It's very much a question of being in the "soup" or not, as well as temperature, as well as speed, as well as atmospheric conditions, as well as aircraft design and capability.

In one of my former lives I flew atmospheric research. One of the keys we looked for was ice production or ice buildup; we made an effort to seek out maximum icing conditions, particularly in thunderstorm penetrations when taking samples or collecting data.

There's a common misconception that ice forms in a narrow range and that a few thousand feet will cure the icing problem. In my experience over the past few decades, this is not true. I've seen many, many times when descent below MEA was impossible and icing conditions existed to MEA and below (often to the surface, especially over mountainous terrain), and well above where I could climb. This can be particularly true of piston and turboprop equipment.

Icing will occur with outside air temperatures above freezing due to several reasons, including supercooled water and freezing rain, local temperature drops to to area pressure changes on the airframe, inaccurate temperature gauges, variability in a parcel of atmosphere, rising or descending air (think orographic lifting, convective activity, etc), and other factors. Icing will occur all the way down to -40C, with the ideal icing range approximately -15C.

Particularly during thunderstorm penetration research, I always picked up my maximum rate of icing around -15C. Icing rate didn't diminish much through 0C and continued up through about 06C in some cases, generally being gone by +10C. It continues with less frequency down through -25C, with liquid water found as low as -40C. I used to fly with the gentleman who discovered liquid water in a research environment at -40C, and not coincidentally one of the first to experience severe icing and airframe damage from hail, at the same time, at -40C. Go figure.

Yes, it's possible that an altitude change of a few thousand feet may remove ice; I've seen that happen. I've seen many more times, however, when making an altitude change of several thousand feet wan't possible, and if possible, woiuldn't remove the icing conditions.

Another popular myth is that ice bridging on boots doesn't exist, and it absolutely does. I've watched it hundreds of times, and there is zero question that it exists. In fact, long ago I lost count of the number of times I popped boots and watched the ice expand outward, change colors as the boots pushed it away, then retracted beneath to leave a hollow shell that continued to build ice as my new leading edge, and coiuldn't be broken by subsequent boot inflations or cycles.

I've seen ice build so rapidly that it couldn't be countered or removed, and have picked up 3/4 to 1" in sixty seconds, and 2-3" in a similarly short period of time. In one case, during a penetration flight, I had to stop the mission and land due to very heavy ice buildup that began to affect controllability and couldn't be removed. I landed at a middle eastern location in the desert, and still had 3/4 inches of ice on the airplane that were peeled off and photographed, more in some areas.

Don't count on it submlimating or being removed by a change of a few thousand feet. Know where your best options are, but also know where they're not. There are a lot of circumstances where the best option is to not be there, and where exiting icing conditions isn't possible.

I think I can count on one hand the number of flights I've had in daylight this year.