Watertown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watertown clear sky chart from 2005-11-05 to 2024-10-14.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 40%	101288
	Night	10% 8% 3 8% 5% 4 5% 4 6% 7% 41%	97453
Transparency	Day	42% 6% 13% 26% 12% 1	101028
	Night	41% 6% 13% 24% 13% 2	97304
Seeing	Day	15% 10% 32% 28% 12% 2	15948
	Night	13% 9% 35% 32% 10% 1	25793

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% 14% 4 11% 8% 6% 8% 5% 7% 6% 21%	2320
	11th to 20th	Day	4 9% 6% 15% 6% 5% 8% 8% 9% 9% 21%	2540
	21st to end	Day	6% 9% 6% 21% 7% 5% 6% 4 7% 7% 22%	3020
February	1st to 10th	Day	7% 12% 5% 11% 7% 7% 6% 4 6% 7% 28%	1710
	11th to 20th	Day	6% 10% 5% 11% 6% 6% 7% 4 6% 7% 31%	1673
	21st to end	Day	8% 10% 6% 11% 7% 6% 5% 4 5% 7% 31%	1570
March	1st to 10th	Day	10% 12% 4 9% 4 4 4 3 5% 7% 37%	3271
	11th to 20th	Day	8% 10% 3 8% 5% 4 4 4 5% 8% 40%	3480
	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	14% 8% 2 6% 3 3 3 3 5% 6% 46%	1965
	11th to 20th	Day	15% 7% 3 5% 3 3 4 3 5% 6% 46%	1830
	21st to end	Day	16% 11% 2 6% 3 3 4 3 4 5% 43%	1852
November	1st to 10th	Day	13% 8% 3 7% 5% 3 3 4 6% 7% 41%	1673
	11th to 20th	Day	8% 11% 4 12% 6% 5% 5% 4 5% 7% 33%	1533
	21st to end	Day	9% 10% 4 9% 5% 5% 4 4 6% 8% 37%	1520
December	1st to 10th	Day	7% 11% 4 11% 9% 8% 7% 5% 5% 6% 28%	1520
	11th to 20th	Day	7% 12% 5% 12% 7% 6% 5% 4 7% 8% 28%	1687
	21st to end	Day	18% 12% 3 13% 6% 4 4 3 4 5% 27%	2442
January	1st to 10th	Night	7% 11% 5% 12% 8% 8% 7% 6% 7% 7% 22%	4632
	11th to 20th	Night	4 8% 4 12% 8% 7% 9% 8% 10% 9% 21%	4430
	21st to end	Night	5% 10% 7% 17% 5% 5% 6% 5% 6% 6% 27%	4644
February	1st to 10th	Night	6% 11% 4 10% 6% 7% 5% 5% 6% 8% 32%	2470
	11th to 20th	Night	6% 9% 4 8% 6% 5% 6% 4 7% 7% 37%	2403
	21st to end	Night	9% 9% 3 8% 7% 6% 5% 4 6% 8% 36%	1892
March	1st to 10th	Night	7% 7% 4 8% 5% 5% 5% 3 5% 7% 43%	3587
	11th to 20th	Night	8% 8% 3 7% 5% 5% 4 3 5% 7% 44%	3480
	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	12% 6% 2 5% 3 3 4 3 4 6% 50%	2195
	11th to 20th	Night	13% 9% 3 4 3 3 4 2 5% 6% 49%	2208
	21st to end	Night	14% 9% 2 6% 3 3 3 3 4 7% 44%	2502
November	1st to 10th	Night	9% 9% 2 6% 4 4 4 3 5% 6% 46%	2405
	11th to 20th	Night	8% 9% 4 10% 6% 4 4 4 5% 7% 39%	2645
	21st to end	Night	8% 8% 4 8% 5% 5% 4 3 5% 7% 43%	2660
December	1st to 10th	Night	7% 10% 4 12% 7% 6% 5% 4 6% 7% 32%	2660
	11th to 20th	Night	9% 11% 3 11% 6% 6% 5% 5% 6% 7% 32%	3508
	21st to end	Night	16% 13% 5% 14% 6% 5% 4 3 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	1710
	11th to 20th	Day	50% 6% 13% 21% 10% 1	1682
	21st to end	Day	53% 6% 14% 16% 9% 1	1570
March	1st to 10th	Day	46% 5% 11% 28% 9% 1	3271
	11th to 20th	Day	42% 5% 12% 27% 11% 2	3480
	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	39% 5% 12% 27% 15% 1	1965
	11th to 20th	Day	38% 5% 11% 28% 15% 2	1830
	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	2470
	11th to 20th	Night	44% 7% 12% 23% 12% 3	2404
	21st to end	Night	46% 6% 12% 20% 12% 4	1892
March	1st to 10th	Night	40% 5% 11% 26% 15% 2	3587
	11th to 20th	Night	38% 5% 12% 30% 13% 1	3480
	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	35% 5% 11% 27% 20% 2	2195
	11th to 20th	Night	37% 3 12% 28% 17% 3	2208
	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% 45% 28% 3	270
	11th to 20th	Day	13% 14% 39% 25% 9%	270
	21st to end	Day	14% 13% 40% 27% 5%	225
March	1st to 10th	Day	19% 13% 40% 26% 1 1	270
	11th to 20th	Day	17% 8% 39% 21% 12% 2	270
	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% 19% 35% 24% 6% 1	279
	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	11% 10% 42% 29% 7% 1	950
	11th to 20th	Night	10% 10% 41% 29% 8% 2	929
	21st to end	Night	13% 10% 35% 31% 9% 2	636
March	1st to 10th	Night	12% 10% 41% 29% 6%	678
	11th to 20th	Night	15% 12% 37% 29% 6%	570
	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	735
	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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