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Weather Systems
| An Introduction to Weather Systems |
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Predicting the weather in the mountains is difficult at the best of times as the dynamics of weather systems are incredibly complex; however, by looking at the individual aspects that make up weather systems, we can at least begin to understand how weather forms. |
| The Atmosphere |
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The earth is surrounded by an atmosphere that protects the planet from the harmful rays of the sun and supports all life on earth. It is made up of air (a mixture of oxygen (21%), nitrogen (78%), carbon dioxide (0.037%) and other gases) and water vapour, and is approximately 800km (500 miles) deep. It is constantly on the move as illustrated by satellite images of the planet, with swirling masses of cloud moving across the continents in what appears to be a random fashion. However, on closer examination, there are distinct patterns to these cloud movements that can be directly related to the influence of the sun. |
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last updated 28-Feb-2008 |
| The Sun and Global Circulation |
The sun is the energy source for the whole planet, but its rays heat up the earth unevenly – i.e. more heat reaches the ground at the equator (where its rays pass through the atmosphere at almost 90 degrees to the earth’s surface) than at the poles. The result is that warm air rises at the equator and flows high in the atmosphere towards the poles, where it then cools and sinks back down to earth to return to the equator at the surface. This is known as “global circulation” and it is the earth’s way of trying to redress this heat imbalance - warm winds spreading excess heat polewards, and cooler air returning to the equator from the poles.
Since the earth rotates on a tilted axis, and there is more land mass in the northern hemisphere than in the southern hemisphere, the actual global pattern is much more complicated than this. Instead of a single-cell of air circulation, the global circulation model consists of three cells for both N and S hemispheres. These three cells are the tropical cell (also called a Hadley cell), the midlatitude cell and the polar cell. In some parts of the world, this circulatory effect gives rise to regular and settled weather patterns. However, in the areas sandwiched between these “competing” polar and equatorial systems, the weather is changeable and much more difficult to predict. The UK, Europe and most of North America falls into this category. |
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last updated 10-Jul-2008 |
| Air Masses and Airstreams |
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Air that remains in contact with the earth’s surface for a prolonged period of time will gradually assume the properties of that surface in terms of temperature and humidity. These air masses are generally to be found in the polar and sub-tropical regions, from where they gently flow out as airstreams. For example, a maritime airstream will become saturated with water vapour, especially in its lower layers, due to its contact with the sea; whereas a continental airstream is likely to be dry. Polar air moving south is warmed from beneath causing it to rise and become unstable, often resulting in clouds and rain. Tropical air moving north is cooled from below and stabilises to create fine weather. Airstreams from a particular source tend to have fairly distinct characteristics, so logic would suggest that the general weather patterns associated with them can be predicted by simply mapping the migration of the airstream. However, weather does not tend to operate logically and so in reality, very few airstreams follow such a simple pattern. |
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last updated 28-Feb-2008 |
| Atmospheric Pressure |
When predicting weather, it is also important to consider the different air pressure belts present in the earth’s atmosphere. Atmospheric pressure is the name given to the weight of air on a given area of the earth’s surface. It tends to be greatest at sea level, where it is saturated with water vapour, and as the air heads to areas of higher elevation, it becomes dryer, less dense and therefore lower in pressure. Low pressure also occurs when air is warmed, as it expands, gets lighter and rises. High pressure occurs when air cools, contracts and becomes heavier (denser) and therefore sinks.
Airstreams flow from regions of high pressure to low pressure as they try to equalise the difference between the two; this is known as the pressure gradient and the resulting movement of air is wind. A large difference in pressure creates strong winds. Strong winds are represented on a weather map by closely packed isobars. |
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last updated 10-Jul-2008 |
| Depressions and Fronts |
Air masses with different characteristics cannot mix. For example, an area of cold polar air cannot mix with warm tropical air. Where different air masses meet, there is a distinct boundary between them known as a “front”.
A weather front can either be cold or warm depending on whether cold air is approaching and replacing warmer air (“cold front”), or warm air is replacing colder air (“warm front”). On a weather map, a warm front is denoted by a line with red semicircles pointing in the direction of travel whilst a cold front is shown by a line with blue triangles pointing in the direction the front is travelling.
Cold air is denser and therefore heavier than warm air, so the lighter, warmer air has a tendency to rise above it. As a warm front approaches, the warm air is pushed up to a point where it cools and condenses into clouds and eventually it rains (hence why you can generally expect rain showers as a warm front passes).
Cold fronts undercut warm air with a more violent thrust than the steady rise of air in a warm front; heaving the warm air upwards. The air associated with a cold front is usually unstable and is conducive to cumulonimbus cloud formation, leading to heavy deluges of rainfall and thunderstorms. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5°C or more within the first hour.
As cold fronts travel faster, so they can catch up with the leading warm front and undercut it, forcing the warm air up and over the cold air. This means the warm air can be forced totally off the ground resulting in what is known as an “occluded front”. If the warm air is forced to rise above the cold air then this is a cold occlusion. If the cold front rises over the warm front, this is known as a warm occlusion.
Depressions are low pressure systems that develop when warm air from the sub-tropics meets cold air from the polar regions. They usually have well defined warm and cold fronts within them and a 3-dimensional aspect, as one of the fronts forces the other off the ground in a similar fashion to an occlusion. There is a favourite meeting place in the north-Atlantic where depressions regularly form and as a result, dictate much of the weather patterns in this part of the world. Depressions tend to bring with them rain, strong winds and changeable conditions and can vary in size from between 200 & 2000 miles in diameter.
High Pressure Systems are the opposite of depressions and so therefore tend to be associated with fine, sunny weather. Because the pressure is high, there is little rising air, the sky is often cloudless and there is little wind. If the pressure remains high for some time, dust and dirt particles in the atmosphere may not be circulated and so the air at lower level can become hazy or “smoggy”.
As with anything weather related, these are textbook descriptions of weather fronts which, of course, are rarely so cut and dried in nature. Sometimes it is difficult to tell exactly where the front is and simply watching the temperature alone can be misleading - if the sun comes out after a cold front has passed, the temperature may actually rise. The presence of clouds, rain and a change in wind direction are much better indicators. |
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last updated 10-Jul-2008 |
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