I will visit tibet on 24 august. what is the weather?

According to weather forecast, it will be rainy on August 24th. The temperature will vary from 50F to 70F. You may pack jackets, shirts, pants, sneakers, and an umbrella.

You can buy a GSM SIM card at China Mobile branches, so that you can connect your phone to the internet. :)

For china mobile branches,what brand do you prefer?
Can i buy at beijing airport?

Well, GSM is a nice choice for you. There are many China Mobile branches. For instance, you may go to the one at the crossroad of Century Avenue and Beijing Avenue, about 900 yards to the east of Lhasa Railway Station.

Yes, the card is avaiable at Capital Airport. You can consult the staff at Enquiry Desk for its exact location. :)

Later i go there the flight from hongkong airport. Can i buy the china mobile at hongkong airport? because i dont have enough time buy the sim card at beijing airport. Can i use the china mobile at guangzhou,xining?

Thank you

Yes! You can buy it at the OK Convenience Stores or 7 Eleven Convenience Stores at HKIA. Yes, it is applicable in Guangzhou and Xining. :)