Introduction to ClimatologyWeather Map Practical1. According to Lamb’s Classification of air flow types (Mayes, & Wheeler, 1997, p. 23), the weather across the British Isles for January 25th 1986 was dominated by Anticyclonic conditions due to a high pressure ridge from a high pressure system situated over the Atlantic Ocean. The ridge is shown in the form of an isobar of 1025mb. The air flow in an anticyclone flows clockwise because the Coriolis force is stronger than the Pressure Gradient Force (PGF) meaning wind is deflected to the right. Consequently, the isobars travel from the south west of Ireland through to the north of Scotland and then south again along the east coast of England. The wind direction follows this isobar causing mainly southerly winds across the west of the British Isles, westerlies at the tip of Scotland as the ridge bends and north westerlies along the east coast. The weather station at Cornwall shows an anomaly as there are northerly winds, however, this could be due to a local factor or the 1030mb isobar to the south of Cornwall as it draws down the wind back into the high pressure system.The weather stations show relatively slow wind speeds across the British Isles with Hull having the strongest wind speed of 18 – 22 Knots. This is because of a higher pressure gradient force in this area shown by the isobars being closer together. North west and south west Ireland both have speeds of 13 – 17 Knots. The rest of England, Wales and Scotland have weak winds of only 8 – 12 Knots which is a typical trend of high pressure systems.There are significant temperature differences across the British Isles with south west Ireland being the warmest at 9 degrees Celsius. The temperature rapidly cools from the south west to the north east of the British Isles with north
Scotland and Glasgow having a low of 2 degrees Celsius and the Shetland Islands being 1 degree Celsius. Temperatures are low as the British Isles are of high latitude (55°N) and away from the equator. Also, it is winter in the northern hemisphere and the British Isles receive less radiation than they would in the summer due to the earth’s tilt. Radiation is less intense as it is dispersed over a larger area due to solar elevation.The high pressure system brought with it a Tropical maritime air mass with homogeneous stable air. It inherits the warmth of its source region ...
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Scotland and Glasgow having a low of 2 degrees Celsius and the Shetland Islands being 1 degree Celsius. Temperatures are low as the British Isles are of high latitude (55°N) and away from the equator. Also, it is winter in the northern hemisphere and the British Isles receive less radiation than they would in the summer due to the earth’s tilt. Radiation is less intense as it is dispersed over a larger area due to solar elevation.The high pressure system brought with it a Tropical maritime air mass with homogeneous stable air. It inherits the warmth of its source region (Mayes, & Wheeler, 1997, p. 20-21) explaining the high temperature in Cornwall. There is presence of cloud cover and it follows a similar decreasing pattern to temperature. North West Ireland is completely overcast and south west Ireland has 7/8 cloud cover. Cornwall and North Wales both have 50% because although the air is stable it has still picked up moisture from its trajectory across the Atlantic Ocean and is rising at the dry adiabatic lapse rate before becoming saturated to form cumulus clouds. The rest of the British Isles has little cover, the maximum being 2/8. Little cloud cover is a trend of anticyclonic conditions. The Shetlands have an anomaly as there is 6/8 cloud cover.There is little present weather across the British Isles on this day, a common feature of an anticyclone. However there is slight drizzle and rain across south west Ireland resulting from an oncoming warm front from a low pressure system above the south of Greenland, which could also be the reason for the high temperature in Cornwall. Oncoming rain from the warm front also explains North West Ireland being over cast. The Shetlands experience some precipitation although it hasn’t reached the station. This is resultant on a sea breeze as they act as triggers for showers explaining the 6/8 cloud cover (Mayes, & Wheeler, 1007, p. 105).2. The 3 cell model shows the expected circulation patterns across the globe without the influence of seasons, continents and oceans. It suggests that at90°N there should be a polar high, at 60°N there should be sub-polar lows and at 30°N there should be sub-tropical highs and at the equator we should see the ITCZ (Guyot, 1998, p. 269). The Intertropical Convergence Zone (ITCZ) follows maximum solar radiation. Due to seasonality the ITCZ moves north during the summer and south during the winter as it follows the sun. This explains why the ITCZ is not on this particular map as it has moved below the equator because it is winter. There is a high over the North Pole showing that this area does fit the 3 cell model. However the distribution of high pressure cells and low pressure cells does not fit the model. At 60°N there is a number of low pressure cells yet there are also high pressure cells e.g. north of Iceland (70°N, 20°E). 30°N is dominated by high pressure supporting the 3 cell model but there are areas of low pressure e.g. Mexico (26°N, 105°W). This shows that the model breaks down towards the poles. The reason for this distribution is because air cannot flow directly from the equator to the poles due to Angular Momentum. With respect to the Earth’s surface, air speeds up as the radius of the Earth decreases, therefore different pressure systems move at different speeds dependent upon their latitude.High pressure cells lay over the Tibetan Plateau (40°N, 90°E) and the Sahara Desert in North Africa (30°N, 15°E) both of which are vast areas with little relief of the land. They retain perfect conditions for high pressure cells as there conditions are tranquil. In mountainous areas it is common to find areas of low pressure to the west e.g. the Rockies in N. America display a high pressure system to the east and a low to the west. This suggests that relief can affect the location of pressure systems.The model also suggests that from 90°N to 60°N there should be Polar easterlies and from 60°N to 30°N there should be mid-latitude westerlies. Polar easterlies can be found to the west of Greenland (70°N, 60°W) and west of Norway (70°N, 0°) shown by low pressure systems where the air moves from east to west. Mid-latitude Westerlies are across the Atlantic Ocean (40°N, 20°W) and over central America (40°N, 110°W) shown by high pressure systems and weather stations where the air from west to east.3. Pressure contours show the height at which there is 500mb pressure. In the upper air chart, there is a low pressure system over the North Pole and Polar Regions and the height at which there is 500mb is 4880m above sea level. The height of 500mb pressure gradually increases towards the equator. In the mid-latitudes the contour lines show the height to be 5440m and at approximately 20°N in the sub-tropics the height of 500mb is 5840m. The graph shows the latitudinal transect along the Greenwich meridian of the 500mb pressure level. It illustrates a trend of increasing height with decreasing latitude. This is due to the temperature of the Earth’s surface. At the North Pole the air is colder and denser with higher pressure while at the equator the air is warmer and lighter with lower pressure. As a result the altitude at which there is 500mb pressure is lower at the poles and higher at the equator due to the density of the air.Air flows from an area of high pressure to an area of low pressure, the greater the difference the faster the air flows. This is called the Pressure Gradient Force (PGF), creating Gradient winds as it forces air motion to the left. However, it does not take into account the rotation of the Earth. The Coriolis force is the deflection of air due to the Earth’s rotation. In the Northern Hemisphere the air is deflected to the right with respect to the surface of the Earth. If the forces are balanced and the isobars are straight they create Geostrophic wind where the wind flows along the contour lines from west to east. If the isobars are curved and the forces are imbalanced then there is a resultant centripetal force which causes cyclonic and anticyclonic conditions in the lower atmosphere.There are 6 Rossby waves across the Northern Hemisphere defined from ridge to trough to ridge. The Rossby waves in the westerlies are zonal with high index; this is caused by temperature as surface heating produces pressure differences at higher levels. Although there is oscillation, it is weak. It is weak because there is not a great difference between the ridges and the troughs, the largest fluctuation being above the western Pacific Ocean from 65°N (the top of the ridge) to 20°N (the bottom on the trough). The ridges are narrower than the troughs as they are more compacted due a smaller radius of the Earth’s surface. The widest trough is over the Pacific Ocean and Eastern Asia (60°N, 150°E). Other troughs can be found at (30°N, 70°E), (37°N, 15°E), (50°N, 55°W), (35°N, 105°W) and (30°N, 150°W). The ridges are situated at (58°N, 90°E), a small ridge is over Iraq due to an old low pressure system (48°N, 48°E), (70°N, 10°), another small ridge over Hudson Bay, Canada (56°N, 85°W), (60°N, 130°W) and (64°N, 165°W). Although the Rossby waves are zonal, one cut-off low can be found over the Atlantic Ocean (30°N, 37°W) however it may be part of a new set of Rossby waves. Another cut off-low is being created at (40°N, 150°W) due to a blocking high forcing its way north over Alaska (55°N, 160°W).4. BibliographyGuyot, G. (1998). Physics of the Environment and Climate. West Sussex: John Wiley & Sons Ltd.Mayes, J., & Wheeler, D. (1997). Regional Climates of the British Isles. London: Routledge.
