Is there a simple interface from which I can extract average summer temperature and humidity for a list of latitude and longitudes?

Question

I am interested in finding the summer average (either climatalogical mean or for a specific year) relative humidity and temperature for a set of globally distributed latitude and longitude coordinates.

What I need are mean values for June, July, and August.

I would prefer to use actual observed values, but a reanalysis (such as NARR or CRU-NCEP would suffice). Is there a web-based interface from which I can obtain this information?

The closest that I have come to this

Wolfram Alpha, but they only provide rainfall and temperature, not relative humidity
worldclim.org looks promising, but requires me to download all of the data before using another program to extract information; it also does not have relative humidity.

Mapperz · Accepted Answer · 2012-10-17 13:41:24Z

Weather Underground users can now access daily weather conditions dating back six years in most cities around the world. Historical weather information such as maximum and minimum temperatures, precipitation, humidity and winds are available for use by researchers, travelers, event planners and weather buffs free of charge. The databank was recently expanded to include hourly weather figures beginning with the 1999 archives in most markets.

http://www.wunderground.com/about/pr/news.asp?date=19990412

Use the API to access this data

Historical Request Example (San Fransisco Airport) on 18th October 2010.

http://api.wunderground.com/api/YOUR_API_KEY/history_20101018/q/SFO.json

Share Response
HTTP/1.1 200 OK

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    "termsofService": "http://www.wunderground.com/weather/api/d/terms.html",
    "features": {
      "history": 1
    }
  },
  "history": {
    "date": {
      "pretty": "October 18, 2010",
      "year": "2010",
      "mon": "10",
      "mday": "18",
      "hour": "12",
      "min": "00",
      "tzname": "America/Los_Angeles"
    },
    "utcdate": {
      "pretty": "October 18, 2010",
      "year": "2010",
      "mon": "10",
      "mday": "18",
      "hour": "19",
      "min": "00",
      "tzname": "UTC"
    },
    "observations": [
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          "mday": "18",
          "hour": "00",
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        },
        "utcdate": {
          "pretty": "7:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
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          "hour": "07",
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          "tzname": "UTC"
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        "tempm": "15.0",
        "tempi": "59.0",
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        "metar": "METAR KSFO 181056Z 22005KT 10SM BKN017 BKN120 13/08 A3004 RMK AO2 SLP173 T01280083 $"
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        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181856Z 33005KT 10SM FEW010 SCT160 BKN180 18/11 A3006 RMK AO2 SLP178 T01830106"
      },
      {
        "date": {
          "pretty": "12:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "12",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "7:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "19",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "300",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.1",
        "pressurei": "30.04",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181956Z 30013KT 10SM FEW010 18/10 A3004 RMK AO2 SLP171 T01830100"
      },
      {
        "date": {
          "pretty": "1:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "13",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "8:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "20",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "37.0",
        "wgusti": "23.0",
        "wdird": "300",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1016.7",
        "pressurei": "30.03",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182056Z 30013G20KT 10SM R28R/2200VP6000FT FEW010 18/10 A3003 RMK AO2 SLP167 T01830100 58013 $"
      },
      {
        "date": {
          "pretty": "2:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "14",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "9:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "21",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.9",
        "tempi": "66.0",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "56",
        "wspdm": "20.4",
        "wspdi": "12.7",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "290",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1016.1",
        "pressurei": "30.01",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182156Z 29011KT 10SM FEW010 19/10 A3001 RMK AO2 SLP161 T01890100 $"
      },
      {
        "date": {
          "pretty": "3:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "15",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "10:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "22",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "310",
        "wdire": "NW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1015.7",
        "pressurei": "30.00",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182256Z 31013KT 10SM FEW010 18/10 A3000 RMK AO2 SLP157

truncated for GIS Stack Exchange.

http://www.wunderground.com/weather/api/ (can be setup in 5 minutes)

For Lat/Lngs you need to add the GeoLookUp request:

http://api.wunderground.com/api/YOUR_API_KEY/geolookup/q/37.776289,-122.395234.json
http://api.wunderground.com/api/YOUR_API_KEY/geolookup/q/CA/San_Francisco.json

That is a phenomenal resource! And there is a command-line interface too github.com/sramsay/wu! Could you give an example of how I could query, e.g., daily average temperature and relative humidity for a given lat/lon? — Abe, Commented Oct 16, 2012 at 22:18
Have created a json historical request/response example (San Fransisco Airport) on 18th October 2010. See updated answer (full response was truncate as GIS SE has a limitation on the amount of characters you can have in an answer) — Mapperz, Commented Oct 17, 2012 at 13:37
This is working very well ... but the closest that I have gotten to what I need is the "planner" feature to get climatealogical means for a date range, e.g. api.wunderground.com/api/MY_API_KEY/planner_06010831/q/…. It returns Dew Point instead of Relative Humidity; but there is a sufficient way to estimate RH. — Abe, Commented Oct 17, 2012 at 16:24
argggh ... it seems that the conversion from dewpoint to RH is non-trivial (as discussed by Lawrence 2005) — Abe, Commented Oct 17, 2012 at 16:50

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