The weather part 1

  “Sunshine is delicious, rain is refreshing, wind braces us up, snow is exhilarating; there is really no such thing as bad weather, only different kinds of good weather” J. Ruskin 

When my son was younger he would run into the kitchen shouting “dad, dad your favourite TV programme is on” and I would rush into the lounge to watch the latest episode of…The ‘weather forecast; obsessively flipping channels to find the one with the best outlook for the day. Every serious outdoor enthusiast is obssessed by the weather and how it is going to affect them when they are out and about. Smart phones, ipads and computers mean that we have instant access to hundreds of weather forecasts and updates, with hour by hour breakdown of wind speeds, temperature and rainfall for the exact point where we are standing. But such easy access to information means that fewer people understand the causes of weather, why is it so fickle and how the mountains can change it.

Over 3 parts I will examine why our small island has such fickle weather, what causes local and global weather, how to interpret weather maps, how to predict the weather and how the mountains of the world affect the weather. The weather may seem chaotic and it is a massively complex subject, but despite this patterns exist that allow us to identify and predict common types of weather behaviour. Of course, it is not necessary to have an in-depth understanding of weather science to enjoy the outdoors, but a model of the way weather works will definitely help you to make predictions from weather maps and understand what meterologists are saying. To understand weather charts, jet stream images, and predict the weather from them, you will first need to understand how the earth’s weather is formed. With apologies to meteorologists what follows is a simplified picture,.

The sun causes the earths weather?

The world’s fickle weather is ultimately generated by the unequal heating of the earth’s oceans and land by the sun; the warm earth in turn heats the thin atmosphere  above it (the troposphere). Close to the equator the suns rays are at their strongest while at the poles they are at their weakest. This uneven heating is exacerbated by the white of the snow at the poles that reflects 90% of the sun’s heat energy, while the equator absorb much more.

But, weather isnt just generated on a global scale it also created on small local scales but the principle are is the same - different surfaces (such as forests, ice sheets, or man-made objects) absorb or reflect heat and have differing moisture content. An example is a coastal breezes - as the warm air rises on the land the space left must be occupied by other air which is sucked off the sea bringing with it moisture laden air. Another is warm moist air being forced over a mountain range increases the wind speed.

Globally, the uneven heating of the ocean and land causes huge pockets of warm, moist and high energy air to rise. When meteorolgists examined the global paterns of rising and falling air they identified three large cyclical ‘cell’s’ of rising and falling air - The ‘Hadley’, ‘Ferrel’ and ‘Polar’ cells. It is at the barrier between these huge and different temperature masses of air, that our dramatic weather systems are born. 

Air has pressure

At sea level there is at least 17km of air above you (it less at the poles and more at the equator) this means there is a  large number of air molecules pressing down on the earth - this is what meterologists refer to as air pressure. The average air pressure at sea level is set at a convenient 1000 mb.  The uneven heating of the earth causes warm air to rise and denser cold air to sink which results in lower and higher-pressure air masses or ‘highs’ and ‘lows’. There is not a particular pressure that makes a mass of air a high or low; it is the relative differences between air masses that counts.

Air that contains water will form clouds

As warm air rises it takes with it, moisture from any body of water - oceans, lakes and even the ground (the water molecules fit more easily fits into the larger gaps between the air molecules when it is warm). Warm air passing over the ocean absorbs huge amounts of moisture and this explains why the warm ocean currents circulating the earth affect our global weather picture so much. When warm air rises it cools and the air then starts to descend. As the air cools the molecules become more packed (dense) and water molecules are ‘squeezed’ out to finally gather around grains of dust forming water droplets - the result…Clouds. When the droplets become heavy enough they fall as rain, or if it is very cold they form snow flakes. Nearly all the rain during the winter is actually melted snowflakes and when it is cold enough at ground level they land as snow.

We therefore have three ways that air is made to rise with its associated cloud formation and rain.

At a local level












     Convectional Rainfall - Heating the land from below makes warm moist air rise and when it cools hey presto we get rain

· 












     Orographic or Relief Rainfall - Air being forced to rise over the hills and mountains

2  At a Global level

·  ‘ Frontal lifting’ - When a mass of warm rising air meets a mass of cold sinking air the warm air is forced up over the boundary or ‘front’ of cold air resulting in cooling and hence clouds. This meeting of air masses is how weather is formed on a global scale and explains the weather systems that frequently bombard the UK.

This is examined next.

The global rising and falling of air masses  

Localised and global rising and falling of air are ‘interdependent’ i.e. they are separate, but moderate or exacerbate each other. To better understand why we get large weather systems and associated ‘frontal lifting’ we must look more closely at the global circulation of air masses.

If the Earth was static and not rotating, warm air would simply rise at the equator and head towards the poles, then cool, sink and move back southwards at ground level. However, because the Earth is spinning,  the air returning to the equator is deflected towards the west (easterlies) due to something known as the ‘Coriolis Effect’. To better understand this imagine yourself standing on the north pole (you will be spinning slowly), now throw a ball; because the earth is spinning the ball will actually take a curved route to the right as the earth turns underneath it. If you are standing on the equator you will move with the earth and the ball will travel in a straight line in the direction you are travelling. Only large bodies of water or air are affected, so the idea of water going down a plug hole in opposite directions north and south of the equator is  just a magic trick. Understanding the ‘Coriolis Effect’ is an important step in getting to grips with how ‘depressions’ form.

The much smaller and indistinct ‘Ferrel’ cell found in the gap between the ‘Hadley’ and ‘Polar’ cells has been described as the ‘ball bearing’ between the ‘Hadley’ and ‘Polar’ cells. Air in the Ferrel cell travels with the rotation of the earth as it is to small and indistinct to be affected by the Coriolis effect i.e. eastwards forming the ‘westerlies’.. This movement is therefore in the opposite direction to the Polar and Hadley cells

The Polar cell and the Ferrel cell, meet not-quite-head-on and are actually sliding past one another. At the boundary between the two air masses a sharp gradient in temperature occurs called ‘The Polar Front’. The Polar front is turbulent and follows a forever changing wave pattern around the Earth it even breaks up and disappears alltogether. In winter, as the suns warms the equator less effectively and the warmer air becomes less powerful the polar front pushes towards the Equator, whereas in the summer the increasing mass of warm air pushes the polar front northwards.

The abrupt change in temperature north and south of the Polar Front causes a large pressure difference between the polar cell and the ferrel cell with the polar cell being dense and high pressure. The air in the Ferrel cell is pushed upwards resulting in air moving very quickly because it has to keep up with the mass of air circulating the globe at the surface.This phenomenon is called the ‘Jet Stream’ and it is a good indicator of the strength of the polar front’s position.

Depressions and associated fronts

The up and down movement of the air in the cells and the effect of mountain chains creates colossal waves or kinks in the polar front  called ‘Rossby’ waves. In the UK we can blame the Canadians for all our poor weather because as the Polar front runs over the top of the Canadian Rockies it causes kinks in the barrier between the two air masses and it is these that start the  process of creating the swirling masses of air called depressions or low pressure systems.

Now is the time to remember the air is flowing in opposite directions in the two air masses. The warmer air in the ferrel cell to the south of the Polar front is moving slowly eastwards and the cold air in the polar cell is moving slightly to the west.  When a kink forms it sets up an anticlockwise rotation around the kink.  The leading edge of the warm area is called the ‘Warm Front’, and behind it - the back of the kink – is the cold air moving in to fill the space - the Cold Front. A warm and cold front is depicted on weather charts using warm red semicircles or cold blue triangles. The phrases cold front and warm front describes the condition of the air behind them.   

At the cold and warm fronts, warm air rises above cold and you know what that creates - clouds and rainfall.  Therefore that if you look at what surface the air is travelling over you can estimate how much moisture it contains and whether the air is cold or warm. For example if the air mass is circulating over southern France it will be warm and dry. Unfortunately the air mass that most often affects us comes from the atlantic which is often warm and wet!

The larger the difference in pressure difference between the two masses of air the more powerful the depression will be, the stronger the circulating winds will be and the more rain it will contain. The warm front is slow moving and the gradient of the boundary is shallow, giving prolonged and increasingly heavy rainfall. Conversely, cold fronts are steeper and faster moving bringing shorter but significantly heavier rainfall.  The warm sector inbetween is invariably warmer and drier. The cold front eventually catches up and overtakes a warm front. When this happens it lifts all the warm air above the cold air and is called an ‘Occluded front. Eventually the cold air fills the void and the kink or wave eventually closes up and disappears.

This is the formation and destruction of a Depression – also called a Low-pressure system - something, which is constantly happening along the polar front. 

In part 2 I will examine how we can predict the movement of depressions, how to recognise them when out and about and what they look like on a weather map.

 
 

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