1B.2 Weather Systems Flashcards
Define air mass.
A large body of air with similar temperature and moisture characteristics all the way through it.
Describe polar maritime air masses.
These come from the north-west and originate over the north Atlantic. They bring cool, moist conditions. In summer they bring sunny intervals with scattered showers and temperatures of 10-14°C. In winter temperatures are much lower (4-8°C) and sleet showers may occur. Winds may be strong as these air masses follow behind a depression.
Describe polar continental air masses.
These come from the east and originate over Scandinavia and Russia. They bring cold conditions in winter, with temperatures below freezing. It may snow in eastern Britain as moisture is picked up over the North Sea. In summer conditions are drier and warmer with fresh winds.
Describe tropical maritime air masses.
These come from the south-west and originate over the Azores or the Caribbean. They bring mild, damp, cloudy weather as they pick up moisture from the Atlantic Ocean. In winter they bring temperatures of around 8-12°C and in summer 14-18°C. The air is often humid as it lies within the warm sector of a depression.
Describe tropical continental air masses.
These come from the south and originate over dry northern Africa. They bring hot, dry weather and summer heatwaves with temperatures of 20°C. They may reach the south coast of England during summer but the cooling effect of the Atlantic Ocean reduces its influence on Northern Ireland. The air masses can last for several days and often end with thunderstorms.
Describe arctic air masses.
These come from a northerly direction and bring cold, snowy weather in winter and cool, damp weather in summer.
What is meant by a variable climate?
The weather changes from day to day, which makes it difficult to forecast.
What does an equable climate mean?
Extremes of heat or cold, drought or prolongued rainfall are rare.
What is a front?
The boundary between warm and cold air masses.
What is a depression?
A system or area of low pressure that can bring cloud, rain and windy weather.
Describe fully how a depression forms.
- The warm, moist Tropical Maritime air meets the colder, moist Polar Maritime air along the polar front, over the Atlantic Ocean.
- The warm air, being lighter and less dense than the colder air, starts to rise over the cold air. A low pressure is created on the ground.
- The warm air is sucked into the low pressure area to create a warm sector. The cold air is sucked in behind the warm air. This causes the entire air mass to start swirling in an anticlockwise direction.
- Where the warm air starts to rise over the cold air, is called the warm front. Where the cold air pushes in behind the warm air, is called the cold front. Between the fronts is a mass of tropical maritime air known as the warm sector.
What does a depression look like on a weather map?
A circular pattern of isobars (lines which join equal areas of pressure) with the lowest pressure in the centre and the winds blowing into the centre.
Describe and explain how temperature changes during the passing of a depression.
❄ As a depression approaches, the temperature remains low because colder, polar air dominates.
❄ As the warm sector passes over, the temperature rises because this is a pocket of tropical maritime, warm, southerly air.
❄ As the cold front passes, the temperature falls back because the wind swings from the north-west, bringing cold, polar air back once more.
Describe and explain how precipitation changes during a depression.
❄ Ahead of the warm front, there is higher pressure, so the air sinks, no clouds form and no rain occurs.
❄ As the warm front arrives, air is forced to rise over the dense, cold air. It cools, condenses and heavy rain occurs. There is less rain within the warm sector because the air is not forced to rise.
❄ At the cold front the cold sector is undercutting the warm sector forcing air to rise, cool and condense. This rapid uplift creates high clouds and heavy rain. Scattered showers follow the cold front because the polar maritime air crosses the ocean, picking up moisture.
Describe and explain how cloud type changes during a depression.
❄ As the warm front approaches, clear skies change to cirrus clouds due to the warm air rising, cooling and condensing along the front. These soon develop into thick nimbostratus clouds.
❄ In the warm sector, lower stratus clouds dominate as the uplift of air is less.
❄ At the cold front, air is forced to rise steeply because of the undercutting by cold air. This rapid uplift creates towering cumulonimbus clouds. After the cold front there is less uplift of air so the clouds are smaller, pathy cumulus.
Describe and explain how wind speed changes during a depression.
❄ Ahead of the warm front, the isobars are far apart so the pressure gradient is gentler and the wind speed is lower.
❄ As the depression moves overhead, the pressure decreses and the isobars become closer together. This creates a steep pressure gradient and stronger winds. Gusty winds are especially significant along the cold front.
❄ When the depression passes, wind speed drops once more.
Describe and explain wind direction changes during a depression.
❄ Before the depression, the air has an anticlockwise movement around the centre of the low pressure system. This means winds are southerly.
❄ Within the warm sector the rotation brings air from the tropical maritime south-west.
❄ After the cold front, the rotation brings air from the polar north, with a direction of north-west.
How does an occluded front form?
This happens when the cold front moves eastwards faster than the warm front and so cold air pushes the warm front off the ground.
What is an anticyclone?
A system of high pressure that produces calm, settled weather.
How does an anticyclone occur?
- In the centre of the system, the air is sinking slowly from great heights.
- As the air sinks it swirls in a clockwise direction and spreads out at the surface.
- The sinking air is compressed and warms up as it nears the ground. This means clouds do not form and it is less likely to rain.
- The isobars are spaced well apart and so the pressure gradient is gentle, meaning wind speeds are low and there is usually dry, bright weather.
Fully describe the weather patterns associated with a summer anticyclone.
❄ Descending air means no clouds so warm, sunny conditions are common during the day with temperatures perhaps over 20°C in July or August.
❄ At night, clear skies allow heat to escape into space through radiation. Therefore, temperatures can drop rapidly to 8°C.
❄ As air next to the ground cools at night, condensation can occur so dew and mist may form. however these should clear in the morning as the sun’s energy evaporates the moisture.
❄ Heatwave conditions and rapid uplift of warm air currents can trigger afternoon thunderstorms as convectional rainfall occurs.
Fully describe the weather patterns associated with a winter anticyclone.
❄ The sun is very low in the sky between November and February. Its low angle and lack of energy keeps temperatures low, approaching 0°C. However the rapid loss of heat under the clear evening skies means that nights can be very cold, below freezing.
❄ Condensation at night near the ground surface can produce fog which may, due to the sun’s lack of heat, persist all day. This creates dangerous conditions for motorists.
❄ The weather should be dry and bright because descending air warms meaning no clouds form.
❄ It may be frosty because the ground radiates long wave radiation into the clear sky and surface temperatures fall rapidly. This means that any condensation freezes to create frost.
What is a sypnotic chart?
A map summarising the weather conditions at a particular point in time for an area. It records the weather using a set of symbols and shows the fronts of a depression and the variation in the air pressure using isobars.
What is a satellite image?
A photograph taken from space and sent back to Earth. It can show the cloud formations and patterns during depressions, or the clear skies associated with high pressure areas. The visible features on a satellite image is caused by the reflection of sunlight from clouds or the Earth’s surface. Generally the brighter a cloud is, the thicker it is.
