Was December truly warmer?

Here in Iceland we talk a lot about the weather. Often the first subject you discuss about is the weather today, yesterday or tomorrow. We even say, if you don’t like the weather, just wait 5 minutes.

Our feeling is that last December (2016) was much warmer than any other December months in the past. We had little to no snow and the temperature was often above +5°C which is unusual for December. These are just our feelings and nothing scientific behind it, it just felt warmer than last year. But, what do the numbers say?

We are so lucky that we have years’ worth of 10-minute weather data from a nearby station. So, let’s compare December 2015 with December 2016 and see if it was truly warmer.

Intensity Plot

By using Intensity Plot we can view large set of Data where we use color to indicate the size of the values.

  • On the Y-axis we have the days in the month.
  • On the X-axis we have the hours in each day.

The color scale is from -10°C to +12°C.

By viewing the 2 months we can clearly see that 2016 was much warmer than 2015.


By viewing the two months in a Histogram we see the following.

  • December 2015: Most readings (9.32%) were for 1-1.5°C
  • December 2016: Most readings (7.77%) where for 7-7.5°C

Overlay Graph

Overlay graph allows you to plot and compare days, months or years.

By plotting December 2016 (Black) and December 2015 (Red) we can see that 2015 was most of the time warmer by few degrees.

Diurnal plot

Diurnal Plot is great to see long term behaviour in the data sets. It takes all readings between each hour and calculates the average for the whole time period.

On the X-axis are the hours in the day. From 00:00 to 24:00

The two plots are showing all data for December 2015 and 2016. The point at 08:00 is the Average value for all readings between 07:00 and 08:00 for the whole month.

Here we can clearly see that 2016 is much warmer and specially in the afternoon where the average temperature is 3.88°C

Download Data

At any moment we can easily download any data from the database. We select our Air Temperature and download a monthly summary instead of the Raw data.

We include the Average, Maximum and Minimum. VDV creates a Excel file where we have the numerical data and can quickly compare the two months and get the following information.

  1. Lowest value 2.53°C  lower 2015 than 2016. (-10.34°C vs. -7.82°C)
  2. Highest value 2.86°C higher 2016 than 2015. (7.05°C vs. 9.915°C)
  3. Average value 3.74°C higher 2016 than 2015. (-0.16°C vs. 3.58°C)

Trend Lines

We can also simply plott the two months using the Historical Data. We can quickly see that the Average Air Temperature is 3.58°C 2016 vs. -0.16°C 2015.


Higher average Air Temperature

The conclusion is that our feeling was indeed correct. December 2016 was much warmer than the year before. This article shows different ways on how data can be viewed and analysed in VDV to get to the bottom line.

One thing is to have all the data, it’s a completely different thing to be able to have the data telling you what is really happening.