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Weather The Atmosphere Thin layer of gas that clings to earth’s surface ( as thick as the skin on an apple ) Water vapour varies from less than 1% to 4% of atmosphere. This is because of the water cycle – evaporation and condensation. Air density How much air is present in a cubic metre Varies with height above sea level Sea level 1.2 kg air per cubic metre – top of Everest approx 1/3 of this. The Earth's atmosphere is almost completely contained within 80 kilometers, or 50 miles. As air is highly compressible, air at lower altitudes is compressed by air above and so is more dense. From the diagram below, it can be seen that 50% of the Earth's atmosphere is found below 5km. Air pressure Weight of the air vertically above an area At mean sea level the air pressure averages just over 1000 hectopascals ( hPa) whilst at the top of Everest ( nearer 300hPa The decreasing pressure with increase in height has key influence on Weather. Expanding air As air rises from the surface it experiences lower surrounding pressure. This allows the rising air to expand which in turn causes the air to cool at a rate approx 1 C per 100m When air is compressed it tends to heat up ( bike pump) When air descends it experiences increased pressure and so is compressed and heats up. The Suns Rays The amount of sunlight hitting the earth has an effect on weather. The more sunlight the greater the heating that occurs Equator most heat, poles least heat How clouds are formed. When rising air rises and cools enough it will reach saturation point. This is when water vapour will condense around a tiny dust speck and form a droplet. These will stay up in the air as the are incredibly small ( 200 per millimetre ) They fall slower than the surrounding air is rising and so stay airbourne. As the droplets collide or increase in size they will eventually become too large to stay airbourne and fall as rain. This is one way rain forms and is known as warm rain. NZ rain More commonly rain forms from ice crystals. Once ice crystals from within a cloud they attract water vapour much more easily than other water droplets. As this happens very quickly the water vapour cools quickly, the saturation point falls, more water condenses on the ice crystal which grows – the cycle quickly gets quicker and quicker. As the large crystal falls it bumps into other droplets that freeze onto it, and it increase in size even further. Eventually it falls into warmer air and melts falling as rain. Rain and the Weather map To find where rain is to fall search the weather map for areas where air is rising or going to rise in the atmosphere What is a weather map An analysis map is a map that shows the CURRENT situation at the specified time A prognosis map is a prediction of the future – a forecast The weather map ISOBARS Isobar – join places where air pressure is the same. Lines drawn at 4 hPa intervals Aound NZ pressur varies from 960 hPa ( low pressure ) to 1040 hPa ( high pressure ) Wind Is caused by the movement of air from high to low pressure. From the pattern of the isobars should be able to work out what the wind is doing at different places. Air moves from high to low pressure Sometimes air is deflected by obstacles in its way. Wind Wind blows almost parallel to the isobars crossing them at a small angle at low pressure. Over the open ocean the angle is approx 15 degrees Over the land the angle is 30 degrees due to friction Coriolis effect Lower pressure at tropics as heat rises, lower pressure at poles as cold air sinks. This continually happens. As this happens the movement is deflected westerly by the earths rotation – this is called the coriolis effect Why do: lows / depressions / Cyclones Highs / anticylcones form? Even though tropics recieve the most sun, temperatures do not continue to rise as wind and oceans transport heat towards the poles. This gives rise to anticyclones and depressions that we see on our weather map. Air moves from tropics to poles due to pressure differences. higher pressure at poles as cold air sinks. This continually happens. Lower pressure at tropics as heat rises, Covection – Tricellular model The tricellular model found by Ferrel is still the best model we have. In this model the air, after crossing the warm oceans in the trade winds and becoming warm and moist, arrives at the Equator (at the ITCZ, inter-tropical convergence zone) and is heated, causing it to rise. The unstable air rises to form very high cumulo-nimbus clouds and afternoon thunderstorms and low-pressures are found. The equator is an area with very gentle winds called the doldrums. Polar Front Jet Stream (PFJS):Sub Tropical Jet Stream (STJS): Wind Patterns around the world – seasonal changes will have some effect Pessure gradient – wind speed. In basic terms the bigger the difference in pressure between the High and the Low the stronger the winds will be. The smaller the difference the lighter the winds. Winds are named according to the compass direction of their source. Thus, a wind from the north blowing toward the south is called a northerly wind. The diagram describes the sixteen principal bearings of wind direction. Most meterological observations report wind direction using one of these sixteen bearings. Buys ballot – geostrophic winds In the upper troposphere the air is unaffected by friction and we can see that there is a balance between the pressure gradient force and the coriolis force. The resultant wind is called the geostrpohic wind. The geostrophic wind blows parallel to the isobars. Buys Ballot was a Dutchman who recognised the existence of the geostrophic wind in 1857. His law states that if you stand with your back to the wind the low pressure is always on the left and the high pressure on the right. (Placing of low and high pressures tell you which direction the pressure gradient wind is blowing. The coriolis is in the opposite direction to the pressure gradient force. So you can work out the direction of the resulting wind.) Wind direction In southern hemisphere the wind travel clockwise in a low and anti clockwise in a high In northern hemisphere the flow is reversed. Wind strength In general the closer the isobars the stronger the wind. Does depend on lattitude. Straight isobars at 30 degrees south would cause twice as much wind as at 50 degrees south . Where winds curve tightly around a low the wind speed may be much less than the isobars suggest. Low pressure Areas of low pressure are marked with an L Associated with rising air As air rises it rapidly expands and cools thereby causing clouds and usually rain. Coriolis and the roundabout The coriolis force can be explained by using an example of a roundabout. If we consider person a standing in the middle of the roundabout and trying to throw a ball to person B standing at the edge of the roundabout. If A throws the ball straight at B then by the time the ball has reached the edge of the roundabout B has further round. To the people on the roundabout it looks like the ball has curved round to the right. High Pressure Associated with high pressure and sinking air. Marked with a H Anticyclone Air sunks compressing and warming – clouds evaporate Produces fine settled weather – summer In winter low cloud and drizzle Thunderstorms common in summer Air diverges at high altitude, converges low altitude. Highs / Anti cyclone Formed when air converges aloft. The air descends, compresses and heats – pressure increases. Water vapour remains the same, but this air becomes relatively dry as warm air can hold more vapour. Any cloud rapidly evaporates. Normally air does not reach the surface but spreads out at around 1500m leaving a layer of colder, denser air between it and the surface Water vapour at the edge of the cold air often form cloud. In summer this cloud burns off over the land during the day In winter this cloud may stay all day as the sun is not strong enough to heat the air to burn it off. This is common in Coastal areas. Highs and mountains. Highest temperatures in NZ are caused by a combination of sunlight heating the air plus warm air aloft getting even warmer as it descends after a mountain range Canterbury plains example Sun heats ground and air Warm air descends to surface after crossing mountains Cloud Cold dense air is stopped by mountain Blocking high Important characteristics of highs or anticyclones is the way they sometimes become stationary or slow moving for a week or more. This is called BLOCKING In NZ this can have an effect on the weather depending on where the blocking high is situated. If the high is centred to the west it will mean prolonged south / southwest winds bringing cold air up from the Antartic. Showers If the high is east then warm North or Northwestly winds will be experienced. Heavy rain If the high is centre over the islands then weather will be mainly dry apart from coastal drizzle in winter. Prolonged blocking highs in summer can lead to drought. Sept 1 2008 All the same info on three maps Infra red Satellite Photo Synoptic chart Fronts Mark the boundaries between air of different temperatures and origin. Places where upward air movement occurs. These cut across isobars Fronts have broad bands of cloud and narrow bands of rain Frontal symbols Cold front Air marked with blue triangles signify cold fronts. Triangle points direction that front is travelling. Signifies cold air advancing The arrival of a cold front Cold front in profile Warm Front Shown by red half moons side by side and facing the same way. Warm front in profile Occluded front Where traiangle and half moon shapes are side by side and facing the same way No marked difference between temperature on either side at sea level Stationary front Symbols are placed on either side of the front Shows that the front has slowed as is not moving. If the line at the front is dashed it means it is weak. Troughs and ridges Trough - Dashed line without triangles signifies a trough ( associated with lows ) frequently contains showers or thunderstorms Ridge - Area of high pressure extending away from a high. Not usually marked on a map. Weather similar to that of a high. Who gets the rain? If there is a front near by or strong winds blowing from seas to land likely to be rain. This is because all have or cause rising air that cools, condenses to produce rain. In NZ our mountains have a lot to do with the rainfall Depending on what side of the ranges you live will determine weather you receive the rain. Prevailing Westerlies Prevailing – where it comes from most often One of the most common weather events is a cold front moving over NZ from the tasman sea with NW winds ahead and SW winds behind Southern Alps hokitika Christchurch What the Map shows Strong winds in Canterbury– isobars close together ahead of the front Cold front pushing warm air up, also over the Alps will cause heavy rain on west coast. Deflection due to southern alps Cloud types - Lenticular Lenticular clouds, technically known as altocumulus standing lenticularis, are stationary lens-shaped clouds that form at high altitudes, normally aligned at rightangles to the wind direction. Where stable moist air flows over a mountain or a range of mountains, a series of largescale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves. Under certain conditions, long strings of lenticular clouds can form, creating a formation known as a wave cloud. High cloud Cirrostratus”halo cloud” Cirrus Cirrocumulus Clouds Mid Layer Cloud Altocumulus Clouds Cumulonibus (Mamma) Clouds Alto stratus – grey sheet Low Cloud Stratocumulus Clouds Stratus Stratus Fratus Clouds Vertical development Clouds ( any height ) cumulus Growing Congest Cumulus Clouds Cumulonimbus Cumulonimbus – usually anvil shaped Cumulus Shower cloud Look fluffy Can contain heavy rain Last for maybe an hour or more Created by strong upward air motion Warming by the sea Most commonly in NZ when a cold air front from the south moves north usually behind a cold front. Warming by the Land Happens most in the summer Showers can be produced Favoured are places where land can heat up quickly – ploughed fields, rocky surface, cities Can develop into thunderstorms Sometimes fires Cooling Aloft Sometimes caused by divergence common in NZ Winds up high often stronger than down below. FOG Simply cloud resting on earth’s surface Humid air next to ground is cooled enough to reach saturation point so water vapour starts to ocndense Most common on clear nights with light or no wind. Heat radiates into space from the land cooling the air from below. If the ground temperature is below freezing before the vapour starts to condense a frost will form. Can also form when humid air is blown over a cold surface – sometimes on the coast with on shore winds, or at seas with different temperatures in currents. This sometimes happens in Wellington with the cooler Cook strait. Fog can clear by evaporation, droplets falling to ground without new vapour condensing to replace, wind change Snow Results from cold air from south meeting warm air from the north. Heavy snow if humid warm air from the north is undercut by vary cold air from the south Many snow storms start with a couple of hours of rain. The snow melting before reaching the ground takes latent heat from air which cools until eventually air cools sufficiently for snow to settle. The more snow falling at higher levels the faster this happens. Hail Hail is a large frozen raindrop produced by intense thunderstorms, where snow and rain can coexist in the central updraft. As the snowflakes fall, liquid water freezes onto them forming ice pellets that will continue to grow as more and more droplets are accumulated. Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm. Hail continued As the ice pellets once again fall through the cloud, another layer of ice is added and the hail stone grows even larger. Typically the stronger the updraft, the more times a hail stone repeats this cycle and consequently, the larger it grows. Once the hail stone becomes too heavy to be supported by the updraft, it falls out of the cloud toward the surface. The hail stone reaches the ground as ice since it is not in the warm air below the thunderstorm long enough to melt before reaching the ground. Freezing Rain supercooled droplets freezing on impact Ice storms can be the most devastating of winter weather phenomena and are often the cause of automobile accidents, power outages and personal injury. Ice storms result from the accumulation of freezing rain, which is rain that becomes supercooled and freezes upon impact with cold surfaces. Freezing rain is most commonly found in a narrow band on the cold side of a warm front, where surface temperatures are at or just below freezing. Freezing rain continued Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing. However, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled drops"). When the supercooled drops strike the frozen ground (power lines, or tree branches), they instantly freeze, forming a thin film of ice, hence freezing rain. Sleet freezing rain is defined as frozen raindrops that bounce on impact with the ground or other objects. The diagram shows a typical temperature profile for sleet with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing. Sleet continued Sleet is more difficult to forecast than freezing rain because it develops under more specialized atmospheric conditions. It is very similar to freezing rain in that it causes surfaces to become very slick, but is different because its easily visible. La Nina / El Nino La Nina – NW winds. Wet weather to eastern areas, Bay of plenty, East cape El Nino – W winds. Wet weather west coast, dry east coast - canterbury Katabatic / adiabatic Katabatic where air cooled at the top of mountains and descends Adiabatic where air warms at bottom of mounatins and rises. Lapse rates is the term given as to the temperature at given points of altitude. Katabatic Most katabatic winds (except the Foehn) are more or less the result of air in contact with upper level ground is cooled by radiation, increases in density, and flows downhill and along the valley bottom. For example radiation cooling during nighttime can cause a katabatic flow in the early morning when a pool of cold, high elevation air begins to descend beneath warmer, less dense air. Adiabatic lapse rate The temperature of the air at sea level on the west side of the mountain begins at 30 degrees celsius. The air begins to rise over the mountain cooling at the dry adiabatic rate of -10 degrees/1000 meters. Measuring the weather Check out http://www.metservice.co.nz Weather stations measure wind speed, temp, pressure, rainfall and othe rthings such as humidity etc