What meteorological significance is there for 39 mph as the lower limit for tropical storms and 74 mph...?

Question

for hurricanes? What is significant about those wind speeds such that 39 mph is the lower threshold for tropical storms and 74 mph for hurricanes? What's the difference between, say, a 73 mph tropical storm and a 75 mph hurricane in terms of their effects? It just seems from a layman's perspective that those wind speeds are arbitrary, but I figure there must be some reason meteorologically that 39 and 74 mph are considered the lower limits for tropical storms and hurricanes. Any meteorologists out there who can shed some light on why these wind speeds are the cutoffs for tropical storms or hurricanes?

Answer 1

It has been found out that if the winds exceed 35 knots, the clouds associated with the storm become more organised and produce a spiral formation when viewed through radars and satellites.The difference between the central pressure and the outward pressure should have a particular value to produce this minimum wind.

Answer 2

It's 35 knots and 65 knots for the thresholds. There was no concrete reason for these two numbers to have been the cut-offs. It's a derivative from the old Beaufort scale, which ranked winds from Force zero to 12, with 12 being hurricane force. Later on the old International Meteorological Committee assigned wind speeds to these forces, and since it was heavily nautical in nature, it turned out that 65 knots was assigned for the threshold of Force 12. Force 8 (gale force) turned out to be 35 knots, I believe.