Risk of frozen plants in planes : how cold it gets with altitude.

  I am in Texas , can I safely import live plants from Thailand in April and May ? 
 
Thailand is like Texas in the northern hemisphere and the planes take routes in this hemisphere, where temperature get warmer during these months.

But at anytime of the year the risk of freezing in planes exists when they fly, and this risk is between any two airports, whether it be between 2 distant towns of the same country or between two countries  ( we are not considering temperature at the airports where cargo are downloaded, or in Post office buildings and warehouses etc. where temperature are warmer in spring ).  Indeed at the altitude where planes fly the temperature is less than on ground by more or less 6.5 Celsius per km ( 3 F per 1,000 ft. ) of altitude.
For instance if the temperature in your garden is 25 Celsius in April or May then 9 km above ( where planes can fly ) the temperature is 25 - ( 9 x 6.5 ) = - 33.5 Celsius ( - 92.3 Fahrenheit ) .

In aircrafts the temperature normally flows from the cabin to the hold so temperature and pressure are about the same in cargo holds and cabins : usually holds are only a couple degrees cooler than the main cabin.  But this setup can change from aircraft model to aircraft model and a reader on a http://www.airliners.net, made this comment : " it doesn't usually go below freezing. If you're worried about too hot, that's a little more iffy. "
Adding foam in cartons can be an option, but the cost of transport increases accordingly.

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