My script to use the DarkSky API to tell me when it's going to start raining in the next hour. When I bike to work, I run this on a launchd job every 10 minutes from 4:00-6:00.

dark_sky_growler.rb

#! /usr/bin/env ruby
# encoding: UTF-8

require 'rubygems'
require 'active_support/core_ext'
require 'rest_client'
require 'json' unless defined?(JSON)
require 'growl'

API_KEY = ENV["DARKSKY_API_KEY"]
LAT_LON = "44.982953,-93.246889"
URL = "https://api.darkskyapp.com/v1/forecast/#{API_KEY}/#{LAT_LON}"

def spark_char_for_ratio(x)
  @spark_chars ||= %w(▁ ▂ ▃ ▄ ▅ ▆ ▇ █)
  @spark_chars[(x * (@spark_chars.count - 1)).floor]
end

def growl_summary_and_hour_forecast(forecast)
  message = ""
  forecast["hourPrecipitation"].each_slice(5) do |mins|
    ratio = mins.sum {|m| m["probability"].to_f } / mins.count
    message << spark_char_for_ratio(ratio)
  end
  
  title = forecast["currentSummary"]
  if forecast["hourSummary"] != forecast["currentSummary"]
    title << ", " << forecast["hourSummary"]
  end

  Growl.notify message, :title => title,
                        :icon => "~/Pictures/Icons/Dark Sky/darksky_light.png"
end

forecast = JSON.parse(RestClient.get(URL))

if forecast["isPrecipitating"] || forecast["minutesUntilChange"] > 0
  growl_summary_and_hour_forecast(forecast)
end