Around the world meteorological weather balloons are launched twice daily, and continuously transit weather telemetry to a ground station using something called a radiosonde. The RTL-SDR software defined radio combined with a decoding program can be used to intercept this telemetry, and display it on your own computer. You will be able to see real time graphs and data of air temperature, humidity, pressure as well as the location and height of the balloon as it makes it’s ascent.
Note that currently, this won’t work in the USA, as there are no decoding programs available for use with the telemetry protocol used in the states. The USA uses the Mark II Sippican radiosondes, while most of the rest of the world use Graw, Vaisala, Meteomodem or Meteolabor radiosondes, all of which are decodable. However, it seems that the USA is undergoing a Radiosonde Replacement System (RRS), where they are switching to Vaisala radiosondes.
This tutorial is also applicable to other software defined radios such as the Funcube dongle, Airspy, HackRF, BladeRF or even hardware radios with discriminator taps, but the RTL-SDR is the cheapest option that will work.
In this example YouTube user Superphish shows a radiosonde being received and decoded using a RTL-SDR, SDRSharp and SondeMonitor.