GEOGRAPHY OPTIONAL by SHAMIM ANWER LEARNING GEOGRAPHY - A NEVER BEFORE EXPERIENCE PREP SUPPLEMENT CLIMA T OLOGY NOT FOR SALE | Off : 57/17 1st Floor, Old Rajender Nagar 8026506054, 8826506099 | Above Dr, Batra’s Delhi - 110060 Also visit us on www.keynoteias.com | www.facebook.com/keynoteias.in KEYNOTE IAS PHYSICAL GEOGRAPHY CLIMATOLOGY INDEX 1. CIRCULATION OF THE ATMOSPHERE ................................................................1-13 Local Winds; Observed Distribution of Pressure and Winds and Idealized Zonal Pressure Belts; Monsoons, Westerlies and Jet Streams; EL Nino and La Nina. 2. MOISTURE AND ATMOSPHERIC STABILITY ..................................................14-24 Atmospheric Stability and Instability 3. FORMS OF CONDENSATION AND PRECIPITATION ........................................25-38 Types and Global Distribution of Precipitation 4. AIR MASSES...........................................................................................................39-53 Polar-Front Theory; Fronts; Cyclone Formation; Cyclonic and Anticyclonic Circulation PREP-SUPPLEMENT: CLIMATOLOGY 1 1. CIRCULATION OF THE ATMOSPHERE Atmospheric circulation and wind Macroscale Winds: The largest wind patterns, called macroscale winds, are exemplified by the Winds are generated by pressure differences that westerlies and trade winds. These planetary-scale arise because of unequal heating of Earth's surface. flow patterns extend around the entire globe and Global winds are generated because the tropics can remain essentially unchanged for weeks at a receive more solar radiation than Earth's Polar time. Regions. Thus, Earth's winds blow in an unending attempt to balance inequalities in surface Asomewhat smaller macroscale circulation is temperatures. Because the zone of maximum solar called synoptic scale, or weather-map scale. Two heating migrates with the seasons-moving well-known synoptic scale systems are the northward during the Northern Hemisphere individual traveling cyclones and anti-cyclone that summer and southward as winter approaches-the appear on weather maps as areas of low and high wind pattern that make up the general circulation pressure, respectively. These weather producers also migrate latitudinally are found in the middle latitudes where they move from west to east as part of the larger westerly Wind movement in the atmosphere may be flow. Furthermore, these rotating systems usually classified into 3 broad categories: Primary, persist for days or occasionally weeks and have a secondary and tertiary circulation. Primary horizontal dimension of hundreds to thousands of circulation includes the planetary wind system kilometers. Anticyclones, in contrast, are areas of which are related to the general arrangement of subsidence associated with outward flow near the the pressure belts on the earth surface. Trade surface. Somewhat smaller macroscale systems are winds, westerlies, and the polar easterlies together the tropical cyclones and hurricanes that develop form the primary circulation. Secondary in late summer and early fall over the warm circulation consists of air masses, cyclones, anti- tropical oceans. Airflow in these systems is inward cyclones, and monsoons. Tertiary includes all the and upward as in the larger mid-latitude cyclones. local winds which are produced by local causes. However, the rate of horizontal flow associated with hurricanes is usually more rapid than that of their more poleward cousins. Mesoscale Winds generally last for several minutes and may exist for hours. These middle- size phenomena are usually less than 100 kilometers (62 miles) across. Further, some mesoscale winds-for example, thunderstorms and tornadoes-also have a strong vertical component Land and sea breezes, as well as mountain and valley winds, also fall into this category Microscale Winds The smallest scale of air motion is referred to as microscale circulation. These small, often chaotic winds normally last for seconds or at most minutes. Examples include simplegusts which hurl debris into the air and small, well-developed vortices such as dust devils. Large- and Small-Scale Circulation PREP-SUPPLEMENT: CLIMATOLOGY 2 Time Scales for Atmospheric Motions Scale Time scale Distance scale Examples Macroscale Planetary Weeks or longer 1000-40,000 km Westerlies and trade winds Synoptic Days to weeks 100-5000 km Mid-latitude cyclones, anticyclones, and hurricanes Mesoscale Minutes to hours 1-100 km Thunderstorms, tornadoes, and land-sea breeze Microscale Seconds to minutes <1 km Turbulence, dust devils, and gusts Local Winds depend on the location and the time of year. Tropical areas where intense solar heating is DUST DEVILS continuous throughout the year experience more A common phenomenon in arid regions of the frequent and stronger sea breezes than do mid- world is the whirling vortex called the dust devil - latitude locations. The most intense sea breezes Although they resemble tornadoes, dust devils are develop along tropical coastlines adjacent to cool generally much smaller and less intense than their ocean currents. In the middle latitudes, sea breezes destructive cousins. Most dust devils-are only a are most common during the warmest months, but few metersin diameter andreach heights no greater the counterpart, the land breeze, is often missing, than about 100 meters (300 feet). Further, these for the land does not always cool below the ocean whirlwinds are usually short-lived microscale temperature. In the higher middle latitudes the phenomena. Most form and die out within minutes. frequent migration of high- and low-pressure In rare instances dust devils have lasted for hours. systems dominates the circulation, so land and sea Unlike tornadoes, which are associated with breezes are less noticeable. convective clouds, dust devils form on days when Illustration of a sea breeze and a land breeze, (a) clear skies dominate. Further, these whirlwinds during the daylight hours the air above the land form from the ground upward, exactly opposite of tornadoes. Because surface heating is critical to their formation, dust devils occur most frequently in the afternoon when surface temperaturesare highest. Land and Sea Breeze The daily temperature contrast between the land and the sea, and the pressure pattern that generates a sea breeze.Land is heated more intensely during daylight hours than is an adjacent body of water. As a result, the air above the land surface heats and expands, creating an area of low pressure. A sea breeze then develops, as cooler air over the heatsand expands, creating an area of lower water moves onto the land .At night, the reverse pressure. Cooler and denser air over the water may take place; the land cools more rapidly than moves onto the land, generating a sea breeze, (b) the sea and a land breeze develops At night the land cools more rapidly than the sea, The sea breeze has a significant moderating generating an offshore flow called a land breeze. influence on coastal temperatures. Smaller-scale sea breezes can also develop along the shores of large lakes. In many places sea breezes also affect the amount of cloud cover and rainfall. The peninsula of Florida, for example, experiences a summer precipitation maximum caused partly by the convergence of sea breezes from both the Atlantic and Gulf coasts. The intensity and extent of land and sea breezes 3 Mountain and Valley Breezes common on warm summer days in the mountains. A daily wind similar to land and sea breezes occurs After sunset the pattern is reversed. Rapid radiation in many mountainous regions. During the day, air heat loss along the mountain slopes cools the air, along mountain slopes is heated more intensely which drains into the valley below and causes a than air at the same elevation over the valley floor mountain breeze. Similar cool air drainage can This warmer air glides up along the mountain slope occur in regions that have little slope. The result and generates a valley breeze. The occurrence of is that the coldest pockets of air are usually found these daytime upslope breezes can often be in the lowest spots, a phenomenon one likely have identified by the isolated cumulus clouds that experienced while walking in hilly terrain. develop over adjacent mountain peaks. This also Consequently, low areas are the first to experience causes the late afternoon thundershowers so radiation fog and are the most likely spots for frost damage to crops. Like many other winds, mountain and valley Chinook (Foehn) Winds breezes have seasonal preferences. Although Warm, dry winds sometimes move down the east valley breezes are mostcommon during the warm slopes of the Rockies, where they are called season when solar heating is most intense, chinooks, and the Alps, where they are called mountain breezes tend to be more frequent during foehns. Such winds are often created when a strong the cold season. pressure gradient develops in a mountainous Valley and mountain breezes, (a) Heating during region. As the air descends the leeward slopes of the daylight hours warms the air along the the mountains, it is heated adiabatically (by mountain slopes. This warm air rises, generating compression). Because condensation may have a valley breeze, (b) after sunset, cooling of the air occurred as the air ascended the windward side, near the mountain can result in cool air drainage releasing latent heat, the air descending the into the valley, producing the mountain breeze. leeward side will be warmer and drier than at a similar elevation on the windward side. Although