The GY-21P is an interesting module in that it combines a BMP280 sensor and an SI7021 sensor. The on-board BMP280+SI7021 sensor measures atmospheric pressure from 30kPa to 110kPa as well as relative humidity and temperature. Basically, anything you need to know about atmospheric conditions you can find out from this tiny breakout. 
Features :
High quality and durable.
Can measure humidity,temperature and barometric pressure.
Portable design,easy to carry.
Easy to install.
Precision sensor form , is soldered onto PCB.
It uses both I2C and SPI (supports 3-, 4-wire SPI) interface.
Not only the pressure and the temperature, this sensor can measure humidity.
Aplication :
indoor/outdoor navigation
weather forecasting
home automation
personal health
wellness monitoring
Specifications :
Operation Voltage: 3.3V
I2C & SPI Communications Interface
Supply Voltage: 1.8 – 5V DC
Interface: I2C (up to 3.4MHz), SPI (up to 10 MHz)
Operational Range:      -Temperature: -40 to +85°C      – Humidity: 0-100%      – Pressure: 300-1100 hPa
Measurement accuracy      -Temperature: ±1°C      – Humidity: ±3%      – Pressure: ±1Pa
Item size:13x 11x1mm
Item weight:1g