You used the same routine on Monday and on Thursday. On Monday your curls were defined and springy. On Thursday they looked like someone else's hair. Nothing in the bottle changed, but the air did. Before you buy another product for frizz, it helps to know how much of that change the air can explain.
Hair takes water from the air
Hair is hygroscopic: it takes up and releases water as the humidity around it changes. Robbins' reference on human hair describes how water molecules break and reform the hydrogen bonds in keratin, which is one reason the fiber's shape is not fixed.
The clearest measurement comes from Gao (2007). Water regain in hair fibers rose roughly linearly as relative humidity climbed from 40% to 85%, and elasticity fell as humidity rose. In plain terms, hair holds more water as the air gets wetter, and it becomes less springy while it does.
Curious how a routine is built from numbers like these? See how Rizzado works, step by step.
See how Rizzado worksFrizz is partly a friction problem
Frizz is what people notice, and curl shape is what makes it visible. A 2024 study in the Journal of Oleo Science measured friction in the root direction on straight, curly and wavy hair. Curly and wavy hair measured higher than straight hair, with friction coefficients of 0.51 and 0.54 against 0.47. The differences are small, but they point the same way.
A lab measurement does not tell you how your hair will feel on a humid morning. It does explain why the same curls can feel rough without any change in your routine.
What a product can and cannot do
Humectants draw water toward themselves, so how much they help depends on how much water is in the air. That is general chemistry rather than a single trial result, and it is one reason the same product can feel different on different days.
Penetration is a separate question. A 2025 study in the International Journal of Cosmetic Science used NanoSIMS imaging and solid-state NMR to show that panthenol enters hair and concentrates in the cortex, where it appears to form hydrogen bonds with protein and improve tensile properties. That is a mechanistic study. It shows where the molecule goes, not how much curl definition you will gain at home.
What this means for your routine
Log the air next to the result. For two to three weeks, write down the day's humidity or weather, the product you used, and how the curls looked. Patterns usually appear before the end of that window.
Test one change at a time. If a product needs to work on humid days, test it on a humid day, not on the first dry morning. If humidity changes a lot across the year, keep a stable routine and adjust the product amount or the drying step on the days that need it.
CITY DATA
Compare a published local source
These pages keep the utility or public authority source next to the figure. They are a starting point for a question, not proof of what is happening to your hair.
Browse every city with a primary source