Understanding weather

Atmosphere

The atmosphere is a thin layer of mixed gases that cover the Earth and help it from becoming too hot or too cold. Its circulation, the heat (terrestrial radiation) and light (solar radiation) which pass through it, and the processes which go on in it, all affect the climate.

The atmosphere is about 800 km (500 miles) deep and is made up of 21% oxygen, 78% nitrogen, 0.041% carbon dioxide, and other gases including methane, hydrogen, helium, neon, argon, krypton, xenon, and water vapour.

In order moving away from the Earth, the atmosphere's layers are: Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

Refraction

Why is the sky blue? What causes a sunrise or sunset to be so colourful? What makes a rainbow? Answer = refraction.

When light enters the atmosphere from the Sun it contains all the different colours of the rainbow, which when they are all together appear as white light. In the atmosphere, the gases in it cause the light to slow down, change direction and scatter, for most colours the effect of this is very little, but for blue it is scattered the most, which is why it is the one we see when we look at the sky.

The same things happens at sunet and sunrise, but because the sun is lower in the sky, the angle that light approaches causes different colours to be scattered.

It is very similar for rainbows, except this time the rain, or something like a waterfall, is causing the light we see to be split up into all of its seperate colours. You can only see a rainbow when the sun is behind you with water droplets falling in front of you, because of this the rainbow is unique to you and where you are standing so no one else is seeing quite the same rainbow as you!

Air masses

Air masses are parcels of air that bring distinctive weather features to the country. An air mass is a body or 'mass' of air that have very little change temperature and humidity within them. That is to say the air making up the mass is very uniform in temperature and humidity.

An air mass is separated from another body of air by a weather front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

High pressure (or anticyclone)

In an anticyclone (also referred to as a 'high') the winds tend to be light and blow in a clockwise direction. Also the air is descending, which stops the formation of clouds. The light winds and clear skies can lead to overnight fog or frost. If an anticyclone persists over northern Europe in winter, then much of the British Isles can be affected by very cold east winds from Siberia. However, in summer an anticyclone in the vicinity of the British Isles often brings fine, warm weather.

Low pressure (or depression)

In a depression (also referred to as a 'low'), air is rising. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). There are usually frontal systems associated with depressions.

Weather fronts

A weather front is simply the boundary between two air masses.

There are three different types of weather front

1. Cold front

This is the boundary between warm air and cold air and is where cold air is replacing warm air at a point on the Earth's surface.

On a synoptic chart (a weather chart) a cold front is a blue line with blue triangles on it.

The presence of a cold front means cold air is moving in and pushing underneath warmer air. This is because the cold air is 'heavier' or denser, than the warmer air so sinks below it. So cold air is replacing warm air at the surface. The triangles on the front point to the direction that the front is moving.

You can tell when a cold front has passed by a rise of pressure, a fall of temperature and dew point, and a change in wind direction. Rain occurs with most cold fronts and may extend some 100 to 200 km ahead of or behind the front. Some cold fronts give only a shower at the front, while others give no precipitation. Thunder may occur at a cold front.

2. Warm front

This is the boundary between cold air and warm air and is when warm air replaces cold air at a point on the Earth's surface.

On a synoptic chart a warm front is a red line with red semi-circles on it.

The presence of a warm front means warm air is moving in and rising up over cold air. This is because the warm air is 'lighter' or less dense, than the colder air. So warm air is replacing cold air at the surface. The semi-circles on the front point to which direction the front is moving.

As a warm front approaches, temperature and dew-point within the cold air gradually rise and pressure falls more quickly. Precipitation usually occurs in a wide belt about 400 km just ahead of the front. When the front has passed, the will be a steadying of the barometer (pressure will stop falling), a jump in temperature and dew point, a change of wind direction, and precipitation will stop, or mostly stop.

3. Occluded front

These are more complex than cold or warm fronts. An occlusion is formed when a cold front catches up with a warm front. When a cold front catches up with a warm front, the warm air in the warm sector is forced up from the surface.

On a synoptic chart an occluded front is a purple line with purple semi-circles and triangles.

Not all occlusions follow the same rules, as some will be more similar to warm fronts and some will be more like cold fronts, so some will cause it to feel warmer once it has passed, others cooler, some will lower the dew point, others will cause it to increase. All occlusions will cause a rise in pressure, a change in wind direction and precipitation will slowly clear when they have passed.

Weather charts

Weather can change on a daily basis especially at middle to high latitudes (parts of the earth that are quite far north or south of the Equator) where it is controlled by weather systems, depressions and anticyclones. On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure), depressions (areas of low pressure), troughs and ridges which are associated with particular kinds of weather.

Temperature differences

Temperature affects other weather elements including air pressure, wind, cloud formation, humidity and precipitation.

Factors affecting temperature:

• Latitude - warmer closer to the Equator and cooler moving away towards the poles
• Altitude - getting colder as you go higher up
• Distance from the sea - temperatures inland are usually higher than the coast during the summer and lower than the coast during winter. This is because land heats up and cools down more quickly than the sea
• North-facing slopes in the southern hemisphere and south-facing slopes in the northern hemisphere receive more sunlight than the opposite slopes and are warmer
• Wind - generally makes the air feel cooler

Wind

The movement of air around the earth from high pressure to low pressure is what brings about winds. The direction given for the wind refers to the direction from which it comes. For example, a westerly wind is blowing from the west towards the east.

Measurements of wind strength are made at 10 metres (33 feet) above the ground. A specified height has to be used so that all the measurements taken across the world can be compared and because at ground level there are lots of obstacles like trees and buildings that can cause the wind to get stronger or weaker.

In the UK winds are measured in knots (nautical miles per hour). However, forecast winds are often given in miles per hour (where 1 knot is equivalent to 1.15 mph) or in terms of the Beaufort Scale.

There can be rapid variations in the speed of the wind - these are referred to as gusts. Gusts are higher inland than over the sea or windward coasts, although the mean wind speeds tend to be lower inland. Typically, gusts can be 60% higher than the mean speed, although in the middle of cities this can reach 100%. Northerly winds tend to be gustier than southerly ones. In general, the weather is strongly influenced by the wind direction, so information about the wind provides an indication of the type of weather likely to be experienced.

• Northerly winds tend to bring relatively cold air from polar regions to the British Isles.
  As cold polar air moves southwards over an increasingly warm sea, the heating of the air by the sea causes cumulus clouds to form. These clouds may grow sufficiently for showers to develop and, consequently, winds from the north-west, north or north-east usually bring cold, showery weather to the British Isles.
• Southwesterly wind bring warm air from the tropics, which is cooled from below as it moves northwards over a gradually cooling sea. Sometimes the cooling is sufficient for sea fog or a thin layer of stratus to form. The cloud can become thick enough for drizzle, especially on windward coasts and over high ground. In general, winds from the west or southwest are associated with overcast, wet weather.
• Winds from the south and southeast mainly occur in summer and these bring warm, dry weather. However, southerly winds can sometimes bring hot, thundery weather.
• Easterly winds in winter bring very cold air to the British Isles. The characteristics and path of the air determine whether it is cloudy (with perhaps rain, sleet or snow) or fine and sunny. In summer, an easterly wind will mean it is cool on the east coast but warm elsewhere, usually with clear skies.

Clouds

A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.

Cirrus - a tuft or filament (e.g. of hair)

Cumulus - a heap or pile

Stratus - a layer

Nimbus - rain bearing

Clouds form when moist air is cooled to such an extent it becomes saturated.

The main mechanism for cooling air is to force it to rise. As air rises it expands - because the pressure decreases with height in the atmosphere - and this causes it to cool.

Eventually it may become saturated and the water vapour then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.

The ten main types of cloud can be separated into three broad categories according to the height of their base above the ground: high clouds, medium clouds and low clouds.

High clouds

High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet (5,500 and 14,000 metres).
Cirrus - white filaments
Cirrocumulus - small rippled elements
Cirrostratus - transparent sheet, often with a halo

Medium clouds

Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals, and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).
Altocumulus - layered, rippled elements, generally white with some shading
Altostratus - thin layer, grey, allows sun to appear as if through ground glass
Nimbostratus - thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy

Low clouds

Low clouds are usually composed of water droplets - though cumulonimbus clouds include ice crystals - and have a base below 6,500 feet (2,000 metres).
Stratocumulus - layered, series of rounded rolls, generally white with some shading
Stratus - layered, uniform base, grey
Cumulus - individual cells, vertical rolls or towers, flat base
Cumulonimbus - large cauliflower-shaped towers, often 'anvil tops', sometimes giving thunderstorms or showers of rain or snow