The Rain in Maine

One of the side-effects of global warming is that warmer air can hold more water vapor. In particular, storm clouds can hold more water so they can dump more water on the land. Sometimes it’s true that “when it rains, it pours.”

Not everyplace is going to get more rainfall, however. Also, some places that get the same amount of rain will nonetheless get drier because more heat means more evaporation. That’s one of the chief reasons global warming is such a big factor in the ongoing California drought. Their precipitation has been low lately (2013 was an especially bad year), but they’ve seen just as bad spells in the past. What they haven’t seen is a drought as bad as this one, and that’s because drought brought on by reduced rainfall has been made worse by increased evaporation due to the heat.

But rather than talk about California, let’s consider Maine, and how much rain can fall in a single day.

There are 12 USHCN (U.S. Historical Climate Network) stations in Maine. Looking at daily rainfall, we can see that at some locations patterns have changed over the years. One of the first questions which springs to mind is, does it rain more or less often? I’m not talking about the amount of rain, just whether or not it happens at all. The answer for Portland is yes:


With a p-value of 0.000376, the increase is statistically significant. There’s been a statistically significant increase in the number of days per year with at least some rain for 8 of the 12 USHCN stations in Maine, while none show a statistically significant decrease.

Another question is, when it does rain, does it rain more or less? I compared the data before 1990 to that since 1990, and found that for Portland, the answer is yes: the mean amount when it does rain has increased from 0.33 in to 0.37 in.

Portland has seen an increase in the number of days with relatively heavy rain as well. I counted the number of days per year with at least 1 inch but not as much as 2, with at least 2 inches but not as much as 3, and with 3 inches or more of rain in a single day. It turns out, all three classes of day have gotten more frequent:


The solid lines are trend estimates using Poisson regression. The number of days per year in Portland with 3 inches or more of rain in one day, has increased from about 0.25 (only once per 4 years) to about 1.24 (more than once per year on average).

Portland isn’t the only Maine town to show such increase. Again, 8 of 12 USHCN stations show a significant increase in at least one of these size classes of rain. Particularly striking is the increases in Eastport:


I also wondered, is the increase of 1-inch rain days and 2-inch rain days and 3-inch rain days due only to the increase in the average rainfall per rainy day? To investigate, I took all the days on which some rain fell in Portland, no matter how much, then split it into the time spans before and after 1990. For each, I divided the daily rainfall by the mean daily rainfall for that time span (0.33 in. pre-1990, 0.37 in. post-1990) to define normalized rainfall. Then I computed the survival function (the probability of being that high or higher) for each time span (pre-1990 in black, post-1990 in red):


There seems to be a divergence, with daily rainfall above the 5-times-average value being more common after 1990 than before.

We’ve known for some time about the changes in California because its drought has been so much in the news. But I was a bit surprised how plain and clear are some of the changes in precipitation patterns on the opposite side of the country, in Maine. But they’re there. It’s a clear sign that the warming of Earth, its increase of water vapor in the atmosphere, and hence the change of rainfall patterns, is already upon us. Climate change is here already.

As for the impact on human life, changes in precipitation can bring some of the most profound. Increases, especially of heavy-rain days, can make flood more likely, sometimes far more likely. Decreases make drought more likely. Unfortunatly, it seems right now that the worst combination is happening, that the dry places are getting dryer and the wet places wetter. We need to be ready to deal with it, and we should do all we can to prevent its getting as bad as it will if we just keep burning fossil fuels as usual. Maybe we need a new saying: “When it rains it pours, when it dries it crumbles to dust.”

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25 responses to “The Rain in Maine

  1. Annual average precip in Holland rose about 30% over the past half century (25% inland up to 35% on coast). Number of days <50mm detonated (statistically spoken).

    Warm air holds more water, it has to drop somewhere. And it has to evaporate from somewhere – else, usually.

  2. Where does the rain in Maine, mainly rain?

    [Response: On the plain, of course.]

  3. I am a regular reader and greatly appreciate your blogs. I thought you might be interested in this:

  4. Even if the wet places just received the same amount of rain, if more of it falls as intense precipitation, that’s not a good thing. In Montana the NOAA Storm Atlas dates to 1973. Could that be a problem in engineering calculations for the 10/25 year storm events. Yes.

  5. Seem to be related:

    Click to access nclimate1828-aop.pdf

    “Trends in hourly rainfall statistics in the United States under a warming climate” (2013); DOI: 10.1038/NCLIMATE1828
    “Contribution of large-scale circulation anomalies to changes in extreme precipitation frequency in the United States” (2016); doi:10.1088/1748-9326/11/4/044003

  6. You really have to hand it to the Chinese, this hoax is just far more elaborate than we ever could have thought up ourselves.

  7. methane madness

    The other day in South Australia there was a storm that stripped fruit trees of a hundred million dollars worth of product. The Abom attributed it’s severity to increased atmospheric carbon dioxide.

    It would seem that there is an increase in the difference of temperature from day to day here in south australia. The variability normally associated with spring seems to have become more extreme, change is more rapid, larger temperature differences over shorter periods.

  8. methane madness

    The magnitude and frequency of variability seems to have increased is what I”m trying to say.

  9. Whachamacallit

    Very intriguing, if a bit confusing for one reason; one of my friends lives in Maine and had said that the state was experiencing the worst drought in 50 years. When I was there for a few years, I had definitely noticed that the springs seemed to lack rainfall, at least in flawed comparison to the springs in the Midwest. Is my friend’s claim inaccurate, or does your new saying regard when it doesn’t rain really play a part in this hearsay drought?

    • You can have more bursts of heavy rain at the same time as less total rain. We have had similar drought issues in Massachusetts (although I’m not sure if there was a measurable increase in heavy rain here), due to a dry winter and spring.

      On top of that, rain can only soak into the ground so fast. Heavy bursts of rain tend to result in water running across the surface to streams, storm drains etc. where it quickly flows away from where it is needed for irrigation towards the ocean/lakes, etc.

    • Paul-san, remember that drought is a consequence of both rainfall and soil evaporation. Rain can be up, but if evaporation is up more, you still get dryer soil.

  10. Thanks for another excellent post. This one especially interesting for those of us in Maine. I was surprised that increase in <2 inch rain was not greater. Used to be rare and seems like it happens about once a month during summer now, but that's just anecdotal impression, no data.
    Request: where appropriate, duplicate Y axis labels on the right side of chart.
    Maine Note: "Facing the Surge" 30 min. movie about coastal flooding in Norfolk Va., free showing at Husson Univ. in Bangor, 7pm, Wed., Nov. 30 with audience Q&A to follow. Sponsored by Citizens Climate Lobby.

  11. methane madness

    Yesterday in Melbourne, Australia two people died from Thunderstorm Asthma…. new to me too.. another fun fact for the increased extremity’s file.

  12. I suspect this would make an interesting global animation. Basically a map showing the frequency of extreme rainfall events over time. A bit tricky because a 2 inch day in Maine might be similar in frequency to a half inch day somewhere else. Maybe dots whose area is equal to the inverse probability of an event over some baseline period.

    It could turn out that this is the most damaging effect of global warming in the short term. You can hear the denialists now, “Why did noone warn us?”.

  13. here in the UK we have had our first floods of the Autumn / Winter season

    BBC radio were interviewing a representative from the insurance industry, she said that the yearly flooding events we get, are now are factored into the insurance models as normal weather events

    flooding IS the new normal – for the UK insurance industry

  14. Pujalte et al. have documented the great increase in erosion at the PETM attributed to more intense rainfall. Bump up the intensity of rainfall, and the runoff will scour topsoil away until a different stable slope and root layer can develop. Look up “tunnel gully” for one pattern that happens — a quick increase in intense rainfall can turn a stable swale that has carried runoff easily, into a rapidly eroding site with more intense rainfall.

  15. What’s the precipitation “event” in the first figure? Portland averages ~130 days/year with precip>=0.01 (the NWS official definition).

    The large increase in heaviest precip in the northeast was talked about in the National Climate Assessment (and preceding documents):

    • That’s for all days with measurable precip, liquid and frozen. Maybe the 40-50 days/yr on the figure is just for liquid precip? Still seems low.

  16. Timothy (likes zebras)

    When I was studying for my MSc a bit more than a decade ago we were asked to perform some elementary statistical analysis of the England and Wales Precipitation series, which is one of the longest rainfall records in existence.

    What I found was that there was a positive trend in winter total precipitation and a negative trend in summer total precipitation. This suggests that there could be an increased risk of both drought and flooding in the same place.

    (It’s a bit more complicated than that because dry winters are an important factor in creating drought conditions in England and Wales, and flooding is sensitive to the intensity of rainfall on shorter time periods than a season – though if it has been a wet season then less additional rainfall is required to create flooding because the ground will be waterlogged.)

    So I’m afraid the situation might be a bit worse than, “…the dry places are getting dryer and the wet places wetter.”

    Looking at the seasonal changes could be interesting.