MarkerCluster

HTML

<!DOCTYPE html>
<html>
<head>
  <title>Google Maps Marker Example</title>

<script src="https://maps.googleapis.com/maps/api/js" async defer></script>
        <script src="https://unpkg.com/@googlemaps/markerclustererplus/dist/index.min.js"></script>
   <script type="text/javascript" src="mark.js"></script>
  <script>
  
	
  </script>
  <style>
    #map {
      width: 500px;
      height: 500px;
      border: thin solid black;
    }
  </style>
</head>
<body onload="loadMap();">
  <h1>Google Maps Marker Example</h1>
  <div id="map"></div>
</body>
</html>

JavaScript

var map;

function loadMap() {

    map = new google.maps.Map(document.getElementById('map'), {
        center: {
            lat: -41.75,
            lng: 172.75
        },
        zoom: 4
    });
	
    addMarkers();
}
var WeatherStations = [{
        "stn": 719460,
        "wban": 99999,
        "StationName": "FORT SIMPSON",
        "FIPS": "CA",
        "Latitude": 61.759998322,
        "Longitude": -121.2369995,
        "Elevation_m": 169,
        "NOAAReadings": 14952
    }, {
        "stn": 935970,
        "wban": 99999,
        "StationName": "CAPE CAMPBELL AWS",
        "FIPS": "NZ",
        "Latitude": -41.71699905,
        "Longitude": 174.26699829,
        "Elevation_m": 32,
        "NOAAReadings": 12174
    },
    {
        "stn": 934360,
        "wban": 99999,
        "StationName": "WELLINGTON INTL",
        "FIPS": "NZ",
        "Latitude": -41.32699966,
        "Longitude": 174.80499268,
        "Elevation_m": 12,
        "NOAAReadings": 9914
    }, {
        "stn": 937090,
        "wban": 99999,
        "StationName": "HAAST AWS",
        "FIPS": "NZ",
        "Latitude": -43.86700058,
        "Longitude": 169,
        "Elevation_m": 5,
        "NOAAReadings": 14371
    },
    {
        "stn": 935270,
        "wban": 99999,
        "StationName": "FAREWELL SPIT AWS",
        "FIPS": "NZ",
        "Latitude": -40.54999924,
        "Longitude": 173,
        "Elevation_m": 3,
        "NOAAReadings": 11575
    }, {
        "stn": 935460,
        "wban": 99999,
        "StationName": "NELSON AERODROME AWS",
        "FIPS": "NZ",
        "Latitude": -41.29999924,
        "Longitude": 173.19999695,
        "Elevation_m": 6,
        "NOAAReadings": 10557
    },
    {
        "stn": 937800,
        "wban": 99999,
        "StationName": "CHRISTCHURCH INTL",
        "FIPS": "NZ",
        "Latitude": -43.48899841,
        "Longitude": 172.53199768,
        "Elevation_m": 38,
        "NOAAReadings": 14933
    }, {
        "stn": 938050,
        "wban":...