From High to Low, Look out Below!
by Russ Still
Published
Updated
One of the most confusing issues for pilots involves pressure levels and the airplane’s height above sea level - its true altitude. When plowing through the books, every student encountered the sayings “hot to cold, look out below” or “high to low, look out below”. They understood the sayings, but didn’t quite get how, or why, it works.
Let’s take it apart and see what’s inside.
Pressure Levels and Altitude
Recall this fact about the atmosphere. It should be common knowledge for every pilot. Air pressure decreases by one inch of Hg (mercury) with each 1000 feet of altitude gained. If the altimeter setting on the ground is 30.92, at one thousand feet, the air pressure would be reduced by one inch - 29.92.
The context is that you are flying along with a constant indicated altitude and a constant barometric pressure setting in the Kollsman window. Everything you see on the altimeter remains unchanged.
As a reminder, the atmosphere tends to be cooler and less dense the higher we go. And conversely, it tends to be warmer and more dense at lower altitudes.
So we start out with some barometric pressure setting at the surface and as the air pressure around the airplane changes during our flight, the altimeter responds by either moving up or down.
Assume that we have a correct pressure setting in the altimeter and we’re flying along with 5,000 feet indicated. Over distance, if the pressure levels stay where they are, we cruise along and our height above the surface, or above mean sea level to be specific, remains constant.
If we intentionally descend from 5,000 feet to 4,000 feet, we are physically moving into air that is more dense, and our altimeter reacts by showing the change in indicated altitude.
Change in Altimeter Settings
When atmosphere changes and the pressure levels expand and rise upward, or contract and move downward, we must adjust the barometric pressure in the altimeter.
That’s why controllers provide us with local altimeter settings during cross-country flights.
If the pressure levels change over distance and we fail to reset the altimeter with the new pressure settings, we will end up either higher or lower, in terms of height above sea level, than we realize. And that’s where the “high to low, look out below” comes in.
Assuming travel into a lower pressure area, the air column is compressing and the lower pressure air above us is trending lower. Because we failed to adjust the altimeter setting, we continue flying along, keeping 5,000 feet on the altimeter face. We’ll be descending and riding the pressure level down toward the surface without even knowing it.
So, when flying from high to low, if you haven’t been resetting your altimeter, definitely look out below. You’re probably closer to the earth than you think you are.
An Example Flight
You are currently in flight at 1,000 feet MSL, traveling from Point A to Point B, near Miami, Florida.
At your current location, the altimeter setting (showing ground barometric pressure, normalized) is 30.92. Since the altimeter is set properly, the altimeter indicates 1,000 feet. So far, so good.
Remember that air pressure drops one inch per thousand feet, therefore the pressure at your altitude is 29.92.
Now, imagine that the altimeter setting at Point B, at the surface, is 29.92, but your altimeter is still set to 30.92. If you fail to reset your altimeter while approaching Point B, your altimeter will continue to indicate 1,000 feet, but you will fly lower and lower until you meet the surface. (Hopefully there was a runway right there.)
In this example, you followed the 29.92 pressure level from 1,000 feet MSL all the way to the ground (which happens to be at sea level). When traveling from higher pressure to lower pressure without resetting your altimeter, look out below!