Is my emoji dry?

05AB1E, 18 17 15 bytes

Code:

|…-_-123:)ø€J€ï

Explanation:

|                  # Take all input as a list of strings.
 …-_-              # 3-char string, which results into "-_-".
     123:)         # Replace "-_-" with 123.
          ø        # Zip, resulting into the columns of the 2D array.
           €J      # Join each of them.
             ە    # For each, convert to integer. If this is not possible, it will ignore
                     the result.
                   # Implicitly output the array.

Uses the CP-1252 encoding. Try it online! (make sure to pad all lines with spaces to the same length..

JavaScript (ES6), 95 bytes

a=>[...a[n=0]].map((_,i)=>a.map(s=>(c=s[i])>"-"&c<"_"?p=1:n+=!++c,p=0)|p<!c&&o.push(n),o=[])&&o

Input should be an array of strings, with each line padded with spaces to form a square. Output is an array of 1-indexed numbers.

Explanation

var solution =

a=>
  [...a[n=0]].map((_,i)=>  // n = current index of emoji, for each column i of input
    a.map(s=>              // for each line s
      (c=s[i])             // c = character in column
      >"-"&c<"_"?p=1       // p = 1 if column is protected from rain
      :n+=!++c,            // increment n if emoji character found, c = 1 if last line
                           // contained a space in the current column
      p=0
    )
    |p<!c&&o.push(n),      // if the emoji is not protected in the current column
    o=[]
  )
  &&o

<textarea id="input" rows="6" cols="40">   /\   
  /  \  
        
     -_-</textarea><br />
<button onclick="result.textContent=solution(input.value.split('\n'))">Go</button>
<pre id="result"></pre>

JavaScript (ES6), 92 bytes

a=>a.map(s=>s.replace(/\S/g,(c,i)=>c>'-'&c<'_'?u[i]=3:++n&u[i]||r.push(n)),n=0,u=[],r=[])&&r

Accepts a ragged array of lines and returns a 1-indexed result. Explanation:

a=>a.map(               Loop through all lines
 s=>s.replace(/\S/g,    Loop through all non-whitepsace
  (c,i)=>c>'-'&c<'_'    If it's part of the umbrella
   ?u[i]=3              Mark that column as dry
   :++n&                Add 1 to the emoji index
     u[i]||             If the column is not dry
      r.push(n)         Add the emoji index to the result
  ),n=0,u=[],r=[]       Initialise variables
 )&&r                   Return result