KaiserReo:
I already figure out how to use the sprintf() function and I found out that it only work well with positive integers.
Sorry, I was forgetting that sprintf() does not work for double data type on Uno.
But dtostrf is also a good suggestion.
Also possible is to send the double data value in its binary format (I think its only 4 bytes on Uno).
As tkbyd suggests, why not just send the Celsius version of the temp and convert to that other old-fashioned scale on the receiving Uno?