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 2025-03-12.

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% 40%	112407
	Night	9% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	115355
Transparency	Day	43% 6% 13% 25% 12% 1	112147
	Night	42% 5% 13% 24% 14% 2	115206
Seeing	Day	15% 10% 33% 28% 12% 2	16395
	Night	13% 9% 35% 32% 9% 1	26501

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	8% 13% 3 14% 11% 8% 8% 5% 6% 6% 18%	3336
	11th to 20th	Day	4 8% 5% 14% 7% 6% 9% 8% 11% 8% 20%	3640
	21st to end	Day	4 7% 6% 19% 7% 6% 5% 4 6% 6% 30%	4334
February	1st to 10th	Day	7% 13% 5% 11% 7% 7% 5% 4 6% 7% 28%	1800
	11th to 20th	Day	6% 9% 5% 11% 6% 6% 7% 4 6% 7% 31%	1764
	21st to end	Day	8% 10% 5% 11% 7% 6% 5% 4 5% 7% 32%	1650
March	1st to 10th	Day	10% 12% 3 9% 4 4 4 3 5% 7% 38%	3376
	11th to 20th	Day	8% 10% 3 8% 5% 4 4 4 5% 8% 40%	3491
	21st to end	Day	21% 15% 5% 12% 6% 4 3 3 4 5% 22%	3814
April	1st to 10th	Day	16% 10% 3 7% 4 3 4 3 5% 7% 39%	2194
	11th to 20th	Day	18% 12% 4 9% 5% 4 3 2 4 5% 34%	2439
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 6% 36%	2470
May	1st to 10th	Day	16% 7% 2 6% 4 4 5% 4 5% 7% 40%	4120
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 46%	4165
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 6% 9% 38%	4925
June	1st to 10th	Day	12% 5% 2 6% 4 3 5% 4 7% 7% 45%	2850
	11th to 20th	Day	13% 6% 3 6% 5% 4 4 4 6% 8% 42%	2850
	21st to end	Day	10% 5% 2 5% 3 4 5% 4 6% 8% 48%	2850
July	1st to 10th	Day	12% 4 1 5% 4 5% 5% 5% 6% 10% 44%	4750
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 8% 60%	4558
	21st to end	Day	5% 4 2 6% 5% 5% 6% 4 7% 8% 48%	4598
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 3 5% 7% 47%	4103
	11th to 20th	Day	14% 7% 2 7% 4 4 5% 4 6% 8% 40%	4041
	21st to end	Day	10% 4 2 5% 3 4 4 3 5% 7% 52%	4532
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 5% 7% 53%	2243
	11th to 20th	Day	10% 6% 2 5% 4 3 4 4 5% 8% 50%	2090
	21st to end	Day	13% 7% 2 4 2 2 3 3 5% 7% 53%	2090
October	1st to 10th	Day	10% 6% 2 4 3 3 3 3 4 5% 57%	3264
	11th to 20th	Day	11% 7% 3 6% 4 4 4 3 4 5% 50%	3140
	21st to end	Day	14% 10% 2 6% 4 4 4 3 5% 7% 41%	3199
November	1st to 10th	Day	13% 9% 3 7% 4 3 3 4 6% 8% 39%	1763
	11th to 20th	Day	8% 11% 4 11% 6% 5% 5% 4 5% 7% 33%	1613
	21st to end	Day	9% 9% 4 10% 6% 5% 4 4 6% 8% 36%	1600
December	1st to 10th	Day	6% 10% 6% 12% 8% 8% 6% 4 6% 7% 28%	2610
	11th to 20th	Day	7% 14% 7% 14% 7% 7% 7% 4 6% 6% 22%	2647
	21st to end	Day	14% 13% 4 16% 7% 4 4 3 5% 6% 24%	3498
January	1st to 10th	Night	5% 10% 4 15% 10% 9% 8% 6% 7% 7% 20%	6626
	11th to 20th	Night	3 6% 4 11% 8% 7% 11% 8% 10% 9% 24%	6340
	21st to end	Night	4 9% 6% 14% 5% 5% 6% 4 6% 6% 35%	6641
February	1st to 10th	Night	6% 11% 4 10% 6% 7% 5% 5% 6% 8% 32%	2600
	11th to 20th	Night	5% 9% 4 9% 6% 5% 6% 4 7% 8% 37%	2533
	21st to end	Night	9% 9% 3 8% 7% 6% 5% 4 6% 8% 36%	1988
March	1st to 10th	Night	7% 7% 4 8% 5% 5% 5% 3 5% 7% 43%	3702
	11th to 20th	Night	8% 8% 3 7% 5% 5% 5% 3 5% 7% 44%	3497
	21st to end	Night	21% 13% 4 8% 7% 4 4 3 6% 6% 25%	3817
April	1st to 10th	Night	13% 10% 3 7% 5% 3 4 3 6% 7% 39%	1980
	11th to 20th	Night	17% 8% 4 8% 4 3 4 3 4 7% 39%	1689
	21st to end	Night	18% 8% 3 6% 5% 4 5% 3 4 7% 37%	1710
May	1st to 10th	Night	18% 6% 2 4 3 3 3 3 5% 7% 47%	2850
	11th to 20th	Night	9% 6% 2 6% 3 4 4 4 6% 7% 49%	2738
	21st to end	Night	14% 6% 2 6% 4 4 4 4 7% 8% 42%	2658
June	1st to 10th	Night	10% 4 1 4 3 3 4 4 5% 10% 52%	1330
	11th to 20th	Night	7% 5% 2 4 4 3 4 3 8% 8% 51%	1330
	21st to end	Night	7% 4 1 4 2 3 3 3 5% 7% 59%	1330
July	1st to 10th	Night	10% 6% 2 5% 3 3 5% 4 7% 10% 45%	2220
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 7% 70%	2408
	21st to end	Night	4 2 1 5% 4 4 5% 4 7% 11% 54%	2964
August	1st to 10th	Night	8% 7% 2 5% 4 3 4 3 5% 8% 50%	2836
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	2798
	21st to end	Night	8% 3 2 4 3 3 4 3 6% 8% 54%	3135
September	1st to 10th	Night	8% 4 1 3 3 3 4 3 4 7% 59%	1934
	11th to 20th	Night	8% 7% 1 5% 3 3 4 2 6% 7% 54%	2090
	21st to end	Night	12% 5% 1 4 2 3 3 3 4 5% 57%	2090
October	1st to 10th	Night	8% 4 2 3 2 2 3 2 4 6% 65%	3663
	11th to 20th	Night	10% 6% 2 4 2 3 4 3 7% 7% 53%	3779
	21st to end	Night	12% 8% 3 6% 4 3 4 3 5% 7% 45%	4341
November	1st to 10th	Night	9% 9% 2 6% 4 4 5% 3 5% 6% 46%	2535
	11th to 20th	Night	8% 9% 4 10% 6% 4 3 4 5% 7% 40%	2785
	21st to end	Night	8% 8% 4 9% 6% 5% 4 3 5% 7% 42%	2800
December	1st to 10th	Night	4 8% 4 13% 6% 8% 6% 4 6% 7% 33%	4580
	11th to 20th	Night	8% 11% 4 12% 7% 8% 6% 4 6% 7% 29%	5558
	21st to end	Night	13% 14% 5% 14% 7% 6% 4 3 5% 5% 24%	7480

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	65% 6% 11% 12% 6% 1	3337
	11th to 20th	Day	55% 5% 11% 17% 11%	3640
	21st to end	Day	53% 7% 16% 14% 9%	4334
February	1st to 10th	Day	55% 4 12% 18% 10% 1	1800
	11th to 20th	Day	50% 5% 12% 20% 10% 1	1773
	21st to end	Day	52% 6% 15% 17% 9% 1	1650
March	1st to 10th	Day	45% 5% 12% 28% 9% 1	3376
	11th to 20th	Day	42% 5% 12% 27% 11% 2	3491
	21st to end	Day	65% 5% 11% 13% 5% 1	3814
April	1st to 10th	Day	45% 5% 12% 25% 12% 1	2194
	11th to 20th	Day	53% 5% 8% 20% 12% 1	2442
	21st to end	Day	49% 6% 11% 24% 9%	2470
May	1st to 10th	Day	42% 7% 9% 26% 16%	4120
	11th to 20th	Day	37% 6% 11% 30% 13% 3	4169
	21st to end	Day	42% 7% 12% 26% 12%	4925
June	1st to 10th	Day	36% 7% 16% 29% 12% 1	2850
	11th to 20th	Day	40% 7% 14% 24% 13% 2	2850
	21st to end	Day	32% 8% 16% 28% 14% 2	2850
July	1st to 10th	Day	36% 6% 19% 26% 12% 1	4676
	11th to 20th	Day	21% 6% 15% 36% 19% 3	4479
	21st to end	Day	32% 7% 15% 28% 16% 2	4452
August	1st to 10th	Day	36% 6% 11% 30% 15% 2	4090
	11th to 20th	Day	42% 6% 14% 30% 7% 1	4080
	21st to end	Day	32% 6% 17% 29% 14% 2	4531
September	1st to 10th	Day	31% 6% 11% 35% 17% 1	2243
	11th to 20th	Day	33% 4 17% 33% 12% 1	2090
	21st to end	Day	32% 5% 12% 31% 18% 2	2090
October	1st to 10th	Day	30% 3 8% 30% 27% 2	3264
	11th to 20th	Day	37% 4 9% 28% 19% 3	3140
	21st to end	Day	43% 4 14% 23% 16% 1	3199
November	1st to 10th	Day	42% 6% 11% 25% 13% 3	1763
	11th to 20th	Day	49% 5% 14% 21% 10% 1	1613
	21st to end	Day	46% 4 15% 23% 10% 2	1600
December	1st to 10th	Day	54% 5% 16% 19% 6% 1	2610
	11th to 20th	Day	61% 6% 15% 14% 4	2644
	21st to end	Day	61% 4 9% 17% 8% 1	3498
January	1st to 10th	Night	60% 6% 10% 16% 7%	6626
	11th to 20th	Night	51% 5% 14% 16% 12% 2	6340
	21st to end	Night	46% 7% 14% 21% 11%	6641
February	1st to 10th	Night	48% 5% 13% 19% 13% 2	2600
	11th to 20th	Night	43% 7% 12% 23% 12% 3	2534
	21st to end	Night	45% 6% 12% 21% 13% 4	1988
March	1st to 10th	Night	40% 5% 11% 27% 15% 2	3702
	11th to 20th	Night	38% 5% 13% 30% 13% 1	3497
	21st to end	Night	58% 6% 13% 17% 5% 1	3817
April	1st to 10th	Night	44% 5% 12% 27% 12% 1	1980
	11th to 20th	Night	46% 4 9% 24% 15% 2	1689
	21st to end	Night	47% 5% 10% 24% 12% 1	1710
May	1st to 10th	Night	37% 7% 11% 27% 15% 4	2850
	11th to 20th	Night	32% 8% 11% 26% 21% 2	2739
	21st to end	Night	38% 7% 14% 29% 11% 1	2659
June	1st to 10th	Night	28% 6% 16% 31% 15% 3	1330
	11th to 20th	Night	28% 7% 18% 27% 17% 3	1330
	21st to end	Night	24% 6% 16% 32% 20% 2	1330
July	1st to 10th	Night	34% 7% 21% 22% 15% 2	2185
	11th to 20th	Night	15% 5% 13% 44% 20% 3	2365
	21st to end	Night	23% 7% 19% 29% 19% 3	2871
August	1st to 10th	Night	33% 6% 15% 27% 16% 4	2826
	11th to 20th	Night	35% 5% 17% 30% 12% 1	2825
	21st to end	Night	28% 6% 17% 28% 18% 3	3135
September	1st to 10th	Night	25% 6% 12% 33% 22% 2	1934
	11th to 20th	Night	30% 5% 15% 30% 19% 1	2090
	21st to end	Night	30% 4 10% 33% 20% 2	2090
October	1st to 10th	Night	22% 3 7% 29% 35% 4	3663
	11th to 20th	Night	31% 3 13% 29% 22% 2	3779
	21st to end	Night	39% 4 13% 28% 15% 1	4341
November	1st to 10th	Night	38% 5% 12% 24% 19% 3	2535
	11th to 20th	Night	43% 6% 14% 21% 15% 2	2785
	21st to end	Night	42% 5% 16% 24% 12% 1	2800
December	1st to 10th	Night	49% 4 17% 21% 8%	4580
	11th to 20th	Night	53% 6% 16% 20% 5%	5560
	21st to end	Night	63% 4 9% 18% 6% 1	7480

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% 30% 12% 2	300
	11th to 20th	Day	13% 13% 34% 31% 8% 1	300
	21st to end	Day	14% 13% 42% 18% 12% 1	330
February	1st to 10th	Day	12% 15% 44% 26% 3	300
	11th to 20th	Day	13% 14% 39% 24% 10%	300
	21st to end	Day	15% 12% 41% 27% 5%	249
March	1st to 10th	Day	18% 13% 40% 26% 2 1	300
	11th to 20th	Day	17% 8% 39% 22% 12% 2	273
	21st to end	Day	23% 14% 25% 24% 11% 2	297
April	1st to 10th	Day	25% 18% 30% 21% 5%	374
	11th to 20th	Day	25% 15% 30% 20% 8% 1	640
	21st to end	Day	23% 13% 32% 22% 9% 1	650
May	1st to 10th	Day	20% 11% 35% 25% 8% 1	653
	11th to 20th	Day	19% 12% 30% 26% 12% 2	675
	21st to end	Day	19% 9% 31% 29% 11% 2	734
June	1st to 10th	Day	10% 8% 27% 33% 19% 3	680
	11th to 20th	Day	15% 10% 35% 26% 11% 3	680
	21st to end	Day	13% 9% 31% 30% 14% 3	680
July	1st to 10th	Day	9% 4 27% 34% 21% 6%	671
	11th to 20th	Day	6% 6% 29% 39% 17% 3	668
	21st to end	Day	5% 4 26% 41% 19% 4	739
August	1st to 10th	Day	9% 4 26% 36% 22% 4	676
	11th to 20th	Day	14% 5% 25% 34% 17% 5%	665
	21st to end	Day	8% 5% 29% 34% 20% 4	748
September	1st to 10th	Day	13% 7% 34% 27% 15% 4	453
	11th to 20th	Day	13% 7% 37% 26% 16% 3	300
	21st to end	Day	20% 14% 27% 26% 9% 3	300
October	1st to 10th	Day	19% 9% 33% 24% 12% 4	300
	11th to 20th	Day	14% 17% 37% 25% 6% 1	300
	21st to end	Day	21% 21% 34% 17% 6% 1	330
November	1st to 10th	Day	23% 11% 32% 25% 8% 1	300
	11th to 20th	Day	16% 16% 44% 20% 5%	300
	21st to end	Day	14% 18% 38% 23% 7% 1	300
December	1st to 10th	Day	13% 9% 44% 26% 8%	300
	11th to 20th	Day	14% 14% 46% 21% 6%	300
	21st to end	Day	25% 9% 39% 20% 6% 1	330
January	1st to 10th	Night	11% 11% 41% 30% 6% 2	1000
	11th to 20th	Night	12% 10% 41% 31% 5% 1	1000
	21st to end	Night	14% 13% 41% 26% 7%	1100
February	1st to 10th	Night	12% 11% 42% 28% 7% 1	1000
	11th to 20th	Night	10% 11% 41% 29% 8% 1	979
	21st to end	Night	13% 10% 36% 31% 9% 1	668
March	1st to 10th	Night	13% 11% 40% 30% 6%	713
	11th to 20th	Night	15% 12% 37% 29% 6%	575
	21st to end	Night	17% 9% 31% 30% 11% 1	627
April	1st to 10th	Night	19% 9% 37% 30% 5%	570
	11th to 20th	Night	21% 12% 29% 27% 9% 1	563
	21st to end	Night	20% 11% 31% 28% 8% 1	570
May	1st to 10th	Night	18% 9% 33% 28% 11%	570
	11th to 20th	Night	16% 8% 34% 31% 9% 1	561
	21st to end	Night	14% 8% 34% 33% 11% 1	617
June	1st to 10th	Night	12% 6% 25% 40% 17% 1	570
	11th to 20th	Night	9% 6% 38% 37% 9%	570
	21st to end	Night	8% 4 28% 45% 14% 1	570
July	1st to 10th	Night	6% 2 26% 44% 20% 2	555
	11th to 20th	Night	5% 3 29% 48% 15% 1	552
	21st to end	Night	4 3 23% 53% 16% 1	624
August	1st to 10th	Night	8% 4 22% 47% 17% 1	567
	11th to 20th	Night	11% 3 23% 39% 22% 3	564
	21st to end	Night	6% 3 30% 41% 18% 2	627
September	1st to 10th	Night	10% 4 31% 39% 14% 2	570
	11th to 20th	Night	10% 6% 33% 36% 13% 3	570
	21st to end	Night	14% 9% 35% 31% 11% 1	570
October	1st to 10th	Night	15% 9% 35% 29% 11%	675
	11th to 20th	Night	19% 11% 37% 26% 7% 1	760
	21st to end	Night	19% 12% 33% 29% 6%	969
November	1st to 10th	Night	13% 11% 38% 29% 8%	975
	11th to 20th	Night	13% 12% 38% 27% 8% 1	1000
	21st to end	Night	12% 13% 38% 31% 7%	1000
December	1st to 10th	Night	12% 13% 40% 27% 8% 1	1000
	11th to 20th	Night	17% 10% 41% 26% 6% 1	1000
	21st to end	Night	21% 11% 38% 24% 5% 1	1100

Page generated at 2025-03-09 07:08:45. Version ac07

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