Winds, air masses and fronts
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This document discusses winds, air masses, and fronts in the atmosphere. It begins by introducing winds and how they are caused by pressure differences driven by temperature variations. It describes various forces that influence winds, including pressure gradient force, Coriolis force, and friction. Several global wind systems are then outlined, such as trade winds and westerlies. Local wind systems like sea breezes and mountain/valley breezes are also summarized. These local winds develop due to uneven heating and cooling of land and water surfaces.
Winds, air masses and fronts
- 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2/ 47 Introduction Winds Air mass Fronts ENV 111: Introduction to Meteorology Lecture 4 Winds, air masses and fronts ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2/ 47 Introduction Winds Air mass Fronts Presentation outline 1 Introduction 2 Winds in the atmosphere How does air move? Large scale winds Local wind systems 3 Air masses 4 Frontal systems ndettoel@2016 ENV 111: Introduction to Meteorology
- 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3/ 47 Introduction Winds Air mass Fronts Introduction Figure 2 : Winds and its effects We all experience winds blowing Sometimes, like a pool of cool air, making us shiver In other circumstances, air with different properties meet and cause certain weather conditions Is this usual for the atmosphere? Wind is an exhibition that air moves, and cause other weather phenomena ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4/ 47 Introduction Winds Air mass Fronts Winds in the atmosphere Figure 3 : Sea breeze Wind is simply air in motion. The atmosphere creates kinetic energy (wind) from heat energy Energy source is the temperature differences between: ⇒ the poles and the equator ⇒ the upper and the lower atmosphere Air is set into motion by pressure imbalance due to temperature difference ndettoel@2016 ENV 111: Introduction to Meteorology
- 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5/ 47 Introduction Winds Air mass Fronts Winds Figure 4 : Daily variation of pressure Pressure exhibits daily fluctuations; The largest difference occurs near equator Mainly due to the absorption of solar energy by: ⇒ ozone in the upper atmosphere ⇒ water vapour in the lower atmosphere ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6/ 47 Introduction Winds Air mass Fronts Forces that influence winds Figure 5 : A balance between pressure force and gravity The motion created is modified by friction, the Earth’s rotation, and gravity (in the vertical direction) So, pressure distribution (see illustration) is the primary factor for wind formation Since P1 < P2, pressure (gradient) force is directed upward and balances with the gravity force, g which is directed downward The air is then in hydrostatic equilibrium ndettoel@2016 ENV 111: Introduction to Meteorology
- 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7/ 47 Introduction Winds Air mass Fronts Forces . . . With no balance, vertical accelerations occur and convective currents are created However, mean vertical velocity over large areas are negligibly small Since pressure varies also in horizontal, the difference in pressure will tend to drive the cube horizontally in the direction from high to low pressure As no component of gravity in horizontal, the created horizontal pressure force maintains the motion ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8/ 47 Introduction Winds Air mass Fronts Forces . . . So the large wind systems are mainly horizontal currents Mathematically, pressure gradient force (PGF) is given as: PGF = ∆ p/(ρ∆z) Winds blow due to pressure gradient force, but modified by the effects of the Earth’s rotation and friction The Earth’s rotation causes a deflecting force (Coriolis force), acts everywhere at right angles to the path and to the right of the direction of the motion. ndettoel@2016 ENV 111: Introduction to Meteorology
- 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9/ 47 Introduction Winds Air mass Fronts Forces . . . Figure 6 : A balance between pressure and Coriolis forces This force varies with wind speed, latitude (φ) and equals to: FCo = 2V ωsinφ The force is maximum at the pole and decreases to zero at the equator If a unit of air moves under the influence of the pressure force and the deflecting force; its motion eventually becomes parallel to the isobars Such motion is steady; not accelerating, called geostrophic wind ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10/ 47 Introduction Winds Air mass Fronts Forces . . . Figure 7 : Geostrophic wind flow The geostrophic wind blows along the isobars with high pressure to the right and low pressure to the left of the direction of the motion The wind has neither divergence nor convergence It blows between the isobars in the same manner as the water streams in a river i.e. the narrower the cross section of the river, the faster is the flow ndettoel@2016 ENV 111: Introduction to Meteorology
- 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11/ 47 Introduction Winds Air mass Fronts Forces . . . Figure 8 : Actual wind drifts across isobars due to frictional force, F However, in practice, a slight drift across the isobars that is actually observed in nature, occurs. It means another force (friction) affects wind Under the influence of friction (see illustration), pressure force (G) balances the resultant force (R) of the deflecting force (D) and friction force (F) Since the pressure force is perpendicular to the isobars, the wind (V) must blow to the left of the isobars ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12/ 47 Introduction Winds Air mass Fronts Forces . . . The angle between the isobar and the wind direction increases with increasing friction; it is greater over land than at sea. Its range is 25 − 35◦ under normal conditions Actual wind near the Earth’s surface deviates to the left of the direction of the geostrophic wind Wind velocity increases with height as the frictional force decreases. So the wind direction also change to be more parallel to the isobars The influence of friction is palpable up to a height of 1 km; the layer is called the friction layer (planetary boundary layer) ndettoel@2016 ENV 111: Introduction to Meteorology
- 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13/ 47 Introduction Winds Air mass Fronts Global wind systems Figure 9 : Atmospheric general circulation depicting the global wind systems The wind velocity continues to increase in the free atmosphere since pressure force varies with elevation The maximum value is at the tropopause and then the velocity decreases The atmospheric general circulation is thus an idealized diagram of pressure distribution and prevailing global winds ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14/ 47 Introduction Winds Air mass Fronts Global winds . . . Around the equator, the winds are light and variable; the belt is called the doldrums The belts of high pressure (subtropical anticyclones) separates the easterly winds (trade winds) on their equatorial sides and westerly winds on their poleward sides The trade winds converge at the doldrums bringing there frequent showers, thunderstorms, and heavy rainfall The anticyclones are characterized by divergence and subsiding air currents, low relative humidity, almost clear sky, and less rainfall ndettoel@2016 ENV 111: Introduction to Meteorology
- 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15/ 47 Introduction Winds Air mass Fronts Global winds . . . They move slightly poleward in summer and toward the equator in winter The belts of high pressure are referred to as horse latitudes by sailors The westerlies increase in strength as the latitude increases and are bounded on their poleward sides by a subpolar low The energy imbalance that exists between high and low latitudes cause air to move fast in a narrow channel, called a Jet stream ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16/ 47 Introduction Winds Air mass Fronts Global wind systems: Jet streams Figure 10 : Main jetstreams They occur due to horizontal variations in temperature and pressure The main jet streams at a global scale are: ⇒ Polar Front Jet ⇒ Subtropical Jet ⇒ Tropical easterly jet ndettoel@2016 ENV 111: Introduction to Meteorology
- 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17/ 47 Introduction Winds Air mass Fronts Upper winds: Classification Figure 11 : Upper wind flows Upper winds exhibit various flows: ⇒ Geostrophic, where it blows in a straight path parallel to evenly spaced lines ⇒ Gradient, where it blows parallel to curved contour lines ⇒ Meridional, where it flows in large, looping meanders in a north-south trajectory ⇒ Zonal, where it blows in a west-to-east direction ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18/ 47 Introduction Winds Air mass Fronts Local wind systems Figure 12 : Scales of motion ndettoel@2016 ENV 111: Introduction to Meteorology
- 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19/ 47 Introduction Winds Air mass Fronts Local wind systems Figure 13 : Clouds developing at Lukobe hill in Morogoro ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20/ 47 Introduction Winds Air mass Fronts Sea and land breezes Local wind systems form due to thermal circulation caused by heating and cooling of the atmosphere near the ground Such thermal pressure systems are: ⇒ shallow, ⇒ weakens with height, ⇒ but are mainly maintained by local surface heating and cooling Examples: ⇒ sea and land breezes ⇒ mountain and valley breezes ⇒ katabatic wind ⇒ chinook (Foen) wind ⇒ dust devils or whirlwinds ndettoel@2016 ENV 111: Introduction to Meteorology
- 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21/ 47 Introduction Winds Air mass Fronts Sea and land breezes Figure 14 : Sea and land breezes The sea and land breezes are mesoscale coastal winds system It is caused by uneven heating rates of land and water During the day, intensive heating of the air above land produces a shallow thermal low Whereas a shallow thermal high exists above the water ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22/ 47 Introduction Winds Air mass Fronts Sea and land breezes Thus sea breeze blows at the surface from the sea toward the land The strongest winds typically occur right near the beach and diminish inland Sea breezes are strongest in the afternoon due greatest contrast in temperature between land and water At night the system reverses by surface breeze blowing from the land toward the water hence a land breeze ndettoel@2016 ENV 111: Introduction to Meteorology
- 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes Figure 15 : Mountain and valley breezes During the day, air in upslope is warmer than the air of the same altitude above the valley. It rises as a gentle upslope wind known as a valley breeze The flow reverses at night since the mountain slopes cool quickly, chilling the air in contact with them. The cooler, more-dense air glides downslope into the valley, providing a mountain breeze (also known as gravity, or nocturnal drainage winds) ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes The upslope winds begin early in the morning, reach a peak speed by midday, and reverse direction by late evening While the downslope mountain breeze reaches its peak in the early morning hours usually before sunrise A well developed upslope wind with sufficient moisture, reveals itself by cumulus clouds at summits Hence cloudiness, showers, and even thunderstorms are common over mountains during the warmest part of the day Very strong downslope winds usually on the order of 10 knots or less.are referred to as katabatic winds ndettoel@2016 ENV 111: Introduction to Meteorology
- 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes The ideal setting for a katabatic wind is an elevated plateau surrounded by mountains, with an opening that slopes rapidly downhill Such winds in excess of 100 knots occur: ⇒ Northern Adriatic coast, bora - a cold, gusty; ⇒ Mistral winds into the Rhone Valley of France ⇒ Downslope winds off the ice sheets in Greenland and Antarctica ⇒ Columbia Plateau of Idaho, Oregon, and Washington Whereas, a warm, dry, downslope wind that descends the eastern slope of the Rocky Mountains is specifically termed as chinook (Foehn) winds ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes Foehn winds are characterized by sharp temperature rise, sometimes 20◦ C per hour, and a corresponding sharp drop in the relative humidity Chinook winds are warm due to compressional heating as the air descends is compressed and warms at the dry adiabatic rate (10 ◦ /km) Clouds and precipitation on the mountain’s windward side enhance the chinook ndettoel@2016 ENV 111: Introduction to Meteorology
- 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes Figure 16 : Chinook winds St. Ana wind is a warm, dry wind blowing downhill from the east or northeast into southern California In desert areas, huge dust storms (textitdesrt winds) form, where strong winds are able to lift and fill the air with particles of fine dust ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28/ 47 Introduction Winds Air mass Fronts Mountain and valley breezes Figure 17 : Dust devils or whirlwinds Whereas, dust devils or whirlwinds are rotating vortices on a small scale area that form on clear, hot days over a dry surface The spin of a dust devil around its central core is either cyclonic or anticyclonic, depending on the nature of the topographic feature ndettoel@2016 ENV 111: Introduction to Meteorology
- 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29/ 47 Introduction Winds Air mass Fronts Monsoon winds Figure 18 : Monsoon winds Monsoon winds are examples of thermal circulations that are much larger than those of the more local sea and land breezes. Monsoon is a seasonally changing wind system ndettoel@2016 ENV 111: Introduction to Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30/ 47 Introduction Winds Air mass Fronts Wind measurements Figure 19 : A sonic anemometer Wind is measured in terms of direction and speed. Wind direction is measured by using a wind vane Anemometer is used to measure wind speed. The oldest type of anemometer is the pressure plate anemometer developed by Robert Hooke in 1667 Sonic anemometer is the modern and most accurate instrument to determine wind speed and direction ndettoel@2016 ENV 111: Introduction to Meteorology