How much does land expand on a hot day?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 September 2012

David Kariuki has noted that the ambient temperatures fluctuate continuously and he is wondering how this affects land surfaces: “My question is based on the fact that all matter expands when heated. How much expansion would happen to the earth on a hot day? Would I get more surface area if I measured my land when hot?”

The answer is yes. You would get a larger area if you measured the dimension on a hot day. But the bigger question is: how much bigger?.

Even though the thermal expansion of materials is of great concern in engineering, the amounts are miniscule. Typically, solids expand by only of a few ten thousandths of a per cent per degree rise in temperature.

Thus if you have a piece of land measuring, say 100m by 100m (one hectare, or about 2.5 acres),  its size will only increase by only a few tenths of a millimetre for every one degree celsius! Considering that average daytime temperatures fluctuate by at most 20 degrees, then it turns out that this parcel of land may expand by a few millimetres only.

Now if you are in the business of selling land, do you think it is worthwhile to wait for the hottest day in order to gain a few millimetres per hectare? 

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Philip Kaqichu’s question is somewhat bewildering. He writes, “Two weeks ago on a flight to Tanzania we departed 20minutes late. [However,] Mid-air, the pilot apologised for the late departure and he assured us he would try to compensate for the lost time. It did not come to pass, since we took the stated flight time of 50minutes. [My] question is: is it possible to recover lost time mid-air by say, increasing altitude?”

I suppose Philip is thinking about flying lower in order to reduce the flight distance. Well, this might work, but again the gain will be too small to be even measurable. The better way to reduce flight time is the obvious one: by increasing the cruising speed of the aeroplane!

However, flying speed alone does not guarantee a shorter flight time. A joke is told in civil aviation circles that the first commercial passenger flight from London to Paris took about one hour flying at about 300km/h. Today, jetliners cruise at three times that speed (900km/h) but they take one and a half hours to do the same journey!

How can that be possible? How can a faster plane take longer to cover the same distance? The answer is that there is an air traffic jam in the skies above both cities.

In a related matter, did you know that it take longer to drive from Nairobi city centre to Jomo Kenyatta Airport than to fly to Mombasa? And the same happens when you arrive at Moi International Airport?

It just goes to show that travel times are not dictated by the speed of movement alone.