The Climate of Ethiopia and the Horn of Africa

CHAPTER FIVE THE CLIMATE OF ETHIOPIA AND THE HORN 5.1. Introduction Ethiopia as a large country in the Horn of Africa, is characterized by a wide variety of altitudinal ranges and diverse climatic conditions. In addition, because of its closeness to the equator and the Indian Ocean, the country is subjected to large temporal and spatial variations in elements of weather and climate. The climate of Ethiopia is therefore mainly controlled by the seasonal migration of the Intertropical Convergence Zone (ITCZ) and associated atmospheric circulations as well as by the complex topography of the country. Weather is the instantaneous or current state of the atmosphere composing temperature, atmospheric pressure, humidity, wind speed and direction, cloudiness and precipitation. Weather parameters are measured using various instruments. In general, the weather that impacts the surface of the Earth and those that live on the surface takes place in the troposphere. Climate refers the state of the atmosphere over long time periods, decades and more. It is the composite of daily weather conditions recorded for long periods of time. Climate also takes into account the extremes or variations that may occur beyond the average conditions. Hence, in this chapter, climate of Ethiopia and the Horn will explicitly be discussed. Elements and controls of weather and climate, spatiotemporal patterns of weather and climate, agroecological zones, climate and its implication on Ethiopian biophysical and socio-economic aspects, and climate change will be the major sub sections of this chapter. Objectives Upon the completion of this chapter, you will be able to:  Distinguish between weather and climate,  Explicate the spatiotemporal patterns and distribution of temperature and rainfall in Ethiopia,  Analyse climate and its implications on biophysical and socioeconomicaspects,  Comprehend the causes, consequences andresponse mechanisms of climate change. 5.2. Elements and Controls of Weather and Climate What elements do you consider to define weather condition of your locality? All weather conditions may be traced to the effect of the Sun on the Earth. Most changes in weather involve large scale horizontal motion of air which is called wind. Weather is expressed by a combination of several elements. Here are lists of major elements and controls of weather and climate. The climate of a region is ultimately determined by the radiation, its distribution and temporal fluctuations. The long-term state of the atmosphere is a function of a variety of interacting elements. Table 5.1.Elements and controls of weather and climate Elements Controls 1. Temperature 1 Latitude/angle of the Sun 2. Precipitation and humidity 2 Land and water distribution 3. Winds and air pressure 3 Winds and air pressure 4 Altitude and mountain barriers 5 Ocean currents 5.2.1. Controls of Weather and Climate What do you think is the source of summer rainfall in Ethiopia? Have you ever noticed varying lengths of days and nights by seasons? What do you think is the reason behind? The climate of any particular location on earth is determined by a combination of many interacting factors. These include latitude, elevation, nearby water, ocean currents, topography, vegetation, and prevailing winds. Moreover, the global climate system and any changes that occur within it also influence local climate. Hotness or coldness, rainy or cloudiness, sunniness, windiness or calmness, of air you are feeling on the daily base in your current location are expressions of weather. Now the question one should inquire here is what determines the variations in weather and climate between places and seasons. Hence, these determining factors are called controls of weather and climate or climatic controls. Some of the major controls are discussed below. a. Latitude Latitude is the distance of a location from the equator. The sun shines directly on equator for more hours during the year than anywhere else. As you move further away from the equator towards the poles, less solar insolation is received during the year and the temperature become colder. Ethiopia’s latitudinal location has bearings on its temperature. Latitudinal location of Ethiopia and the Horn resulted in;  high average temperatures,  high daily and small annual ranges of temperature,  no significant variation in length of day and night between summer and winter. b. Inclination of the Earth's Axis The earth’s rotation axis makes an angle of about 66 ½ ° with the plane of its orbit around the sun, or about 23 ½ ° from the perpendicular to the ecliptic plane. This inclination determines the location of the Tropics of Cancer, Capricorn and the Arctic and Antarctic Circles. As the earth revolves around the sun, this inclination produces a change in the directness of the sun's rays; which in turn causes the directness of the sun and differences in length of day and seasons. Equinoxes and Solstices Anequinox is the instant of time when the sun strikes the plane of the Earth's equator. During this passage the length of day and night are equal. Moreover, revolution of the earth along its orbit, the inclination of its axis from the plane of that orbit, and the constant position (parallelism) of the axis causes seasonal changes in the daylight and darkness periods. Equinox appears twice a year. Let’s see two major equinoxes’;  The Vernal (spring) equinox: is the day when the point of verticality of sun’s rays crosses the equator northwards. This equinox experiences in Northern Hemisphere when the sun is exactly above the equator. During this period, the length of day and night are equal. Vernal (spring) equinox marks the beginning of Spring season. March 21 marks the offset of the vernal equinox.  The Autumn equinox: appears to happen when the sun crosses equator giving approximately equal length between day and night. It appears to happen when the visible sun moves south across the celestial equator on 23rd of September. It marks the beginning of Autumn season. Solstice is an event when the overhead sun appears to cross northern or southern points relative to the celestial equator resulting in unequal length of days and nights in the hemispheres. Both hemispheres during this event has either the most or least sunlight of the year.  The summer Solstice: on June 21st, the northern hemisphere has maximum tilt towards the sun experiencing longest daylight of the year. It is the astronomical first day of summer in the Northern Hemisphere. The sun is at its highest position in the noonday sky, directly above 23 ½ in the Tropic of Cancer.  The winter solstice: 22nd of December is the day when the maximum southward inclination is attained in the Southern Hemisphere. In this event the sun travels shortest length causing longest night and shortest daylight. In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23 ½ ° south of the equator. Figure 5.1. The apparent path of the sun at different latitudes Source: Ahrens & Henson, 2019 c. Altitude Do you have any experience of visiting high elevation places? What differences have you felt between the lowest and highest elevations? Altitude is the height of location above the sea level. Under normal conditions there is a general decrease in temperature with increasing elevation.