| Ångströms (Å) | Hectometers (hm) |
|---|---|
| 1 Ångström | 1 × 10-12 hm |
| 2 Ångströms | 2 × 10-12 hm |
| 3 Ångströms | 3 × 10-12 hm |
| 4 Ångströms | 4 × 10-12 hm |
| 5 Ångströms | 5 × 10-12 hm |
| 10 Ångströms | 1 × 10-11 hm |
| 20 Ångströms | 2 × 10-11 hm |
| 25 Ångströms | 2.5 × 10-11 hm |
| 50 Ångströms | 5 × 10-11 hm |
| 100 Ångströms | 1 × 10-10 hm |
| Reference | Ångströms (Å) | Hectometers (hm) |
|---|---|---|
| A sheet of A4 paper (long side) | 2.97 × 109 Å | 0.00297 hm |
| Average adult human height | 1.7 × 1010 Å | 0.017 hm |
| A football pitch (length) | 1.05 × 1012 Å | 1.05 hm |
| A marathon | 4.2195 × 1014 Å | 421.95 hm |
| Height of Mount Everest | 8.849 × 1013 Å | 88.49 hm |
The ångström is a unit of length equal to one ten-billionth of a metre, or 0.1 nanometres. It takes its name from Anders Jonas Ångström, the Swedish physicist who used it in his 1868 map of the solar spectrum. The symbol is Å, a letter borrowed from the Swedish alphabet.
The ångström survives because it matches the scale of atoms. A hydrogen atom has a radius of about 0.5 Å. A carbon-carbon single bond measures roughly 1.5 Å. Expressing these figures in nanometres produces awkward decimals, so crystallographers, spectroscopists and structural biologists continue to prefer the older unit. Protein structures deposited in public databases are still described by their resolution in ångströms, and a structure resolved to better than 2 Å is considered high quality.
Wavelengths of visible light also fall in a convenient range. Red light sits near 7000 Å and violet near 4000 Å. X-ray wavelengths cluster around 1 Å, which is precisely why X-ray diffraction reveals atomic spacing: the probe and the target are the same size.
The ångström is not part of the International System of Units. The BIPM lists it among units that are accepted for use with SI but discourages new applications, preferring the nanometre or picometre. That guidance has had limited effect in the fields where the unit is entrenched. Semiconductor manufacturing offers a clear illustration. Process nodes were named in nanometres for decades, but as features shrank the industry began quoting gate oxide thicknesses in ångströms, and Intel named a generation of its technology the Angstrom era.
Reading older scientific literature requires care. Before the ångström was tied to the metre it was defined against a specific spectral line of cadmium, and figures published in the early twentieth century may differ slightly from modern values. The International Astronomical Union adopted that spectroscopic definition in 1907, and it stood until the metre itself was redefined against krypton in 1960. The discrepancy is small, but it is real, and it matters when comparing historical spectral measurements against current ones.
Converting is straightforward. One ångström equals 10-10 metres, 0.1 nanometres, or 100 picometres. Ten ångströms make a nanometre.
The hectometre is one hundred metres, written hm. It uses the SI prefix hecto, from the Greek hekaton meaning hundred. Like the decametre and the decimetre it is a legitimate unit that ordinary usage has largely bypassed in favour of metres and kilometres.
Its clearest application is in land measurement, where it appears indirectly. A square hectometre is 10,000 square metres, which is exactly one hectare. The hectare is the standard unit of land area in agriculture, forestry and property registration across most of the world, so the hectometre is embedded in daily practice even though the length itself is seldom named. A square field measuring one hectometre on each side covers one hectare.
Rail and road infrastructure provides the other common use. Several European railway networks mark distances with hectometre posts, placing a marker every hundred metres along the track. These give maintenance crews and signallers a precise location reference, and incident reports commonly cite a kilometre and hectometre figure. Dutch and Belgian motorways use hectometre markers in the same way, and drivers are directed to quote them when reporting a breakdown.
Athletics offers a familiar length without using the name. The straight of a standard outdoor track is one hectometre, and the 100 metres is the shortest standard sprint distance. Nobody calls it the one-hectometre sprint.
Meteorology uses the unit for cloud base height in some aviation reporting formats, where reporting in hundreds of metres or hundreds of feet keeps the figures compact.
Nautical charts and aviation both work in comparable steps without adopting the name. Runway visual range is reported in hundreds of metres in many countries, and visibility in aviation weather reports is given in metres up to five thousand, effectively in hectometre increments. Where the unit is used explicitly, as on Dutch motorways, its advantage is precision: a hectometre marker locates an incident to within fifty metres.
Radio engineering names a whole band after it. The international classification calls waves between 100 and 1000 metres hectometric, corresponding to frequencies from 300 kilohertz to 3 megahertz, and that band is the medium wave used for broadcasting since the 1920s. A transmitter working at 1000 kilohertz radiates a wave three hectometres long, which is why medium-wave aerials are tall masts rather than the short whips that serve higher frequencies. The naming continues in both directions: the band below is kilometric and the one above decametric, so the ladder of metric prefixes maps directly onto the divisions of the radio spectrum.
One hectometre equals 100 metres, 10 decametres, or 0.1 kilometres. It is approximately 328.084 feet, or 109.361 yards.