这个句子不完整.
好像应该是:Under some conditions,snowflakes can take the form that are five centimeters across.
或者:Under some conditions,snowflakes can be in the form that are five centimeters across.
或者:Under some conditions,snowflakes can form what are five centimeters across.
或者:Under some conditions,snowflakes that are five centimeters across can form .