Can the weather really affect how long it takes to hit gel stage? Most definitely yes!
Today we use candy thermometers to help us gauge whether or not we’ve hit gel stage (220 F). Sometimes that can happen quickly and sometimes it seems impossible to get the temperature up that high. To understand the variability behind why this happens we need to understand what happens to water when it boils, what affects the boiling point of water, and why it’s difficult to raise the temperature of any liquid above the boiling point.
When we boil water, we are using heat from the stove to convert water into gas. As water reaches the boiling point, it vaporizes and escapes so water never actually increases in temperature once it begins to boil. Because water only boils when the pressure inside the water molecules matches the atmospheric pressure, many things can affect the temperature at which water boils, e.g., atmospheric pressure (stormy versus sunny weather), altitude, pot shape, and adding other ingredients to the water such as salt or sugar.
During cloudy or stormy weather, atmospheric pressure decreases–which means a lower boiling point for water. Although it comes to a boil more rapidly, water boils at a lower temperature so food cooks slower. This explains the old adage that you can’t make divinity on a cloudy day. At higher altitudes the same thing happens–water boils at a lower temperature. Therefore foods cook faster at sea level where the boiling point is higher (meaning hotter cooking.)
Using a high sided, narrow pan versus a short broad pan will increase the pressure placed on the water, giving you a higher boiling temperature. Adding things like sugar or salt will give you a higher boiling temperature. And using hard water versus soft water will also give you a higher boiling temperature.
Because water stops increasing in temperature once it hits the boiling point, we need to cook things like marmalade (or other jams to which we’ve added no commercial pectin) sufficiently to remove enough water in the mixture (thereby increasing the ratio of sugar to water) to actually increase the temperature of our marmalade to the gel stage.
Therefore, achieving a temperature of 220 F is not only a measure of physical heat but also a way for us to know that sufficient water has cooked out of the jam or marmalade to cause it to gel.
Canning Across America contributor Annette Cottrell lives in Seattle with her husband, two young boys, hairy dog and backyard chickens. She has devoted the front and side yards of her quarter acre city lot to growing enough fruits and vegetables to feed her family year-round. She blogs at Sustainable Eats about thoughtful, sustainable eating and provides tools and resources to others who want to make the journey from supermarket to local, farm fresh food one step at a time. In her spare time she runs Pollywog Baby, a website full of practical solutions for infant reflux and colic.