Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage.
The diagram below shows a cross section through a warm front, with associated cloud, temperature and weather changes. A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface.
Therefore cold fronts occur where cooler air is replacing warmer air at the surface. The passage of weather associated with a cold front is much shorter lived than that with a warm front. As there is often a lot of cloud in the warmer air ahead of the cold front, there is often little indication of the approaching cold front. As the front passes temperatures fall and there is often a short spell of very heavy rain, sometimes with inbedded thunderstorms and cumulonimbus clouds.
Behind the front the weather is much brighter with broken clouds but occasional showers. Winds veer with the passage of the cold front and are often strong and gusty, especially near showers. Pressure rises throughout the approach and passage of the cold front. The diagram below shows a cross section through a cold front, with associated cloud, temperature and weather changes.
In a mature depression the warm front normally precedes the cold front. Cold fronts generally travel much quicker than warm fronts, and eventually it will catch up with the warm front.
Where the two fronts meet, warm air is lifted from the surface and an occlusion is formed. An occlusion can be thought of as having similar characteristics to both warm and cold fronts.
The weather ahead of an occlusion is similar to that ahead of a warm front, whilst the weather behind is similar to that behind a cold front. Most depressions have a warm and cold front, more mature depressions may also have an occluded front. The diagram below shows a cross-section through a depression, showing the warm and cold fronts and an indication of the associated weather.
Using this information on the passage of depressions you should now be able to complete worksheet 3 , and worksheet 4. Web page reproduced with the kind permission of the Met Office. By clicking any link on this page you are giving your consent for us to set cookies. More info. Strictly Necessary Cookie should be enabled at all times so that we can save your preferences for cookie settings. If you disable this cookie, we will not be able to save your preferences. This means that every time you visit this website you will need to enable or disable cookies again.
Home » Teaching Resources » Weather Systems. Weather Systems. Anticyclones, Depressions and Fronts By the end of the lesson, you will be able to: Understand the characteristics of depressions and fronts and the sequence of associated weather Understand the characteristics of anticyclones and the contrast between those in summer and in winter. Part A Anticyclones, Depressions and Fronts Part A — Anticyclones and Depressions High pressure systems A high pressure system, also known as an anticyclone occurs when the weather is dominated by stable conditions.
Winter anticyclones In winter the clear, settled conditions and light winds associated with anticyclones can lead to frost and fog. Summer anticyclones In summer the clear settled conditions associated with anticyclones can bring long sunny days and warm temperatures. Low pressure systems A low pressure system, also known as a depression occurs when the weather is dominated by unstable conditions. What to do next Using this information on pressure systems you should now be able to complete worksheet 1.
A low pressure system is a whirling mass of warm, moist air that generally brings stormy weather with strong winds. When viewed from above, winds spiral into a low-pressure center in a counterclockwise rotation in the Northern Hemisphere. A low pressure system is represented as a big, red L.
L What is a dew point? The dew point is the temperature at which water starts to condense out of a particular air mass. The dew point temperature changes only when the moisture content of the air changes. The higher the dew point, the greater the moisture content is in the air.
What is humidity? Humidity is the amount of water vapor in the air. What is relative humidity? Relative humidity is the ratio of water vapor contained in the air compared to the maximum amount of moisture that the air can hold at that specific temperature and pressure. Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature.
If the air is at percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator.
This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward. On a weather map, you may notice a blue H, denoting the location of a high pressure system.
How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon.
This means the density of air is high inside the balloon. When the density of air is high, the air pressure is high. The pressure of the air pushes on the balloon from the inside, causing it to inflate. If you heat the balloon, the air pressure gets even higher.
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