How many square metres of a field of wheat are required to make a loaf of bread?

Many variables, including yield per hectare, variety of wheat grown, flour yield, type of bread, recipe quantities etc, have to be taken into account when trying to come up with an answer to this one and there will, necessarily, be a spread of values around the mean.

Using typical UK values for the main variables I have estimated the following:

An average yield for wheat in the UK is about eight tonnes, or 8,000kg per hectare. Taking a flour yield of 80 per cent (conservative) and a bread to flour ratio of 2.75:1 (remember that bread is much more moist than flour and is therefore much heavier) the resultant bread yield per hectare is 17,600kg. In “old money” the standard loaf was 2lb which equals 0.9kg. This means we get some 19,500 loaves per hectare or, two loaves per square metre.

In the great wheat-growing plains of the US, Canada and Australia, yields are much lower and typically 2-4 tonnes per hectare as production is much less intensive; consequently you would need one to two square metres to provide the wheat for your loaf. By comparison, the better wheat growers in the UK frequently exceed ten tonnes per hectare.

We used to import a lot of hard wheat from the US and Canada for our bread-making but since the general adoption of the Chorleywood bread-making process, much more home-grown wheat can now be used.
Barry Linton, Reading

We were taught at school that hot air rises. So why does it get cooler as you go up a mountain?

We were taught at school that air pressure and temperature increase and decrease together (Charles’ Law). Atmospheric pressure declines with altitude because, as height increases, there is less air above it to compress it. Thus, in a stable atmosphere, taking these two effects together, the air gets cooler as you ascend a mountain. Another result of this reducing atmospheric pressure is that a kettle will boil at a lower temperature because the emerging water vapour then finds it easier to escape.
Roy McKenzie, Derbyshire

Much faith is now placed on carbon dating. On what is this based and how do scientists know that the age of the item is correct?

The procedure of what has become carbon-14 was developed in America in the late 1940s. Every living being absorbs the isotope C-14 contained in carbon dioxide during its life, it stops at the moment of death. Fibrous tissue was taken from Otzi’s left hip and sent for independent investigation. Calculations were made by the Research Laboratory for Archaeology in Oxford and also the Swiss Federal Institute of Technology in Zurich. Botanical material was subject to examination by Svedberg Laboratoriet, University of Uppsala and Centre des Faibles Radioactives in Paris. The data from the samples led to very similar results: the Iceman lived between 3350 and 3100BC. The Iceman therefore lived 5,000 years ago. When the first Egyptian pyramid was built Otzi had already lain buried under the glacier for 600 years. It has been calculated that 500 years after the Iceman’s death the building of Stonehenge commenced.
Harry Brimson, Shorne, Kent

Why is Mark Phillips often given the title “Captain”? My understanding is that only Major and above carry their rank into civilian life.
R. P. Jones, Old Windsor

The Bible refers many times to 40-day periods, and to the Israelites’ 40 years in the wilderness. Could this be linked to early Babylonian knowledge of the planet Ishtar (Venus) moving in regular 40-year cycles, and of the Plaeidies star cluster disappearing for 40 days a year?
Adrian Sandes, Bath

What put the great in Great Britain?
Mark Wakeling, Barnet, Herts