Component-I (A) - Personal Details Role Name Affiliation Principal Investigator Prof. Masood Ahsan Siddiqui Department of Geography, Jamia Millia Islamia, New Delhi Paper Coordinator, if any Dr. Ramashray Prasad Associate Professor Dr B.R. Ambedkar College (University of Delhi Content Writer/Author Dr. Prabuddh Kumar Mishra Assistant Professor in (CW) Geography, Shiva Ji College, DU Content Reviewer (CR) Dr. Ramashray Prasad Associate Professor Dr B.R. Ambedkar College (University of Delhi Language Editor (LE) Component-I (B) - Description of Module Items Description of Module Subject Name Geography Paper Name Climatology Module Name/Title FRONTOGENESIS PROCESS: WORLD PATTERNS AND ASSOCIATED WEATHER Module Id CL-21 Pre-requisites Objectives define fronts, identify the frontal zones, discuss characteristics of fronts, explain the classification of fronts, distinguish between cold and warm front, and; describe the distribution of fronts over the globe and weather phenomena Keywords 1 Introduction Learning Objectives Definition of Front Frontal Zone Frontogenesis Conditions for Frontogenesis Temperature Difference Convergence of Air Masses Development of Fronts Frontolysis (Dissipation of Fronts) Frontal Characteristics Classification of Fronts Warm Front Cold Front Stationary Front Occluded Front Cold Occlusion Warm Occlusion World Distribution Pattern of Fronts Associated Weather with Warm Front Associated Weather with Cold Front Summary and Conclusion Multiple Choice Questions Answer to MCQs References Web References Introduction The literal meaning of the term ‘front’ is the forward margin of something. It a sort of boundary between two air bodies. These two bodies are quite different from each other. It is not always possible to find definite boundaries between two different air masses. More often a gradual transition is observed for many kilometers. However when one air mass moves rapidly into another a distinct boundary becomes established. Such a boundary is called a front. Therefore, it can be stated that a narrow strip along two different bodies is generally termed as front. Another word ‘genesis’ means beginning or method of formation or evolution of something. You must be aware that the air mass is divided into different groups based on different criteria. When two different bodies of air masses are coming into direct contact with each other, fronts are originated. Therefore, the process of the formation of front is known as frontogenesis. Frontogenesis is very commonly found in the higher latitudes. They have evolved a unique pattern over the globe and have modified the weather of the area under their influence very 2 significantly. Opposite to frontogenesis, there is term called ‘frontolysis’ meaning the decaying of fronts. In this module, an attempt is made to study the process of the formation of fronts, their distribution as well as associated weather conditions. Definition of Front Fronts are three dimensional sloping boundary across which the horizontal gradients are sleek. In other words, fronts are defined as ‘zone of mixing’ or ‘discontinuity between two air masses’. They have contrasting characteristics in terms of temperature, density, relative humidity, wind direction, and pressure. Fronts are interface between cold and warm air masses interacting along the temperate convergence zone or subtropical low which help maintaining latitudinal heat balance. Thus, it can be said the front is a line of discontinuity that represents the transitional zone between two air masses having contrasting characteristics. Frontal surface separates these two contrasting air masses. The first real advancement in our understanding of the mid-latitude weather variations was made with the discovery that many day-to-day changes are associated with the formation and movement of boundaries or front, between air masses. The term ‘front’ was first proposed by a group of meteorologists which was led by Vilhelm Bjerknes who was working in Norway, during the First World War in 1918. The three meteorologists V.J. Bjerknes, H. Solberg, and T. Bergeron are considered important for creating an in-depth knowledge and advancement of the fronts. Their ideas are still a very integral part of the forecasting and weather analysis in the middle and the high latitudes. Later, in 1975 F.W. Cole defined fronts as an interface or transition zone between two air masses of different density. However, it should be noted that fronts are not only the transition zone between two air masses of different density. The transition zone might also be of different temperature, humidity, wind direction, and pressure. Frontal Zone Frontal zone is the vast transitional zone between two converging air masses. It depicts the discontinuity zone of opposing and contrasting air masses. Frontal zones are inclined at a very low angle towards the ground and are never parallel or vertical to the ground surface. Frontal zone is a dynamic and mobile zone, therefore it drifts with prevailing winds. Frontogenesis The term frontogenesis was introduced by Tor Bergeron (Norwegian meteorologist) for describing the formation of new fronts. However, later the term was extended to include the process of formation of both new fronts and the regeneration of the old and decaying fronts that already exists in the atmosphere. The region of frontogenesis represents the region that has the 3 convergence of the contrasting air masses. On contrary, we use the term frontolysis means the destruction or dying of fronts. Hence, frontolysis is the opposite of frontogenesis. Fronts do not come into existence all of a sudden; rather they appear only after the process of frontogenesis has been in operation for quite some time. The act of weakening or vanishing of the existing fronts is not accompanied suddenly. The process of frontolysis must continue for some time in order to destroy an existing front. Conditions for Frontogenesis When the distribution of frontogenesis was studied, it was noticed that this process doesn’t occur everywhere. Frontogenesis requires certain characteristics for processes to occur. They are: Temperature Difference: The two opposing air masses that converge to form a front must have a contrasting temperature. If one air mass is warm, moist and light, a front can only be created when the other air mass is cold, dry and dense. What happens when these two contrasting air masses converge? The cold and dense air mass obviously invades the area occupied by the warm and light air mass. As a result, the warm and light air mass is pushed upward and this generates a front. This also explains why no frontogenesis takes place in the equatorial region. Two air masses also converge at the equator (trade winds) but the temperature of both these air masses is uniform. So, the temperature difference is the missing factor here because of which no frontogenesis takes place at the equator. Convergence of Air Masses: In the very definition of frontogenesis, the word ‘convergence’ has been used and thus it is understood that it is the pre-requisite for the frontogenesis process. When two air masses having different temperature converge, they try to invade the space of each other and this leads to the formation of the fronts. For frontolysis to occur, the air masses have to diverge or get diluted by mixing and the contrast being removed. Development of Fronts When two contrasting air masses converge and spread horizontally along the axis of dialation, the probability of creation of front increases. However, there are other necessary conditions for the actual formation of the frontogenesis. When the convergence occurs, the formation of the fronts depends on the angle between the axis of dialation and the isotherms. Fronts are not formed when this angle is more than 450. Frontogenesis is only activated when the angle between the axis of dialation and isotherms decreases. Due to this decrease in angle, a point comes when the isotherms become almost parallel to the axis of dialation and this is when the frontogenesis takes place. The intensity and steepness of the front depend on the temperature gradient of the two air masses. A stationary front is formed when convergence between two air masses parallel to each other occurs. In such a situation, no upward displacement of the air occurs. Such kind of 4 stationary front doesn’t result in cloud formation and precipitation. Stationary fronts are rarely formed because a slope mostly occurs in the fronts due to the Coriolis force. Frontolysis (Dissipation of Fronts) Frontolysis (frontal decay) represents the final phase of a front’s existence, although it is not necessarily linked with occlusion. When contrasting air masses lose their characteristics and difference decay occurs. In other words, when the air masses move away from each other or when the temperature contrast between the adjacent air masses diminishes due to one reason or another the fronts dissipate or start declining. The dissipation of fronts takes place in three ways: (a) through front’s stagnation over a similar surface; (2) as a results of both the air masses cold and warm moving on parallel tracks at the same speed; (3) by the system entering air of the same temperature. Frontolysis happens in the area of Siberia, Northern America etc. Frontal Characteristics The character of frontal weather depends upon the vertical motion in the air masses. If the air in the warm sector, it is rising relative to the frontal zone. The fronts are very active and are termed ana-fronts. The term "ana-front" describes boundaries which show instability, meaning air rises rapidly along and over the boundary to cause significant weather changes. Sinking to warm air relative to cold air masses gives rise to less intense Kata-fronts (Berry et at., 2010 pp-238). "Kata-front" is weaker, bringing smaller changes in temperature and moisture, as well as limited rainfall. Fronts may differ in their types, location or a real extent, but the following characteristics are more or less common to all of them. Fronts are the zone of mixing of cold and warm air masses and it covers certain area.