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 2024-07-31.

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% 11% 3 8% 5% 5% 4 3 5% 6% 32%	90011
	Night	19% 12% 4 8% 5% 4 4 3 5% 5% 30%	89813
Transparency	Day	52% 8% 12% 19% 9% 1	89730
	Night	55% 7% 11% 17% 9% 1	89648
Seeing	Day	24% 17% 29% 19% 8% 2	14178
	Night	25% 14% 29% 23% 8% 1	23250

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	18% 19% 5% 13% 6% 5% 4 3 4 5% 19%	2517
	11th to 20th	Day	15% 17% 8% 13% 7% 6% 4 3 4 4 18%	2520
	21st to end	Day	17% 20% 7% 12% 7% 7% 4 3 4 5% 15%	2882
February	1st to 10th	Day	18% 16% 5% 13% 6% 5% 4 3 5% 5% 22%	1710
	11th to 20th	Day	16% 16% 6% 10% 7% 4 4 3 4 5% 25%	1699
	21st to end	Day	20% 17% 4 10% 6% 5% 3 3 4 5% 22%	1570
March	1st to 10th	Day	22% 12% 4 9% 4 4 4 3 5% 5% 28%	3116
	11th to 20th	Day	14% 18% 5% 9% 5% 5% 4 3 4 4 29%	3443
	21st to end	Day	15% 9% 3 8% 5% 4 4 3 4 6% 40%	4057
April	1st to 10th	Day	22% 12% 3 6% 5% 4 4 3 4 5% 32%	2280
	11th to 20th	Day	22% 11% 3 6% 4 4 3 2 4 5% 36%	2254
	21st to end	Day	23% 10% 4 7% 4 4 4 3 4 5% 32%	2374
May	1st to 10th	Day	21% 9% 3 8% 5% 4 5% 4 5% 6% 31%	4426
	11th to 20th	Day	18% 10% 3 8% 5% 4 5% 3 5% 6% 33%	4398
	21st to end	Day	14% 7% 2 6% 4 5% 5% 4 6% 9% 38%	4837
June	1st to 10th	Day	22% 8% 2 7% 5% 4 4 4 6% 7% 31%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 6% 4 4 7% 7% 37%	2660
	21st to end	Day	20% 7% 3 6% 5% 6% 5% 3 5% 8% 33%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 7% 8% 41%	2660
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 39%	2660
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 5% 8% 9% 39%	2887
August	1st to 10th	Day	13% 6% 3 6% 5% 5% 5% 5% 6% 8% 39%	2477
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	2246
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	2376
September	1st to 10th	Day	15% 6% 2 5% 4 4 6% 4 5% 7% 42%	2160
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 4 7% 46%	2144
	21st to end	Day	19% 6% 2 5% 4 4 5% 4 6% 8% 38%	1949
October	1st to 10th	Day	22% 7% 2 6% 5% 4 4 3 5% 6% 34%	1800
	11th to 20th	Day	21% 11% 3 9% 5% 4 5% 4 6% 5% 27%	1800
	21st to end	Day	27% 11% 3 8% 4 4 4 3 4 6% 26%	1827
November	1st to 10th	Day	17% 9% 4 7% 6% 4 5% 4 5% 7% 32%	1671
	11th to 20th	Day	18% 13% 3 8% 6% 5% 5% 3 4 5% 29%	1573
	21st to end	Day	22% 14% 5% 11% 6% 5% 5% 3 4 5% 21%	1520
December	1st to 10th	Day	25% 19% 5% 10% 5% 4 4 3 4 5% 15%	1520
	11th to 20th	Day	22% 19% 6% 8% 5% 4 4 3 3 5% 22%	1906
	21st to end	Day	27% 17% 5% 11% 5% 4 4 2 3 2 20%	2772
January	1st to 10th	Night	19% 19% 6% 12% 5% 4 3 3 4 5% 21%	4429
	11th to 20th	Night	14% 18% 7% 15% 7% 5% 6% 3 4 4 16%	4440
	21st to end	Night	20% 19% 7% 11% 7% 5% 4 3 3 3 19%	4760
February	1st to 10th	Night	19% 16% 6% 13% 7% 5% 4 3 4 5% 19%	2470
	11th to 20th	Night	15% 15% 5% 12% 6% 5% 4 3 4 5% 26%	2365
	21st to end	Night	17% 17% 5% 10% 6% 4 4 3 3 5% 27%	1884
March	1st to 10th	Night	21% 13% 4 7% 5% 4 4 3 3 5% 32%	3724
	11th to 20th	Night	21% 18% 5% 8% 4 4 4 3 3 4 27%	3513
	21st to end	Night	17% 12% 3 8% 4 4 4 2 4 4 38%	3574
April	1st to 10th	Night	21% 11% 4 7% 5% 4 3 3 5% 6% 31%	1900
	11th to 20th	Night	21% 10% 2 7% 5% 4 3 2 5% 6% 34%	1875
	21st to end	Night	22% 10% 3 7% 4 3 4 3 4 6% 33%	1822
May	1st to 10th	Night	23% 9% 3 7% 5% 4 5% 3 5% 6% 29%	2514
	11th to 20th	Night	20% 9% 3 7% 5% 3 4 3 5% 6% 35%	2495
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 8% 40%	2738
June	1st to 10th	Night	18% 8% 2 6% 5% 6% 5% 3 7% 8% 31%	1520
	11th to 20th	Night	16% 6% 2 5% 5% 5% 4 3 6% 8% 40%	1520
	21st to end	Night	21% 8% 1 7% 5% 4 5% 4 5% 8% 33%	1520
July	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 44%	1520
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 9% 42%	1520
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	1662
August	1st to 10th	Night	12% 6% 2 7% 4 4 5% 4 6% 8% 41%	1457
	11th to 20th	Night	14% 7% 3 7% 4 4 4 4 5% 6% 42%	1636
	21st to end	Night	12% 7% 3 7% 4 3 4 4 5% 7% 44%	1980
September	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 7% 44%	1800
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 5% 7% 49%	1815
	21st to end	Night	15% 7% 3 6% 4 4 3 4 5% 6% 42%	2008
October	1st to 10th	Night	20% 8% 3 7% 4 4 5% 3 6% 7% 34%	2160
	11th to 20th	Night	20% 10% 3 7% 5% 5% 4 3 6% 6% 32%	2160
	21st to end	Night	27% 10% 3 7% 5% 4 4 3 5% 6% 27%	2528
November	1st to 10th	Night	19% 11% 3 6% 4 4 4 3 4 5% 35%	2407
	11th to 20th	Night	18% 12% 4 7% 5% 4 4 3 5% 5% 32%	2605
	21st to end	Night	22% 15% 4 8% 5% 5% 4 3 4 6% 23%	2660
December	1st to 10th	Night	23% 19% 6% 10% 5% 4 5% 3 4 4 16%	2660
	11th to 20th	Night	23% 16% 4 10% 5% 4 4 2 4 4 25%	3288
	21st to end	Night	31% 15% 5% 11% 4 4 3 2 4 3 17%	4884

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	66% 11% 10% 8% 4 1	2518
	11th to 20th	Day	67% 8% 12% 7% 6% 1	2520
	21st to end	Day	73% 6% 10% 7% 3 1	2882
February	1st to 10th	Day	65% 5% 8% 17% 3 1	1710
	11th to 20th	Day	61% 7% 10% 14% 7% 1	1708
	21st to end	Day	64% 7% 10% 11% 8% 1	1570
March	1st to 10th	Day	58% 7% 10% 17% 7% 1	3119
	11th to 20th	Day	57% 8% 12% 13% 8% 2	3443
	21st to end	Day	45% 6% 9% 20% 16% 3	4057
April	1st to 10th	Day	54% 6% 7% 19% 11% 3	2280
	11th to 20th	Day	51% 5% 8% 19% 15% 2	2254
	21st to end	Day	54% 5% 10% 15% 14% 3	2374
May	1st to 10th	Day	52% 9% 9% 19% 10% 1	4440
	11th to 20th	Day	50% 9% 12% 20% 9% 1	4402
	21st to end	Day	40% 9% 12% 24% 13% 1	4837
June	1st to 10th	Day	50% 8% 12% 18% 11% 1	2660
	11th to 20th	Day	44% 7% 12% 24% 11% 2	2660
	21st to end	Day	50% 9% 11% 19% 11% 1	2660
July	1st to 10th	Day	37% 7% 13% 28% 14% 1	2590
	11th to 20th	Day	40% 8% 16% 27% 9%	2585
	21st to end	Day	38% 10% 17% 25% 9%	2734
August	1st to 10th	Day	42% 8% 17% 23% 10%	2463
	11th to 20th	Day	41% 8% 17% 25% 10%	2246
	21st to end	Day	40% 8% 15% 26% 11%	2376
September	1st to 10th	Day	40% 8% 15% 24% 13%	2160
	11th to 20th	Day	37% 7% 12% 31% 13% 1	2144
	21st to end	Day	43% 7% 14% 26% 9%	1949
October	1st to 10th	Day	51% 6% 13% 20% 11%	1800
	11th to 20th	Day	56% 8% 15% 15% 5% 1	1800
	21st to end	Day	60% 7% 12% 14% 8%	1827
November	1st to 10th	Day	50% 8% 14% 21% 7% 1	1671
	11th to 20th	Day	57% 7% 12% 14% 9% 1	1573
	21st to end	Day	65% 8% 12% 10% 4	1520
December	1st to 10th	Day	71% 8% 11% 8% 2	1520
	11th to 20th	Day	67% 6% 12% 12% 3	1906
	21st to end	Day	72% 5% 7% 13% 3	2772
January	1st to 10th	Night	65% 9% 13% 9% 3	4429
	11th to 20th	Night	70% 9% 10% 7% 4	4440
	21st to end	Night	71% 6% 8% 9% 4 1	4760
February	1st to 10th	Night	67% 5% 10% 12% 5%	2470
	11th to 20th	Night	60% 6% 10% 18% 6%	2367
	21st to end	Night	61% 5% 8% 15% 9% 1	1884
March	1st to 10th	Night	54% 6% 10% 20% 9% 1	3734
	11th to 20th	Night	59% 7% 15% 11% 7% 1	3514
	21st to end	Night	49% 6% 7% 22% 14% 3	3574
April	1st to 10th	Night	54% 5% 9% 19% 12% 1	1900
	11th to 20th	Night	51% 5% 9% 18% 14% 1	1875
	21st to end	Night	52% 5% 10% 16% 13% 4	1822
May	1st to 10th	Night	53% 9% 11% 16% 9% 2	2520
	11th to 20th	Night	48% 9% 12% 20% 10% 1	2496
	21st to end	Night	39% 8% 14% 20% 17% 2	2739
June	1st to 10th	Night	49% 8% 14% 17% 11% 1	1520
	11th to 20th	Night	42% 6% 13% 20% 16% 2	1520
	21st to end	Night	48% 8% 14% 17% 13% 1	1520
July	1st to 10th	Night	36% 8% 14% 25% 16% 2	1480
	11th to 20th	Night	36% 8% 14% 29% 13%	1472
	21st to end	Night	37% 10% 15% 23% 13% 1	1574
August	1st to 10th	Night	40% 10% 16% 22% 12%	1447
	11th to 20th	Night	42% 7% 14% 23% 13% 1	1636
	21st to end	Night	38% 8% 14% 27% 11%	1980
September	1st to 10th	Night	41% 7% 13% 26% 13% 1	1800
	11th to 20th	Night	34% 7% 15% 31% 12% 1	1815
	21st to end	Night	42% 8% 11% 28% 11% 1	2008
October	1st to 10th	Night	49% 8% 11% 21% 11% 1	2160
	11th to 20th	Night	51% 7% 15% 18% 8%	2160
	21st to end	Night	58% 8% 13% 13% 8% 1	2528
November	1st to 10th	Night	51% 6% 14% 18% 10% 1	2407
	11th to 20th	Night	53% 8% 13% 16% 8% 2	2605
	21st to end	Night	62% 7% 13% 13% 5%	2660
December	1st to 10th	Night	71% 7% 10% 10% 2	2660
	11th to 20th	Night	65% 5% 10% 14% 5%	3288
	21st to end	Night	72% 7% 8% 11% 2	4884

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	29% 22% 29% 14% 6%	180
	11th to 20th	Day	32% 21% 32% 11% 4 1	180
	21st to end	Day	25% 23% 29% 19% 4	231
February	1st to 10th	Day	35% 27% 26% 10% 1	270
	11th to 20th	Day	28% 29% 33% 9%	306
	21st to end	Day	34% 21% 30% 13% 1	382
March	1st to 10th	Day	27% 22% 31% 16% 3	460
	11th to 20th	Day	30% 20% 35% 12% 3 1	460
	21st to end	Day	32% 21% 26% 13% 6% 1	506
April	1st to 10th	Day	34% 19% 28% 14% 4	460
	11th to 20th	Day	34% 20% 24% 15% 5% 3	452
	21st to end	Day	32% 18% 29% 12% 7% 2	460
May	1st to 10th	Day	25% 18% 30% 18% 8% 1	463
	11th to 20th	Day	20% 18% 34% 18% 9% 2	487
	21st to end	Day	19% 17% 28% 26% 9% 1	529
June	1st to 10th	Day	27% 16% 28% 21% 7% 1	490
	11th to 20th	Day	17% 15% 35% 23% 8% 3	490
	21st to end	Day	26% 14% 33% 17% 10% 2	490
July	1st to 10th	Day	16% 9% 29% 31% 13% 2	485
	11th to 20th	Day	14% 12% 32% 27% 13% 3	484
	21st to end	Day	14% 9% 33% 31% 10% 2	529
August	1st to 10th	Day	16% 9% 25% 33% 14% 3	446
	11th to 20th	Day	15% 6% 28% 28% 15% 7%	439
	21st to end	Day	14% 12% 34% 21% 12% 8%	495
September	1st to 10th	Day	16% 10% 30% 23% 13% 8%	450
	11th to 20th	Day	15% 8% 24% 28% 14% 10%	450
	21st to end	Day	18% 15% 28% 22% 14% 4	450
October	1st to 10th	Day	26% 16% 24% 14% 14% 4	450
	11th to 20th	Day	27% 21% 27% 18% 5% 1	450
	21st to end	Day	34% 15% 27% 20% 5%	418
November	1st to 10th	Day	26% 21% 28% 18% 6% 1	365
	11th to 20th	Day	29% 18% 31% 15% 6% 1	233
	21st to end	Day	30% 22% 33% 9% 6%	180
December	1st to 10th	Day	34% 19% 28% 14% 4	180
	11th to 20th	Day	32% 31% 26% 10% 2	180
	21st to end	Day	37% 22% 25% 12% 4 1	198
January	1st to 10th	Night	30% 19% 29% 18% 4	760
	11th to 20th	Night	31% 21% 26% 18% 4	760
	21st to end	Night	23% 17% 33% 22% 5%	836
February	1st to 10th	Night	28% 20% 31% 19% 2	760
	11th to 20th	Night	25% 19% 36% 17% 3	743
	21st to end	Night	27% 18% 32% 20% 3	628
March	1st to 10th	Night	29% 16% 29% 22% 4	760
	11th to 20th	Night	29% 20% 30% 18% 4 1	760
	21st to end	Night	27% 15% 30% 22% 6%	836
April	1st to 10th	Night	28% 16% 30% 23% 3	760
	11th to 20th	Night	29% 17% 28% 18% 7%	751
	21st to end	Night	28% 14% 29% 21% 7% 1	682
May	1st to 10th	Night	27% 9% 30% 26% 8% 1	380
	11th to 20th	Night	19% 11% 36% 28% 6%	374
	21st to end	Night	14% 11% 39% 27% 10%	411
June	1st to 10th	Night	19% 9% 30% 34% 7%	380
	11th to 20th	Night	16% 11% 30% 30% 12% 1	380
	21st to end	Night	23% 7% 30% 28% 11% 1	380
July	1st to 10th	Night	11% 7% 30% 40% 12%	370
	11th to 20th	Night	15% 5% 27% 37% 15% 1	368
	21st to end	Night	12% 6% 31% 41% 9% 1	416
August	1st to 10th	Night	16% 5% 24% 39% 15% 1	383
	11th to 20th	Night	14% 5% 23% 37% 19% 1	572
	21st to end	Night	14% 6% 31% 30% 16% 2	792
September	1st to 10th	Night	18% 7% 27% 31% 16% 1	720
	11th to 20th	Night	17% 8% 22% 32% 18% 3	720
	21st to end	Night	18% 11% 26% 25% 18% 2	720
October	1st to 10th	Night	25% 11% 30% 19% 14% 1	720
	11th to 20th	Night	26% 13% 29% 22% 9%	720
	21st to end	Night	32% 14% 27% 22% 5%	792
November	1st to 10th	Night	27% 15% 31% 19% 8% 1	740
	11th to 20th	Night	27% 17% 30% 17% 8%	760
	21st to end	Night	31% 19% 26% 18% 6%	760
December	1st to 10th	Night	34% 19% 26% 18% 3	760
	11th to 20th	Night	30% 20% 30% 15% 3 1	760
	21st to end	Night	36% 17% 27% 18% 3	836

Page generated at 2024-07-28 09:52:12. Version ac07

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