Both sentences are grammatically correct.
There are many different meanings and uses of 'know (of)', but I think the meanings of the sentences above can best be seen as the following two from the (very long list in the) OED:
"There are too many words that I don't know."
To (come to) apprehend, be or become conversant with or aware of; to learn.
That is to say, 'there are many words that I do not understand'.
"There are too many words that I don't know of."
Intr. To be aware or cognizant of (a person or thing as existing,
an event as having occurred, etc.)
That is to say, 'I am not aware of the existence of a great number of words.'
The difference between the sentences with 'words' is, I think, negligible. However, if you substitute 'words' with 'people', the difference becomes clearer:
"There are too many people that I don't know."
Which could mean that you don't know the people (at, for example, an event) personally.
"There are too many people that I don't know of."
Which could mean that you haven't even heard (a lot) about many people, and you might be oblivious to their existence.
The first use of 'know' can actually also mean 'be aware of', but it is made more explicit with 'know of'.
SHORT VERSION:
Only if you change the first statement to
I have experience in something
does it have the same meaning as the second statement.
LONG VERSION:
Make sure you understand the difference between (2) and (3) in the treatment of the noun here:
2.
a. Active participation in events or activities, leading to the accumulation of knowledge or skill . . .
b. The knowledge or skill so derived.
3.
a. An event or a series of events participated in or lived through.
as well as the difference between the adjective experienced:
- Having had experience in an activity or in life in general: a highly experienced traveler.
- Skilled or knowledgeable as the result of active participation or practice: consulted an experienced investment counselor.
and the past tense of the verb, experience(d) (below the noun entries linked above):
To participate in personally; undergo; experience a great adventure; experienced loneliness
Now to address your examples:
"I've had an experience in something"
is unusual phrasing. It looks like an awkward cross between "I had an experience" (noun, 3) and "I have experience with _____" (noun, 2).
Normally, "an experience" specifies one event or occasion, as in definition 3a above. This is singular and therefore distinct from the idea of ongoing experience.
But experience "in something" suggests practical exposure, as in definition 2a.
"I'm experienced in something"
This is much clearer than your previous phrase, because the adjective experienced only corresponds to Definition 2 of the noun; there is no equivalent adjective "experienced" that corresponds to the noun's Definition 3.
Best Answer
The phrase 'marginal conditions' here is used to describe the weather as in previous sentence, there is the note of 'Mother Nature'. And it is all about maintaining condition with enough snow for skiing.
As Jim commented, that's the weather condition right on the edge between acceptable and unacceptable.
I could find the meaning of 'marginal weather' in this context.