Weather is fairly chaotic and unpredictable on our planet. Climate is more stable.
The climate is what we expect, weather is what we get.
- The Normal
Meteorologists all over the world measure temperatures, wind and precipitation at certain times of the day every day throughout the year.
Based on long time series of such data at thousands of places around the world, it is possible to calculate the average weather at local places or regions over time.
- According to an international meteorological convention, a normal is calculated for 30 year periods.
The first such normal period was 1901 to 1930. The next normal period was from 1931 to 1960.
The current normal is based on the average data from 1961 to 1990. In other words, the normal is the average weather over thirty years.
The normal can also be calculated over hundred years, but that is more unusual.
It is therefore possible to say what the normal temperature, wind and precipitation are e.g. 20 March in New York, Beijing, Oslo and thousands of other places.
- Most meteorologists will tend to compare today's temperature and precipitation with the normal for their location.
When consulting these normals, you can find out what kind of weather to expect if you travel to Cairo or Anchorage in March.
You will know that you are likely to get warm, dry and sunny weather in Egypt at that time, while if you go to Alaska, you should prepare for cold winds and maybe snow. These expectations are based on the normals - the average weather over a long time period, in other words; the climate.
You never know the weather, though. You might to your surprise get snow in Cairo and a warm and nice spring day in Anchorage.
The weather from one day to the next, or even from one year to the next can of course deviate quite a lot from the normal.
The weather one or even 3-4 years in a row does not influence much on the average weather over a thirty years period.
Oscillations like El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) typically have substantial effect on the weather, but usually little effect on the climate.