of these phenomena can be a serious threat to life and property, the tornado is the most danger- ous of all, and is capable of bringing total devastation to settlements and development lying in its path. Tornadoes are liable to form when the wind, temperature and humidity conditions through a deep layer of the atmosphere are such as to generate strong convection of air near the ground. Although such conditions are favourable for tornado genesis, it is by no means certain that a tornado will automatically form. The mechanism involved is not yet sufficiently under- stood. Nevertheless an analysis of the meteorological elements which determine the vertical structure of the atmosphere will give frequencies of occurrence of conditions which might result in the formation of a tornado in a given area. For the purposes of a realistic assessment of tornado hazard, these statistics should be used in conjunction with records of actual occasions when tornadoes were experienced in the area. 5.3 River Floods Excessive rainfall is the basic cause of a river flood but there are simultaneously other con- tributory factors. These may include structural failures such as the collapse of the walls of a reservoir or the embankment of a river proving insufficiently robust to contain the strong flow of water. When rainfall is of very high intensity, the resulting flood may be of sudden onset, usually described as a flash-flood. This phenomenon is particularly dangerous because it leaves very little time for any adequate warning or evacuation. If a river flood takes place near the coast, the haza.rd may be enhanced if, at the same time, strong onshore winds cause a storm surge (see 5.4 below). In order to describe river flood hazards, hydrologists undertake the preparation of two basic maps. One map delineates areas liable to fiooding on average once every 10 years; the 
**Analysis**
The text discusses natural disasters, specifically tornadoes and river floods, and their impact on settlements and development. The language used is technical and scientific, indicating that the text is focused on understanding and predicting these phenomena. The text also mentions the preparation of maps to identify areas prone to flooding, suggesting a connection to environmental and spatial analysis.

**Category**
Natural resources and environment