Adam Bertram explains how to use environment variables in Powershell:
Environment variables are exposed with a PowerShell drive known as “$env:”. It’s possible to browse through all of the environment variables by typing $env: at the console and hitting the tab key. This will allow you to see the names of each environment variable in alphabetical order.
The $env: drive is the recommended place to refer to any environment variables with PowerShell. However, it’s possible also to read the variables via .NET in PowerShell by using the GetEnvironmentVariable static method on the Environment class. This is essentially the same task.
Read on to see how you can use these in your scripts.