Saturday, May 24, 2014

Thunderstorms and Dewpoint

In Brian's post the other day, we saw that lightning activity in the Fairbanks area ramps up extremely rapidly in early June, with lightning being very unusual in May but quite frequent by mid-June.  I was struck by this (can't help the pun) because climatological changes in weather phenomena are rarely defined so rigidly by calendar date.  I was curious about the physical explanation for the rapid change and decided to look up the humidity data to see if a rapid rise in average humidity in early June might be a factor.  In other words, is there - as reader Eric speculated - a humidity threshold that must be met before thunderstorms become reasonably common?

The first chart below simply shows the climatological frequency of thunderstorm days, based on the hourly observations from Fairbanks airport since 1981.  This chart is closely akin to Brian's Figure 4, except that Brian showed an even more rapid rise in the total count of lightning strikes.  This makes sense, because as thunderstorms become more frequent they also become stronger on average, with more lightning strikes per storm.  In any case, we still see a very rapid jump in thunderstorm frequency after June 1, while the decline in late July and August is considerably more gradual.

The second chart, below, examines how the thunderstorm frequency depends on the daily (midnight to midnight) maximum in dewpoint temperature.  We see that the majority of thunderstorms in Fairbanks occur when there is rather high humidity, with the daily probability of thunder reaching nearly 30% for daily maximum dewpoints near 15-16 °C (60 °F).  The drop-off in thunderstorm frequency at the highest humidity levels may be an artifact of having too small a sample with such high humidity.  It is interesting that we do NOT see a pronounced threshold that must be satisfied before thunder can occur; the probability rises relatively smoothly with the humidity level.

Finally, I looked at the climatological frequency of days on which the daily maximum of dewpoint temperature is 50 °F or higher, which encompasses the great majority of thunderstorm events.  The chart below shows a rapid increase in early June, as for the thunderstorm frequency, but also shows that the climatological peak is much later than for thunder, as high dewpoints are common well into August.

What can we conclude?  It's very clear that the low-level humidity (dewpoint) is not the only factor determining thunderstorm activity, as the storm frequency drops off markedly even while the humidity is peaking in late July.  Moreover, there does not appear to be a hard dewpoint threshold that clearly defines thunder versus no-thunder days.  However, the rapid rise in both thunderstorm frequency and humidity in early June does suggest that the summer influx of humidity is an important contributor to the onset of summer storms.  A next step in the investigation might be to look at climatological changes in the vertical stability profile to see how the rising low-level humidity acts to destabilize the environment and favor deep convective overturning.

As an aside, the highest dewpoint observed so far this year in Fairbanks is 45 °F; and no thunder has yet been reported at the airport.


  1. Excellent Richard. I had long assumed qualitatively that May's relative lack of moisture input and humidity led to fewer thunderstorms, and now it seems that has some validity. Paint now or forever wish you had. Just holding a paint brush or roller is enough to precipitate a shower later in the summer.

    But there's likely more. As you mention the seasonally variable stability of the air column must play a factor. Last summer we had few TRW's for a change. Even Rick mentioned that in a Blog. Other years bring an abundance of storms any lightning.

    On a good year for activity, my ham radio is alive with local static crashes, similar to the daily QRN that radio folks near the equator experience on a daily basis. I've never read much relative to frequency by latitude, but if I point my directional beam antenna south (or towards the the Sun) during the summer it's typically noisier than to the north over the Pole.


  2. Thunderstorms need three things: heat, moisture, and unstable air. Fairbanks gets the first two easily. The unstable air requires some help from air masses. While last summer was hot and dry because of the deep high pressure, the summer was anything but. In June there would develop these huge, miles-wide storms that I had to outrace on my bike from work. They would usually form east and move west over Fairbanks. It seemed that some overarching air mass was having fun and creating some unstable air to rain upon us.

    1. (How do you edit a posted post?) I was describing the summer of 2012.

  3. When spring finally pops in interior Alaska, it really pops! The top graph (of thunderstorms days at Fai) is so well correlated to sun with the exception of the left side chopped way down. Even with the mostly clear weather in May the sun has to melt any snow left from the winter and any frost from the previous night (see before it can heat the land enough to think about thunderstorms.

  4. Jim, yes - it's amazing how long it takes to get the atmosphere destabilized.

    I am wondering if the sudden increase in thunderstorm frequency at Fairbanks is one of the most rapid climatological changes for any weather phenomenon in the world. In my experience it is rare to see a climatological change so rigidly tied to the calendar.

  5. Another observation. It tends to remain warm over a 24 hour period from now on through maybe mid-August. That's a rough hypothesis I know, but when it stays warm through the evening, and moisture is available, I think that's when we get the most thunderstorm activity. Cold evenings may reset the thunderstorm cycle (?).

    Maybe having a look at 6pm-6am temps (if that's possible), daily means, or daily high-low spread might support my contention.


  6. Here's a couple of good references to Interior Alaskan Thunderstorms:

    There's more devoted to the relationship of lightning and fires, probably funded by fire management agencies.