why did we have so many tropical depressions in the 70s?

[year_2005]

In the 1970s we had many years where the number of depressions was significantly more than in the last decade or so.

in 1971,1973, 1974, 1975 (27 depressions!), 1976, 1979(26 depressions) had more than 20 TDs , although few of these turned into storms,
the 1980s dont ompare to this in numbers of tropical depressions and neither do the 1990s,

in the 2000s only 2 years went over 20 TDS. and many of those in the 70s were over 24 TDs.

[Macrocane]

I have the same question. I don't know why there were so many depression, maybe they had a different definition of Tropical Depression at that time.

[brunota2003]

Probably has to do with us better understanding what exactly a TC is, and having better tools to determine strength. In the '70s, they had some sat images, along with some recon...but I'd imagine some of the depressions were probably not entirely tropical in nature, and the weak storms (that hit 40 or 45 mph) may of been labeled as a depression and never upgraded (again, better tools today).

[djones65]

I believe back in the '70's the term "tropical depression" was really a very loosely defined term and many or maybe even "most" of the depressions were actually what we now call Invests. We have between 20 and 30 invests each season give or take and that would probably account for the huge disparity between now and forty years ago. They didn't use "invest" to describe broad low pressure areas that did not meet the organization of a named tropical storm yet, thus they called them depressions. They didn't even "number" the advisories, it wasn't even tropical depression advisory #1, etc... it was simply tropical depression advisory. So, in my humble opinion, most of the depressions in the 70's were simply invests that never attained tropical storm force, and very few of them probably achieved the organization now required to be classified as a depression.