Kathy said it as well as it can be said but to put it another way...
It seems there have been a lot of questions lately that have warranted a response similar to this from me. Almost enough that I want to put it in my signature
(not being sarcastic at all)
Almost everything East and or South of I-95 in South Carolina is swampland. It is either actively swampland or has been "reclaimed" for use as whatever (residential, commercial, etc). That's why it floods, that's why we can't have basements for the most part, that's why we have gators, bugs, lizards, frogs, etc.
To answer the humidity question, even the "dry land" is wet when compared to other parts of the country.
Adding to the already wet land here, we have a big honkin' ocean right next door. The mid Atlantic is warm and pleasant to swim in when compared to the Pacific. I was visiting my dear old Grandpa in LA when I was 12 and we went to the beach. I got about waist deep in the Pacific in August and said "**********" because the water was about 30 degrees too cold for me.
The mid Atlantic also churns out hurricanes which are fueled by warm water. They either go north or south. If they go north, even if they don't hit us directly, they dump a lot of that warm water on us. If they go south, they throw warm water from the gulf of Mexico at us in the form of rain (any one remember last week?).
Coastal South Carolina has what is commonly known as a sub tropical climate. It doesn't get as hot or as humid as one would expect in a tropical paradise but it does get pretty warm here and there is a lot of water near the surface ready to evaporate.
Kathy is right, humidity can make all the difference. There have been days when I went to walk my dog and expected a sauna but it was ok despite high temperatures because the humidity was relatively low. Other days with lower temperatures and higher humidity and or no wind were unbearable and the walks were mercifully short.