In the local altimeter setting observation, what triggers the 35-minute observation interval?

• A. A change of 0.02 inch Hg or more since last ALSTG value
• B. A change of 0.03 inch Hg or more since last ALSTG value
• C. A change of 0.01 inch Hg or more since last ALSTG value
• D. A scheduled hourly update

Answer: (C)

The local altimeter setting observation uses a small pressure-change trigger to keep altimeter readings accurate. If the atmospheric pressure changes by as little as one-hundredth of an inch of mercury since the last ALSTG value, that signal indicates a detectable shift that could affect altitude calculations. Because of that, a new ALSTG observation must be performed within about 35 minutes after that change, ensuring pilots have an up-to-date setting.

Smaller changes than 0.01 inHg don’t trigger a prompt update, and larger thresholds like 0.02 or 0.03 inHg aren’t the trigger for this specific 35-minute cadence. A scheduled hourly update isn’t the trigger either—the 0.01 inHg change is.