Many types of candy are made by heating sugar and water (sometimes with butter and other ingredients like corn syrup) mixtures to temperatures above the boiling point of water, which as all of us know is 212ºF at sea level with a normal barometric pressure of 14.7 pounds per square inch (psi). Variations in elevation affect barometric pressure, which in turn will modify the temperature at which water boils. See below.
In general, when we consider making sugar based candy the cooking process eliminates water present in the ingredients in accordance with the temperature to which we elevate the mixture above 212 degrees F. It is very useful to know just how high to heat the temperature of a candy mixture to achieve different physical characteristics related to softness or hardness.
The descriptions below are very useful when you decide to make candy, for they provide exactly the information you need to achieve different levels of softness or hardness. Candy examples are given and that is particularly useful for you to home in on the normal temperature range appropriate to what you plan to make. I found the descriptions on the Internet and I believe they are very important to include in Food Nirvana, and I am grateful to the source, whose identity I cannot possibly remember.
Note that no information was provided to address any elevation considerations. If you live at a high elevation (1000 feet or higher above sea level) you should do Internet research to further understand the effect of elevation on making candy. The information provided in the descriptions below is for elevation ranges from sea level to roughly 1000 feet. But here is a handy rule of thumb: Subtract two degrees Fahrenheit from a stated temperature range shown below for every 1,000 feet you are above sea level.
Thread: 230ºF to 235ºF Sugar Concentration: 80%
Thread is the early stage of sugar syrup that forms thin strands when drizzled and cooled. The syrup can be used to make meringue and syrups of different types in this stage.
Softball: 235ºF to 240ºF Sugar Concentration: 85%
At softball stage the sugar syrup is viscous, and when dropped in water it forms a ball that quickly loses its shape when removed from the water. Sugar syrup at this stage is right for fondant, fudge, pralines and penuche.
Firmball: 245ºF to 250ºF Sugar Concentration: 87%
The sugar syrup is very tacky and thick, and if you drop some in water it will form a loose ball that is sticky to the touch. Caramel candies are cooked to this stage.
Hardball: 250ºF to 265ºF Sugar Concentration: 90%
At the hardball stage the sugar forms a ball and holds its shape when dropped into cold water, and it has resistance to shape change and it has buoyancy. Nougat, marshmallow and gummy candies are created at this stage.
Soft Crack: 270ºF to 290ºF Sugar Concentration: 95%
Soft crack is the stage where the sugar is pliable and can be pulled with your fingers (when cooled to a safe handling temperature). Chewy, sticky candies like taffy, butterscotch and toffee are made at this stage.
Hard Crack: 300ºF to 310ºF Sugar Concentration: 97%
Lollipops, brittles and hard candies are created at the hard crack stage. If dropped into cold water the sugar syrup will completely solidify.
Light Caramel: 320ºF to 335ºF Sugar Concentration: 100%
At this temperature range the sugar mixture starts to break down and it takes on a light brown color. At this temperature caramel takes on a bright nutty flavor. Remove the mixture from the heat quickly once it reaches this temperature range and you will be able to make a golden caramel sauce (with heavy cream).
Dark Caramel: 340ºF to 350ºF Sugar Concentration: 100%
This is the point at which to remove the syrup from the heat immediately and mix in heavy cream to make a deeply colored robustly flavored caramel sauce. Do not cook further.