. At higher altitudes, there is very little vapour or dust, so there is a rapid transfer of heat through the air which results in the fall of temperature of the particular air.
Distance from the sea
Heating properties of land and water surface vary considerably because they react differently to solar radiation. The land surface heats and cools faster than the water surface. This is because in water there are fast moving mobile matters and redistribution of heat occurs mainly through turbulence. On the land surface, there is less presence of mobile matters and redistribution of heat is done through molecular heat conduction and proceed by moving from particle to particle.
Normally, water surfaces reflect a lot of light because they are flatter Some of the light, however, penetrates deep into the water causing a large volume of water to be heated. When water moves with circulating currents, it helps to disperse the heat to the land surface. The specific heat of water is greater than the specific heat of land surfaces. Therefore, it takes more heat to raise up the temperature of water.
Onshore winds bring warmth to coastal regions during the winter. This warming influence is confined to a narrow coastal belt because the warm air rapidly loses its heat on the colder land. During summer, land surface is warmer than the water surface and the air over the land is, therefore, warmer than that of over the sea. For that reason, coastal regions are cooler than inland regions during the summer.
Prevailing winds
Prevailing winds are winds that blow consistently in a given direction over a particular region on Earth. This is caused by several factors such as uneven heating from the Sun and the Earth’s rotation. Winds tend to transfer heat and moisture over the land. If winds are warm and blown from a hot area, they will raise temperatures. If winds blow from cold areas, they will lower the temperature of a local area. For example, local winds such as Sirocco and Mistral tend to influence changes in temperature.
During the day, the Sun heats up both the ocean and the land surface. Land, however, absorbs solar radiation faster than water does. Land, thus, absorbs much of the Sun’s energy as well. Because water heats up more slowly than land, air above the land will be warmer than the air over the ocean. The warm air over the land will rise throughout the day, causing low pressure on the land surface. Over the water, high surface pressure will be formed because of the cold air. As a result, the air will sink over the sea or ocean. Winds will, therefore, blow from the higher pressure zone over the water surface to lower pressure zone over the land, hence causing a sea breeze. Strength of the sea breeze will vary depending on the temperature difference between the land and the sea or ocean. At night, the situation operates in reverse. The air over the sea or ocean is now warmer than the air over the land. The land loses heat quickly after the Sun goes down and the air above cools too. This can be compared to a tarmac road. During the day, the tarmac road heats up and becomes very hot to walk on. At night, however, the tarmac road loses its heat and becomes cool. The ocean, however, takes time to lose its heat after the Sun sets. This causes the low surface pressure to shift over the sea or ocean during the night and the high surface pressure to move over the land. This causes a small temperature inversion between the sea or ocean surface and nearby land at night. Consequently, the wind blows from the land to the sea or ocean hence creating a land breeze. Figure 23 shows how the land and sea breezes affect the temperature of the coastal areas:
Ocean currents
An ocean current is a continuous and directed movement of sea water generated by a number of forces acting upon the water which includes winds, temperature of the sea or ocean, the shape of coastline salinity and density differences of the sea or ocean. Coastal areas are influenced by warm or cold ocean currents. Both ocean currents and winds affect temperature by transporting their heat or coldness to adjacent regions. Warm currents raise temperatures while cold currents lower the temperature of land surfaces when the winds are onshore. Warm currents move towards the pole carrying tropical warmth into the high latitudes, for example, the Brazil warm ocean current. Ln tropical latitudes onshore winds, crossing warm ocean currents blow this warmth and moisture to the land to raise both the temperature and precipitation. Cold ocean currents have less effect on temperature because they usually lie under off-shore winds, which give rise to mist or fog along the coast, for example, the Benguela cold ocean currents. Figure 2.4 shows how ocean currents affect temperature:
Aspect
Aspect refers to the direction in which a slope faces the Sun. Figure 2.5 shows how aspect affects temperature. For example in the Northern Hemisphere, southward facing slopes are wanner than northward facing slopes because the northward slopes never receive direct sunshine as this region never gets overhead Sun. The southward slopes, on the other hand, are warmer and, therefore, carry moisture winds which bring rainfall to this area whereas the northward facing slopes experience dry winds with no moisture and, hence, no rainfall.
Vegetation
Vegetation affects the temperature of a particular place significantly. Areas with dense forest cover, such as the Congo and the Amazon Basin receive less insolation towards the Earth’s surface. Due to this, they are often cooler than areas in open spaces.
Soils
Light soils reflect more heat than darker soils, which are better heat absorbers. Such soil differences may cause slight variations in the temperature of the region. Dry soils such as sands are very sensitive to temperature changes, whereas wet soils such as clay retain much moisture and tend to warm up or cool more slowly.
Human influence
Human activities such as settlements, agriculture, industries and construction of dams influence climate change as people tend to clear forests, drain water and cultivate wetlands. These activities lead to climate change. For example, as population increases, land use increases as well. Consequently, trees arc cut down in large numbers for socio-economic activities such as forming, settlement, lumbering or timber production, firewood and charcoal production. These activities leave the land bare and expose it to foe agents of soil erosion, such as running water and winds. It also affects hydrological cycle systems, hence leading to drought in an area
Temperature
Temperature is the degree or intensity of heat present in a substance or object. Temperature is expressed in a comparative scale measured by a thermometer or simply perceived by a touch.
Temperature on the Earth’s surface varies from one place to another. Places near the Equator experience high temperature throughout the year whereas places near the poles experience cold temperature throughout the year.
Tn the context of Geography, temperature is a mean temperature. It can be the daily or diurnal mean temperature, mean monthly temperature, and mean annual temperature. It is important, therefore, to understand the following concepts of temperature:
Daily mean temperature: This is the average of the maximum temperature and minimum temperature recorded during a day.
Mean monthly temperature: This is the average of temperature of a month obtained by taking the sum of daily temperature of a calender month divided by the number of days in a month.
Mean annual temperature: This is the average of maximum and minimum temperatures of a year. It is the average between the coldest and the hottest months in a year.
Annual range of temperature: This is the difference between the temperature of the hottest month and that of the coldest month in a year.
Diurnal range of temperature: This is the difference between the hottest and coldest parts of the day i.e., maximum day and minimum night temperatures.
Atmospheric pressure
Atmospheric pressure is the pressure within the atmosphere. It is the force exerted on a surface by the air above it as the gravity pulls it to the Earth. Therefore, it is also important to understand the following related concepts of atmospheric pressure:
Depression or cyclone: This is a large scale air mass that rotates around a strong centre of low atmospheric pressure (L) in anti-clockwise direction north of the Equator and in a clockwise direction to the south. Cyclonic winds move across nearly all the regions of the Earth except the equatorial belt and are generally associated with rain or snow. Cyclones are characterised by inward spiraling winds that rotate about a zone of low pressure. Figure 2.6 is an example of a cyclone:
Figure 2.6: Cyclone
Anticyclone: This is a system of winds that rotate clockwise about the centre of high atmospheric pressure (H) in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. As air sinks, no clouds or rain are formed. This is because when air sinks it becomes warm and it cannot hold more water. This situation causes settled weather conditions during the summer. There is a clear sky and high temperature. Figure 2.7 illustrates an anticyclone:
Figure 2.7: Anticyclone
Inter-Tropical Convergence Zone (ITCZ): This is a low-pressure belt, which is found between two trade wind systems. The ITCZ is known by sailors as the doldrums or the calm zone or area because of its monotonous and windless weather. It is an area where the northeast and southeast trade winds converge. Trade winds converge in this region resulting in heavy rains, which are accompanied by lightning and thunderstorms. The ITCZ moves northwards and southwards following the apparent movement of the Sun. Figure 2.8 shows the Inter-tropical convergence zone:
Rainfall
Rainfall may be defined as the droplets of water falling from the atmosphere after condensation. When water vapour rises, it cools at a high altitude until dew point is reached. A dew point is the temperature rate at which the atmosphere is saturated with water vapour. Condensation takes place to form clouds after a dew point has been reached. Moist air contains small particles of matter called nuclei that are made of dust, salt, ice and soot. Large water droplets join the nuclei to form raindrops. The following are types of rainfall:
Convectional rainfall: This occurs when the energy of the Sun heats the surface of the Earth causing water to evaporate to form water vapour. When the land heats up, it warms the air above it, hence raising the currents of air so high in the atmosphere where it cools down very fast, condenses, becomes saturated and falls as heavy rain. This type of rainfall is very common in regions with equatorial climate where there is constant high temperature and humidity. Figure 2.9 shows formation of convectional rainfall:
Relief or topographic rainfall: Occurs when moisture carrying winds are forced to rise over mountains lying along a coast at right angles to onshore winds. Presence of mountains acts as a barrier to the movement of moist air, hence forcing air to rise. The ascending moist air cools and condenses to form clouds which bring rain on a windward side. After crossing the mountain, the winds lose most of the moisture and cool down and dry. As such, no rain or fog may occur on the leeward side. Figure 2.10 shows the formation of relief rainfall:
Cyclonic or depression rainfall: It is formed when masses of warm air are forced over and chilled by wedges of cold air. The line where the two air masses meet is known as a “front” which explains why sometimes this type of rainfall is known as “frontal rainfall”. The collision of two air masses of different characteristics can cause some air to rise; as a result, it leads to the occurrence of rainfall. Here the warm air is forced to ascend or rise over cold air, resulting in moisture and heavy rains. Figure 2.11 shows the cyclonic rainfall:
The winds that may cause cyclonic or depression rainfall are:
Sirocco winds: Hot winds with clouds of desert dust blowing from the Sahara Desert northwards across the Mediterranean Sea into Italy. They arc so humid that they bring unpleasant, irritating heat and dryness.
Mistral winds: A mass of cold air which blows southwards from the Alps and Central Massif across the Mediterranean Sea into Northern Africa.
Harmattan winds: Dust-laden winds blowing southwards to the Coast of West Africa from the Sahara Desert.