Clear ice forms fastest on aircraft in flight between 0 C and -15 C due to large supercooled droplets freezing on contact

In which temperature range is the most rapid accumulation of clear ice on an aircraft likely to occur in flight?

• A. Between -15 C and -30 C
• B. Between 0 C and -15 C
• C. Above 0 C
• D. Between -30 C and -45 C

Answer: (B)

The most rapid accumulation of clear ice on an aircraft during flight typically occurs within the temperature range of 0 degrees Celsius to -15 degrees Celsius. This is primarily because the conditions in this temperature range enable large supercooled water droplets to exist, which are capable of freezing upon contact with the aircraft's surfaces. At temperatures just above freezing (0 degrees Celsius), water droplets can remain in a supercooled state, meaning they have not yet frozen despite being below freezing temperatures. When these droplets collide with the aircraft, they can freeze rapidly, forming clear ice, which is dense and heavy. As temperatures drop below 0 degrees Celsius, the size of the supercooled droplets tends to decrease, leading to more rime ice formation rather than clear ice. Additionally, in the colder ranges, particularly from -15 degrees Celsius and below, conditions become less conducive for large droplet accumulation, leading to slower rates of icing. Therefore, the conditions between 0 degrees Celsius and -15 degrees Celsius are optimal for rapid clear ice formation due to the presence of larger supercooled droplets and the dynamics of freezing upon contact with the aircraft's surfaces.

Outline

• Title: The 0°C to -15°C Window: Why Clear Ice Forms Fast in Flight

• Opening: Ice isn’t just a weather word—it's a real risk pilots feel in their hands and instruments.

• What ice is and why it matters: clear ice versus other types, and how it shifts flight dynamics.

• The sweet spot: 0°C to -15°C — what makes it the prime range for rapid clear ice buildup.

• Outside that window: what happens when it’s warmer or colder, and how droplets behave.

• Real-world angles: how pilots read clouds, moisture, and temperature to stay safe.

• Tools and rules: what to watch for in weather data, charts, and cockpit indicators.

• Quick takeaway: the temperature range to remember and why it matters.

The 0°C to -15°C window: a quick map for ice awareness

Ice in flight isn’t a folklore tale. It’s a measurable phenomenon that can change the way a plane handles in minutes. When we talk about “clear ice,” we mean a dense, glassy layer that forms when very cold water droplets freeze after striking a surface. It’s different from the milky, rough frost called rime ice, which forms in drier, colder conditions and tends to stick differently to wings and props. Clear ice is heavy, smooth, and deceptively slippery until you notice the change in lift and stall characteristics.

Let me explain the science in plain terms. Water droplets in the air exist in clouds as tiny droplets. In some flying conditions, especially when the air temperature sits around freezing, there are droplets that remain liquid even though the air is below 0°C. These are called supercooled droplets. They’re not frozen yet, but they’re ready to freeze the moment they touch a solid surface—like an aircraft wing or a propeller blade. When those larger droplets hit the aircraft, they spread out and freeze slowly, leaving a smooth, clear ice layer. That’s the essence of the “clear ice” you hear about in training and in the hangar talk.

Why the range 0°C to -15°C?

Here’s the thing: within roughly 0°C down to -15°C, conditions often line up so you get a lot of surface contact with those big supercooled droplets. Large droplets mean a bigger volume of water to freeze on contact, so you get a thicker, more continuous sheet of ice. That ice can cling to the leading edges and other critical surfaces, changing the wing’s shape ever so slightly and robbing you of lift. It’s slick—like ice on a car after a fresh spray of sleet, but up on a wing with flight dynamics to worry about.

As temperatures dip below -15°C, the air tends to hold fewer large supercooled droplets, or the droplets aren’t as abundant in the right places to create the same rapid buildup. The icing tends to be less aggressive, or the ice that does form is more likely to be rime-like—brittle, rough, and not as dense as clear ice. Conversely, at surface temperatures above 0°C, you start losing those supercooled droplets because the air can hold more liquid water or the droplets freeze too quickly or not at all in the same way. The magic happens in that mid-range window where big droplets and the right thermal conditions meet.

A closer look at icing types helps anchor the idea

• Clear ice: glossy, heavy, and slowly freezing. It builds up in wetter cloud layers with droplets that don’t freeze instantly on contact.

• Rime ice: white, opaque, and bubbly. It forms from smaller droplets that freeze rapidly and don’t flow before freezing.

• Mixed ice: a combination, with patches of clear and rime textures.

In this 0°C to -15°C zone, you’re most likely to encounter clear ice if you’re flying through measurable moisture in that sweet spot of temperature. It’s not that clear ice can’t happen elsewhere, but the odds—and the potential for rapid accumulation—are highest here.

Why this matters in real flight

Icing isn’t just a rumor from weather class. It changes how a plane flies within minutes. A smooth, icy surface reshapes the air flow, increasing drag and reducing lift. The wing’s angle of attack may be impacted, and a pilot might see a decrease in indicated airspeed, a shift in the stall warning behavior, or an unusual pull on the control yoke. If you’ve ever felt a car slip a bit on an icy road and then regained traction, you know the pattern pilots try to avoid: sudden changes in handling while you’re trying to land or climb.

That’s why pilots keep a weather eye on more than just the temperature. They watch:

• Visible moisture in the air: clouds, freezing drizzle, mist, or low-hanging fog can be precursors to icing.

• Temperature trends at altitude: in the 0°C to -15°C range, icing is more probable if there’s a genuine layer of supercooled droplets present.

• Aircraft instruments and deicing systems: these systems aren’t a magic wand; they’re a tool that helps you manage risk, not eliminate it.

A natural tangent: clouds, droplets, and a touch of meteorology trivia

Think of the air as a theatre stage. The scene changes with temperature, moisture, and altitude. Stratus clouds, with their flat, sheet-like appearance, can set up long layers of mist and drizzle. Cumulus clouds—those fluffy giants—often bring more dynamic water content and stronger updrafts, which can carry droplets through a temperature window like the 0°C to -15°C range. If you’ve ever watched fog lift off a hill on a chilly morning, you’ve caught a glimpse of how moisture behaves in a cool film of air. In aviation, that moisture becomes icing potential when the droplets are the right size and the temperature is just right.

What pilots and students watch in the cockpit and in the weather briefing

• Cloud types and bases: if a flight path slices through a layer of moist air near freezing, the odds rise for clear ice on the leading edges.

• Temperature profiles: a vertical temperature chart gives away the chance of hitting that critical 0°C to -15°C slice.

• Particle size hints: larger droplets in the cloud mean bigger ice potential on contact.

• Ground and air density: not just about the air being cold; the humidity and the moisture level aloft matter too.

Practical takeaways you can apply when you’re reading a weather briefing

• If you see temperatures flirting with 0°C in clouds capable of producing visible moisture, be alert for slow-forming, heavy ice on leading edges.

• In the 0°C to -15°C band, expect clear ice to form more rapidly than in the far colder ranges. That doesn’t mean icing can’t occur elsewhere, but this is the zone to watch first.

• If you’re unsure, assume icing is possible and plan accordingly with route choices, fuel margins, and available deicing or anti-ice resources.

Tools, data sources, and how to sharpen your instincts

• METARs and TAFs: real-time weather observations and forecasts. They can hint at moisture availability and temperature trends along your route.

• A weather radar view and icing charts (where available): these aren’t perfect, but they help map layering of moisture and the likelihood of icing in the forecast window.

• PIREPs (pilot weather reports): first-hand reports from pilots in the air can reveal icing conditions that ground observers might miss.

• Official meteorology sources: national weather services, aviation authorities, and the advisory bulletins can be your reliable guide for understanding icing risks.

A few words about training and mindset

Icing awareness isn’t a one-topic thing you “check off.” It’s a disciplined habit: constantly cross-referencing temperature, moisture, and cloud form, and matching that to the performance envelope of your aircraft. The best pilots treat icing as a weather puzzle—one that’s solvable with careful observation, solid data, and prudent decision-making. And yes, while the math behind droplet behavior can be abstract, the practical takeaway is simple: in the 0°C to -15°C range, be mindful of rapid clear ice buildup and plan accordingly.

Turning a concept into competence

If you’re building a mental library for icing, start with this: remember the 0°C to -15°C window as the prime time for rapid clear ice formation, driven by the presence of larger supercooled droplets. Everything else—that is, temperatures above 0°C or much colder than -15°C—changes the game in predictable ways, but the sweet spot is the one you’ll see most often in icing stories and training scenarios.

To close, a quick recap you can carry in your pocket

• Clear ice forms rapidly in flight when large supercooled droplets strike a surface and freeze on contact.

• The most favorable temperature range for this rapid clear ice buildup is between 0°C and -15°C.

• Outside this window, ice can still form, but the rate and type tend to differ (rime ice becomes more common at colder temps; at warmer temps, the water content in clouds behaves differently).

• Understanding the temperature-moisture relationship helps you anticipate icing risks, choose safer routes, and use cockpit icing protections effectively.

Final thought

Weather is a living partner in flight. When you know the climate patterns that drive icing, you’re not just reacting to a forecast—you’re staying a step ahead. And that sense of preparedness—knowing what to watch, where the risks lie, and how the ice might behave—can make all the difference between a smooth leg and an awkward moment.

If you’d like, I can tailor a few quick-reference notes or a simple checklist focused on the 0°C to -15°C window, so you have a handy reminder for the next flight or study session.