Human beings judge how warm they are by how quickly heat leaves the body (we have specialised cells called thermoreceptors that do this job), and maintain temperature by controlling sweating. As sweat evaporates from the skin, it takes heat away from the body and therefore the rate of evaporation affects the rate of heat loss.

When the air is very humid, it is more difficult for evaporation to take place, and therefore humidity affects people’s perception of the temperature: when the air is very humid it feels warmer than it really is. Humidex is an index used by Canadian meteorologists to calculate this apparent temperature.

The dew point is the temperature at which water condenses out of the air and is linked to the relative humidity. If the relative humidity is 100% then the dew point is equal to the current temperature; when the current temperature is below the dew point then water will leave the air (the dew point cannot be higher than the current temperature).


As you can see, as the dew point increases the perceived temperature increases. When the air temperature is 20ºC and the dew point is also 20ºC  then the perceived temperature will be 27.6ºC.

The hottest and most humid parts of the world are the coastal areas around the Arabian Peninsula. On July 8th 2003 in Dharan, Saudi Arabia the air temperature was 42.2ºC with a dew point of 35ºC; this yields a humidex of an almost-unbearable 68.9ºC. If we combined the hottest recorded temperature of 56.7ºC in Death Valley, California with this Saudi Arabian dew point record that would yield a humidex of 83.4ºC, which would have killed people: luckily the Death Valley area is a desert valley and the dew point is usually quite low.

Lift Test Towers

How do you test a lift? Build a big tower, of course.


The world’s tallest lift test tower is Hitachi’s G1 Tower in Ibaraki, Japan. It is 218.5 metres tall (which would make it the fifth tallest building in the UK) and has room for seven different elevator shafts operating at speeds over 1000 feet per minute or 5.5 metres per second.

The world’s fifth largest lift test tower is the National Lift Tower in Northampton, at 127.5 metres tall.


Hydraulic Fuses

A hydraulic fuse acts to stop the excessive flow of a hydraulic fluid, in the same way that an electrical fuse acts to stop the excessive flow of electrical current. They are commonly found in “mission critical” hydraulic systems, such as those that operate the flight control surfaces (ailerons, flaps, rudder) on aeroplanes.

There are two types of hydraulic fuse. The first operates as a pressure relief valve, and vents fluid in case of a build-up of pressure. The second operates to prevent the loss of hydraulic fluid, for example if a fluid line is severed, and operates as a check valve – allowing fluid to flow only in one direction.



In the example of the second type of fuse shown above, excessive flow through the inlet will push the piston between the two metering plate housings and into the outlet, preventing fluid from passing through the fuse. The spring prevents the fuse from operating too early, pulling the piston to the left against the pressure to the right.

The fuse plugs used in hydroelectric dams can be viewed as a type of hydraulic fuse. They are usually constructed from across dam spillways, preventing water from exiting the reservoir along the spillway. In the event that the water level rises too high, the fuse plug is washed away by the increased water pressure.


The fuse plug at the Warragamba Dam in Australia.

The fuse plug is the large dark grey structure in the bottom-centre of the map. In the event that the level of water in Lake Burragorang on the left rises too high, the excess water will wash away the fuse plug and run down the light brown spillway towards the top-right.

Firearm Types

The term “firearm” refers to any sort of portable device that uses a barrel to fire a projectile, and they come in many forms.



A German Walther P99 handgun

As its name suggests, a handgun is designed to be held only in the hand, without the use of shoulder support. They come in two basic types: pistols, like the Walther P99 above, which have the chamber that holds the bullet integrated into the barrel, and revolvers which separate the barrel and chamber (usually possessing six or more separate chambers).

Handguns are usually semi-automatic, which means that each time the trigger is depressed one round is fired. Some fully automatic handguns (usually referred to as machine pistols) do exist, in which the gun will continue to fire for as long as the trigger is held down until all the ammunition is expended, but these are rare.


A rifle is a firearm with a long rifled barrel; the long barrel and the rifling give the weapon increased range and accuracy.

Assault Rifle


A Swiss SIG SG 550 assault rifle

An assault rifle is a selective fire rifle; this means that the weapon can fire in both semi-automatic and fully-automatic modes, and many assault rifles also offer a burst mode (usually a three-round burst) in which multiple rounds are fired each time the trigger is depressed. Assault rifles fire rounds that are between pistol and rifle rounds in terms of power, and use detachable magazines which hold around thirty rounds.

Assault rifles are the primary weapon issued to most armed forces, though most also issue a handgun as a backup side arm. Assault rifles were created when it was realised that most engagements occurred at ranges below the maximum range of battle rifles, and that a weapon in between the handgun and the battle rifle was required.

Battle Rifle


A Belgian FN FAL battle rifle

Battle Rifles fire full-powered rifle rounds, and thus have greater range than assault rifles, but because they are higher-powered they are more difficult to control in fully-automatic mode.




A carbine is a shorter and lighter version of an assault rifle (the H&K G36C carbine shown above is a version of the H&K G36 assault rifle). Because they are lighter and more manoeuvrable they are favoured where space is restricted (e.g. for use by helicopter pilots or armoured personnel vehicle drivers) or where high mobility is key, e.g. by special forces or hostage-rescue teams.

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The Composition of Earth’s Atmosphere With Elevation

In researching a post about the Kármán Line I discovered the NASA MSIE E-90 atmosphere model (thanks to Rhett Allain) which models the composition of Earth’s atmosphere up to an elevation of 1000?km. I found it very interesting.


Up to around 100?km the composition is fairly “normal”, in that it’s what we surface-dwellers would expect: mostly molecular nitrogen (N2 rather than N) and molecular oxygen (O2) with a small amount (0.93%) of argon and traces of some other gases (carbon dioxide, neon, etc.).

After 100?km the percentage of molecular nitrogen and molecular oxygen decrease sharply, and there is a similarly sharp increase in monatomic and triatomic oxygen, better known as ozone (i.e. this represents the “ozone layer”). There is also a small increase in the percentage of monatomic nitrogen and nitrogen compounds, and argon disappears entirely.

By 200?km ozone dominates, and this continues to about 650?km where helium takes over as the predominant component. Monatomic nitrogen and nitrogen compound concentration peaks at around 500?km, with an overall concentration of 1.6%.

By the time we reach an elevation of 1000?km helium makes up 93% of the atmosphere. This is due to the fact that helium is an unreactive and very light atom (with a mass about one-eighth of oxygen) and thus isn’t held tightly by Earth’s gravitational field. (Helium is so light that it can escape Earth’s gravity entirely.) The bulk of the remainder is hydrogen, also prevalent due to its low mass (about one-sixteenth of oxygen’s).


The concentration of “normal” gases in the atmosphere with elevation.


The concentration of less common gases in the atmosphere with elevation.

It’s important to note that the graphs above all show concentration as a percentage of the total number of particles of gas in the atmosphere, rather than by mass or volume. The atmosphere becomes incredibly thin at high elevations, so that particles of gas may travel many kilometres between collisions, and if absolute concentrations were used instead, the graph would look very different (and be completely unusable, which is why I haven’t included it here).