空气是否需过饱和才能发生凝华？兼询凝结尾迹与化学凝结尾迹的持久性差异

Hey Noah, great questions to unpack for your pre-research on contrails and the chemtrail conspiracy—let’s break these down with solid atmospheric science:

Short answer: Yes, under almost all real-world conditions.

Deposition (the phase change from water vapor directly to ice) requires a driving force to overcome the energy barrier of forming ice nuclei. At equilibrium, saturated air means the rate of water molecules leaving ice equals the rate joining it—no net growth or shrinkage happens. For deposition to actually happen (i.e., ice forms and grows), the air needs to be supersaturated with respect to ice: there’s more water vapor present than the air can hold in equilibrium with ice.

If there are no pre-existing ice nuclei or particles to act as seeds (like the tiny soot/ash particles in jet exhaust), the air needs to be highly supersaturated for spontaneous ice formation. But with jet exhaust providing plenty of ready-made nuclei, the required supersaturation is lower—though it still needs to be above the saturation point for ice to grow instead of sublimating away.

First, let’s get this straight: the idea of "chemtrails" is a conspiracy theory with no scientific evidence to back it up. The "persistence gap" conspiracy theorists claim is entirely explained by atmospheric conditions, not secret chemical sprays.

Here’s why contrails vary so much in how long they last:

• Short-lived contrails (seconds to minutes): These form when the upper atmosphere is dry or only weakly saturated with respect to ice. The ice crystals from jet exhaust quickly sublimate (turn back to water vapor) because there’s not enough surrounding moisture to sustain them. The air can’t keep the ice stable, so the trail vanishes fast.
• Long-lived contrails (minutes to hours): These form when the upper atmosphere is supersaturated with respect to ice (and usually at temperatures below -40°C, where ice is more stable than liquid water). The ice crystals from the exhaust don’t just stay—they grow by depositing more water vapor from the surrounding air, and can even spread out to form cirrus clouds. This is a normal, well-documented atmospheric process, not a sign of chemical tampering.

Jet engines produce plenty of tiny particles (soot, metal oxides) that act as excellent ice nuclei, which is why contrails form in the first place—but none of these are "secret chemicals." The persistence has everything to do with the air’s moisture content and temperature, not what’s coming out of the plane.

内容的提问来源于stack exchange，提问作者Noah Krasser