Weather – Nonstandard Temperature | Training Blog

What Effect Does Nonstandard Temperature Have on Altimeter Readings?

​An altimeter uses static atmospheric pressure to infer altitude above a pressure reference; it does not directly measure height above terrain.

Because the altimeter is calibrated to the standard atmosphere, temperatures that are significantly warmer or colder than standard can create a difference between:

Indicated altitude — what the altimeter shows
and
True altitude — the aircraft’s actual height above mean sea level

The most important relationship to remember is:
Cold air = True altitude lower than indicated
Warm air = True altitude higher than indicated

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🌡️ Why This Matters (Terrain + Obstacle Clearance Reality)

The magnitude of error depends on the temperature departure from standard and the aircraft’s height above the reporting source.

In very cold conditions, especially when operating:

• Near terrain
• Near obstacles
• At low altitudes
• On instrument approaches

…the difference between indicated altitude and true altitude can become operationally significant.

Your altimeter may say you are at the correct altitude.
The terrain doesn't care what your altimeter says.

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❄️ Colder Than Standard Temperature

When the atmosphere is colder than standard, pressure levels become compressed closer together.

As a result:
Your true altitude is lower than your indicated altitude.

In other words:
The altimeter says you're higher than you actually are.

This is the dangerous side of temperature error because your actual clearance above terrain and obstacles is less than the indicated altitude suggests.

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Example
Suppose you're flying 1,000 feet above the airport with an outside air temperature of -10°C.

Depending on the atmospheric conditions, your aircraft could be approximately 100 feet lower than the altitude indicated on the altimeter.

Your altimeter might indicate that you're 1,000 feet above the airport.
Your actual height could be closer to 900 feet above the airport.

​That difference matters when terrain or obstacles are nearby.

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☀️ Warmer Than Standard Temperature

When the atmosphere is warmer than standard, pressure levels expand farther apart.

As a result:
Your true altitude is higher than your indicated altitude.

The altimeter says you're lower than you actually are.

While this generally provides additional terrain clearance, it can still matter when precise altitude control is required.

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🧠 Think of Pressure Levels

A useful way to visualize temperature error is to imagine layers of atmospheric pressure.

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Cold Air
Pressure levels are compressed closer together.

When you fly to an indicated altitude, you haven't actually climbed as high as you would have in standard conditions.

True altitude is lower than indicated.

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Warm Air
Pressure levels are expanded farther apart.

When you fly to the same indicated altitude, you've actually climbed higher than you would have in standard conditions.

True altitude is higher than indicated.

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🗣️ Remember: “From Hot to Cold, Look Out Below”

For nonstandard temperature, remember:
“From hot to cold, look out below.”

When flying in air that is colder than standard, your true altitude is lower than your indicated altitude.

In other words:
The altimeter may indicate that you are higher than you actually are.

The colder the temperature—and the greater your height above the altimeter-setting source—the greater the potential difference between indicated and true altitude.

Note: Don’t confuse this with the similar mnemonic used for changes in atmospheric pressure: “Going from high to low, look out below.”

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🛩 Operational Scenarios

Scenario 1
You're flying at an indicated altitude of 3,000 feet on an extremely cold winter day (colder-than-standard conditions).

Is your true altitude exactly 3,000 feet?

No.
Your true altitude is lower than indicated.

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Scenario 2
You're flying over mountainous terrain in very cold temperatures (colder-than-standard conditions).
Your altimeter indicates adequate terrain clearance.

Should temperature error matter?

Absolutely.
Your actual altitude may be lower than indicated, reducing terrain and obstacle clearance.

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Scenario 3
The temperature is significantly warmer than standard.

How does true altitude compare to indicated altitude?

True altitude is higher than indicated.

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⚠️ Common Training Mistakes

• Thinking a correct altimeter setting eliminates temperature error
• Confusing temperature error with pressure-setting error
• Forgetting that cold temperatures reduce true altitude
• Assuming a small altitude error never matters
• Failing to consider temperature when operating near terrain or obstacles

The colder the temperature and the greater the height above the reporting source, the larger the potential error can become.

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🧩 The Big Takeaway

Nonstandard temperature affects the relationship between indicated altitude and true altitude.

Colder than standard: True altitude is lower than indicated.
Warmer than standard: True altitude is higher than indicated.

The most important safety concern is cold weather.

When the temperature drops, your aircraft may be closer to terrain and obstacles than the altimeter suggests.

Remember: "From High to Low, Look Out Below."

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🗓 Next Week

Airport Operations – Traffic Patterns

At what traffic pattern altitude should piston aircraft and large or turbine-powered aircraft fly?

Next week, we'll compare the recommended traffic pattern altitudes for typical piston aircraft and for large or turbine-powered aircraft:
Piston aircraft: 1,000 feet AGL (or as published)
Large or turbine-powered aircraft: 1,500 feet AGL (or 500 feet above published)

We'll also look at why different aircraft may operate at different pattern altitudes and why pilots should verify the published traffic pattern altitude for the specific airport rather than assuming every airport uses the same altitude.

​Because knowing where to look for traffic is much easier when everyone knows where they're supposed to be.