'at very high altitude', 'at very high altitudes': both are okay in this sentence?

Question

I'd like to know and ask whether both the sentences below are correct.

• At very high altitude you will have difficulty breathing.
• At very high altitudes you will have difficulty breathing.

My thoughts are as follows: since the word altitude has both countable and uncountable uses as described below by OALD, either will be okay and the two sentences seem to have no difference at all.

OALD's second definition for altitude(noun):

​[countable, usually plural, uncountable] a place that is high above sea level, e.g. Snow leopards live at high altitudes e.g. The athletes trained at altitude in Mexico City

Answer 1

The use of the plural altitudes depends on whether you mean more than one altitude.

'An altitude' is a specific measurement, so any given altitude is singular.

For example:

The plane is flying at an altitude of 30,000 feet.

However, you could refer to a variety of different measurements, in which case you would use the plural.

For example:

The plane is capable of flying at high altitudes.
(ie it could fly at 32,000ft 34,000ft, 36,000ft etc)

In your example, either work - although I would go for the plural, because it makes it a broader statement - 'high altitudes' is a range where this would occur.