Weather forecasts try to predict how rapidly changing, rotating masses of air will affect certain places whereas climate forecasts, using other information, predict the average conditions over much larger areas. More ….
 
Weather is what we experience today, tomorrow or in a few days time whereas climate represents the average conditions experienced over a number of years. Weather depends on the distribution of low-pressure and high-pressure weather systems across the Earth’s surface which form an apparently ever-changing pattern. Climate on the other hand is a steady or slowly changing feature of our environment. It is hard to be sure a climatic change has happened on shorter time scales because of the natural variability caused by the weather, ocean currents, volcanic eruptions and other factors.
 
Weather forecasts are made using computer models that start with real measurements (like pressure and temperature). The models use mathematical equations that include the movement of the air and transfer of heat between the air and the ground or the sea. Even so measurements are not available everywhere, some may be inaccurate and it is only possible to calculate future conditions at a grid of points which may be kilometres apart. Even if a series of models are run from almost the same starting point predictions begin to differ after a few days because of the chaotic nature of the atmosphere.
 
Climate forecasts are different because they are attempts to predict what the average conditions will be over much larger areas of the Earth’s surface many years in the future. Climate modelling is constrained by our knowledge, or estimates, of long-term changes in the Earth’s orbit, the output of the Sun, the distribution of vegetation and ice and even human activities as well as by the slowly warming or cooling deep ocean. Because these forecasts look so far ahead they smooth out the random variability of weather and can predict trends far into the future. Even so, climate forecasts can differ because of the different assumptions made.