AIRMET Zulu: Icing and the Freezing Level
AIRMET Zulu is the ice advisory, and it is the only one of the three that also publishes a plain measurement: the height of the freezing level.
That pairing is the point. The ice warning tells you there is a problem. The freezing level tells you where the problem starts and which way to go to leave it.
What triggers an AIRMET Zulu
The icing trigger is moderate airframe icing over a wide area.
Moderate means the rate of build is high enough that even short exposure is a problem. Ice protection has to be used. A change of level or a diversion is called for.
Severe icing is not in this product. That is a SIGMET, because the build rate beats what the equipment can handle, and that threatens every aircraft rather than some of them.
The freezing level part is issued regardless. Even on a day with no icing forecast at all, the advisory carries the freezing level across the region.
Why the freezing level is half the product
Ice needs liquid water that is colder than zero. That is the whole recipe.
Below the freezing level the water in cloud is warmer than zero, so it does not freeze onto your airframe. Above it, the water can be supercooled, and supercooled water freezes the moment it touches something.
Go high enough and the risk drops again. Around minus twenty most cloud water has already turned to ice crystals, and dry crystals bounce off a cold airframe rather than sticking.
So the icing band has a floor and a roof. The freezing level gives you the floor. The advisory text and the cloud tops give you a sense of the roof.
Reading the altitude band
The most useful line in a Zulu is the height range, because it is the line that gives you an escape.
A body reading moderate icing between the freezing level and 16,000 feet is telling you two things. Ice is likely in that slab, and above 16,000 it is not.
If the freezing level is 6000 feet, the band runs 6000 to 16,000. Below 6000 you are in above-zero air. Above 16,000 you are out the top.
That is a real plan. Cruise at 4000 and stay warm, or climb through and get on top, depending on terrain, aircraft and where the cloud tops are.
Without the band you are only told that ice exists somewhere, which is much less use.
Multiple freezing levels
Sometimes the advisory gives more than one freezing level for the same place, and that is a warning in its own right.
The air normally gets colder with height. When a warm layer slides in above colder air, the temperature crosses zero twice. That is a multiple freezing level.
The danger there is freezing rain. Rain falls from the warm layer into the subzero air underneath, staying liquid but colder than zero, and freezes on whatever it hits.
Freezing rain produces clear ice fast, and clear ice is the heavy, sticky kind that runs back behind the protected surfaces. It is one of the few conditions where the right answer is to leave immediately rather than to manage it.
Two freezing levels in a Zulu should always make you look at the surface reports for freezing rain and freezing drizzle.
Using it with other products
A Zulu covers a wide area over six hours, so it is coarse by design. Two other products sharpen it.
PIREPs give you what pilots actually met, with an altitude band and an ice type. A run of negative icing reports through the advisory band is real evidence that the layer is currently clean.
The freezing level charts give a map rather than a text description, which makes the slope of the freezing level much easier to see. That slope matters on a long route, because a level that is safe at one end may be inside the band at the other.
What it does not tell you
A Zulu says ice is likely somewhere in a slab of air. It does not say the slab is full of it.
Ice needs cloud. Clear air inside the band is dry and holds nothing to freeze onto you. So a flight at a level well inside the icing band can be entirely clean if it stays out of cloud.
The advisory also says nothing about ice type. Rime and clear ice behave very differently, and only a PIREP or the cloud type will hint at which you would meet.
Read it as a map of where ice is possible, then use the cloud forecast to work out where it is likely.
Verdict on AIRMET Zulu
Read AIRMET Zulu as two products stapled together. The icing half warns you. The freezing level half gives you the way out.
Work out the band, check it against your planned levels, and know before you depart whether your escape is up or down. Then look for PIREPs in the same block of air, because one pilot who flew it beats six hours of area forecast.
Frequently asked questions
What does AIRMET Zulu cover?
Two things. Moderate airframe icing over a wide area, and the height of the freezing level across that area. The freezing level part is issued whether or not any icing is expected.
Why does AIRMET Zulu include the freezing level?
Because the freezing level is where icing becomes possible. Knowing where zero degrees sits lets you work out which levels could hold supercooled water, and which levels are a safe place to go if you pick up ice.
Is severe icing covered by an AIRMET Zulu?
No. Severe icing is a SIGMET. Zulu stops at moderate, which is icing that needs the ice protection running and calls for a change of level, but that the equipment can still handle.
Can there be icing above the freezing level only?
Icing needs supercooled liquid water, which exists below zero. So airframe ice forms above the freezing level in height terms, in air colder than zero. Below the freezing level the water is above zero and does not freeze on contact.
What is a multiple freezing level?
Air where the temperature crosses zero more than once with height, usually a warm layer sandwiched between colder air. It creates freezing rain, because raindrops fall from the warm layer into subzero air below.