Method isThundering

src/main/java/net/minecraft/world/World.java:3555–3558

Returns true if the current thunder strength (weighted with the rain strength) is greater than 0.9

()

     * Returns true if the current thunder strength (weighted with the rain strength) is greater than 0.9
     */
    public boolean isThundering()
    {
        return (double)this.getThunderStrength(1.0F) > 0.9D;
    }

    /**
     * Returns true if the current rain strength is greater than 0.2