So-and-so-many square stadia?
Well, not far from where I live there is a street named Claribel.
I always figured that was a measure of clarity, perhaps commonly used for describing the clarity of fine wines. “This fine 1948 vintage Beaujolais has 12e15 claribels.”
Okay, a (slightly) more serious remark.
The knot, a measure of linear velocity, is one nautical mile per hour. A nautical mile, being based on some fractional measure of the earth’s surface, seems sort-of related to the metric system, in which the meter was (originally) defined based on some fraction of the earth’s surface.
It turns out that nautical miles and knots are MUCH easier to work with, at least approximately, than the usual English measures of miles and miles-per-hour. Why?
A nautical mile is currently defined as 1853.2480 meters, or 6080.21 feet (6080.2 in the United States). This is very close to exactly 6000 feet. So, for quick ball-park approximations, one can use 6000 feet. This makes various kinds of feet-to-miles or miles-to-feet very easy to do mentally. And since speed is commonly measured in miles per hour or feet per minute or second, and and hour is 60 minutes and a minute is 60 seconds and one miles is 6000 feet, the conversion factors are easy to work with.
Here is a example of a real-life problem that one might need to figure out quickly in one’s head: A certain sailplane has a glide ratio of 36:1, meaning it can glide 36 feet forward for every foot of altitude it sinks. To give ourselves a BIG margin of error for safety, we assume half that: It can glide 18 feet for every foot of sink. If a glider is at 5000 feet above the ground, and it must arrive at the nearest airport at 1000 feet to fly a proper landing patter, and that airport is 10 (nautical) miles away, can it get back to the airport? Work this out in your head!
Yes.
I’m questioning the OP to begin with. Is the bel a metric unit?
change all of those units to metres or kilometres, the calculation is no harder.
I think you didn’t read the problem very carefully. Note that most of the measures are given in feet, but the distance to the airport is given in nautical miles. You can’t just change all the measures to meters or kilometers.
Furthermore, the problem as given is a real problem. The specific distances are all realistic, as are the choices of unit of measure to use. If you change, say, 5000 feet of altitude to 5000 meters, you are now at about 15000 feet, so the problem is not whether you can reach any nearby airport, but how much supplemental oxygen you should be breathing.
And anyway, the point was that nautical miles are easier to work with than statute miles.
The problem boils down to: Can a glider that goes 18 feet horizontally for one foot of vertical sink get to a point ten miles away if it has 4,000 feet to sink? Or, to put it geometrically: Will a right triangle whose shorter leg is 4,000 feet have a longer leg that’s shorter or longer than ten miles if that leg is 18 times as long as the shorter one? Or, to boil it down to the absolute algebraic core: Is ten miles more or less than 4,000 feet multiplied by 18? That’s all there is to it. I can see that it’s easier to calculate this mentally with nautical miles that are approximated to be 6,000 feet long than with statute miles that has 5,280 feet. But that difference is not enormous. And, as Novelty Bobble has pointed out, the problem becomes absolutely trivial if altitude is given in metres and distance in kilometres.
Decibels are measured on a logarithmic scale, so the wide range of meaningful values (in nature) can be indicated with a relatively compact range of numerical outputs, precluding the need for a series of prefixes that indicate magnitude. 20 decibels has ten times the power of 10 decibels, and 30 decibels has 100 times the power, and so on. This is just my guess as to why prefixes are not often used with decibels.
Other dimensions such as the length and mass are indicated linearly and prefixes are handy for sifting through the possible lengths and masses we might encounter.
Coming back to the OP, I think it deserves to be mentioned that the litre as the metric unit of volume is defined as one cubic decimetre. So the decimetre is used a lot, but not under that name.
To me, that is an oddity in the metric system. Since one litre of water weighs one kilogram, it has the consequence that 1/1,000 litre weighs a gram. And 1/1,000 of a litre is properly called a millilitre. So the millilitre weighs one gram, or 1,000 milligrams, and a milligram is 1/1,000 millilitre, which you may call a microlitre. I would find it more logical if the base unit of volume were the cubic metre, not the cubic decimetre. Then the prefixes for volumes of water would coincide with the prefixes for the corresponding mass.
How about, hecatomb? You never hear about a measly dekatomb or an insane kilotomb.
A kilogram is basically the base unit, though. The oddity is that it already comes with a prefix (they changed the name from grave).
True that it’s not the mass of a cubic m of water, but then it was always based on a cubic dm, so there was never really the matching you are looking for. From the beginning, they had to have practically realizable standards. A cubic metre of water is a metric tonne.
Sure, I understand that. For everyday life, the litre is more useful than the cubic metre simply because you’re seldom going to buy a cubic metre of any liquid (except running water - that is often billed in cubic metres in metric countries). My concern was simply that I never quite got around the fact that a milligram of water is not a milli-whatever unit. I just intuitively feel that the system would be a bit smoother if that were true.
A milligram of water is about 1 cubic millimeter, so what do you mean here? Anyway you can’t massage units in such a way as to take away the power of 3 in [unit of volume] = [unit of length][sup]3[/sup], so not clear what extra smoothing is possible.
“Cubic metre” may not be common because “kilolitre” exists. My water bill is in kL, not m[sup]3[/sup]
That’s the joy of metric.
OK, I think you mean: 1 stere of water weighs 1 ton, so 1 microstere of water weighs 1 microton. 1 litre of water weighs 1 grave, except now it’s called kilogram, and you can’t have a millikilogram nor are they going to change the name back.
You’ll be OK if you call them 10[sup]-3[/sup] l and 10[sup]-3[/sup] kg; no problem there…
m3 is very common when specifying volume of gases. Gases like natural gas, propane, CO2 …
The prefix used in the metric system is N (normal) while the US units use S (Standard). Standard is not the same as Normal 
You do in France. House/building plots are always quoted in ares, Until of course they get big, so large surfaces such as fields or farms are in hectares.
Oh, and cubic metre is used for larger water volumes (e.g pools) and on water bills.
What I mean is that the unit of volume (litres and derivatives of it) takes another prefix than the unit of mass; the two scales are off each other by a factor of 1,000. The system would be smoother if they were the same (for water, on which the system was based). A millilitre of water weighs one gram; milligram exists as a unit of mass, but in water it doesn’t correspond to a millilitre but to 1/1,000 of it. The system would be “smooth” if the base unit of volume were the cubic centimetre, not the cubic decimetre. Then the mass of one base unit of water would be one gram, the milligram would correspond to a milli-base unit, etc. It would also work upwards: One kilogram of water would correspond to a kilo-base unit of volume. The two scales would be aligned with respect to the prefixes they take.
Personally, I’m a fan of the meter-ton-second system. That way, none of the base units has a prefix, and you still get to keep the density of water at a convenient 1 unit.
And I stand corrected on both μmHg and on ares.