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Weather Silver and Gold Objectives • Understand and read an Isobaric Weather map • Clouds • Recognise 6 different cloud types • Associated weather • Understand how terrain affects the weather. • Make a weather forecast using: • your own observations • an Isobaric weather map Understand an Isobaric Weather map Air Masses • A volume of air defined by its temperature and water vapour content. • Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Isobars • Lines of equal atmospheric air pressure at sea level • Iso = Equal (Like Isossoles Triangle) • Bar = Pressure Isobars and Wind • Winds blow almost directly along the isobars. • CLOCKWISE around LOWS • ANTI-CLOCKWISE around HIGHS. • The closer the isobars, the stronger the winds • If you LOOK into the wind, the LOW pressure is on your LEFT Front Cold Front Cold fronts push in underneath the warmer air ahead of them, forcing the warm air upwards and making cloud and areas of rain. Its surface position is marked by a line with triangles pointing to where it is moving As a cold front passes by: any rain clears but showers may appear, humidity drops, air temperature usually drops, pressure rises and the wind changes direction. Warm Front The advancing warm air rises over a zone of retreating As a warm front passes by: any rain cooler air, making a cloud bank that slopes forwards from becomes patchy but humidity remains ground level upwards, often bringing prolonged steady rain. high, air temperature may rise a little, pressure steadies, and the wind changes direction. Stationary Front Neither air-mass is making much progress. It takes a while for a stationary front to pass by Occluded Front occurs when a cold front overtakes a warm front, so that all that remains of the original warm air is trapped above, where it cools making dense cloud and rain. Ridge When isobars turn a sharp corner around a High they form what is called a ridge of high pressure, which is often shaped like a tongue extending from the High centre : any rain clears only slowly and temperature and pressure do not change much. The weather in a ridge is an extension of the weather in the High. H – High Pressure (Anticyclone) • where isobars enclose an area of high pressure associated with sinking air. • This air warms as it descends, causing clouds to evaporate and producing fine weather L – Low Pressure (Cyclone) • A low pressure system is like a giant funnel of wind spiralling inwards and upwards forcing warmish air in the centre to rise. As air rises it cools and clouds form. • This air cools and often condenses into clouds and rain. Todays Map from Metservice.com Clouds • Recognise 6 different cloud types • The weather associated with the cloud type • https://youtu.be/FMagDRCpJ14 Clouds Clouds High Level Clouds (Above 6km) Cloud Type Description Weather Cirrus Cirrus clouds are feathery, wispy formations made up of very minute ice crystals. Cirrus clouds reveal the presence of moisture at great heights. may indicate an onset of bad weather Cirrostratus Cirrostratus clouds are high altitude clouds made up of tiny ice crystals. Cirrostratus clouds have a thin-layered formation through which the sun’s rays pass, creating a halo like effect. They indicate a rainy spell. Cirrocumulus Cirrocumulus clouds are also high altitude clouds but these are formed when two layers of clouds move in directions opposite to each other. They also indicate unsettled weather. Mid Level Clouds (3-6km) Alto = Middle Cloud Type Description Weather Altocumulus Altocumulus Clouds are thick, fluffy, middle altitude clouds that are a patchy white and grey in colour. Through they look like cirrocumulus clouds. Altocumulus clouds indicate sunny spells. Altostratus Altostratus Clouds are clusters of bluish-grey clouds. Indicate that there may be rain head. Low Level Clouds (Below 3 kms) Cloud Type Description Weather Cumulus Cumulus are thick low altitude clouds are usually puffy and have very distinct edges and a noticeable vertical development. They look like heaped up cotton and have interesting shapes. They indicate sunny weather. heaped cloud Stratocumulus flattened heaped cloud Stratus layer cloud Pretty common. This cloud name is a little unusual because the two key words for cloud appearance have been combined, but that's a good description of this cloud type - a "lumpy layer cloud". These low altitude clouds are responsible for dull, They indicate rainy gloomy, overcast days weather. Multi-layered Clouds • Nimbo or Nimbus means rain Cloud Type Description Weather Cumulonimbus The tallest among all clouds, cumulonimbus clouds span all cloud layers and extent above 2000m. These clouds usually have large anvil-shaped tops, which form because of the stronger winds at the higher levels of the atmosphere. Cumulonimbus clouds produce lightning, thunder, heavy rain, hail, strong winds and tornadoes. Nimbostratus Dark grey middle-low level cloud usually associated with a frontal system. The cloud base can be hard to see because of more or less continuously falling rain or snow beneath it. They indicate a long spell of heavy rain or snow. How terrain affects the weather. • Wind • Orographic cloud and rain • Temp vs elevation How terrain affects the weather Wind • accelerates down valleys, bends around headlands, is contorted by the coastline, dips and dives over hills and dales, eddies behind mountains, and gallops through gaps (e.gcook strait). • it may be drawn onshore during a hot day (sea breeze), • or offshore at night (land breeze). • Mountain air cooled at night sinks down valleys and flows out to sea (katabatic wind). These effects do not show up in the isobars on the weather map but can halve or double the local wind speed, and distort its direction (always towards low pressure). How terrain affects the weather. In NZ Temperatures drop about 0.7 °C for every 100 m of altitude. If 15 degrees at St Augs. Petone: • Belmont Trig Approx. 500m so 5 x0.7=3.5degrees colder • Mt Holdsworth Approx. 1500m so 15x0.7=10.5 degrees colder How terrain affects the weather. Orographic Cloud/Rain Resources • https://en.wikipedia.org/wiki/Climate_of_New_Zealand • http://about.metservice.com/our-company/learning-centre/how-to-read-weather-maps/ • http://about.metservice.com/our-company/learning-centre/poster-downloads • http://www.mountainsafety.org.nz/Safety-Tips/The-Outdoor-Safety-Code/Be-aware-of-the-weather.asp • http://www.teara.govt.nz/en/interactive/7589/weather-map-symbols • Isobaric Weather Map for NZ http://metservice.com/maps-radar/maps/tasman-sea-nz
