This element of my weather book is a very lightweight introduction to what weather is. Its purpose is to give a brief explanation of some of the terms and expressions you may encounter elsewhere, either on this site, in weather forecasts or when talking to weather geeks! It is a very simplistic look at weather, forecasting and climate and if you find it piques your interest in which case there are many sites out there which will explain in much greater detail than here (and probably more understandable). Either way, don't take any internet resource as gospel, research and compare and use reputable and educational sites like the BBC, Met Office, NOAA (in the US).
We all think we know what weather is, especially if it is too hot, too cold, too windy or too wet when we are looking to perform some activity. But, like most things, weather is actually more complex than just what we see outside or experience and it has a more complete definition.
Weather is the expression which describes the state of the atmosphere, not just at ground level but also in the troposhere and, to a lesser extent, the stratosphere. In fact most of the weather occurs in the troposhere and down where we are we just experience the effects of that weather in the form of temperature, wind, humidity etc.
The weather we experience is produced by differences in water vapour in the air, air pressure differences between one place and another and the temperature of the air. There are many factors which influence each of these elements for example sea temperature or solar radiation and it is the wide variation in the contributory factors which is what makes weather forecasting so difficult. We will look at forecasting later on and see how we experience the effects of weather, often very locally, in a micro climate.
The sun constantly revolving on its axis means that there is always a place on the surface of the eaarth where the sun is at its meridian; the meridian is the highest point. But, the earth's axis isn't perpendicular to its orbital plane and so at different times the sun's rays fall unequally on the earth's surface and unequal heating occurs at different times of the year and at different latitudes.
How this affects the weather is relatively easy to understand when you take into account that sun heating the earth causes air in the vicinity to rise (warm air rises) and as it goes up it draws in cooler air from nearby and from further afield. This causes wind to flow which in turn drags moisture in the area from one place to another.
How much heat the earth receives is not just related to the sun's influence, the terrain and vegetation also influence the amount of heat which is absorbed and subsequently warms the air. The sea surface temperature (SST) also impacts on the air's temperature and water vapour content adding to the complexity of the weather system. The earth's tilt impacts on the seasons which in turn affects the type of vegetation which is growing and the knock on effect is to modify the heating of the air, the absorption of moisture into the atmosphere, the wind direction and speed.
Unless you have been on another planet or living in a cave you are bound to have heard about climate change. The general consensus is that human factors are causing the climate to change, but what is climate and how is it different to the weather? Once again being simplistic, weather is what happens on a day to day basis and climate is the range of weather experienced over a period of time.
Think of the last few British winters. The earth's temperature has been steadily rising over the past few decades and we had a number of mild winters which some people attributed to global warming then suddenly we get a cold winter with a lot of snow and frosts. Has climate change stopped? No, there are always variations in the weather but the general trend (the climate) is warming up.