Climate of Ethiopia

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1. Weather and Climate: Basic Concepts Weather: Atmospheric conditions (temperature, rainfall, pressure, wind, moisture, cloud cover, humidity) at a specific place over a short period (hours to days). Highly variable. Climate: The pattern of weather conditions experienced in an area over a long period of time . Considers trends and average conditions. A large area can have one type of climate. 2. Elements of Weather and Climate 2.1 Key Elements Precipitation: Any liquid or frozen water falling from the atmosphere (rain, sleet, hail, snow). Temperature: Degree of hotness or coldness of an object. Humidity: Concentration of water vapor in the air. Air Pressure: Force exerted by the air above on a surface due to gravity. Wind: Movement of air caused by uneven heating of the Earth and its rotation. Sunshine: Direct sunlight exposure to an area. Cloud: Visible mass of water droplets or ice crystals suspended in the atmosphere. 3. Controls of Weather and Climate in Ethiopia The spatial and temporal distribution of climatic elements in Ethiopia is determined by various physical factors: Latitude: Angular location affecting the angle of the sun's rays. Ethiopia's tropical location leads to high temperatures, high daily ranges, and small annual ranges. Altitude: Main factor determining spatial temperature distribution. Temperature decreases as altitude increases. Town Latitude Altitude Avg. Annual Temp. Addis Ababa $9^\circ N$ $2,200$ m.a.m.s.l. $16^\circ C$ Bako (West Shewa) $9^\circ N$ $1,800$ m.a.m.s.l. $17^\circ C$ Awash $9^\circ N$ $916$ m.a.m.s.l. $25^\circ C$ Mountain Barriers: Affect rainfall distribution. Windward Side: Faces wind laden with moisture, receives rain. Leeward Side (Rain Shadow): Does not face moisture-laden winds, receives little rain. Revolution of the Earth and Inclination of Earth's Axis: Earth's axis tilt ($23.5^\circ$) and revolution around the sun cause changes in sun's ray angle and day length, leading to seasons and temperature variations. Distance from the Sea: Landmasses absorb/release heat faster than water. Affects coastal areas; less significant inland. Ocean Current: Warm/cold currents affect coastal regions when local winds blow from the sea. Inconsequential for Ethiopia's climate control. Note: Climate of Ethiopia is dominantly controlled by altitude and latitude . 4. Climatic Regions (Agro-climatic Zones) in Ethiopia Based on altitude and temperature: Altitude in meters Traditional Name Global Equivalence Mean Annual Temp. ($^\circ C$) $3,300$ and above Wurch/Kur Alpine or Afro-Alpine $<10$ $2,300 - 3,300$ Dega Temperate $10 - 15$ $1,500 - 2,300$ Woina Dega Subtropical $15 - 20$ $500 - 1,500$ Kolla Tropical $20 - 30$ Below $500$ Bereha Desert $> 30$ 4.1 Characteristics of Zones Wurch-Zone Areas: Highest altitudes, lowest temperatures ($ Dega-Zone Areas: Highland areas, lower altitudes and higher temperatures than Wurch. Historically concentrated human settlement due to secure location, reliable rainfall, absence of tropical diseases. High soil erosion/overgrazing. Woina-Dega-Zone Areas: Contain most agricultural land, main grain-producing areas. High rainfall reliability. Kolla Zone Areas: Warm-to-hot semi-arid climate (tropical lowlands, $500-1500$ m.a.s.l.). Temperatures $20-30^\circ C$. Rainfall $410-820$ mm, highly variable. E.g., Western lowlands. Bereha-Zone Areas: Hot arid climate (desert lowlands, below $500$ m.a.s.l.). Temperatures $>30^\circ C$. Rainfall $ 5. Seasonal Variation in Ethiopia Seasons are defined by Earth's axis inclination and revolution around the sun. 5.1 Seasonal Variation of Temperature High temperatures: March-June. Low temperatures: November-February. Variations due to: Earth's tilt ($23.5^\circ$), overhead sun's apparent north-south movement. Bega Season (Northern Hemisphere Winter): Clear skies, intense solar radiation. High diurnal temperature range (day-night difference). Mean maximum temperatures: Western, northeastern, southeastern lowlands. Lowest temperatures: Semein and Arsi-Bale mountains. Highest temperature ($>45^\circ C$): Danakil Depression (hottest place). 5.2 Seasonal Variation of Rainfall Ethiopia experiences marked spatial and temporal rainfall variations. A. Spatial Variation of Rainfall Result of Inter-Tropical Convergence Zone (ITCZ) oscillation, overhead sun movement, prevailing winds, and barrier effects. ITCZ: Low atmospheric pressure zone, convergence of northeasterly and southeasterly trade winds. Shifts north and south following overhead sun. June (Tropic of Cancer): Ethiopia under Equatorial Westerlies and Easterlies, bringing moisture to highlands. Rain decreases northward. December (Tropic of Capricorn): Northeast Trade winds (dry, continental origin) prevail. Lowland areas near coast (Afar) receive some rain from Red Sea moisture. Dry season for most of Ethiopia. March & September (Equator): Equatorial Easterlies bring rain to southeastern lowlands, central and highlands. B. Temporal Variation of Rainfall Kiremt (Summer): June-August. Belg (Spring): March-May. These two contribute $>90\%$ of rain supply. Meher (Autumn): September-November. Less duration, volume, and coverage than Kiremt/Belg. Bega (Winter): December-February. Generally dry season, small amount of rain only in Afar lowlands. 5.3 Rainfall Regions of Ethiopia (5 types) Year-round rainfall region: Southwestern plateau (Wollega, Kafa, Illubabor, Gamo, Goffa). High rainfall ($1400-2200$ mm) due to equatorial Westerlies from Atlantic Ocean. E.g., Gore, Mizan, Metu, Bonga, Gambella. Summer rainfall region: Largest in country. Northwest Highlands and Western Lowlands. Moisture from Equatorial Westerlies and Easterlies. E.g., Debre Markos, Fitche, Gondar, Bahir Dar. Autumn-and-spring rainfall region: Southeastern highlands and associated lowlands. Moisture from Indian Ocean (Equatorial Easterlies). Rainfall $500-1000$ mm. E.g., Gode, Moyalle, Jigjiga, Yabello. Winter rainfall region: Eastern escarpment of western highlands, middle Rift Valley, Afar subdivision. Moisture from North Easterlies (continental, short Red Sea trajectory). Low annual moisture. E.g., Assaita. Merged spring, summer, and autumn rainfall region: Smallest region, mainly western foothills of Southeastern Highlands. Rainfall $1000-1500$ mm. Corridor from Sidama Highlands to Hararge Plateau. E.g., Assaita, Awash. 6. Measurements of Weather and Climate 6.1 Measuring and Recording Air Temperature Instrument: Thermometer (mercury/alcohol). Scales: Fahrenheit ($^\circ F$): Freezing $32^\circ F$, Boiling $212^\circ F$. Centigrade ($^\circ C$): Freezing $0^\circ C$, Boiling $100^\circ C$. Conversion Formulas: $C = (F - 32) \times \frac{5}{9}$ $F = (C \times \frac{9}{5}) + 32$ Mean daily temperature: $(Max\ Temp + Min\ Temp) / 2$. Daily (diurnal) range of temperature: $Max\ Temp - Min\ Temp$. Mean monthly temperature: Sum of daily averages for month / Number of days. Mean annual temperature: Sum of mean monthly temperatures / $12$. Annual range of temperature: Hottest month temp - Coldest month temp. 6.2 Measuring and Recording Rainfall Instrument: Rain gauge (measures depth in millimeters). Daily readings, summed for monthly and annual totals. Mean monthly rainfall: Sum of daily rainfall for month / Number of days. Total annual rainfall: Sum of monthly rainfall for $12$ months. Mean annual rainfall: Sum of annual rainfall for $30-35$ years / Number of years. 6.3 Measuring and Recording Air Pressure Instrument: Barometer. Units: Millibar (mb) or millimeter (mm) of mercury. Sea level pressure: $1013.25$ mb or $760$ mm. 6.4 Measuring and Recording Wind Speed Instrument: Anemometer. 6.5 Measuring and Recording Wind Direction Instrument: Wind vane. Direction is reported from where the wind is blowing (e.g., a westerly wind blows from the west).