One of the earliest forecasting tools was the use of atmospheric pressure. Soon, after the invention of the barometer, it was found that there were natural fluctuations in air pressure even if the barometer was kept at the same elevation. During times of stormy weather the barometric pressure would tend to be lower. During fair weather, the barometric pressure was higher. If the pressure began to lower, that was a sign of approaching inclement weather. If the pressure began to rise, that was a sign of tranquil weather.