Seaway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seaway Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	131758
	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 28%	130388
Transparency	Day	53% 6% 12% 20% 9% 1	135223
	Night	56% 5% 11% 18% 9% 1	135010
Seeing	Day	25% 17% 30% 19% 8% 2	16288
	Night	25% 14% 29% 23% 8% 1	25508

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% 21% 5% 15% 6% 5% 4 3 4 5% 17%	4661
	11th to 20th	Day	11% 20% 9% 17% 7% 5% 4 3 4 4 16%	4650
	21st to end	Day	11% 17% 7% 14% 9% 9% 5% 3 4 4 18%	5346
February	1st to 10th	Day	17% 15% 5% 14% 6% 5% 4 3 5% 5% 22%	1890
	11th to 20th	Day	15% 16% 6% 11% 7% 4 4 3 4 5% 25%	1883
	21st to end	Day	19% 17% 5% 10% 6% 5% 3 3 4 5% 21%	1730
March	1st to 10th	Day	20% 13% 5% 11% 4 4 5% 3 5% 5% 26%	4576
	11th to 20th	Day	11% 14% 4 9% 5% 6% 5% 4 5% 6% 30%	5038
	21st to end	Day	20% 11% 3 9% 5% 4 4 3 3 5% 35%	5947
April	1st to 10th	Day	22% 13% 3 7% 5% 4 4 3 4 5% 31%	2520
	11th to 20th	Day	23% 11% 3 6% 4 4 3 2 4 5% 35%	2446
	21st to end	Day	23% 10% 4 7% 4 4 4 3 4 5% 32%	2499
May	1st to 10th	Day	25% 11% 4 9% 5% 4 4 4 4 5% 24%	6346
	11th to 20th	Day	18% 10% 3 9% 5% 5% 5% 3 5% 6% 30%	6318
	21st to end	Day	21% 9% 3 7% 4 4 4 3 5% 7% 32%	6949
June	1st to 10th	Day	21% 8% 2 7% 4 4 4 4 6% 7% 32%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 6% 4 4 6% 8% 38%	2800
	21st to end	Day	20% 7% 3 6% 5% 6% 5% 3 5% 8% 33%	2800
July	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 41%	6360
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	6926
August	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 6% 8% 35%	4504
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 39%	4058
	21st to end	Day	17% 6% 2 5% 4 4 4 4 6% 9% 39%	4312
September	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 5% 7% 43%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 47%	2382
	21st to end	Day	19% 6% 2 5% 4 4 5% 4 5% 7% 39%	2165
October	1st to 10th	Day	20% 7% 2 6% 5% 4 4 3 5% 7% 36%	2000
	11th to 20th	Day	20% 11% 3 8% 5% 4 5% 4 6% 5% 29%	2000
	21st to end	Day	26% 11% 3 8% 4 5% 4 3 4 6% 27%	2029
November	1st to 10th	Day	17% 9% 4 7% 6% 4 5% 4 6% 7% 31%	1851
	11th to 20th	Day	17% 13% 3 8% 6% 6% 5% 3 4 5% 29%	1738
	21st to end	Day	22% 15% 5% 10% 5% 5% 5% 3 4 5% 20%	1680
December	1st to 10th	Day	21% 21% 6% 12% 5% 4 4 3 4 5% 14%	2680
	11th to 20th	Day	19% 17% 7% 11% 5% 4 4 3 3 4 22%	3066
	21st to end	Day	21% 15% 5% 13% 5% 5% 4 3 3 3 22%	4048
January	1st to 10th	Night	15% 22% 8% 14% 6% 4 4 2 3 4 18%	8234
	11th to 20th	Night	13% 21% 8% 16% 7% 6% 5% 3 4 4 14%	8235
	21st to end	Night	14% 16% 7% 13% 8% 6% 5% 4 4 4 19%	8821
February	1st to 10th	Night	18% 15% 6% 13% 7% 5% 4 3 4 5% 20%	2730
	11th to 20th	Night	15% 15% 5% 12% 6% 5% 4 3 4 5% 26%	2621
	21st to end	Night	16% 18% 5% 10% 6% 4 4 3 4 5% 26%	2076
March	1st to 10th	Night	21% 15% 4 9% 5% 4 4 2 3 4 29%	5484
	11th to 20th	Night	16% 14% 4 7% 4 4 4 3 5% 7% 31%	5138
	21st to end	Night	19% 13% 4 10% 4 4 4 2 4 4 31%	5226
April	1st to 10th	Night	22% 11% 4 7% 5% 4 3 3 5% 5% 30%	2100
	11th to 20th	Night	22% 11% 2 7% 5% 4 3 2 4 6% 34%	2036
	21st to end	Night	22% 9% 3 7% 4 3 4 3 4 6% 34%	1918
May	1st to 10th	Night	26% 11% 3 8% 5% 4 5% 4 5% 6% 23%	3594
	11th to 20th	Night	17% 10% 3 8% 5% 3 4 3 5% 8% 34%	3575
	21st to end	Night	19% 9% 3 7% 4 5% 5% 4 6% 7% 32%	3926
June	1st to 10th	Night	18% 8% 2 7% 6% 6% 5% 4 7% 8% 31%	1600
	11th to 20th	Night	16% 6% 2 6% 5% 5% 4 4 6% 8% 40%	1600
	21st to end	Night	20% 8% 2 7% 5% 4 5% 4 5% 8% 33%	1600
July	1st to 10th	Night	14% 7% 2 7% 5% 5% 5% 4 6% 9% 37%	3600
	11th to 20th	Night	15% 8% 2 6% 4 3 4 3 6% 7% 43%	3600
	21st to end	Night	10% 6% 3 7% 5% 4 5% 4 6% 7% 44%	3926
August	1st to 10th	Night	15% 8% 3 8% 4 4 4 4 5% 8% 38%	2641
	11th to 20th	Night	15% 8% 3 7% 4 4 4 3 5% 7% 41%	2957
	21st to end	Night	11% 6% 2 7% 5% 4 5% 4 7% 8% 41%	3586
September	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 6% 45%	2000
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 7% 50%	2017
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 6% 41%	2232
October	1st to 10th	Night	19% 8% 3 7% 4 4 5% 3 5% 7% 37%	2400
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 6% 6% 33%	2400
	21st to end	Night	26% 10% 3 7% 5% 4 4 3 5% 6% 28%	2810
November	1st to 10th	Night	20% 12% 4 7% 4 4 4 3 4 5% 35%	2667
	11th to 20th	Night	18% 12% 4 7% 6% 4 4 3 5% 5% 32%	2880
	21st to end	Night	22% 15% 5% 8% 5% 5% 4 3 4 6% 22%	2940
December	1st to 10th	Night	20% 20% 7% 12% 6% 5% 5% 4 4 4 14%	4720
	11th to 20th	Night	24% 15% 4 10% 5% 4 4 2 4 4 24%	5348
	21st to end	Night	25% 16% 5% 12% 4 4 4 3 5% 4 18%	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	69% 7% 9% 12% 3	4662
	11th to 20th	Day	72% 5% 8% 7% 7% 1	4680
	21st to end	Day	71% 4 8% 8% 8% 1	5346
February	1st to 10th	Day	65% 5% 8% 17% 4 1	1890
	11th to 20th	Day	62% 6% 10% 14% 7% 1	1892
	21st to end	Day	65% 6% 9% 11% 8% 1	1730
March	1st to 10th	Day	61% 5% 10% 13% 8% 3	4579
	11th to 20th	Day	54% 6% 16% 14% 9% 1	5038
	21st to end	Day	52% 5% 7% 17% 17% 2	5947
April	1st to 10th	Day	56% 5% 7% 19% 11% 2	2520
	11th to 20th	Day	52% 5% 7% 18% 15% 2	2446
	21st to end	Day	54% 5% 9% 15% 14% 3	2499
May	1st to 10th	Day	61% 6% 8% 16% 7% 1	6357
	11th to 20th	Day	54% 7% 11% 18% 10% 1	6322
	21st to end	Day	50% 7% 12% 22% 10% 1	6949
June	1st to 10th	Day	50% 8% 12% 18% 12% 1	2800
	11th to 20th	Day	44% 6% 12% 25% 11% 2	2800
	21st to end	Day	49% 8% 11% 20% 11% 1	2800
July	1st to 10th	Day	44% 4 14% 27% 10%	6290
	11th to 20th	Day	39% 5% 14% 28% 14%	6285
	21st to end	Day	35% 6% 13% 37% 9%	6728
August	1st to 10th	Day	47% 5% 16% 23% 9%	4490
	11th to 20th	Day	43% 5% 15% 28% 8%	4097
	21st to end	Day	42% 5% 17% 25% 10%	4312
September	1st to 10th	Day	39% 7% 14% 27% 12%	2400
	11th to 20th	Day	36% 6% 12% 32% 13% 1	2382
	21st to end	Day	44% 7% 14% 25% 11%	2165
October	1st to 10th	Day	46% 4 12% 28% 10%	3260
	11th to 20th	Day	49% 5% 16% 24% 6% 1	3240
	21st to end	Day	48% 4 17% 23% 7%	3254
November	1st to 10th	Day	51% 8% 13% 20% 7% 1	1851
	11th to 20th	Day	57% 7% 12% 14% 9% 1	1738
	21st to end	Day	66% 7% 12% 11% 4	1680
December	1st to 10th	Day	73% 5% 11% 10% 1	2680
	11th to 20th	Day	67% 5% 9% 14% 6%	3066
	21st to end	Day	69% 4 8% 15% 4	4048
January	1st to 10th	Night	70% 6% 10% 10% 3	8234
	11th to 20th	Night	74% 5% 6% 8% 5%	8280
	21st to end	Night	69% 4 6% 12% 9% 1	8821
February	1st to 10th	Night	67% 5% 9% 13% 5%	2730
	11th to 20th	Night	61% 5% 10% 18% 6%	2623
	21st to end	Night	62% 5% 8% 16% 9% 1	2076
March	1st to 10th	Night	60% 5% 7% 15% 10% 2	5494
	11th to 20th	Night	51% 6% 19% 14% 7% 3	5139
	21st to end	Night	56% 4 6% 21% 11% 2	5226
April	1st to 10th	Night	55% 5% 8% 19% 11% 1	2100
	11th to 20th	Night	53% 5% 9% 18% 14% 1	2036
	21st to end	Night	52% 5% 9% 17% 14% 3	1918
May	1st to 10th	Night	60% 7% 9% 14% 8% 2	3600
	11th to 20th	Night	48% 6% 13% 19% 12% 1	3576
	21st to end	Night	49% 6% 13% 18% 12% 1	3927
June	1st to 10th	Night	50% 8% 13% 17% 11% 1	1600
	11th to 20th	Night	43% 6% 14% 20% 16% 2	1600
	21st to end	Night	48% 8% 14% 18% 13% 1	1600
July	1st to 10th	Night	44% 4 17% 22% 13% 1	3560
	11th to 20th	Night	40% 4 14% 29% 12%	3552
	21st to end	Night	38% 6% 12% 29% 14%	3809
August	1st to 10th	Night	45% 6% 14% 25% 9%	2631
	11th to 20th	Night	45% 4 14% 27% 9% 1	2984
	21st to end	Night	40% 6% 15% 27% 11% 1	3586
September	1st to 10th	Night	40% 6% 12% 28% 13% 1	2000
	11th to 20th	Night	33% 7% 14% 33% 13% 1	2017
	21st to end	Night	42% 7% 11% 26% 12% 1	2232
October	1st to 10th	Night	44% 5% 13% 28% 10% 1	3920
	11th to 20th	Night	45% 4 16% 25% 10%	3899
	21st to end	Night	50% 5% 17% 20% 8%	4535
November	1st to 10th	Night	52% 6% 13% 19% 9% 1	2667
	11th to 20th	Night	54% 7% 12% 17% 8% 2	2880
	21st to end	Night	63% 7% 12% 14% 5%	2940
December	1st to 10th	Night	74% 4 10% 9% 2	4720
	11th to 20th	Night	66% 5% 8% 14% 7%	5348
	21st to end	Night	69% 6% 6% 12% 7%	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	30% 20% 32% 13% 5%	220
	11th to 20th	Day	32% 23% 31% 10% 3 1	220
	21st to end	Day	26% 24% 28% 17% 5%	283
February	1st to 10th	Day	33% 25% 28% 12% 2	330
	11th to 20th	Day	28% 30% 32% 10%	370
	21st to end	Day	33% 21% 33% 12% 1	446
March	1st to 10th	Day	29% 24% 29% 15% 3	540
	11th to 20th	Day	31% 20% 33% 12% 3 1	540
	21st to end	Day	33% 22% 26% 13% 5% 1	594
April	1st to 10th	Day	36% 20% 27% 13% 4	540
	11th to 20th	Day	36% 20% 23% 14% 5% 3	516
	21st to end	Day	33% 18% 30% 11% 6% 1	500
May	1st to 10th	Day	27% 17% 30% 17% 7% 1	503
	11th to 20th	Day	20% 17% 34% 18% 8% 2	527
	21st to end	Day	21% 17% 27% 25% 9% 1	573
June	1st to 10th	Day	26% 16% 29% 21% 7% 1	530
	11th to 20th	Day	17% 16% 35% 22% 8% 3	530
	21st to end	Day	25% 14% 33% 17% 9% 2	530
July	1st to 10th	Day	16% 10% 30% 30% 13% 2	525
	11th to 20th	Day	14% 12% 33% 27% 12% 2	524
	21st to end	Day	14% 9% 34% 30% 11% 2	576
August	1st to 10th	Day	16% 8% 25% 34% 14% 2	526
	11th to 20th	Day	15% 6% 29% 28% 15% 7%	519
	21st to end	Day	15% 11% 33% 22% 11% 7%	583
September	1st to 10th	Day	17% 10% 30% 22% 14% 7%	530
	11th to 20th	Day	13% 7% 23% 29% 17% 11%	530
	21st to end	Day	19% 14% 29% 22% 14% 3	530
October	1st to 10th	Day	24% 15% 27% 16% 14% 4	530
	11th to 20th	Day	26% 21% 28% 18% 5% 1	530
	21st to end	Day	33% 15% 28% 19% 5%	488
November	1st to 10th	Day	28% 21% 28% 17% 5% 1	425
	11th to 20th	Day	29% 18% 31% 14% 6% 1	278
	21st to end	Day	33% 20% 34% 8% 5%	220
December	1st to 10th	Day	35% 17% 30% 14% 4	220
	11th to 20th	Day	32% 30% 27% 9% 2	220
	21st to end	Day	37% 19% 26% 12% 5%	242
January	1st to 10th	Night	30% 18% 31% 18% 3	840
	11th to 20th	Night	32% 21% 26% 18% 3	840
	21st to end	Night	23% 17% 33% 21% 5%	924
February	1st to 10th	Night	27% 19% 32% 20% 3	840
	11th to 20th	Night	26% 19% 37% 16% 3	823
	21st to end	Night	28% 17% 32% 20% 3	692
March	1st to 10th	Night	30% 16% 29% 21% 3	840
	11th to 20th	Night	29% 20% 30% 18% 3	840
	21st to end	Night	28% 15% 30% 21% 5%	924
April	1st to 10th	Night	29% 16% 29% 23% 3	840
	11th to 20th	Night	31% 17% 28% 18% 6%	816
	21st to end	Night	28% 14% 29% 21% 7% 1	718
May	1st to 10th	Night	28% 9% 30% 25% 8% 1	400
	11th to 20th	Night	19% 10% 37% 27% 7%	394
	21st to end	Night	15% 11% 39% 26% 9%	433
June	1st to 10th	Night	20% 9% 31% 34% 7%	400
	11th to 20th	Night	17% 11% 31% 29% 11% 1	400
	21st to end	Night	23% 7% 31% 28% 11% 1	400
July	1st to 10th	Night	11% 7% 30% 39% 12%	390
	11th to 20th	Night	15% 5% 28% 37% 14% 1	388
	21st to end	Night	12% 6% 31% 42% 9% 1	438
August	1st to 10th	Night	16% 4 25% 40% 15%	427
	11th to 20th	Night	14% 5% 24% 37% 19% 1	637
	21st to end	Night	14% 6% 31% 32% 15% 2	880
September	1st to 10th	Night	18% 7% 28% 31% 15% 1	800
	11th to 20th	Night	15% 7% 21% 32% 21% 5%	800
	21st to end	Night	18% 11% 26% 26% 18% 1	800
October	1st to 10th	Night	23% 11% 30% 20% 15% 1	800
	11th to 20th	Night	25% 12% 31% 23% 8%	800
	21st to end	Night	32% 14% 27% 22% 5%	880
November	1st to 10th	Night	29% 15% 30% 19% 7%	820
	11th to 20th	Night	27% 17% 30% 17% 8%	840
	21st to end	Night	32% 19% 26% 18% 5%	840
December	1st to 10th	Night	35% 18% 26% 18% 3	840
	11th to 20th	Night	31% 21% 30% 15% 3 1	840
	21st to end	Night	36% 17% 27% 18% 3	924

Page generated at 2026-04-14 02:44:24. Version ac07

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