In Scotland we make a kind of fudge that is deliberatly hard and crystallised known as Tablet. This was a popular treat when I was growing up. Essencially the recipe for tablet, soft fudge, toffee and caramel are quite similar. The difference is made by how you cook and treat the mix as it cools.
Essentially you need to know about sugar boiling points. There are two important levels used: soft boil, and hard boil. My cook books suggest using a sugar thermometer to get the perfect boil but I've never found a sugar thermometer in the shops. Instead I rely on a great deal of practice, the colour, the texture and the drip of the mix to gage how hot the sugar is.
In a soft ball (234–240 °F or 112–115 °C), if you drip the mix onto a very cold surface or into cold water, then touch the drip with your finger, it will be soft like caramel. Additionally the colour will change from cream to yellow/tan.
In a hard ball (250–266 °F or 121–130 °C), dripping again onto a cold surface or into cold water, then touching the drip with your finger, it will be firmer or even hard. The colour will darken slightly to a dark yellow or light brown.
For caramel and fudge you go to a soft boil then cool. For tablet and toffee you go to a hard boil.
The next part is the cooling. This is equally important as the boiling. The faster you cool the mix the smaller the crystals become. The slower, the larger. If you want a caramel or a toffee, you must avoid agitating the mix as it cools. For fudge you should stir the gently as it cools slightly before pouring onto the try. For tablet, the mix should be vigorously stirred until stiff then poured onto a try.
Best Answer
I think you have it backwards. They weren't measuring humidity to calculate temperature. Rather, for home cooks, temperature is used to determine humidity.
In fudge making, the goal of cooking is simply to reduce the water in the sugar syrup to a specific concentration. Sugar syrups produce different hardness of candy at different concentrations.
When cooking other things- such as meat- the temperature is important because bad bugs are killed or meat proteins denatured at specific temperatures. Sugar, on the other hand, isn't changed at the temperatures used in candy making. Water is just boiling away. It's usually a bad thing when the temps get hot enough to start changing the sugar, ie caramelization.
Sugar syrups happen to boil at higher temperatures as they become more concentrated. Therefore the temperature can be used to determine the concentration. Temperature is much easier to measure than water concentration. Unfortunately, the boiling temperature of water changes dramatically depending on elevation. I moved from sea level to the mountains and the boiling point of water changed by 9 degrees F. That is a significant difference in candy concentration and I have to reduce the cooking time for my recipes.
In a commercial setting, it is worth the cost and effort to measure the syrup humidity directly. It's more foolproof. However, for home cooks, thermometers are cheap and easy to use. Therefore, most recipes are written in terms of temperature and not sugar concentration.