Monthly Archives: November 2011

The speed of jet lag

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Jet lag (ICD-10: G47.2) occurs when the body’s internal clock (its circadian rhythm) gets out of sync with the time of day.

Example: London to Los Angeles

Leaving London at 1200 you will arrive in Los Angeles ten hours later and your body will feel like the time is 2200. The actual time will be 1400 and so your body expects it to be late night, but it’s actually the middle of the day: an offset of eight hours. Travelling back, leaving Los Angeles at 1200 you will arrive in London ten hours later and again feel like the time is 2200, but it will actually be 0600 the next day; your body expects late evening but gets early morning: an offset of sixteen hours. The difference in these offsets is what gives rise to the fact that travelling west to east causes worse jet lag than travelling in the opposite direction.

Jet lag only occurs when travel causes a difference between the internal and real clocks. If you take anything more than one hour to travel a time difference of one hour then jet lag does not occur. Also, flying north to south doesn’t cross any time zones and therefore jet lag does not occur; flying from Cape Town to Stockholm, for example, is safe for your body clock.

The Earth rotates once per day and therefore contains twenty-four time zones, spaced evenly apart. Turning through 360° in twenty-four hours is equivalent to 15° per hour. At the equator, fifteen degrees of longitude is equivalent to 1670 kilometres so an aeroplane flying along the equator would have to travel at a speed of at least 1670 kilometres per hour (over 1000 mph) for jet lag to occur. At a latitude of 45° (north or south) this 15° is only 1180 kilometres, reducing the speed of jet lag to 734 mph.

Both of the situations above assume that plane fly directly along lines of latitude, but this never happens. In reality planes fly “great circle” paths (see the previous post about geodesics) and travelling along great circle paths, especially those that fly close to or over the poles where time zones are “thinner”, lowers the speed of jet lag to below the 500-600 mph speed of an aeroplane.

The narrowing of time zones at northern latitudes is obvious in this map of Western Europe.

Types of Desert

A desert is defined as an area that receives a very small amount of precipitation: these areas come in three main forms.

The most recognisable type of desert is the subtropical desert, typified by the Saharan and Arabian deserts. They are the hottest deserts and any rain that does fall often evaporates before it hits the ground.

The Sahara Desert in North Africa.

The Earth has two polar deserts, the Arctic and the Antarctic. At 13.9 million square kilometres the Antarctic Desert is by far Earth’s largest, more than one and a half times the size of the next largest desert, the Sahara.

The Antarctic Desert at the South Pole

Cold winter deserts, like the Gobi Desert in China and Mongolia, are often created by the rain shadow effect in which a tall mountain range causes warm moist air to rise and cool. As the air cools water vapour condenses out and falls as rain or snow, leaving the air dry and creating a desert on the lee (upwind) side of the mountain. For example, the Gobi Desert is created by the Himalaya Mountains; the Patagonian Desert in South America by the Andes; and the Great Basin Desert in the western United States by the Sierra Nevada.

The Gobi Desert, north-east of the Himalaya Mountains.

UK electricity import and export

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The UK doesn’t have enough electricity.

The amount of electricity that the UK produces (from various sources) is not enough to meet demand, and the UK relies heavily* on imports from France and the Netherlands in order to meet its needs. This energy gap is due to the closure of coal-fired power stations that cannot meet emission standards and the shutdown of aging nuclear power stations.

The import and export of electricty uses submarine high voltage direct current (HVDC) cables. HVDC cables waste less electricity than AC cables (about 3% per 1000km) and are simpler to construct and operate. A 73 km cable connects Bonningues-lès-Calais in France to Sellindge in Kent; a 55 km cable connects Auchencrosh in Scotland and Ballycronan More in Ireland; and a 260 km cable connects Maasvlakte in the Netherlands to Grain in Kent.

The graph below shows electricity import and export (in GWh) for the eight months since February†, when the Britned Interconnector began operating. When the value is positive (when the line is above the origin) the UK is importing electricity and when it is negative electricity is being exported.

One of the most interesting features of the graph is the change in the import/export to Ireland via the Moyle Interconnector. On 26th June at 0417 one of the 250 megawatt cables failed, halving the cable’s capacity, and at 1409 on 24th August the cable failed entirely and has not transferred any electricity since.

The cause of the failure is unknown and the MV North Sea Giant, the world’s longest offshore construction vessel, is currently moored in the Irish Sea investigating the fault. The cable has been dug out of the seabed 200m below the surface, cut in half, and both ends raised to the surface so the cause of the fault can be investigated. The fault is expected to take up to six months to fix and were a similar fate to befall the Cross-Channel or Britned interconnector the UK would have a very serious energy problem.

If it is to close the growing energy gap the UK must accelerate the pace of construction of energy infrastructure, in particular the construction of safe, zero-carbon nuclear power stations.

* The UK is the world’s sixth largest importer of electricity despite being eleventh in terms of electricity production and twenty-second in terms of population.

† The graph shows a seven day moving average of import/export values. For half-hourly data you can download the full dataset as an Excel spreadsheet.

Why does metal feel cold?

One adjective commonly used to describe metals, along with the adjectives like “shiny” and “silvery”, is “cold”.

But this doesn’t makes any sense when you take the Zeroth Law of Thermodynamics into account. Over a long enough period, everything in the same location will tend* to the same temperature, so any metal must be at the same temperature as its surroundings.

So why does metal feel cold?

Metals feel cold because they are very good conductors. Both the metal blade and wooden handle of a shovel left out in the Sun will be at the same temperature but the blade will feel colder because the metal is a good conductor: it “sucks” the heat out of your fingers and this heat leaving your fingers is what makes them feel cold.


Fifteen minute timelapse of melting ice cubes.

In the video above identical ice cubes placed on a wooden board and a metal heatsink removed from a broken laptop computer melt at vastly different rates. The wood is a poor conductor and so the ice cube takes a long time to melt; the opposite is true for the metal heatsink.

* I’m using “tend” in the physics sense of “to approach” rather than the general public sense of “to occur frequently” or “to look after”.