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 2025-02-06.

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	18% 12% 4 8% 5% 4 4 3 5% 7% 30%	108586
	Night	18% 14% 5% 10% 5% 4 4 3 4 5% 27%	109646
Transparency	Day	54% 5% 12% 20% 8% 1	112125
	Night	59% 5% 10% 17% 8% 1	114271
Seeing	Day	26% 18% 30% 17% 7% 2	14799
	Night	26% 17% 28% 22% 7% 1	24087

By Month

Percentage Cloud Cover NE (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	14% 21% 11% 16% 6% 4 3 2 3 4 15%	3597
	11th to 20th	Day	12% 22% 8% 23% 8% 4 3 2 3 4 11%	3570
	21st to end	Day	14% 18% 9% 14% 9% 6% 4 3 4 5% 14%	4280
February	1st to 10th	Day	15% 20% 6% 13% 7% 5% 3 4 5% 5% 17%	1755
	11th to 20th	Day	15% 19% 5% 13% 8% 5% 4 2 3 5% 23%	1687
	21st to end	Day	17% 17% 7% 11% 6% 4 4 3 5% 5% 21%	1570
March	1st to 10th	Day	22% 15% 5% 9% 4 3 3 3 3 4 28%	3143
	11th to 20th	Day	13% 22% 5% 9% 5% 4 4 3 4 5% 27%	3480
	21st to end	Day	16% 10% 3 7% 5% 4 4 3 5% 7% 36%	4006
April	1st to 10th	Day	21% 14% 4 5% 4 3 4 3 4 5% 32%	2280
	11th to 20th	Day	22% 10% 2 6% 4 4 3 3 5% 6% 36%	2254
	21st to end	Day	23% 10% 3 6% 4 4 5% 3 5% 6% 32%	2331
May	1st to 10th	Day	24% 11% 3 7% 4 4 4 3 4 5% 31%	4426
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	4398
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 7% 8% 39%	4837
June	1st to 10th	Day	21% 8% 3 6% 4 4 5% 3 6% 7% 32%	2660
	11th to 20th	Day	18% 5% 2 6% 4 4 5% 4 6% 7% 37%	2660
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 5% 8% 34%	2660
July	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 7% 9% 41%	4440
	11th to 20th	Day	15% 6% 2 5% 4 4 4 4 6% 9% 42%	4440
	21st to end	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 40%	4814
August	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 37%	4243
	11th to 20th	Day	19% 6% 2 6% 4 4 5% 4 6% 8% 35%	3953
	21st to end	Day	18% 6% 2 6% 4 3 4 4 6% 9% 39%	4180
September	1st to 10th	Day	16% 5% 3 6% 4 4 5% 3 5% 8% 42%	2280
	11th to 20th	Day	15% 6% 2 5% 4 4 3 3 5% 8% 46%	2280
	21st to end	Day	19% 6% 3 6% 4 4 4 4 5% 8% 37%	2029
October	1st to 10th	Day	23% 10% 3 8% 3 4 3 3 6% 6% 32%	1900
	11th to 20th	Day	23% 11% 3 7% 5% 4 5% 4 5% 6% 28%	1900
	21st to end	Day	28% 10% 3 7% 4 4 5% 3 5% 6% 24%	1971
November	1st to 10th	Day	16% 13% 4 7% 6% 4 4 4 6% 7% 29%	1761
	11th to 20th	Day	16% 15% 4 9% 6% 5% 4 3 4 6% 27%	1655
	21st to end	Day	20% 22% 4 10% 5% 5% 5% 3 5% 5% 16%	1600
December	1st to 10th	Day	20% 24% 6% 12% 6% 5% 4 3 4 4 13%	2600
	11th to 20th	Day	20% 23% 7% 12% 5% 4 3 2 3 5% 15%	2986
	21st to end	Day	23% 18% 6% 12% 6% 4 4 3 3 4 18%	3960
January	1st to 10th	Night	14% 25% 9% 14% 6% 4 3 2 3 3 17%	6349
	11th to 20th	Night	11% 21% 12% 22% 8% 5% 4 3 3 2 9%	6315
	21st to end	Night	15% 19% 10% 16% 9% 6% 4 2 3 3 13%	6674
February	1st to 10th	Night	17% 18% 6% 15% 8% 4 4 3 4 4 18%	2536
	11th to 20th	Night	14% 17% 7% 12% 6% 5% 5% 3 4 4 23%	2378
	21st to end	Night	17% 16% 6% 10% 5% 5% 4 3 4 5% 25%	1884
March	1st to 10th	Night	20% 12% 4 8% 5% 5% 3 3 4 5% 31%	3695
	11th to 20th	Night	21% 20% 5% 9% 4 3 3 2 4 5% 24%	3479
	21st to end	Night	19% 11% 3 8% 5% 4 3 2 4 5% 35%	3612
April	1st to 10th	Night	23% 13% 3 7% 4 4 3 3 4 5% 31%	1900
	11th to 20th	Night	22% 10% 3 7% 4 4 4 3 5% 7% 32%	1875
	21st to end	Night	23% 10% 3 7% 4 4 3 4 5% 6% 33%	1863
May	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 5% 6% 28%	2514
	11th to 20th	Night	21% 11% 3 7% 4 3 4 3 5% 7% 33%	2495
	21st to end	Night	15% 7% 2 5% 4 3 4 4 6% 7% 41%	2738
June	1st to 10th	Night	18% 9% 2 7% 5% 4 5% 4 6% 6% 35%	1520
	11th to 20th	Night	17% 5% 2 5% 4 4 5% 3 6% 9% 40%	1520
	21st to end	Night	17% 7% 3 5% 6% 4 5% 4 6% 7% 35%	1520
July	1st to 10th	Night	16% 6% 2 6% 5% 4 5% 4 6% 8% 39%	2520
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 4 8% 42%	2520
	21st to end	Night	14% 7% 2 5% 4 3 5% 4 6% 7% 44%	2739
August	1st to 10th	Night	16% 8% 3 6% 4 3 4 3 6% 8% 39%	2683
	11th to 20th	Night	16% 7% 3 8% 5% 5% 5% 3 6% 8% 36%	2844
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	3476
September	1st to 10th	Night	16% 7% 2 6% 3 3 4 5% 5% 6% 43%	1900
	11th to 20th	Night	13% 6% 2 5% 4 3 4 2 5% 6% 51%	1900
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 7% 40%	2147
October	1st to 10th	Night	22% 8% 3 9% 4 4 4 3 4 5% 33%	2280
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	2280
	21st to end	Night	27% 12% 3 9% 5% 4 4 3 5% 4 24%	2627
November	1st to 10th	Night	18% 13% 4 7% 5% 4 4 4 5% 6% 31%	2537
	11th to 20th	Night	18% 13% 5% 8% 5% 4 4 3 5% 6% 29%	2742
	21st to end	Night	23% 19% 4 8% 5% 4 4 3 5% 5% 20%	2800
December	1st to 10th	Night	19% 26% 7% 13% 5% 4 4 3 3 4 11%	4580
	11th to 20th	Night	23% 22% 6% 12% 4 3 3 2 3 3 19%	5208
	21st to end	Night	25% 17% 5% 11% 5% 4 4 2 3 5% 17%	6996

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	75% 3 7% 11% 4	3598
	11th to 20th	Day	81% 3 5% 6% 4 1	3600
	21st to end	Day	75% 6% 8% 7% 5% 1	4280
February	1st to 10th	Day	72% 4 8% 12% 3 1	1755
	11th to 20th	Day	68% 3 8% 13% 7% 1	1696
	21st to end	Day	66% 6% 8% 13% 7%	1570
March	1st to 10th	Day	61% 3 10% 17% 8% 1	3146
	11th to 20th	Day	60% 5% 15% 12% 7% 1	3480
	21st to end	Day	49% 4 10% 24% 11% 2	4006
April	1st to 10th	Day	56% 4 8% 19% 11% 2	2280
	11th to 20th	Day	50% 4 8% 22% 13% 2	2254
	21st to end	Day	54% 4 10% 17% 11% 3	2331
May	1st to 10th	Day	54% 6% 10% 20% 9% 1	4440
	11th to 20th	Day	49% 8% 13% 21% 8% 1	4402
	21st to end	Day	41% 8% 13% 25% 12% 1	4837
June	1st to 10th	Day	51% 5% 13% 19% 11% 1	2660
	11th to 20th	Day	45% 6% 13% 25% 10% 1	2660
	21st to end	Day	47% 7% 15% 20% 10% 1	2660
July	1st to 10th	Day	40% 5% 14% 29% 12%	4370
	11th to 20th	Day	38% 6% 17% 31% 8%	4365
	21st to end	Day	40% 8% 15% 28% 9%	4661
August	1st to 10th	Day	46% 5% 15% 23% 10%	4229
	11th to 20th	Day	46% 7% 17% 23% 7%	3992
	21st to end	Day	41% 6% 15% 30% 7%	4180
September	1st to 10th	Day	42% 5% 13% 27% 12% 1	2280
	11th to 20th	Day	39% 5% 11% 29% 15%	2280
	21st to end	Day	46% 6% 14% 24% 10%	2029
October	1st to 10th	Day	48% 4 11% 28% 8%	3160
	11th to 20th	Day	49% 4 14% 22% 10%	3140
	21st to end	Day	51% 5% 15% 23% 6%	3222
November	1st to 10th	Day	55% 7% 15% 16% 7% 1	1761
	11th to 20th	Day	59% 6% 10% 15% 7% 3	1655
	21st to end	Day	72% 5% 10% 9% 4	1600
December	1st to 10th	Day	77% 6% 11% 6% 1	2600
	11th to 20th	Day	74% 5% 7% 12% 2	2986
	21st to end	Day	70% 5% 7% 14% 4	3960
January	1st to 10th	Night	76% 4 8% 9% 3	6349
	11th to 20th	Night	84% 4 4 6% 2	6360
	21st to end	Night	78% 4 7% 7% 5%	6674
February	1st to 10th	Night	73% 5% 8% 10% 4	2536
	11th to 20th	Night	66% 5% 8% 15% 6%	2380
	21st to end	Night	61% 6% 12% 15% 6%	1884
March	1st to 10th	Night	58% 4 10% 19% 9% 1	3705
	11th to 20th	Night	63% 6% 10% 15% 6%	3480
	21st to end	Night	53% 4 6% 23% 13% 2	3612
April	1st to 10th	Night	57% 3 9% 20% 9% 2	1900
	11th to 20th	Night	54% 5% 8% 19% 14% 1	1875
	21st to end	Night	52% 5% 10% 18% 13% 2	1863
May	1st to 10th	Night	56% 5% 10% 16% 10% 2	2520
	11th to 20th	Night	51% 6% 12% 21% 8% 2	2496
	21st to end	Night	42% 6% 12% 21% 19% 1	2739
June	1st to 10th	Night	48% 7% 12% 22% 9% 1	1520
	11th to 20th	Night	41% 5% 15% 20% 17% 2	1520
	21st to end	Night	46% 7% 13% 22% 11% 1	1520
July	1st to 10th	Night	43% 6% 12% 21% 17% 1	2480
	11th to 20th	Night	41% 5% 13% 31% 9%	2472
	21st to end	Night	38% 7% 13% 29% 13% 1	2651
August	1st to 10th	Night	43% 6% 14% 26% 10% 1	2673
	11th to 20th	Night	47% 7% 13% 26% 7%	2871
	21st to end	Night	36% 8% 13% 34% 9% 1	3476
September	1st to 10th	Night	39% 8% 13% 27% 11% 1	1900
	11th to 20th	Night	35% 4 13% 30% 16% 1	1900
	21st to end	Night	44% 6% 13% 25% 11% 1	2147
October	1st to 10th	Night	49% 5% 10% 24% 12% 1	3800
	11th to 20th	Night	48% 4 13% 22% 11% 1	3779
	21st to end	Night	57% 4 10% 23% 5% 1	4326
November	1st to 10th	Night	54% 6% 12% 17% 9% 2	2537
	11th to 20th	Night	57% 6% 10% 17% 7% 2	2742
	21st to end	Night	68% 7% 9% 12% 4	2800
December	1st to 10th	Night	79% 5% 8% 7% 1	4580
	11th to 20th	Night	74% 4 7% 11% 4	5208
	21st to end	Night	72% 5% 6% 10% 6% 1	6996

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	31% 27% 26% 13% 4	200
	11th to 20th	Day	32% 25% 28% 15% 1 1	200
	21st to end	Day	31% 26% 30% 11% 2	290
February	1st to 10th	Day	38% 28% 26% 7%	285
	11th to 20th	Day	34% 31% 29% 6%	294
	21st to end	Day	35% 27% 28% 9% 1	382
March	1st to 10th	Day	31% 21% 32% 12% 4	460
	11th to 20th	Day	32% 23% 32% 10% 3	460
	21st to end	Day	37% 21% 26% 10% 4 1	506
April	1st to 10th	Day	38% 21% 25% 12% 4	460
	11th to 20th	Day	36% 21% 21% 14% 4 3	452
	21st to end	Day	34% 21% 27% 12% 4 2	460
May	1st to 10th	Day	30% 20% 26% 15% 7% 1	463
	11th to 20th	Day	22% 14% 35% 21% 7% 1	487
	21st to end	Day	20% 15% 34% 18% 12% 1	529
June	1st to 10th	Day	27% 15% 32% 19% 6% 1	490
	11th to 20th	Day	21% 14% 34% 21% 7% 2	490
	21st to end	Day	25% 14% 35% 19% 6% 1	490
July	1st to 10th	Day	15% 12% 33% 26% 13% 2	485
	11th to 20th	Day	18% 10% 35% 25% 10% 3	484
	21st to end	Day	15% 10% 38% 27% 8% 1	532
August	1st to 10th	Day	16% 10% 32% 28% 10% 4	486
	11th to 20th	Day	17% 10% 27% 28% 13% 5%	479
	21st to end	Day	17% 12% 33% 22% 11% 5%	539
September	1st to 10th	Day	17% 14% 30% 22% 12% 4	490
	11th to 20th	Day	14% 11% 26% 21% 19% 9%	490
	21st to end	Day	18% 16% 28% 23% 12% 3	490
October	1st to 10th	Day	29% 16% 26% 17% 9% 3	490
	11th to 20th	Day	30% 21% 28% 14% 6% 1	490
	21st to end	Day	37% 20% 24% 16% 3	476
November	1st to 10th	Day	30% 21% 28% 14% 6%	395
	11th to 20th	Day	29% 21% 34% 10% 5% 1	255
	21st to end	Day	33% 32% 26% 9% 2	200
December	1st to 10th	Day	44% 17% 25% 13% 2	200
	11th to 20th	Day	37% 28% 29% 6% 1 1	200
	21st to end	Day	32% 22% 31% 11% 4	220
January	1st to 10th	Night	31% 26% 26% 15% 2	800
	11th to 20th	Night	28% 26% 27% 16% 3	800
	21st to end	Night	23% 23% 31% 19% 4	880
February	1st to 10th	Night	28% 27% 27% 16% 1	781
	11th to 20th	Night	27% 21% 34% 15% 2	743
	21st to end	Night	26% 25% 31% 16% 2	628
March	1st to 10th	Night	26% 20% 32% 19% 3	760
	11th to 20th	Night	31% 22% 26% 16% 3	760
	21st to end	Night	28% 17% 31% 19% 5%	836
April	1st to 10th	Night	32% 18% 29% 20% 2	760
	11th to 20th	Night	28% 18% 27% 21% 5%	751
	21st to end	Night	30% 14% 26% 22% 7% 1	723
May	1st to 10th	Night	26% 12% 27% 27% 7% 1	380
	11th to 20th	Night	18% 11% 36% 28% 6%	374
	21st to end	Night	15% 10% 32% 30% 12% 1	411
June	1st to 10th	Night	22% 9% 29% 32% 8%	380
	11th to 20th	Night	18% 8% 30% 31% 13%	380
	21st to end	Night	18% 10% 31% 33% 7%	380
July	1st to 10th	Night	12% 6% 26% 39% 15% 1	370
	11th to 20th	Night	14% 6% 27% 33% 18% 2	368
	21st to end	Night	13% 7% 30% 38% 11%	416
August	1st to 10th	Night	16% 6% 27% 37% 14% 1	477
	11th to 20th	Night	17% 6% 20% 35% 19% 3	597
	21st to end	Night	13% 8% 28% 33% 16% 1	836
September	1st to 10th	Night	18% 9% 26% 32% 14% 2	760
	11th to 20th	Night	16% 7% 24% 29% 20% 4	760
	21st to end	Night	18% 14% 23% 26% 17% 2	760
October	1st to 10th	Night	27% 14% 25% 18% 14% 1	760
	11th to 20th	Night	28% 16% 26% 22% 7% 1	760
	21st to end	Night	35% 17% 27% 19% 2	836
November	1st to 10th	Night	29% 17% 28% 19% 7%	780
	11th to 20th	Night	27% 19% 30% 16% 7% 1	800
	21st to end	Night	36% 22% 23% 15% 4	800
December	1st to 10th	Night	36% 24% 23% 14% 3	800
	11th to 20th	Night	32% 27% 27% 12% 2 1	800
	21st to end	Night	35% 23% 27% 14% 2	880

Page generated at 2025-02-03 03:05:50. Version ac07

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