Watertown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watertown clear sky chart from 2005-11-06 to 2024-02-10.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 8% 3 7% 6% 3 5% 5% 6% 7% 39%	78058
	Night	9% 8% 4 7% 6% 3 5% 5% 6% 7% 40%	80953
Transparency	Day	43% 6% 13% 24% 12% 1	77759
	Night	42% 6% 13% 24% 13% 2	80776
Seeing	Day	15% 10% 32% 28% 13% 2	14908
	Night	13% 9% 35% 32% 10% 1	25006

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 13% 4 10% 10% 4 8% 6% 7% 6% 21%	2320
	11th to 20th	Day	4 9% 6% 13% 9% 4 7% 10% 9% 9% 21%	2540
	21st to end	Day	6% 9% 6% 20% 9% 4 5% 6% 6% 7% 22%	3020
February	1st to 10th	Day	8% 10% 6% 10% 8% 5% 7% 4 6% 8% 29%	1701
	11th to 20th	Day	6% 9% 5% 11% 7% 6% 7% 6% 7% 7% 30%	1583
	21st to end	Day	9% 9% 6% 10% 8% 5% 5% 5% 5% 7% 29%	1480
March	1st to 10th	Day	12% 10% 5% 10% 7% 3 5% 4 5% 7% 31%	1876
	11th to 20th	Day	12% 10% 4 8% 7% 3 5% 5% 4 7% 34%	1980
	21st to end	Day	14% 13% 6% 8% 7% 3 4 4 6% 6% 27%	2178
April	1st to 10th	Day	17% 9% 3 6% 5% 2 4 4 5% 7% 39%	2078
	11th to 20th	Day	19% 12% 4 7% 6% 2 4 2 4 5% 34%	2309
	21st to end	Day	19% 9% 3 5% 5% 2 4 4 5% 6% 37%	2340
May	1st to 10th	Day	17% 8% 3 6% 5% 3 5% 4 4 7% 37%	2340
	11th to 20th	Day	14% 8% 3 6% 6% 3 5% 4 6% 7% 39%	2343
	21st to end	Day	16% 6% 3 5% 5% 3 5% 4 5% 8% 39%	2774
June	1st to 10th	Day	12% 5% 2 5% 5% 2 5% 6% 6% 7% 45%	2700
	11th to 20th	Day	12% 5% 3 5% 5% 3 5% 5% 5% 8% 43%	2700
	21st to end	Day	9% 5% 2 5% 4 3 4 6% 6% 8% 49%	2700
July	1st to 10th	Day	7% 3 1 4 4 2 5% 5% 6% 10% 54%	2700
	11th to 20th	Day	5% 3 2 3 4 2 4 5% 7% 8% 58%	2592
	21st to end	Day	6% 3 2 4 4 3 5% 6% 7% 9% 51%	2626
August	1st to 10th	Day	8% 5% 3 5% 6% 3 5% 6% 6% 8% 44%	2328
	11th to 20th	Day	11% 4 2 5% 5% 3 4 4 5% 8% 49%	2301
	21st to end	Day	9% 3 2 3 3 2 4 4 5% 7% 58%	2574
September	1st to 10th	Day	11% 5% 2 4 4 3 4 5% 5% 7% 52%	2126
	11th to 20th	Day	10% 6% 2 5% 4 3 4 5% 6% 8% 48%	1980
	21st to end	Day	14% 7% 2 4 3 1 3 3 5% 7% 51%	1980
October	1st to 10th	Day	15% 8% 3 5% 5% 2 3 4 5% 6% 45%	1862
	11th to 20th	Day	15% 7% 4 4 4 3 4 4 5% 6% 46%	1800
	21st to end	Day	16% 10% 2 5% 4 2 4 4 4 6% 43%	1852
November	1st to 10th	Day	13% 8% 3 7% 5% 2 3 4 6% 8% 41%	1673
	11th to 20th	Day	8% 10% 5% 10% 8% 4 4 6% 5% 7% 34%	1533
	21st to end	Day	10% 9% 3 8% 7% 3 5% 5% 6% 8% 37%	1520
December	1st to 10th	Day	7% 10% 4 9% 10% 6% 9% 6% 5% 6% 28%	1520
	11th to 20th	Day	8% 11% 6% 10% 8% 4 5% 5% 7% 8% 28%	1687
	21st to end	Day	18% 11% 4 12% 8% 3 3 5% 4 5% 27%	2442
January	1st to 10th	Night	7% 11% 5% 11% 9% 6% 7% 8% 7% 7% 22%	4632
	11th to 20th	Night	4 7% 4 11% 9% 5% 9% 10% 9% 10% 21%	4430
	21st to end	Night	5% 10% 7% 15% 7% 4 6% 6% 6% 6% 27%	4644
February	1st to 10th	Night	6% 10% 4 9% 7% 5% 6% 5% 6% 8% 33%	2464
	11th to 20th	Night	6% 9% 5% 7% 7% 4 6% 6% 7% 7% 36%	2273
	21st to end	Night	10% 9% 3 8% 8% 5% 5% 5% 5% 8% 34%	1783
March	1st to 10th	Night	10% 8% 5% 8% 6% 4 6% 5% 5% 8% 36%	2083
	11th to 20th	Night	12% 10% 4 7% 7% 4 5% 3 4 5% 40%	1980
	21st to end	Night	12% 9% 5% 7% 6% 4 5% 5% 7% 8% 33%	2178
April	1st to 10th	Night	13% 9% 3 6% 6% 3 4 4 6% 7% 40%	1876
	11th to 20th	Night	17% 8% 4 7% 5% 2 4 3 4 7% 38%	1599
	21st to end	Night	18% 8% 4 5% 6% 3 5% 4 4 7% 37%	1620
May	1st to 10th	Night	14% 6% 3 5% 4 3 4 4 5% 8% 43%	1620
	11th to 20th	Night	12% 7% 3 5% 4 3 4 4 6% 7% 45%	1550
	21st to end	Night	13% 5% 2 6% 5% 3 5% 5% 7% 9% 40%	1498
June	1st to 10th	Night	10% 4 1 4 3 2 4 4 5% 10% 52%	1260
	11th to 20th	Night	7% 3 3 4 4 3 3 4 8% 10% 52%	1260
	21st to end	Night	6% 4 1 4 3 2 3 4 5% 8% 60%	1260
July	1st to 10th	Night	4 2 2 3 4 2 4 5% 6% 10% 58%	1260
	11th to 20th	Night	4 2 1 2 2 2 3 5% 4 9% 66%	1364
	21st to end	Night	4 2 1 3 4 3 4 5% 5% 10% 59%	1691
August	1st to 10th	Night	8% 4 2 5% 5% 2 5% 5% 6% 9% 50%	1611
	11th to 20th	Night	9% 3 2 3 4 3 3 4 4 8% 59%	1600
	21st to end	Night	5% 2 1 3 3 2 3 4 5% 6% 65%	1782
September	1st to 10th	Night	9% 4 1 3 4 2 4 4 4 8% 58%	1831
	11th to 20th	Night	8% 6% 2 5% 4 2 4 3 6% 6% 54%	1980
	21st to end	Night	13% 5% 2 3 3 2 3 3 4 5% 56%	1980
October	1st to 10th	Night	13% 6% 3 5% 4 2 4 4 4 7% 48%	2079
	11th to 20th	Night	14% 8% 4 4 3 2 4 3 4 6% 49%	2160
	21st to end	Night	15% 8% 3 6% 4 2 3 4 4 8% 44%	2502
November	1st to 10th	Night	9% 8% 3 5% 5% 3 4 4 5% 7% 47%	2405
	11th to 20th	Night	8% 9% 4 9% 7% 3 4 5% 5% 7% 39%	2645
	21st to end	Night	9% 7% 4 7% 6% 4 5% 4 5% 7% 43%	2660
December	1st to 10th	Night	7% 10% 4 10% 9% 4 6% 5% 5% 7% 32%	2660
	11th to 20th	Night	9% 9% 4 9% 7% 4 5% 6% 6% 8% 32%	3508
	21st to end	Night	16% 12% 5% 12% 7% 3 4 4 5% 6% 25%	5225

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	60% 7% 11% 14% 7% 1	2321
	11th to 20th	Day	55% 6% 10% 22% 6% 1	2540
	21st to end	Day	59% 9% 10% 16% 6%	3020
February	1st to 10th	Day	54% 5% 12% 19% 10% 1	1701
	11th to 20th	Day	51% 6% 12% 20% 9% 2	1592
	21st to end	Day	54% 6% 15% 15% 9% 1	1480
March	1st to 10th	Day	53% 6% 14% 19% 7% 1	1876
	11th to 20th	Day	50% 6% 12% 23% 9%	1980
	21st to end	Day	57% 6% 14% 17% 5% 1	2178
April	1st to 10th	Day	45% 5% 12% 24% 12% 1	2078
	11th to 20th	Day	54% 5% 8% 20% 12% 1	2312
	21st to end	Day	49% 6% 11% 25% 9%	2340
May	1st to 10th	Day	48% 6% 11% 22% 12%	2340
	11th to 20th	Day	43% 6% 15% 24% 11% 1	2347
	21st to end	Day	44% 6% 14% 24% 11%	2774
June	1st to 10th	Day	36% 7% 16% 29% 11% 1	2700
	11th to 20th	Day	39% 8% 14% 25% 13% 2	2700
	21st to end	Day	31% 8% 17% 28% 14% 2	2700
July	1st to 10th	Day	26% 7% 17% 32% 16% 2	2626
	11th to 20th	Day	23% 7% 15% 30% 21% 5%	2513
	21st to end	Day	27% 8% 15% 29% 17% 3	2480
August	1st to 10th	Day	35% 9% 14% 26% 13% 4	2314
	11th to 20th	Day	33% 8% 14% 32% 12% 2	2301
	21st to end	Day	25% 7% 14% 33% 18% 4	2574
September	1st to 10th	Day	32% 7% 11% 33% 17% 1	2126
	11th to 20th	Day	34% 5% 17% 32% 12% 1	1980
	21st to end	Day	34% 5% 13% 31% 17% 1	1980
October	1st to 10th	Day	40% 5% 13% 27% 14% 1	1862
	11th to 20th	Day	38% 5% 11% 28% 15% 2	1800
	21st to end	Day	43% 5% 12% 26% 13% 1	1852
November	1st to 10th	Day	41% 6% 11% 24% 14% 3	1673
	11th to 20th	Day	49% 6% 14% 20% 10% 1	1533
	21st to end	Day	45% 4 16% 23% 10% 2	1520
December	1st to 10th	Day	55% 7% 13% 16% 8% 1	1520
	11th to 20th	Day	52% 6% 17% 19% 6%	1684
	21st to end	Day	60% 5% 9% 20% 7% 1	2442
January	1st to 10th	Night	56% 8% 12% 17% 8%	4632
	11th to 20th	Night	53% 6% 12% 18% 9% 1	4430
	21st to end	Night	52% 10% 12% 17% 8% 1	4644
February	1st to 10th	Night	48% 5% 13% 20% 12% 1	2464
	11th to 20th	Night	44% 7% 12% 22% 12% 3	2274
	21st to end	Night	48% 6% 10% 20% 12% 4	1783
March	1st to 10th	Night	46% 6% 15% 22% 9% 1	2083
	11th to 20th	Night	47% 4 10% 25% 13% 1	1980
	21st to end	Night	46% 7% 15% 23% 7% 2	2178
April	1st to 10th	Night	43% 5% 12% 27% 11% 1	1876
	11th to 20th	Night	47% 4 9% 23% 15% 2	1599
	21st to end	Night	48% 5% 10% 25% 11% 1	1620
May	1st to 10th	Night	39% 7% 13% 27% 14%	1620
	11th to 20th	Night	36% 9% 14% 25% 15% 1	1551
	21st to end	Night	39% 7% 16% 23% 14% 1	1499
June	1st to 10th	Night	29% 6% 17% 32% 15% 2	1260
	11th to 20th	Night	27% 7% 18% 28% 16% 3	1260
	21st to end	Night	23% 6% 17% 32% 20% 2	1260
July	1st to 10th	Night	21% 8% 16% 32% 19% 3	1225
	11th to 20th	Night	16% 7% 15% 33% 23% 6%	1321
	21st to end	Night	21% 7% 16% 32% 21% 5%	1597
August	1st to 10th	Night	30% 8% 15% 29% 14% 4	1601
	11th to 20th	Night	25% 6% 17% 36% 16% 1	1600
	21st to end	Night	19% 6% 13% 34% 23% 5%	1782
September	1st to 10th	Night	26% 6% 13% 30% 23% 2	1831
	11th to 20th	Night	30% 5% 15% 30% 19% 1	1980
	21st to end	Night	31% 4 11% 33% 20% 2	1980
October	1st to 10th	Night	37% 5% 12% 27% 18% 2	2079
	11th to 20th	Night	38% 3 11% 27% 17% 3	2160
	21st to end	Night	40% 5% 14% 25% 14% 1	2502
November	1st to 10th	Night	37% 5% 12% 23% 19% 3	2405
	11th to 20th	Night	44% 6% 14% 20% 14% 2	2645
	21st to end	Night	41% 5% 17% 24% 12% 1	2660
December	1st to 10th	Night	50% 5% 16% 19% 9%	2660
	11th to 20th	Night	47% 7% 19% 20% 6%	3510
	21st to end	Night	61% 4 8% 18% 7% 1	5225

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 11% 31% 29% 13% 1	270
	11th to 20th	Day	13% 13% 34% 32% 8% 1	270
	21st to end	Day	15% 11% 41% 19% 13% 1	297
February	1st to 10th	Day	10% 14% 44% 28% 3	267
	11th to 20th	Day	14% 15% 38% 24% 8%	240
	21st to end	Day	14% 15% 41% 26% 5%	198
March	1st to 10th	Day	19% 13% 40% 25% 1 1	240
	11th to 20th	Day	18% 8% 39% 20% 13% 3	240
	21st to end	Day	22% 13% 24% 25% 13% 3	264
April	1st to 10th	Day	26% 17% 31% 20% 6%	338
	11th to 20th	Day	25% 14% 31% 20% 8% 1	590
	21st to end	Day	22% 13% 33% 22% 9% 1	600
May	1st to 10th	Day	20% 10% 34% 25% 9% 1	603
	11th to 20th	Day	19% 12% 31% 26% 11% 2	625
	21st to end	Day	18% 9% 30% 29% 11% 2	679
June	1st to 10th	Day	9% 8% 27% 33% 20% 3	630
	11th to 20th	Day	13% 9% 36% 28% 11% 3	630
	21st to end	Day	12% 10% 31% 31% 14% 3	630
July	1st to 10th	Day	8% 4 27% 34% 21% 5%	621
	11th to 20th	Day	6% 6% 30% 39% 16% 3	618
	21st to end	Day	6% 4 27% 41% 19% 4	684
August	1st to 10th	Day	9% 3 25% 36% 23% 4	626
	11th to 20th	Day	13% 5% 24% 35% 18% 6%	615
	21st to end	Day	8% 5% 29% 33% 22% 4	693
September	1st to 10th	Day	13% 8% 34% 27% 15% 4	416
	11th to 20th	Day	14% 5% 36% 27% 16% 3	270
	21st to end	Day	22% 13% 29% 25% 10% 1	270
October	1st to 10th	Day	20% 9% 33% 24% 11% 4	270
	11th to 20th	Day	14% 18% 35% 25% 6% 1	270
	21st to end	Day	21% 22% 33% 17% 6% 1	297
November	1st to 10th	Day	23% 10% 33% 25% 9% 1	270
	11th to 20th	Day	13% 17% 44% 20% 5%	270
	21st to end	Day	14% 18% 37% 23% 7% 1	270
December	1st to 10th	Day	12% 9% 45% 26% 8%	270
	11th to 20th	Day	13% 13% 46% 21% 6%	270
	21st to end	Day	26% 10% 38% 20% 5% 1	297
January	1st to 10th	Night	11% 11% 41% 29% 6% 2	950
	11th to 20th	Night	12% 10% 40% 32% 5% 1	950
	21st to end	Night	14% 12% 40% 26% 7%	1045
February	1st to 10th	Night	12% 10% 42% 28% 7% 1	948
	11th to 20th	Night	11% 11% 41% 28% 8% 1	879
	21st to end	Night	13% 10% 35% 30% 10% 2	599
March	1st to 10th	Night	13% 10% 41% 30% 6%	643
	11th to 20th	Night	16% 10% 38% 30% 6%	540
	21st to end	Night	16% 9% 31% 31% 11% 2	594
April	1st to 10th	Night	19% 9% 37% 30% 6%	540
	11th to 20th	Night	21% 12% 29% 27% 10% 1	533
	21st to end	Night	20% 10% 32% 28% 9% 1	540
May	1st to 10th	Night	18% 9% 33% 29% 11%	540
	11th to 20th	Night	17% 8% 35% 30% 9% 1	531
	21st to end	Night	14% 8% 33% 33% 11% 1	584
June	1st to 10th	Night	12% 5% 24% 41% 18% 1	540
	11th to 20th	Night	9% 5% 37% 39% 10%	540
	21st to end	Night	8% 4 28% 45% 14% 1	540
July	1st to 10th	Night	5% 2 26% 46% 19% 2	525
	11th to 20th	Night	5% 3 30% 48% 14% 1	522
	21st to end	Night	4 2 23% 55% 15% 1	591
August	1st to 10th	Night	8% 4 21% 48% 18% 1	537
	11th to 20th	Night	10% 3 22% 40% 22% 3	534
	21st to end	Night	6% 3 29% 43% 18% 2	594
September	1st to 10th	Night	10% 4 31% 38% 14% 2	540
	11th to 20th	Night	10% 5% 32% 37% 13% 2	540
	21st to end	Night	14% 9% 35% 31% 9% 1	540
October	1st to 10th	Night	16% 9% 35% 29% 10%	639
	11th to 20th	Night	19% 11% 37% 25% 7% 1	720
	21st to end	Night	19% 12% 32% 30% 6%	918
November	1st to 10th	Night	12% 12% 38% 29% 8%	925
	11th to 20th	Night	13% 12% 38% 28% 9% 1	950
	21st to end	Night	12% 12% 39% 31% 6%	950
December	1st to 10th	Night	13% 13% 39% 27% 8% 1	950
	11th to 20th	Night	17% 9% 41% 27% 6% 1	950
	21st to end	Night	22% 11% 37% 24% 5% 1	1045

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