# What is the reasoning behind this question about the correct altitude to avoid clear icing?

This question has been killing me for a while and it's an original PPL question:

You are approaching to an airmass with a temperature of 6 C at 10000ft. Which altitude should you be in order to avoid clear icing?

A) 14000ft
B) 12000ft
C) 6000ft
D) 9000ft

The answer is C and I have no clue why.

• Are you sure it's not MINUS 6 degrees? Because +6 doesn't make a lot of sense. Mind you, -6 is also fine when there's no visible moisture around.
– Ben
Commented Nov 1, 2019 at 0:43
• @Ben, +6 does make sense—the air is moving faster around the aircraft, which causes temperature to drop due to adiabatic expansion. So icing risk starts a bit higher than freezing point. Commented Nov 5, 2019 at 6:53

The definition for icing conditions is visible moisture at a TAT of less than 10°C. From the Boeing 737 NG FCOMv1 (SP.16.1 Supplementary Procedures - Adverse Weather):

Icing conditions exist when OAT (on the ground) or TAT (in-flight) is 10°C or below and any of the following exist:

• visible moisture (clouds, fog with visibility less than one statute mile (1600m), rain, snow, sleet, ice crystals, and so on) is present, or
• ice, snow, slush or standing water is present on the ramps, taxiways, or runways.

To avoid icing, you should descend to an altitude where the temperature is above 10°C. The standard lapse rate of the atmosphere according to the International Standard Atmosphere (ISA) is 2°C per 1,000 ft:

As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 K/km (3.56 °F or 1.98 °C/1,000 ft) from sea level to 11 km (36,090 ft or 6.8 mi).

This would result in the following expected temperatures at the given altitudes:

• A) 14,000 ft: -2°C
• B) 12,000 ft: 2°C
• C) 6,000 ft: 14°C
• D) 9,000 ft: 8°C

This only leaves answer C) with a temperature above 10°C and therefore outside of icing conditions.

Note: The lapse rate of 2°C per 1,000 ft is only an average, the actual lapse rate could be somewhat different. Wikipedia says:

A typical value is around 5 °C/km, (9 °F/km, 2.7 °F/1,000 ft, 1.5 °C/1,000 ft).

Maybe the question assumes a lower lapse rate of 1°C per 1,000 ft, where an altitude of 6,000 ft is exactly where you would get 10°C.

• Thank you for the detailed explanation. Very well explained. Commented Nov 1, 2019 at 19:05
• An observation-- the question doesn't ask about avoiding "icing conditions" which apparently is a term that has been defined at least in relation to the aviation world as having something to do with 10 degrees C, regardless of the extreme improbability of actually encountering airframe ice at that temperature. Rather, the question simply asks about avoiding "clear icing". Commented Jan 6, 2022 at 21:31
• @quietflyer: avoiding icing conditions and avoiding clear icing are the same (clear ice is an icing condition). Your aircraft can accumulate ice at and near 10C (with visible moisture). Supercooled rain droplets can adhere to the airframe, engine cowling intake, etc. Then ice can accumulate because of the characteristics of the atmosphere surrounding the airframe.
– user22445
Commented Jan 7, 2022 at 1:12

It all seems a bit dubious to me. This source linked in this answer to a related ASE question suggests that ice formation is not likely below plus 2 degrees C. So, 9000' (corresponding to plus 8 degrees C or 46 degrees Fahrenheit in a standard atmosphere) is supposedly not an acceptable altitude to avoid clear icing?

Still, I have to wonder what could make you think that any of the other three candidate answers might possibly be better than C), 6000', corresponding to 14 degrees C (57 degrees Fahrenheit) in a standard atmosphere?

Perhaps the test question was not really designed to test your knowledge of meteorology, but rather your overall reasoning skills, as evidenced by your approach to multiple-choice questions?