mon, 19-nov-2012, 19:55
Footprints frozen in the Creek

Footprints frozen in the Creek

Reading today’s weather discussion from the Weather Service, they state:

LITTLE OR NO SNOWFALL IS EXPECTED IN THE FORECAST AREA FOR THE NEXT WEEK.

The last time it snowed was November 11th, so if that does happen, it will be at least 15 days without snow. That seems unusual for Fairbanks, so I checked it out.

Finding the lengths of consecutive events is something I’ve wondered how to do in SQL for some time, but it’s not all that difficult if you can get a listing of just the dates where the event (snowfall) happens. Once you’ve got that listing, you can use window functions to calculate the intervals between dates (rows), eliminate those that don’t matter, and rank them.

For this exercise, I’m looking for days with more than 0.1 inches of snow where the maximum temperature was below 10°C. And I exclude any interval where the end date is after March. Without this exclusion I’d get a bunch of really long intervals between the last snowfall of the year, and the first from the next year.

Here’s the SQL (somewhat simplified), using the GHCN-Daily database for the Fairbanks airport station:

SELECT * FROM (
    SELECT dte AS start,
        LEAD(dte) OVER (ORDER BY dte) AS end,
        LEAD(dte) OVER (ORDER BY dte) - dte AS interv
    FROM (
        SELECT dte
        FROM ghcnd_obs
        WHERE station_id = 'USW00026411'
            AND tmax < 10.0
            AND snow > 0
    ) AS foo
) AS bar
WHERE extract(month from foo.end) < 4
    AND interv > 6
ORDER BY interv DESC;

Here’s the top-8 longest periods:

Start End Days
1952‑12‑01 1953‑01‑19 49
1978‑02‑08 1978‑03‑16 36
1968‑02‑23 1968‑03‑28 34
1969‑11‑30 1970‑01‑02 33
1959‑01‑02 1959‑02‑02 31
1979‑02‑01 1979‑03‑03 30
2011‑02‑26 2011‑03‑27 29
1950‑02‑02 1950‑03‑03 29

Kinda scary that there have been periods where no snow fell for more than a month!

Here’s how many times various snow-free periods longer than a week have come since 1948:

Days Count
7 39
10 32
9 30
8 23
12 17
11 17
13 12
18 10
15 8
14 8

We can add one more to the 8-day category as of midnight tonight.

tags: Fairbanks  snow  SQL  weather 
wed, 14-nov-2012, 05:29
Early-season ski

Early-season ski from work

Yesterday a co-worker and I were talking about how we weren’t able to enjoy the new snow because the weather had turned cold as soon as the snow stopped falling. Along the way, she mentioned that it seemed to her that the really cold winter weather was coming later and later each year. She mentioned years past when it was bitter cold by Halloween.

The first question to ask before trying to determine if there has been a change in the date of the first cold snap is what qualifies as “cold.” My officemate said that she and her friends had a contest to guess the first date when the temperature didn’t rise above -20°F. So I started there, looking for the month and day of the winter when the maximum daily temperature was below -20°F.

I’m using the GHCN-Daily dataset from NCDC, which includes daily minimum and maximum temperatures, along with other variables collected at each station in the database.

When I brought in the data for the Fairbanks Airport, which has data available from 1948 to the present, there was absolutely no relationship between the first -20°F or colder daily maximum and year.

However, when I changed the definition of “cold” to the first date when the daily minimum temperature is below -40, I got a weak (but not statistically significant) positive trend between date and year.

The SQL query looks like this:

SELECT year, water_year, water_doy, mmdd, temp
FROM (
    SELECT year, water_year, water_doy, mmdd, temp,
        row_number() OVER (PARTITION BY water_year ORDER BY water_doy) AS rank
    FROM (
        SELECT extract(year from dte) AS year,
            extract(year from dte + interval '92 days') AS water_year,
            extract(doy from dte + interval '92 days') AS water_doy,
            to_char(dte, 'mm-dd') AS mmdd,
            sum(CASE WHEN variable = 'TMIN'
                     THEN raw_value * raw_multiplier
                     ELSE NULL END
               ) AS temp
        FROM ghcnd_obs
            INNER JOIN ghcnd_variables USING(variable)
        WHERE station_id = 'USW00026411'
        GROUP BY extract(year from dte),
            extract(year from dte + interval '92 days'),
            extract(doy from dte + interval '92 days'),
            to_char(dte, 'mm-dd')
        ORDER BY water_year, water_doy
    ) AS foo
    WHERE temp < -40 AND temp > -80
) AS bar
WHERE rank = 1
ORDER BY water_year;

I used “water year” instead of the actual year because the winter is split between two years. The water year starts on October 1st (we’re in the 2013 water year right now, for example), which converts a split winter (winter of 2012/2013) into a single year (2013, in this case). To get the water year, you add 92 days (the sum of the days in October, November and December) to the date and use that as the year.

Here’s what it looks like (click on the image to view a PDF version):

The dots are the observed date of first -40° daily minimum temperature for each water year, and the blue line shows a linear regression model fitted to the data (with 95% confidence intervals in grey). Despite the scatter, you can see a slightly positive slope, which would indicate that colder temperatures in Fairbanks are coming later now, than they were in the past.

As mentioned, however, our eyes often deceive us, so we need to look at the regression model to see if the visible relationship is significant. Here’s the R lm results:

Call:
lm(formula = water_doy ~ water_year, data = first_cold)

Residuals:
    Min      1Q  Median      3Q     Max
-45.264 -15.147  -1.409  13.387  70.282

Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept) -365.3713   330.4598  -1.106    0.274
water_year     0.2270     0.1669   1.360    0.180

Residual standard error: 23.7 on 54 degrees of freedom
Multiple R-squared: 0.0331,     Adjusted R-squared: 0.01519
F-statistic: 1.848 on 1 and 54 DF,  p-value: 0.1796

The first thing to check in the model summary is the p-value for the entire model on the last line of the results. It’s only 0.1796, which means that there’s an 18% chance of getting these results simply by chance. Typically, we’d like this to be below 5% before we’d consider the model to be valid.

You’ll also notice that the coefficient of the independent variable (water_year) is positive (0.2270), which means the model predicts that the earliest cold snap is 0.2 days later every year, but that this value is not significantly different from zero (a p-value of 0.180).

Still, this seems like a relationship worth watching and investigating further. It might be interesting to look at other definitions of “cold,” such as requiring three (or more) consecutive days of -40° temperatures before including that period as the earliest cold snap. I have a sense that this might reduce the year to year variation in the date seen with the definition used here.

tags: Fairbanks  R  SQL  temperature  weather 
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