Barometric Pressure Sensor – MPL115A1
(from Sparkfun)

Technical description: A microelectromechanical systems (MEMS) barometer is very small (<0.1mm) and are used to measure air pressure. They can be used to measure altitude, and are widely used in smartphones. (more on Wikipedia)

Limitations: Weather conditions will affect pressure readings, so this should be compensated for if the sensor is being used for altitude measurements. Each sensor has a limited range of measurements and may not be suited for extreme cases such as vacuums or pressurized gas environments.

Skill: Advanced (the sensor itself is very small and requires some serious SMD soldering skills, unless you use a breakout board. The sensor output requires extra research to take readings and interpret.)

Sample Sensor: Barometric Pressure Sensor – MPL115A1 https://www.sparkfun.com/products/9602

Sample Exercise: https://learn.sparkfun.com/tutorials/bmp180-barometric-pressure-sensor-hookup-/all