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 2026-07-16.

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 8% 5% 4 5% 4 6% 7% 41%	146767
	Night	9% 7% 3 8% 5% 5% 5% 4 6% 7% 41%	143314
Transparency	Day	42% 5% 13% 25% 13% 1	141434
	Night	41% 5% 13% 25% 14% 2	139658
Seeing	Day	15% 10% 33% 27% 12% 2	18366
	Night	13% 9% 35% 32% 9% 1	28244

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	7% 13% 4 15% 10% 7% 7% 5% 6% 6% 19%	4337
	11th to 20th	Day	3 8% 5% 15% 7% 6% 10% 8% 10% 7% 22%	4740
	21st to end	Day	3 6% 5% 16% 7% 6% 7% 5% 7% 7% 31%	5619
February	1st to 10th	Day	7% 12% 5% 12% 7% 7% 6% 4 5% 7% 28%	1890
	11th to 20th	Day	6% 9% 5% 11% 6% 6% 7% 4 6% 7% 32%	1855
	21st to end	Day	8% 10% 5% 11% 7% 6% 5% 4 5% 7% 32%	1730
March	1st to 10th	Day	10% 9% 3 7% 4 4 3 3 5% 8% 45%	4810
	11th to 20th	Day	8% 9% 3 7% 4 4 5% 4 5% 9% 42%	5090
	21st to end	Day	19% 13% 4 12% 6% 5% 4 3 5% 6% 23%	5585
April	1st to 10th	Day	17% 10% 3 6% 4 3 4 3 5% 7% 39%	2426
	11th to 20th	Day	17% 12% 4 8% 5% 4 4 3 4 5% 35%	2699
	21st to end	Day	19% 10% 3 6% 4 4 4 3 5% 6% 37%	2730
May	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 5% 7% 53%	7655
	11th to 20th	Day	18% 9% 2 4 3 3 3 3 4 5% 45%	7780
	21st to end	Day	17% 8% 3 6% 4 4 5% 4 6% 8% 36%	9226
June	1st to 10th	Day	11% 6% 2 6% 5% 3 5% 4 7% 7% 44%	3150
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 8% 42%	3150
	21st to end	Day	10% 6% 2 5% 3 4 5% 4 6% 7% 47%	3150
July	1st to 10th	Day	9% 4 1 5% 4 4 5% 4 6% 10% 47%	6949
	11th to 20th	Day	7% 4 2 4 3 4 4 3 6% 8% 56%	6599
	21st to end	Day	7% 5% 2 7% 4 5% 5% 4 8% 9% 44%	6608
August	1st to 10th	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 42%	5801
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	5821
	21st to end	Day	8% 4 2 5% 3 4 4 3 5% 7% 54%	6490
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 5% 7% 53%	2360
	11th to 20th	Day	10% 6% 2 6% 4 3 4 4 5% 8% 48%	2200
	21st to end	Day	13% 6% 2 4 2 2 3 3 5% 7% 54%	2200
October	1st to 10th	Day	10% 6% 2 4 3 3 3 3 5% 5% 58%	3367
	11th to 20th	Day	11% 7% 3 6% 4 4 4 3 4 5% 50%	3240
	21st to end	Day	14% 10% 3 6% 4 4 4 3 5% 7% 41%	3302
November	1st to 10th	Day	13% 9% 3 7% 5% 3 3 4 6% 8% 40%	1853
	11th to 20th	Day	7% 11% 4 11% 6% 5% 5% 4 5% 8% 34%	1693
	21st to end	Day	9% 10% 4 11% 6% 5% 4 4 5% 7% 35%	1680
December	1st to 10th	Day	6% 10% 6% 12% 8% 7% 6% 4 6% 7% 27%	2690
	11th to 20th	Day	7% 14% 7% 14% 7% 7% 7% 4 6% 6% 23%	2717
	21st to end	Day	14% 13% 4 16% 7% 4 4 3 5% 6% 24%	3575
January	1st to 10th	Night	5% 9% 4 15% 10% 9% 7% 6% 7% 7% 22%	8582
	11th to 20th	Night	3 8% 4 11% 7% 7% 11% 7% 9% 9% 25%	8250
	21st to end	Night	3 7% 5% 12% 5% 5% 6% 5% 7% 8% 37%	8592
February	1st to 10th	Night	6% 11% 4 10% 6% 7% 5% 5% 6% 8% 32%	2730
	11th to 20th	Night	6% 9% 4 9% 6% 5% 6% 4 7% 8% 37%	2663
	21st to end	Night	8% 8% 3 8% 7% 7% 6% 4 6% 8% 36%	2084
March	1st to 10th	Night	8% 7% 3 8% 5% 4 5% 3 5% 7% 45%	5273
	11th to 20th	Night	9% 8% 3 6% 4 4 4 3 6% 8% 44%	5090
	21st to end	Night	18% 11% 3 8% 6% 4 4 4 6% 7% 30%	5588
April	1st to 10th	Night	13% 10% 3 7% 5% 3 4 3 6% 7% 39%	2188
	11th to 20th	Night	15% 8% 4 8% 4 3 4 3 4 7% 39%	1869
	21st to end	Night	19% 8% 3 6% 5% 4 5% 3 4 6% 37%	1890
May	1st to 10th	Night	11% 4 1 4 3 3 3 3 5% 7% 56%	5292
	11th to 20th	Night	13% 8% 2 5% 2 3 3 3 4 6% 51%	5110
	21st to end	Night	15% 7% 2 6% 3 4 4 4 6% 8% 40%	4978
June	1st to 10th	Night	10% 4 1 4 3 3 4 4 5% 9% 52%	1470
	11th to 20th	Night	7% 5% 2 5% 4 3 4 3 8% 9% 50%	1470
	21st to end	Night	8% 4 1 4 3 3 4 4 6% 7% 57%	1470
July	1st to 10th	Night	9% 5% 2 4 3 3 4 3 6% 10% 50%	3250
	11th to 20th	Night	6% 3 1 3 3 3 3 3 4 8% 62%	3488
	21st to end	Night	6% 4 2 6% 4 4 5% 4 7% 11% 49%	4264
August	1st to 10th	Night	12% 8% 2 6% 4 4 4 4 6% 8% 42%	4011
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 50%	4028
	21st to end	Night	7% 3 2 4 3 3 4 4 6% 8% 56%	4487
September	1st to 10th	Night	8% 4 1 4 3 3 4 3 4 8% 58%	2037
	11th to 20th	Night	8% 6% 1 5% 3 4 4 2 6% 7% 53%	2200
	21st to end	Night	12% 5% 1 4 3 3 3 3 4 5% 58%	2200
October	1st to 10th	Night	7% 3 1 3 2 2 3 2 4 6% 65%	3779
	11th to 20th	Night	10% 6% 2 4 2 3 4 3 6% 7% 52%	3899
	21st to end	Night	12% 8% 3 6% 4 4 4 3 5% 7% 45%	4480
November	1st to 10th	Night	9% 9% 3 6% 4 4 5% 3 5% 6% 45%	2665
	11th to 20th	Night	8% 9% 4 10% 6% 4 3 4 5% 7% 39%	2924
	21st to end	Night	8% 8% 4 9% 6% 5% 4 3 5% 7% 41%	2940
December	1st to 10th	Night	4 8% 4 13% 6% 8% 6% 4 6% 7% 33%	4720
	11th to 20th	Night	8% 11% 4 12% 7% 7% 6% 4 6% 7% 29%	5708
	21st to end	Night	13% 14% 5% 14% 7% 6% 4 3 5% 5% 24%	7645

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	64% 5% 11% 12% 6% 2	4339
	11th to 20th	Day	56% 5% 11% 17% 11%	4740
	21st to end	Day	49% 6% 16% 18% 11%	5619
February	1st to 10th	Day	55% 4 12% 18% 10% 1	1890
	11th to 20th	Day	50% 5% 12% 22% 10% 1	1864
	21st to end	Day	52% 6% 15% 18% 9% 1	1730
March	1st to 10th	Day	38% 4 13% 31% 11% 2	4810
	11th to 20th	Day	40% 4 12% 29% 13% 1	5090
	21st to end	Day	62% 5% 11% 17% 5%	5585
April	1st to 10th	Day	46% 5% 12% 24% 12% 1	2426
	11th to 20th	Day	52% 5% 8% 21% 12% 1	2702
	21st to end	Day	49% 5% 11% 24% 10%	2730
May	1st to 10th	Day	30% 5% 9% 30% 25%	7655
	11th to 20th	Day	42% 4 7% 24% 16% 7%	7784
	21st to end	Day	45% 6% 10% 28% 12%	9227
June	1st to 10th	Day	36% 6% 16% 29% 11% 1	3150
	11th to 20th	Day	40% 7% 13% 25% 13% 2	3150
	21st to end	Day	33% 8% 16% 27% 15% 2	3150
July	1st to 10th	Day	32% 5% 21% 29% 12% 1	6876
	11th to 20th	Day	27% 5% 15% 35% 16% 2	6520
	21st to end	Day	35% 6% 18% 26% 13% 1	6419
August	1st to 10th	Day	36% 6% 11% 30% 15% 2	4220
	11th to 20th	Day	42% 6% 14% 30% 7% 1	4210
	21st to end	Day	32% 6% 17% 29% 14% 2	4674
September	1st to 10th	Day	30% 6% 12% 35% 16% 1	2360
	11th to 20th	Day	33% 5% 18% 32% 12% 1	2200
	21st to end	Day	31% 5% 12% 31% 19% 2	2200
October	1st to 10th	Day	29% 3 8% 30% 27% 2	3367
	11th to 20th	Day	37% 4 9% 28% 19% 3	3240
	21st to end	Day	44% 4 13% 23% 16% 1	3302
November	1st to 10th	Day	42% 6% 11% 25% 13% 3	1853
	11th to 20th	Day	48% 5% 14% 21% 10% 1	1693
	21st to end	Day	47% 4 15% 23% 10% 1	1680
December	1st to 10th	Day	55% 5% 15% 19% 5% 1	2690
	11th to 20th	Day	61% 6% 15% 14% 4	2714
	21st to end	Day	61% 4 10% 17% 8% 1	3575
January	1st to 10th	Night	58% 6% 13% 15% 7% 1	8583
	11th to 20th	Night	52% 4 12% 18% 12% 2	8250
	21st to end	Night	42% 6% 14% 24% 13%	8592
February	1st to 10th	Night	49% 5% 14% 19% 12% 1	2730
	11th to 20th	Night	43% 6% 11% 24% 12% 3	2664
	21st to end	Night	45% 5% 12% 21% 12% 4	2084
March	1st to 10th	Night	39% 4 10% 31% 14% 2	5273
	11th to 20th	Night	38% 4 12% 29% 15% 1	5090
	21st to end	Night	53% 5% 13% 23% 6% 1	5588
April	1st to 10th	Night	43% 5% 12% 27% 11% 1	2188
	11th to 20th	Night	45% 4 9% 25% 15% 2	1869
	21st to end	Night	48% 5% 10% 23% 13% 1	1890
May	1st to 10th	Night	28% 5% 9% 36% 20% 2	5293
	11th to 20th	Night	34% 5% 7% 22% 23% 8%	5111
	21st to end	Night	42% 5% 11% 31% 11% 1	4979
June	1st to 10th	Night	29% 6% 16% 32% 14% 3	1470
	11th to 20th	Night	29% 7% 17% 27% 17% 3	1470
	21st to end	Night	26% 6% 16% 31% 19% 2	1470
July	1st to 10th	Night	30% 5% 18% 30% 15% 1	3215
	11th to 20th	Night	22% 4 14% 42% 16% 2	3445
	21st to end	Night	30% 6% 21% 28% 14% 2	4140
August	1st to 10th	Night	34% 6% 15% 27% 15% 4	2916
	11th to 20th	Night	35% 5% 17% 30% 12% 1	2915
	21st to end	Night	28% 6% 17% 29% 18% 3	3234
September	1st to 10th	Night	25% 6% 13% 33% 21% 2	2037
	11th to 20th	Night	31% 5% 16% 30% 18% 1	2200
	21st to end	Night	29% 4 10% 34% 21% 3	2200
October	1st to 10th	Night	22% 3 7% 30% 35% 4	3779
	11th to 20th	Night	32% 3 13% 28% 22% 2	3899
	21st to end	Night	40% 4 13% 28% 15% 1	4480
November	1st to 10th	Night	38% 5% 12% 24% 18% 3	2665
	11th to 20th	Night	44% 5% 14% 21% 14% 2	2924
	21st to end	Night	43% 5% 16% 24% 11% 1	2940
December	1st to 10th	Night	50% 4 17% 21% 8%	4720
	11th to 20th	Night	53% 6% 16% 20% 5%	5710
	21st to end	Night	63% 4 9% 18% 6% 1	7645

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% 32% 30% 11% 2	330
	11th to 20th	Day	13% 15% 35% 28% 7% 1	330
	21st to end	Day	13% 13% 44% 17% 12% 1	363
February	1st to 10th	Day	12% 18% 43% 23% 4 1	330
	11th to 20th	Day	13% 13% 38% 25% 11%	330
	21st to end	Day	14% 15% 41% 26% 4	273
March	1st to 10th	Day	18% 13% 41% 25% 2 1	330
	11th to 20th	Day	19% 10% 39% 20% 10% 2	330
	21st to end	Day	22% 16% 28% 23% 9% 2	363
April	1st to 10th	Day	28% 16% 30% 21% 5%	446
	11th to 20th	Day	24% 16% 31% 20% 7% 1	740
	21st to end	Day	24% 13% 33% 21% 8% 1	750
May	1st to 10th	Day	18% 11% 34% 25% 10% 2	753
	11th to 20th	Day	21% 12% 29% 26% 11% 2	775
	21st to end	Day	19% 9% 30% 28% 11% 2	844
June	1st to 10th	Day	10% 9% 29% 31% 18% 3	780
	11th to 20th	Day	16% 11% 35% 26% 10% 2	780
	21st to end	Day	13% 10% 33% 28% 13% 2	780
July	1st to 10th	Day	8% 3 28% 34% 21% 6%	771
	11th to 20th	Day	6% 6% 29% 40% 16% 3	744
	21st to end	Day	6% 4 28% 41% 18% 4	794
August	1st to 10th	Day	10% 4 26% 35% 21% 3	726
	11th to 20th	Day	14% 5% 26% 33% 17% 5%	715
	21st to end	Day	8% 5% 29% 34% 21% 3	803
September	1st to 10th	Day	12% 7% 34% 27% 15% 4	490
	11th to 20th	Day	14% 7% 37% 25% 15% 2	330
	21st to end	Day	19% 12% 26% 28% 11% 4	330
October	1st to 10th	Day	17% 9% 33% 24% 12% 4	330
	11th to 20th	Day	16% 18% 36% 24% 5% 1	330
	21st to end	Day	21% 20% 35% 17% 6% 1	363
November	1st to 10th	Day	22% 12% 34% 24% 8% 1	330
	11th to 20th	Day	15% 16% 44% 20% 5%	330
	21st to end	Day	13% 18% 39% 22% 7% 1	330
December	1st to 10th	Day	13% 10% 44% 25% 7%	330
	11th to 20th	Day	15% 16% 44% 20% 5%	330
	21st to end	Day	23% 10% 41% 20% 6% 1	363
January	1st to 10th	Night	10% 10% 41% 30% 6% 2	1050
	11th to 20th	Night	11% 11% 41% 31% 5% 1	1050
	21st to end	Night	13% 13% 41% 26% 7%	1155
February	1st to 10th	Night	12% 11% 41% 28% 7% 1	1050
	11th to 20th	Night	10% 11% 41% 29% 7% 1	1029
	21st to end	Night	13% 11% 37% 30% 9% 1	700
March	1st to 10th	Night	12% 11% 40% 30% 6%	749
	11th to 20th	Night	16% 12% 38% 28% 6%	630
	21st to end	Night	17% 10% 34% 29% 10% 1	693
April	1st to 10th	Night	19% 10% 38% 29% 5%	630
	11th to 20th	Night	20% 12% 30% 28% 9% 1	623
	21st to end	Night	21% 11% 31% 28% 8% 1	630
May	1st to 10th	Night	17% 9% 33% 30% 12%	630
	11th to 20th	Night	16% 9% 34% 30% 9% 1	621
	21st to end	Night	14% 8% 33% 34% 11% 1	683
June	1st to 10th	Night	12% 6% 25% 40% 16%	630
	11th to 20th	Night	10% 6% 39% 36% 9%	630
	21st to end	Night	9% 5% 30% 42% 13%	630
July	1st to 10th	Night	6% 3 25% 45% 20% 2	615
	11th to 20th	Night	5% 3 29% 47% 15% 1	597
	21st to end	Night	4 3 24% 53% 15% 1	657
August	1st to 10th	Night	9% 4 22% 46% 17% 1	597
	11th to 20th	Night	11% 3 23% 39% 22% 3	594
	21st to end	Night	6% 3 29% 42% 18% 2	660
September	1st to 10th	Night	10% 4 30% 39% 14% 2	600
	11th to 20th	Night	11% 6% 32% 37% 12% 3	600
	21st to end	Night	13% 9% 34% 31% 13% 1	600
October	1st to 10th	Night	14% 9% 36% 29% 11%	711
	11th to 20th	Night	19% 11% 37% 26% 7% 1	800
	21st to end	Night	19% 13% 34% 29% 6%	1020
November	1st to 10th	Night	13% 12% 39% 29% 8%	1025
	11th to 20th	Night	13% 12% 38% 28% 8% 1	1050
	21st to end	Night	12% 12% 38% 31% 6%	1050
December	1st to 10th	Night	12% 13% 40% 27% 8% 1	1050
	11th to 20th	Night	17% 10% 41% 25% 5% 1	1050
	21st to end	Night	21% 12% 39% 23% 5% 1	1155

Page generated at 2026-07-13 02:14:54. Version ac07

Copyright © 2026 Attilla Danko.