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 2026-06-01.

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 9% 5% 4 4 3 5% 6% 30%	132440
	Night	18% 14% 5% 10% 5% 4 4 3 4 5% 27%	130693
Transparency	Day	55% 5% 11% 20% 8% 1	135931
	Night	60% 5% 9% 17% 8% 1	135289
Seeing	Day	27% 18% 30% 17% 7% 2	16517
	Night	26% 17% 28% 22% 7% 1	25733

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	15% 22% 10% 16% 6% 4 3 2 3 4 14%	4661
	11th to 20th	Day	11% 23% 9% 22% 9% 5% 3 2 3 3 10%	4650
	21st to end	Day	12% 17% 9% 14% 10% 7% 5% 3 4 5% 14%	5539
February	1st to 10th	Day	16% 19% 6% 13% 7% 5% 4 4 5% 5% 17%	1890
	11th to 20th	Day	15% 19% 5% 13% 8% 4 4 3 3 5% 22%	1871
	21st to end	Day	17% 18% 7% 11% 7% 4 4 3 5% 5% 20%	1730
March	1st to 10th	Day	20% 16% 6% 10% 4 3 4 2 3 5% 27%	4618
	11th to 20th	Day	13% 19% 5% 8% 5% 5% 4 3 5% 5% 29%	5090
	21st to end	Day	20% 11% 4 8% 5% 4 4 3 4 5% 33%	5867
April	1st to 10th	Day	21% 15% 4 6% 4 3 4 3 4 5% 31%	2520
	11th to 20th	Day	22% 10% 2 6% 3 4 3 3 5% 5% 35%	2494
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 6% 32%	2576
May	1st to 10th	Day	29% 12% 3 7% 4 4 4 3 4 5% 26%	6486
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 7% 31%	6458
	21st to end	Day	25% 9% 3 6% 4 4 4 3 5% 7% 31%	7103
June	1st to 10th	Day	21% 8% 3 6% 4 4 5% 3 6% 7% 33%	2800
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 6% 7% 36%	2800
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 5% 8% 35%	2800
July	1st to 10th	Day	14% 5% 2 6% 4 4 5% 4 7% 9% 39%	6360
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	6360
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 9% 45%	6926
August	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	4377
	11th to 20th	Day	19% 6% 3 6% 4 4 5% 4 6% 8% 35%	4080
	21st to end	Day	18% 6% 2 6% 4 3 4 4 6% 9% 39%	4312
September	1st to 10th	Day	15% 5% 3 6% 3 4 4 3 5% 8% 43%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 3 3 4 7% 47%	2400
	21st to end	Day	20% 6% 3 6% 4 4 4 4 5% 7% 37%	2135
October	1st to 10th	Day	22% 9% 3 7% 3 4 3 3 5% 6% 34%	2000
	11th to 20th	Day	23% 10% 3 8% 4 4 5% 4 5% 6% 28%	2000
	21st to end	Day	28% 10% 3 7% 4 4 5% 3 5% 6% 25%	2074
November	1st to 10th	Day	16% 14% 4 8% 5% 4 4 4 6% 7% 28%	1851
	11th to 20th	Day	15% 16% 4 10% 6% 5% 4 3 4 6% 27%	1738
	21st to end	Day	21% 22% 4 10% 6% 5% 4 3 5% 5% 17%	1680
December	1st to 10th	Day	20% 24% 6% 12% 6% 5% 4 3 4 4 13%	2680
	11th to 20th	Day	20% 23% 7% 12% 5% 4 3 2 3 5% 15%	3066
	21st to end	Day	22% 18% 6% 12% 6% 4 4 3 3 4 18%	4048
January	1st to 10th	Night	16% 24% 9% 14% 6% 4 4 2 3 3 15%	8234
	11th to 20th	Night	11% 23% 12% 23% 8% 5% 4 3 2 2 8%	8235
	21st to end	Night	13% 16% 8% 18% 9% 7% 5% 3 4 3 15%	8640
February	1st to 10th	Night	17% 17% 6% 15% 8% 5% 4 3 4 4 18%	2730
	11th to 20th	Night	15% 17% 7% 12% 6% 5% 4 3 4 5% 23%	2634
	21st to end	Night	16% 17% 7% 11% 6% 4 4 3 4 5% 24%	2076
March	1st to 10th	Night	19% 13% 4 10% 5% 5% 3 3 4 4 28%	5440
	11th to 20th	Night	17% 16% 4 8% 4 3 4 3 5% 7% 29%	5089
	21st to end	Night	24% 14% 4 8% 5% 4 3 2 3 4 28%	5293
April	1st to 10th	Night	23% 12% 3 8% 4 4 3 3 4 5% 30%	2100
	11th to 20th	Night	22% 11% 3 7% 4 3 4 3 4 6% 32%	2075
	21st to end	Night	22% 9% 2 7% 4 4 3 3 5% 6% 34%	2059
May	1st to 10th	Night	31% 11% 3 7% 4 4 3 3 5% 6% 23%	3674
	11th to 20th	Night	20% 10% 3 7% 4 4 4 3 5% 8% 33%	3655
	21st to end	Night	23% 10% 3 6% 4 3 4 3 5% 6% 33%	4014
June	1st to 10th	Night	18% 9% 2 7% 5% 4 5% 4 6% 6% 35%	1600
	11th to 20th	Night	17% 5% 2 5% 4 4 5% 4 6% 9% 39%	1600
	21st to end	Night	16% 7% 4 5% 6% 4 5% 4 6% 7% 36%	1600
July	1st to 10th	Night	15% 6% 2 6% 5% 4 5% 4 7% 9% 37%	3600
	11th to 20th	Night	15% 8% 2 6% 4 4 3 3 4 7% 43%	3600
	21st to end	Night	12% 6% 2 5% 4 3 4 4 6% 7% 47%	3926
August	1st to 10th	Night	15% 7% 3 6% 4 3 4 3 6% 8% 40%	2768
	11th to 20th	Night	16% 7% 3 8% 4 5% 5% 3 6% 7% 37%	2936
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	3586
September	1st to 10th	Night	15% 6% 2 6% 3 4 4 5% 5% 6% 43%	2000
	11th to 20th	Night	13% 6% 2 5% 4 3 4 2 5% 7% 51%	2000
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 7% 40%	2260
October	1st to 10th	Night	21% 8% 3 9% 4 4 4 3 4 5% 35%	2400
	11th to 20th	Night	21% 10% 3 8% 5% 4 5% 3 5% 6% 30%	2400
	21st to end	Night	27% 12% 3 8% 5% 4 4 3 5% 4 25%	2765
November	1st to 10th	Night	19% 14% 3 7% 4 4 4 3 5% 6% 32%	2667
	11th to 20th	Night	18% 14% 5% 8% 5% 4 4 3 5% 6% 29%	2879
	21st to end	Night	23% 19% 4 8% 5% 4 4 3 5% 5% 19%	2940
December	1st to 10th	Night	19% 26% 7% 13% 5% 4 4 3 3 4 11%	4720
	11th to 20th	Night	23% 22% 6% 13% 4 3 3 2 3 3 19%	5348
	21st to end	Night	25% 17% 5% 11% 5% 4 4 2 3 5% 17%	7150

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	76% 3 7% 10% 3	4662
	11th to 20th	Day	82% 3 4 5% 4 1	4680
	21st to end	Day	74% 5% 8% 8% 4 1	5539
February	1st to 10th	Day	71% 4 8% 12% 4 1	1890
	11th to 20th	Day	68% 3 8% 13% 6% 1	1880
	21st to end	Day	68% 6% 8% 12% 6%	1730
March	1st to 10th	Day	62% 3 9% 14% 9% 2	4621
	11th to 20th	Day	58% 4 16% 13% 8% 1	5090
	21st to end	Day	55% 3 8% 19% 14% 2	5867
April	1st to 10th	Day	58% 4 8% 18% 11% 2	2520
	11th to 20th	Day	52% 4 8% 21% 13% 2	2494
	21st to end	Day	54% 4 10% 18% 12% 3	2576
May	1st to 10th	Day	62% 5% 8% 17% 8% 1	6497
	11th to 20th	Day	52% 7% 11% 20% 10% 1	6462
	21st to end	Day	53% 6% 11% 21% 8% 1	7103
June	1st to 10th	Day	51% 5% 13% 19% 12% 1	2800
	11th to 20th	Day	45% 6% 13% 25% 10% 1	2800
	21st to end	Day	47% 6% 15% 20% 11% 1	2800
July	1st to 10th	Day	41% 5% 14% 29% 11%	6290
	11th to 20th	Day	39% 6% 15% 28% 12% 1	6285
	21st to end	Day	37% 7% 13% 35% 9%	6728
August	1st to 10th	Day	45% 5% 15% 24% 10%	4363
	11th to 20th	Day	46% 7% 16% 24% 7% 1	4119
	21st to end	Day	42% 6% 14% 30% 7%	4312
September	1st to 10th	Day	41% 5% 13% 28% 12% 1	2400
	11th to 20th	Day	38% 5% 11% 30% 15%	2400
	21st to end	Day	46% 6% 14% 24% 10%	2135
October	1st to 10th	Day	47% 4 11% 28% 9%	3260
	11th to 20th	Day	49% 4 14% 22% 10%	3240
	21st to end	Day	51% 5% 14% 22% 6% 1	3325
November	1st to 10th	Day	56% 6% 14% 16% 7% 1	1851
	11th to 20th	Day	59% 6% 11% 15% 7% 2	1738
	21st to end	Day	72% 5% 10% 8% 5%	1680
December	1st to 10th	Day	77% 6% 10% 6% 1	2680
	11th to 20th	Day	74% 5% 7% 11% 2	3066
	21st to end	Day	71% 5% 7% 13% 4	4048
January	1st to 10th	Night	77% 3 7% 9% 4	8234
	11th to 20th	Night	86% 3 3 5% 2	8280
	21st to end	Night	75% 4 8% 7% 6% 1	8640
February	1st to 10th	Night	73% 5% 8% 10% 4	2730
	11th to 20th	Night	66% 4 8% 15% 5%	2636
	21st to end	Night	63% 5% 11% 14% 6%	2076
March	1st to 10th	Night	62% 4 8% 15% 10% 2	5450
	11th to 20th	Night	55% 6% 14% 16% 6% 3	5090
	21st to end	Night	62% 3 4 19% 10% 1	5293
April	1st to 10th	Night	57% 3 9% 20% 9% 1	2100
	11th to 20th	Night	55% 5% 8% 19% 13% 1	2075
	21st to end	Night	51% 5% 9% 19% 14% 2	2059
May	1st to 10th	Night	63% 5% 8% 14% 8% 2	3680
	11th to 20th	Night	50% 6% 12% 20% 11% 1	3656
	21st to end	Night	53% 5% 10% 19% 13% 1	4015
June	1st to 10th	Night	48% 7% 12% 22% 10% 1	1600
	11th to 20th	Night	42% 6% 14% 20% 16% 2	1600
	21st to end	Night	45% 7% 14% 22% 12% 1	1600
July	1st to 10th	Night	42% 6% 15% 22% 15% 1	3560
	11th to 20th	Night	41% 5% 11% 28% 13% 1	3552
	21st to end	Night	36% 6% 11% 33% 13% 1	3809
August	1st to 10th	Night	43% 6% 14% 26% 10% 1	2758
	11th to 20th	Night	46% 7% 13% 26% 8%	2963
	21st to end	Night	36% 8% 13% 33% 9% 1	3586
September	1st to 10th	Night	39% 7% 13% 28% 11% 1	2000
	11th to 20th	Night	34% 5% 13% 32% 16% 1	2000
	21st to end	Night	44% 6% 12% 25% 12% 1	2260
October	1st to 10th	Night	48% 5% 10% 25% 12% 1	3920
	11th to 20th	Night	49% 4 13% 22% 11% 1	3899
	21st to end	Night	56% 4 10% 23% 6% 1	4464
November	1st to 10th	Night	54% 6% 12% 18% 9% 2	2667
	11th to 20th	Night	58% 6% 10% 17% 7% 2	2879
	21st to end	Night	68% 7% 9% 12% 3	2940
December	1st to 10th	Night	79% 5% 8% 7% 2	4720
	11th to 20th	Night	74% 4 7% 11% 4	5348
	21st to end	Night	72% 5% 6% 10% 6% 1	7150

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	32% 26% 25% 14% 3	220
	11th to 20th	Day	35% 24% 27% 13% 1	220
	21st to end	Day	31% 25% 30% 12% 2	319
February	1st to 10th	Day	37% 26% 28% 8% 1	330
	11th to 20th	Day	35% 30% 27% 8%	358
	21st to end	Day	34% 29% 28% 9% 1	446
March	1st to 10th	Day	32% 21% 31% 11% 3	540
	11th to 20th	Day	36% 22% 30% 9% 2	540
	21st to end	Day	40% 21% 25% 10% 4 1	594
April	1st to 10th	Day	40% 21% 24% 11% 3	540
	11th to 20th	Day	37% 21% 21% 14% 4 3	532
	21st to end	Day	34% 22% 26% 11% 4 2	540
May	1st to 10th	Day	33% 20% 26% 14% 6% 1	543
	11th to 20th	Day	24% 16% 35% 18% 6% 1	567
	21st to end	Day	24% 16% 33% 16% 10% 1	617
June	1st to 10th	Day	27% 16% 32% 18% 6% 1	530
	11th to 20th	Day	22% 15% 34% 21% 7% 2	530
	21st to end	Day	25% 13% 36% 19% 6% 1	530
July	1st to 10th	Day	15% 12% 35% 24% 12% 2	525
	11th to 20th	Day	18% 10% 35% 25% 9% 2	524
	21st to end	Day	15% 11% 39% 26% 8% 1	576
August	1st to 10th	Day	14% 10% 32% 29% 11% 4	526
	11th to 20th	Day	17% 10% 28% 27% 14% 4	519
	21st to end	Day	16% 13% 33% 21% 11% 4	583
September	1st to 10th	Day	17% 15% 29% 23% 12% 5%	530
	11th to 20th	Day	13% 10% 26% 24% 18% 9%	530
	21st to end	Day	18% 16% 28% 22% 12% 3	530
October	1st to 10th	Day	27% 15% 26% 19% 9% 4	530
	11th to 20th	Day	29% 22% 28% 14% 5% 1	530
	21st to end	Day	36% 20% 24% 17% 3	513
November	1st to 10th	Day	32% 21% 28% 13% 6%	425
	11th to 20th	Day	30% 22% 32% 10% 6% 1	278
	21st to end	Day	34% 32% 25% 8% 1	220
December	1st to 10th	Day	43% 18% 25% 12% 1	220
	11th to 20th	Day	37% 28% 29% 5%	220
	21st to end	Day	34% 22% 30% 10% 3	242
January	1st to 10th	Night	31% 26% 26% 15% 2	840
	11th to 20th	Night	29% 26% 27% 15% 3	840
	21st to end	Night	23% 24% 31% 18% 4	924
February	1st to 10th	Night	28% 27% 27% 17% 2	840
	11th to 20th	Night	28% 22% 33% 16% 2	823
	21st to end	Night	26% 24% 31% 17% 2	692
March	1st to 10th	Night	27% 21% 31% 18% 3	840
	11th to 20th	Night	32% 22% 27% 16% 3	840
	21st to end	Night	29% 19% 29% 18% 5%	924
April	1st to 10th	Night	32% 18% 29% 19% 2	840
	11th to 20th	Night	30% 18% 26% 20% 5%	831
	21st to end	Night	30% 15% 27% 22% 6% 1	799
May	1st to 10th	Night	29% 12% 26% 25% 7% 1	420
	11th to 20th	Night	19% 11% 36% 27% 6%	414
	21st to end	Night	17% 11% 33% 28% 11% 1	455
June	1st to 10th	Night	23% 9% 29% 32% 8%	400
	11th to 20th	Night	19% 9% 30% 30% 13%	400
	21st to end	Night	18% 10% 33% 32% 7%	400
July	1st to 10th	Night	13% 6% 26% 39% 15% 1	390
	11th to 20th	Night	14% 6% 27% 35% 17% 2	388
	21st to end	Night	13% 8% 29% 38% 11% 1	438
August	1st to 10th	Night	15% 5% 26% 38% 14% 1	502
	11th to 20th	Night	17% 6% 21% 35% 19% 3	629
	21st to end	Night	14% 9% 28% 33% 15% 1	880
September	1st to 10th	Night	17% 9% 26% 32% 15% 2	800
	11th to 20th	Night	15% 7% 24% 30% 21% 4	800
	21st to end	Night	18% 14% 24% 26% 17% 2	800
October	1st to 10th	Night	27% 14% 25% 19% 14% 1	800
	11th to 20th	Night	28% 16% 27% 21% 7% 1	800
	21st to end	Night	35% 16% 28% 20% 2	880
November	1st to 10th	Night	30% 17% 28% 19% 7%	820
	11th to 20th	Night	27% 19% 30% 17% 6% 1	840
	21st to end	Night	36% 22% 23% 15% 4	840
December	1st to 10th	Night	36% 24% 24% 14% 3	840
	11th to 20th	Night	33% 27% 27% 11% 2 1	840
	21st to end	Night	36% 23% 26% 13% 2	924

Page generated at 2026-05-29 01:53:29. Version ac07

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