Cloud formation and properties

Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals. This can happen in three ways:

1. The air is cooled below its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen:

* along warm and cold fronts

* where air flows up the side of a mountain and cools as it rises higher into the atmosphere

* by the convection caused by the warming of a surface by insolation

* when warm air blows over a colder surface such as a cool body of water.

2. Clouds can be formed when two air masses below saturation point mix. Examples are breath on a cold day, aircraft contrails and Arctic sea smoke.

3. The air stays the same temperature but absorbs more water vapor into it until it reaches saturation point.

The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of the relatively warm air holding the droplets is low enough that air currents below and within the cloud are capable of keeping it suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This gives these droplets plenty of time to re-evaporate as they fall into the warmer air beneath the cloud.

History of medicine

The earliest type of medicine in most cultures was the use of plants and animal parts. This was usually in concert with 'magic' of various kinds in which: animism ; spiritualism ; shamanism ; and divination , played a major role.

The practice of medicine developed gradually, and separately, in ancient Egypt, India, China, Greece, Persia and elsewhere. Medicine as it is practiced now developed largely in the late eighteenth century and early nineteenth century in England , Germany and France. The new, "scientific" medicine replaced early Western traditions of medicine, based on herbalism, the Greek "four humours" and other pre-modern theories. Possibly the major shift in medical thinking was the gradual rejection in the 1400's of what may be called the 'traditional authority' approach to science and medicine. This was the notion that because some prominent person in the past said something must be so, then that was the way it was, and anything one observed to the contrary was an anomaly. People like Vesalius led the way in improving upon or indeed rejecting the theories of great authorities from the past such as Galen, Hippocrates, and Avicenna/Ibn Sina, all of whose theories were in time almost totally discredited. Such new attitudes were also only made possible by the weakening of the Roman Catholic church's power in society, especially in the Republic of Venice.