Find words starting with a vowel

#include <iostream>
#include <vector>
#include <string>
using namespace std;

// Take a list of words and return a list of words
// that start with a vowel.

// This function takes a character and returns
// true if it is a vowel. Otherwise, it returns false.
int is_vowel(char letter)
{
    // The character must be one of these.
    string vowels = "AaEeIiOoUu";

    // Go from the beginning to the
    // end of the list of vowels.
    for (size_t i = 0; i < vowels.size(); ++i)
    {
        // If the letter matches that vowel.
        if (letter == vowels[i])
        {
            // The letter is a vowel.
            return true;
        }
    }

    // The letter didn't match any vowel.
    return false;
}

// This function takes a list of words and returns a
// list of the words that start with a vowel.
vector<string> get_vowel_words(vector<string>& word_list)
{
    // These are the words that start with vowels.
    vector<string> words_starting_with_vowels;

    // We are testing this word.
    string current_word;

    // This is the first letter of the word.
    char first_letter = ' ';

    // Go from the beginning to the end of the word list.
    for (size_t i = 0; i < word_list.size(); ++i)
    {
        // Test the next word.
        current_word = word_list[i];

        // Get the first letter of the word.
        first_letter = current_word[0];

        // If the first letter is a vowel,
        if (is_vowel(first_letter))
        {
            // Add the word to the new list.
            words_starting_with_vowels.push_back(current_word);
        }
    }

    return words_starting_with_vowels;
}

int main()
{
    // Create a list of words.
    vector<string> words = { "red", "blue", "orange", "yellow" };

    // Get a list of the words that start with a vowel.
    vector<string> vowel_words = get_vowel_words(words);

    // Display the words that start with a vowel.
    for (size_t i = 0; i < vowel_words.size(); ++i)
    {
        cout << vowel_words[i] << endl;
    }

    return 0;
}