Weather

Earth's Atmosphere

• Composition of the atmosphere - Earth's atmosphere consists of nitrogen (78%), oxygen (21%), small amounts of carbon dioxide, methane, argon, krypton, ozone, neon and other gases such as water vapour.
  
  During the day, the atmosphere regulates the amount of solar radiation reaching the earth and filter out much of the harmful ultra-violet rays.
  
  During the night, the atmosphere, particularly water vapour, absorbs heat radiated from the earth which helps to keep us warm.

• Layers of the atmosphere

1. Troposphere - This layer reaches from sea level to about 10 000 metres above earth and contains much of the weather.
2. Stratosphere - This stretches from 10 000 metres to 50 000 metres above earth and contains the ozone layer that absorbs much of the solar radiation to warm the planet.
3. Mesosphere - This layer reaches from 50 000 metres to 80 000 metres and is much cooler than the lower layers.
4. Thermosphere - This layer stretches from 80 000 metres to about 350 000 metres and contains rarefied air that is very hot (about 1000 °C).

Atmospheric Pressure

• Atmospheric pressure - The pressure or the 'weight' of the air varies from place to place depending on temperature, humidity and altitude.
• Hectopascal - unit of measurement of atmospheric pressure. Equal to 1000 newtons per square metre.
• Average atmospheric pressure at sea level - At sea level, the average atmospheric pressure is 1013 hPa, but this varies according to temperature and water vapour.
• High atmospheric pressure - Cold dry air is denser than other air so it remains close to the earth, giving fine weather.
• Low atmospheric pressure - Hot moist air is less dense than other air so it rises, giving cloudy or rainy weather.

Seasonal Temperatures

• The sun shines at different angles on different parts of the earth. It is directly in line with the equator which has the highest temperatures, whereas further away from the equator the earth becomes cooler.

Water in the Air

• Water cycle - The water cycle is the movement of water to different locations on earth. The parts of the water cycle which relate to weather are:

1. Evaporation - the movement of ocean, river and lake water into the atmosphere
2. Condensation - the process whereby clouds form from the evaporated water vapour
3. Precipitation - falling rain, snow, sleet, hail, fog, mist, dew or frost

• Dew-point - the temperature at which water vapour present in the air changes back to liquid water
• Saturated air - air containing the maximum possible amount of water vapour before precipitation begins
• Humidity - the measure of the amount of water vapour in the air
• Relative humidity - the amount of water vapour the air contains compared with the amount it would need to become saturated at a given temperature (e.g. If the air needs 4 grams of water per cubic metre to become saturated at 20 °C, and the air only contains 2 grams of water, then its relative humidity is 50%.)

Winds

• Wind is moving air.
• Winds blow because of differences in air pressures. Air will always move from a region of high pressure to a region of low pressure to attempt to equalise the pressures.
• Pressure gradient - the rate at which the pressure changes in any direction
• Wind direction - Wind direction is taken as the direction from which the wind is blowing (e.g. If the wind is blowing from the west, it is a westerly wind.)
• Wind speed - The greater the pressure gradient, the greater will be the wind speed.
• Knot - the unit of measure of wind speed. Equal to 1 nautical mile per hour, or 1.852 kilometres per hour

Beaufort Scale of Wind Strengths

Beaufort Number	General Description	Effects at Sea	Effects on Land	Speed in Knots
0	Calm	Sea like a mirror	Calm; Smoke rises vertically	Less than 1
1	Light air	Slight ripples	Smoke drifts; Wind wanes not affected	1 to 3
2	Light breeze	Small wavelets	Wind felt on face; Leaves rustle; Wind vane moves	4 to 6
3	Gentle breeze	Large wavelets; Scattered 'white horses'	Leaves and twigs in constant motion; Flags extended	7 to 10
4	Moderate breeze	Small waves become longer; Frequent 'white horses'	Raises dust and loose paper; Small branches move	11 to 16
5	Fresh breeze	Moderate waves; Many 'white horses'	Small trees sway; Inland waters have crested wavelets	17 to 21
6	Strong breeze	Large waves form; Many white foam crests	Large branches in motion: Umbrellas used with difficulty	22 to 27
7	Near gale	White foam from large waves blows with the wind	Whole trees in motion; Inconvenience when walking against wind	28 to 33
8	Gale	Moderately high waves of greater length; Edges of crests break into spindrift	Twigs break trees	34 to 40
9	Strong gale	High waves; Dense foam blown by wind; Crests roll over	Slight structural damage (e.g. roof tiles removed)	41 to 47
10	Storm	Very high waves with overhanging crests	Trees uprooted; Considerable structural damage	48 to 55
11	Violent storm	Exceptionally high waves; Sea completely covered with foam	Very rare; Widespread damage	56 to 63
12	Cyclone (also known as a hurricane or typhoon)	Air filled with foam and spray; Sea completely white	Extremely rare; Widespread damage	Above 63

Land & Sea Breezes

• Land (soil, rocks etc) heats up rapidly during the day but cools off quickly at night. However water can hold its heat.
• Land breezes - Land breezes blow from the land out to sea. They are strongest on winter mornings.
• Sea breezes - Sea breezes blow from the sea to the land. They are strongest on summer afternoons.

Clouds

• Cloud formation - Water is constantly evaporating from sea and land. The higher the temperature, the greater is the evaporation. Also the higher the temperature, the more water vapour held by the air. Moist air rises up to cooler areas where it condenses onto tiny specks of dust in the air. These droplets combine to form larger droplets which become visible as clouds.
• Cloud types - Clouds are usually named for their shapes such as cirrus = 'curly', cumulus = 'wedge-shaped', stratus = 'spread out', and nimbus = 'cloud'.

Low clouds

1. Stratus - a thin, stretched-out layer of cloud with no rain
2. Cumulus - lumpy white cloud with no rain
3. Large Cumulus - large lumpy whitish cloud with some showers

Middle level clouds

1. Altostratus - a sheet of bluish-grey cloud with light rain
2. Altocumulus - Similar to cirrocumulus but lower in the sky and grey with no rain
3. Nimbostratus - a very thick layer of dark grey cloud with heavy to flood rain

High clouds

1. Cirrus - extremely high and pure white because they are made of tiny ice crystals
2. Cirrostratus - thin, stretched-out layer of very white cloud with no rain
3. Cirrocumulus - thin lumpy clouds in rows with no rain

Precipitation

• Rain - As water droplets condense and join together to each other in warmer areas or to ice particles in colder areas, they attain diameters of 0.5 to 3 millimetres, which are no longer able to be supported by the air. They fall as rain.
• Snow and sleet - If water particles that attach to ice particles in the clouds do not melt while falling, they will fall as snow. Sleet is a combination of both rain and snow.
• Hail - If water droplets are caught in updraughts, they rise higher to cooler areas and freeze in temperatures of –10 °C. The more times they are lifted to the freezing heights by the updraughts of wind, the larger they become. When they can no longer be supported by the air, they fall as ice.
• Fog and smog - Fog is regarded as very low cloud. At night, the earth's surface cools. Air containing moisture may be cooled by contact with the cool earth. Near the sea, fog is usually the result of warm moist air moving over cold sea water. Smog is mixing of fog with industrial or car pollutants in the air.
• Dew and frost - At night, the earth cools down. When water vapour comes in contact with very cool objects such as grass or windscreens, it condenses to form dew or frost if it is very cold.

Fronts

• Cold front- This occurs when a moving mass of cold or cool air moves under a still mass of warm air. It is usually associated with rain and thunderstorms and a wind change from north to south.
• Warm front - This occurs when a moving mass of warm air move over a still mass of cold or cool air. It is usually associated with days of drizzling rain.

Cyclones

• Tropical cyclones are called hurricanes over the North Atlantic Ocean and Northeast Pacific Ocean. They are called typhoons over the Northwest Pacific Ocean.
• Cyclones are vast whirlpools of warm, moist air with very low pressure systems which are usually 100 kilometres across.
• They form over oceans in the tropics, because it involves the rapid rise of hot moist air. The water temperature must be at least 26 °C.
• As the hot moist air rises, it spirals leaving a calm centre called the eye. There is no wind or rain in the eye of a cyclone.
• Winds may reach 300 km/h.
• Cumulonimbus clouds bring torrential rain, formed when the rising hot moist air cools.
• Storm surges where the ocean is lifted beneath the eye cause flooding to low-lying areas as the sea may rise a further 2 metres above the normal tide height.
• Because of the earth's rotation, in the southern hemisphere, cyclones rotate clockwise. In the northern hemisphere, they rotate counter-clockwise.
• Cyclones become less intense as they travel over land or to cooler latitudes.

Tornadoes

• A tornado is the ultimate concentration of cyclonic spinning winds. It is a violent concentrated whirlpool of low pressure air with a funnel-shaped cloud hanging from huge cumulonimbus clouds.
• The base of a tornado is 100 metres but its destructiveness can extend up to several kilometres.
• Winds can be 400 km/h.
• Tornadoes are common in the eastern USA.

Anti-Cyclones

• Anti-cyclones are high pressure systems associated with fine dry weather.
• They rotate slowly and in the opposite direction to tropical cyclones.

Weather-Measuring Devices

• Anemometer - a device for measuring wind speed and direction
• Aneroid Barometer - a device for measuring air pressure
• Hydrograph - an instrument that measures the amount of water vapour in the air
• Thermograph - a device for measuring temperature on rolling graph paper
• Wet and Dry Bulb Thermometer - a device for measuring relative humidity; consists of two thermometers with a wet cloth wrapped around one
• Wind Vane - an instrument for measuring wind direction

Synoptic Charts or Weather Maps

• Synoptic chart - a map showing weather information observed over a large area at the same time.
• Isobar - the lines drawn on a weather map that join areas of equal air pressure
• High winds - These occur where the isobars showing different air pressure areas are close together, and the wind blows from high to low pressure areas.
• Slow winds - These occur where the isobars are spaced further apart.
• Fine weather - usually associated with higher pressure areas over 1013 hPa
• Showers or rain - usually associated with lower pressure areas under 1013 hPa, or
• Some synoptic chart symbols:
  
  High pressure system - H
  Low pressure system or Depressions - L
  Tropical cyclone - T.C.
  Cold front - a bold line with shaded triangles on one side
  Warm front - a bold line with shaded semi-circles on one side
  Trough - an elongated region of low pressure where showers or rain may occur; symbol is a bold dotted line