GE 141 - CLIMATOLOGY

1. GE 141 - CLIMATOLOGY Dr. Donald A. Mwiturubani Geography Department IRA BUILDING OFFICE NO. 316

2. Element of climate • Climatology is the study of the long-term state of the atmosphere. • The long-term state of the atmosphere is a function of a variety of interacting elements. • Solar radiation • Air masses • Pressure systems (and cyclone belts) • Ocean Currents  • Topography

3. Solar radiation • Solar radiation - the most important element of climate. • Solar radiation heats the Earth's surface - determines the temperature of the air. • Solar radiation drives evaporation, so long as there is water available. • Heating of the air determines its stability - affects cloud development and precipitation. • Unequal heating of the Earth's surface creates pressure gradients - result in wind systems.

4. Air masses • Air masses subsumes the characteristics of temperature, humidity, and stability. • Location relative to source regions of air masses in part determines the variation of the day-to-day weather and long-term climate of a place.

5. Pressure systems • Pressure systems have a direct impact on the precipitation characteristics of different climate regions. • Places dominated by low pressure tend to be moist, while those dominated by high pressure are dry. • The seasonality of precipitation is affected by the seasonal movement of global and regional pressure systems. • eg. Climates located at 10o to 15o of latitude experience a significant wet period when dominated by the ITCZ and a dry period when the Subtropical High moves into this region. • Pressure dominance affects the receipt of solar radiation. • Places dominated by high pressure tend to lack cloud cover and hence receive significant amounts of sunshine, especially in the low latitudes. 

6. Ocean currents • Greatly affect the temperature and precipitation of the geographical region. • Those regions bordering cold currents tend to be drier hence inhibit cloud formation and precipitation. • Those regions bordering warm currents tend to be wet hence promote instability and precipitation. • Warm ocean water keeps air temperatures somewhat warmer than locations just inland from the coast during the winter.

7. Topography • Topography affects climate in different aspects. • The orientation of mountains to the prevailing wind affects precipitation. • Windward slopes experience more precipitation due to orographic uplift of the air • Leeward sides of mountains are in the rain shadow and thus receive less precipitation. Air temperatures are affected by slope and orientation - slopes facing the Sun is warmer than those facing away. • Temperature decreases with increase in altitude (Dew point).

8. Elements of weather • Weather is the state of the Earth’s atmosphere recorded in short period of time say a day • The study of weather is with respect to temperature, humidity, rainfall, wind, pressure, and cloudiness. • Temperature – measured by thermometer • Humidity – measured by hygrometer • Rainfall – measured by rain gauge • Pressure – measured by barometer • Wind – measured by anemometer and wind vane

9. Solar Radiation and Heat Balance • solar radiation is the total frequency spectrum of electromagnetic radiation produced by the sun. • This spectrum covers visible light and near-visible radiation, such as x-rays, ultraviolet radiation, infrared radiation, and radio waves. • The visible light and heat of the sun makes life possible, and is called “daylight” or “sunshine.” • The earth’s atmosphere deflects or filters the majority of the sun’s harmful radiation. • Solar radiation is affected by location (latitude and elevation), season, time of day, and cloud cover • Heat balance Equilibrium between the gain and loss of heat at a specific place or for a specific system

10. Importance of Solar Radiation and Heat Balance • Life On Earth • Solar radiation is the basis for all life on earth. • Autotrophs, organisms that produce their own food from the sun (mainly plants), use solar energy along with carbon dioxide and water to produce simple sugars - photosynthesis. • Heterotrophs, organisms that eat other organisms (like animals and fungi), depend on autotrophs to form the bottom level of the food chain. • Heterotrophs couldn’t exist without autotrophs, and autotrophs couldn’t exist without the sun, so life as we know it depends on electromagnetic radiation.

11. Importance cont….. • Seasons and Climate • The Earth’s seasonal climate variation occurs as a result of minute changes in our planet’s distance from the sun during orbit. • Solar radiation is a contributing factor to the process of global warming. • Even before modern human activity and fossil fuels began heating up the atmosphere, the elliptical path of the Earth’s orbit was creating 100,000-year cycles of warming and cooling that led to the ice ages and tropical periods of the distant past. Sunlight affects different parts of the Earth in different ways - equatorial regions and the poles.

12. Importance cont…. • Health • Sunlight keeps us warm, creates food and shelter through plant life, and gives human being light. • Greenhouse gases trap more heat and the ozone layer allows more dangerous UV radiation in - UV rays cause skin cancer in humans and animals. • Biologically, the sun causes our bodies to produce vitamin D. • Global warming – concentration of greenhouse gases in the lower atmosphere - affects environmental resources – food, shelter etc..

13. Terms used in the heat balance • Heat balance of the Earth-atmosphere system: The equilibrium that exists on the average between the radiation received by the Earth and atmosphere from the Sun and that emitted by the Earth and atmosphere. • Heat budget: Relationship between fluxes of heat into and out of a given region or body and the heat stored in. • Heat flux: Amount of heat, transferred in a liquid, in particular in the atmosphere, per unit of time through a unit area. • Heat sink: Process, or region, in which energy is removed from the atmosphere in the form of heat. • Heat source: Process, or region, in which energy is added to the atmosphere in the form of heat • Geophysics: The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere.

14. Transient atmospheric flow and disturbances • Storms • A storm is a temporary disturbance in the atmosphere. Storms are transient, meaning that they generally do not stay in one location for very long. • A storm can bring powerful winds, precipitation, and thunder and lightning. • They move with the flow of air Storms are phenomena that are more limited than the broad-scale wind and pressure systems. • Storms are transient and temporary. • Storms involve the flow of air masses as well as a variety of atmospheric disturbances.

15. Storms……Air Masses • Impacts of storms on landscape. • Positive: promote diversity in vegetative cover, increase size of lakes and ponds,and stimulate plant growth • Negative effect include accelerate erosion, flood valleys, destroy buildings, decimate crops • Air Masses • A large parcel of air that has relatively uniform properties in the horizontal dimension and moves as an entity. • Compose the troposphere.

16. Air Masses ... • Characteristics of an air mass • Air mass must meet three requirements: • Must be large – about 1,000 miles (1,600 kms) across (horizontal) and several miles (several kms) upward to the atmosphere (vertical). • Horizontal dimension must have uniform properties (temperature, humidity, and stability) – are relatively the same at any point within the air mass. • Must be distinct from surrounding air, and when moves, must retain that distinction - traveling across the atmosphere as a single unit. It must be sufficiently strong so as to not break apart as it is pushed along its course.

17. Air Masses ...... • Origin • Air masses form over large areas of the surface of the Earth that have similar characteristics - refer to them as source regions - ocean, a large forest, a desert, or open grasslands. • Formation of air mass occurs if air remains over a uniform land or ocean surface long enough to acquire uniform properties. • Source regions are parts of Earth’s surface that are suited to generate air masses because they are: • Extensive such as arctic and antarctica • Physically uniform • Associated with air that is stationary or anticyclonic

18. Air Masses ....... • Movement of air masses • Air masses leave their source region and begin moving to other areas • Bring with them the conditions that are common in the source region. For example, an air mass that formed over a pole region will be cold, especially during the winter – contain cold temperatures.

19. Fronts • Fronts • A front is a zone of discontinuity between unlike air masses where properties of air change rapidly. • It’s narrow but three-dimensional. • Typically several kilometers wide (even tens of kilometers wide). • Functions as a barrier between two air masses, preventing their mingling except in this narrow transition zone. • Though all primary physical properties are involved in a front, temperature provides the most conspicuous difference.

20. Fronts.............. • Warm fronts • Fronts that are dominated by warm air are referred to as warm fronts. Warm air is lifted upward above the cooler air. As the air in the warm air mass rises it expands, causing it to cool down. As it cools, water vapor can condense, creating precipitation. • Characteristics of warm fronts • Forms by advancing warm air • Slope is gentle, ascends over treating cool air, decreasing adiabatically as the air rises

21. Fronts .................... • Clouds form slowly and not much turbulence (High cirrus clouds, moving towards a altocumulus or altostratus clouds. • Broad precipitation, protracted and gentle • If unstable air, precipitation can be showery or violent. • Cold Fronts • Fronts that are dominated by cold air mass are referred to as cold fronts. • Because cold fronts move along the ground where they encounter friction, they move slower at ground level than they do further up in the atmosphere. For this reason, cold fronts tend to be more sloped than warm fronts.

22. Fronts...... • Characteristics of cold fronts • Forms formed by advancing cold air • Is a steeper front than a warm front with a “protruding nose” • Moves faster than a warm front • Rapid lifting, unstable air, blustering and violent weather • Vertically developing clouds • Precipitation along the leading edge and immediately behind the ground-level position of the front.

23. Fronts................ • Stationary Front • Stationary fronts are formed when two air masses come together but neither displaces the other neither dominates the zone. • Difficult to predict the weather along a stationary front, though it often resembles that of a warm front. • Occluded Front • Occluded fronts are complex fronts formed when cold fronts overtake warm fronts.

24. Atmospheric disturbances • Two types of atmospheric disturbances • Stormy and calm • Mid-latitude cyclone • Large migratory low-pressure system that occurs within the middle latitudes and moves generally with the westerlies. • Also called lows or wave cyclones, depressions. • Probably most significant of all atmospheric disturbances. • Basically responsible for most day-to-day weather changes.

25. Mid-latitude cyclones • Characteristics • Mature mid-latitude cyclone is 1,600 kilometers in diameter; Has oval shape. • In the northern hemisphere circulation pattern converges counterclockwise; • Wind-flow pattern attracts cool air from north and warm air from south hence creates two fronts. • The two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. • Movements • Mid-latitude cyclones move throughout their existence.

26. Mid-latitude cyclones ................ • Lifecycle • Origin to maturity typically takes 3 to 6 days, then another 3 to 6 days to dissipate - Cyclogenesis—birth of cyclones • Most common cause believed to be upper-air conditions in the vicinity of the polar-front jet stream. can also occur on the leeward side of mountains,. • Most begin as waves along the polar front. • Often bring heavy rain or snowstorms • After cyclonic circulation is well developed, occlusion begins. • After occluded front is fully developed, cyclone dissipates.

27. Mid-latitude cyclones ..../ Anticyclones • Occurrence and Distribution • Occur at scattered but irregular intervals throughout the zone of the westerlies. • Route of cyclone is likely to be undulating and erratic, but it generally moves west to east. • Mid-latitude Anticyclones • An extensive migratory high-pressure cell of the mid-latitudes that moves generally with the westerlies. • Typically larger than a midlatitude cyclone, but also moves west to east.

28. Mid-latitude anticyclones... • Characteristics of midlatitude anticyclones • High pressure – has air converging into it from above, subsiding, and diverging at the surface, clockwise rotation. • No air-mass conflict or surface convergence is involved • No fronts involved • Weather clear and dry with little or no opportunity for cloud formation • Wind movement very limited near the center, increases progressively outward • Very low temperatures in the winter • Prone to stagnate and remain over the same region for several days

29. Tropical disturbances • Tropical Disturbances: • Tropical cyclone • Tropical cyclone—a storm most significantly affecting the tropics and subtropics, which is intense, revolving, rain-drenched, migratory, destructive, and erratic. Such • Consists of a prominent low-pressure center that is essentially circular in shape and has a steep pressure gradient outward from the center. • Tropical cyclones provide the only break in weather in low latitudes. • Hurricanes • Develop from incipient low-pressures perturbations in trade-wind flow, called tropical disturbances.

30. Hurricanes ....... • Different names: Hurricanes in North and Central America; Typhoons in western North Pacific; Baguios in Philippines; Tropical cyclones in Indian Ocean and Australia • Characteristics of hurricanes • Prominent low-pressure centers that are essentially circular with steep pressure gradient outward from the center • Converging cyclonic wind pattern “fuel” that powers the storm • Warm, water vapor-laden air spirals into a storm • Intense updrafts within towering cumulonimbus clouds

31. Hurricanes ................. • Origin of Hurricanes • Form only over warm oceans in the tropics at least a few degrees north or south of the equator • Coriolis effect plays key role: it’s at minimum at equator, and no hurricane has been observed to form within 3° of equator or cross over it neither closer than 8° to 10° of equator. • The exact mechanism of formation is not clear, but they always grow from some pre-existing disturbances. • Tropical storm—winds between 63 and 117 kilometers per hour. • Hurricane—winds of 119 kilometers per hour) or more; can double and even triple that minimum.

32. Hurricanes ............. • Movement of hurricanes • Most common in North Pacific basin (origination in Philippines and west of southern Mexico and central America); west central portion of the North Atlantic basin, extending into Caribbean and Gulf of Mexico. • Totally absent from the South Atlantic and from the southeastern part of the Pacific because the water is too cold and because high pressure dominates. • General pattern of movement is highly predictable: • About one-third travel east to west without much latitudinal change. • About two-thirds start off on an east–west path and then curve poleward.

33. Hurricanes .............. • Average hurricane lasts a week; those that remain over tropical oceans can live up to 4 weeks. • Dies down over continents because energy source of warm, moist air is cut off. • Dies down in midlatitudes because cooler environment. • In midlatitudes, can diminish in intensity but grow in size and become a midlatitude cyclone. • Damage and destruction of hurricanes • Damage is influenced by: • High seas – increase speed of storm • Storm size – wind driven water pound on shoreline • Physical configuration of landscape • population size and density of affected area.

34. Localised atmospheric disturbances • Localized Atmospheric Disturbances • Easterly wave • a long but weak migratory low-pressure system in the tropics between 5° and 30° of latitude. • Sometimes intensify into hurricanes. • Bring characteristic weather of trade winds with them. • Thunderstorm • violent convective storm accompanied by thunder and lightning; usually localized and short lived. • Vertical air motion, considerable humidity, and instability combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination. • (2)

35. Thunderstorm • Frequently occur in conjunction with other kinds of storms (hurricanes, tornadoes, fronts [especially cold fronts]) in midlatitude cyclones, and orographic lifting. • Three stages of thunderstorm development • Cumulus stage—updrafts prevail and clouds grow. Rise to above freezing level, where super cooled water droplets and ice crystals coalesce, then fall. Initiate a downdraft. • Mature state—updrafts and downdrafts coexist as cloud continues to enlarge (but precipitation is leaving bottom of cloud). Most active time. • Dissipating state—downdrafts dominate, and turbulence ceases.

36. Thunderstorm ………… • Thunderstorm formation • The action of warm air rising and cold air sinking (convection) plays a key role in the formation of severe thunderstorms. If the warm surface air is forced to rise, it will continue to rise, because it is less dense than the surrounding air. • Two of the most important ingredients for thunderstorm formation are instability (unstable air) and moisture. • Thunderstorms are formed by the uplifting of warm and humid air. • There are many factors that lead to the uplifting of air -solar heating; in the vicinity of low pressure troughs; when two different air streams meet; or when air is forced uphill.

37. Thunderstorm……. • When humid air is lifted, it will be cooled and the moisture in the air will condense to form cloud. • Upon further uplifting, the cloud will extend higher. Water droplets in the cloud continue to grow in size. As the cloud extends further upward, ice crystals may form because of low temperature. • Thunderstorms are produced by cumulonimbus clouds. • Thunderstorms are sometimes preceded or accompanied by heavy rain and squalls. • These arise when the water droplets in the cumulonimbus cloud can no longer be supported by the underlying air and fall down as rain. The downward rushing air produces squalls.

38. Thunderstorm ………… • Types of Thunderstorm • In the process of thunderstorm formation three types of thunderstorms are produced: • Orographic thunderstorms - caused by air that is forced up by a mountain or hillside.   • Air mass thunderstorms - as the result of localized convection in an unstable air mass. • Frontal thunderstorms - occur along the boundaries of weather fronts (e.g. cold front).

39. Lightning • Lightning is a brilliant electric spark discharge in the atmosphere, occurring within a thundercloud, between clouds, or between a cloud and the ground • The flashing of light produced by a discharge of atmospheric electricity • The flashes of light that are produced in the sky during a storm. • Lightning formation • The sky is filled with electric charge. In a calm sky, the positive (+) and negative (-) charges are evenly spaced throughout the atmosphere. Therefore, a calm sky has a neutral charge.

40. Lightning……………….. • Inside a thunderstorm, the electric charge is spread out differently. • A thunderstorm is made up of ice crystals and hailstones. • The ice crystals have a positive charge, and the hailstones have a negative charge. • An updraft pushes the ice crystals to the top of the thunderstorm cloud. • The hailstones are pushed to the bottom of the thunderstorm by its downdraft. • These processes separate the positive and negative charges of the cloud into two levels: the positive charge at the top and the negative charge at the bottom.

41. Lightning ……………. • During a thunderstorm, the Earth's surface has a positive charge. Because opposites attract, the negative charge at the bottom of the thunder cloud wants to link up with the positive charge of the Earth's surface. • Once the negative charge at the bottom of the cloud gets large enough, a flow of negative charge rushes toward the Earth. This is known as a stepped leader. • The positive charges of the Earth are attracted to this stepped leader, so a flow of positive charge moves into the air. When the stepped leader and the positive charge from the earth meet, a strong electric current carries positive charge up into the cloud. This electric current is known as the return stroke and humans can see it as lightning.

42. Tornadoes • Tornado • A localized cyclonic low-pressure cell surrounded by a whirling cylinder of wind spinning so violently that partial vacuum develops within the funnel. • Has the most extreme pressure gradients known (as much as 100-millibar difference between tornado center and air immediately outside funnel). • Most destructive of all atmospheric disturbances • Extreme pressure difference produces winds of extraordinary speed. Maximum estimates speed range from 320 to 800 kilometers per hour. • Usually develops in warm, moist, unstable air associated with midlatitude cyclone.

43. Tornadoes cont……. • Most often develops along a squall line that preceded a rapidly advance cold front, or along the cold front. • Spring and early summer are favorable for development because there’s considerable air-mass contrast present in the mid-latitudes at that time. • Most occur in mid-afternoon, at time of maximum heating. • More than 90% of all reported tornadoes occur in United States

44. Atmospheric moisture • Humidity is the amount of water vapour present in a given volume of air measured by hygrometer. • Absolute humidity is the actual amount of water vapour in a given volume of air at a particular temperature expressed in gramme per cubic metres. • Relative humidity is the ratio between absolute humidity and the maximum amount of water vapour in a given volume of air at a particular temperature expressed in percentage • Relative humidity varies from place to place depending on temperature. That means for any specified temperature there is a definite limit of the quantity of moisture that the air can hold.

45. Rainfall • Rainfall occurs due to the ascent of moisture bearing clouds or a body of moist air and its subsequent cooling, condensation and precipitation. A light rain is called a drizzle. • Types of rainfall • Convectional Rain: • This type of rain is common in the hot and wet tropical rain forests of central Africa, Amazon basin of South America and the islands of south-east Asia. • Continuous heating of the earth in the day causes hot convectional currents to raise Water keeps on evaporating and rising up. • On reaching the upper layers of the atmosphere they

46. cool, condense and from cumulonimbus clouds. In the late afternoon the clouds burst into a heavy downpour, often accompanied by thunder and lighting. It usual occurs at about 4 pm, so convectional rain is called ‘four o’clock rain’ and occurs daily in the equatorial regions