Basic Navigation Lecture 6 ACP 32 Vol 2

Basic Navigation By the end of this lecture you should understand: § Weather § Fronts § Cloud Types

Weather Much of the weather in Britain is caused by massive areas of air at differing pressures. To visualise pressure areas on a chart or map meteorologists join areas of the same pressure together in a line, an ISOBAR.

Weather Think of Isobars as contour lines on a map. Steep/concentrated Isobars indicate high winds. Shallow, spaced out Isobars indicate slower moving air. Areas of LOW pressure, depressions, bring unsettled weather. Areas of HIGH pressure, anti-cyclones, bring calm weather.

Fronts § When masses of cold air and warm air meet a FRONT occurs. § The COLD air/front will bring poor weather. § The WARM air/front will bring stable weather.

Fronts COLD and WARM air meet and the warm rises.

Fronts As the WARM air rises the cold rushes underneath and begins a spin.

Fronts WARM air is trapped above the cold, spinning stops and the front dissipates. This is now known as an OCCLUDED Front

Fronts § In an OCCLUDED Front both the WARM and COLD air are present in the same space

Fronts § Similar to a front is an Anticyclone § This is a mass of high pressure air creating light winds that spin clockwise (in the northern hemisphere) around the centre of high pressure. § They are stable slow moving systems, consisting of warm dry air, bringing long periods of fine clear weather.

Cloud Types § There are 3 main types of cloud: § 1 Cirrus high altitudes and composed of ice crystals. The word cirrus means a thread or hair. § 2 Cumulus a lumpy or fluffy cloud. § 3 Stratus a featureless layer of cloud.

Cloud Types These three words can be used to describe distinct types of clouds. § The words cumulus and stratus on their own identify clouds whose base is below 2000 m. § Cumulus may be combined with nimbus (Latin for rain), to give cumulonimbus - a heaped rain cloud. § A layer of cloud from which rain is falling is nimbostratus.

Cloud Types