Seddon's Corner
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seddon's Corner clear sky chart from 2005-11-06 to 2026-08-15.

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% 9% 4 9% 6% 5% 5% 4 6% 8% 34%	148536
	Night	9% 11% 4 10% 6% 5% 6% 4 6% 7% 32%	142214
Transparency	Day	48% 6% 14% 24% 9% 1	148232
	Night	50% 6% 13% 22% 9% 1	142103
Seeing	Day	15% 10% 31% 28% 13% 3	18783
	Night	15% 7% 31% 33% 12% 1	28634

By Month

Percentage Cloud Cover NW (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	6% 14% 7% 17% 8% 6% 8% 5% 4 7% 17%	4125
	11th to 20th	Day	5% 12% 5% 16% 10% 10% 10% 9% 9% 5% 10%	4119
	21st to end	Day	4 15% 7% 15% 8% 8% 8% 5% 7% 8% 15%	5040
February	1st to 10th	Day	7% 13% 6% 12% 8% 7% 9% 6% 7% 7% 18%	1838
	11th to 20th	Day	7% 10% 4 10% 7% 8% 6% 6% 8% 8% 24%	1844
	21st to end	Day	6% 12% 6% 10% 9% 8% 7% 5% 5% 7% 24%	1594
March	1st to 10th	Day	8% 12% 4 11% 8% 5% 6% 5% 5% 7% 29%	4777
	11th to 20th	Day	9% 10% 5% 13% 9% 6% 6% 5% 6% 7% 24%	5088
	21st to end	Day	10% 13% 5% 12% 6% 5% 4 3 4 6% 32%	5585
April	1st to 10th	Day	11% 11% 4 9% 6% 4 4 4 6% 6% 35%	2551
	11th to 20th	Day	13% 10% 4 7% 5% 4 4 3 6% 7% 35%	2702
	21st to end	Day	14% 10% 3 7% 6% 4 5% 4 6% 7% 35%	2730
May	1st to 10th	Day	11% 10% 4 6% 3 4 4 3 5% 6% 45%	8063
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 4 5% 7% 32%	8503
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 8% 45%	9683
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 5% 8% 35%	3150
	11th to 20th	Day	16% 7% 3 7% 5% 5% 6% 4 5% 8% 34%	3150
	21st to end	Day	13% 7% 2 7% 5% 6% 6% 5% 7% 8% 33%	3150
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 9% 42%	8850
	11th to 20th	Day	11% 5% 2 5% 4 5% 5% 4 6% 9% 43%	8850
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 9% 47%	9605
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	4517
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4224
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 5% 9% 42%	4675
September	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 6% 7% 45%	2477
	11th to 20th	Day	16% 7% 2 8% 5% 4 5% 5% 6% 9% 33%	2225
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Day	13% 10% 3 8% 4 4 4 3 5% 7% 40%	3265
	11th to 20th	Day	12% 11% 4 7% 4 4 5% 3 6% 7% 37%	3240
	21st to end	Day	16% 13% 4 8% 5% 4 5% 4 6% 8% 28%	3159
November	1st to 10th	Day	15% 18% 4 10% 5% 6% 4 3 5% 5% 26%	1647
	11th to 20th	Day	11% 16% 5% 15% 8% 5% 4 3 5% 5% 23%	1680
	21st to end	Day	13% 14% 6% 16% 11% 5% 4 4 5% 5% 19%	1586
December	1st to 10th	Day	12% 15% 7% 17% 11% 10% 5% 4 4 5% 11%	2360
	11th to 20th	Day	9% 15% 6% 14% 6% 4 5% 5% 7% 9% 20%	2709
	21st to end	Day	8% 14% 7% 18% 6% 7% 6% 6% 6% 6% 14%	3575
January	1st to 10th	Night	5% 13% 7% 17% 8% 8% 7% 6% 7% 7% 15%	8799
	11th to 20th	Night	5% 15% 6% 13% 10% 10% 10% 9% 7% 4 11%	8809
	21st to end	Night	5% 14% 6% 12% 7% 6% 9% 5% 9% 7% 20%	9169
February	1st to 10th	Night	7% 13% 5% 11% 7% 6% 7% 5% 6% 6% 24%	2780
	11th to 20th	Night	6% 10% 4 8% 6% 7% 7% 6% 7% 8% 32%	2663
	21st to end	Night	6% 11% 6% 11% 8% 7% 6% 4 6% 6% 29%	2232
March	1st to 10th	Night	8% 13% 5% 11% 7% 5% 6% 4 5% 7% 29%	5308
	11th to 20th	Night	6% 10% 4 9% 6% 5% 5% 4 6% 8% 37%	5090
	21st to end	Night	12% 13% 4 10% 6% 5% 5% 3 5% 5% 33%	5588
April	1st to 10th	Night	12% 10% 3 6% 5% 4 4 4 6% 7% 39%	2066
	11th to 20th	Night	13% 10% 4 8% 5% 4 4 3 4 6% 38%	1869
	21st to end	Night	11% 10% 4 7% 5% 5% 6% 3 5% 5% 38%	1890
May	1st to 10th	Night	11% 7% 2 6% 4 3 4 4 5% 7% 46%	4883
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	4375
	21st to end	Night	12% 7% 2 5% 3 3 3 3 6% 7% 49%	4523
June	1st to 10th	Night	12% 6% 2 6% 5% 4 5% 4 7% 10% 40%	1470
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 9% 41%	1470
	21st to end	Night	13% 6% 2 5% 4 5% 4 4 6% 8% 43%	1470
July	1st to 10th	Night	7% 4 2 6% 5% 4 5% 4 6% 9% 48%	4140
	11th to 20th	Night	9% 4 2 4 4 4 5% 4 6% 10% 47%	4140
	21st to end	Night	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4497
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 8% 10% 47%	2858
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 9% 50%	2924
	21st to end	Night	11% 5% 2 5% 5% 4 5% 4 5% 9% 45%	3234
September	1st to 10th	Night	11% 4 2 6% 4 4 4 3 5% 7% 50%	1922
	11th to 20th	Night	16% 7% 2 7% 3 4 5% 3 4 8% 41%	2172
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 6% 42%	2200
October	1st to 10th	Night	16% 7% 2 5% 4 3 4 3 5% 8% 45%	3882
	11th to 20th	Night	12% 11% 3 8% 4 5% 4 4 5% 6% 38%	3899
	21st to end	Night	11% 12% 4 9% 5% 5% 5% 3 5% 6% 34%	4639
November	1st to 10th	Night	14% 14% 5% 10% 5% 5% 5% 4 5% 5% 28%	2871
	11th to 20th	Night	12% 17% 6% 12% 7% 5% 4 3 4 6% 24%	2940
	21st to end	Night	11% 15% 7% 15% 9% 7% 5% 3 4 5% 18%	3029
December	1st to 10th	Night	9% 19% 7% 18% 11% 6% 5% 3 4 4 13%	5050
	11th to 20th	Night	8% 13% 6% 14% 8% 5% 5% 7% 7% 7% 20%	5718
	21st to end	Night	8% 12% 7% 20% 8% 6% 6% 5% 6% 5% 17%	7645

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	68% 5% 8% 13% 6%	4125
	11th to 20th	Day	70% 5% 9% 13% 4	4140
	21st to end	Day	65% 4 8% 18% 5%	5040
February	1st to 10th	Day	61% 5% 13% 13% 7% 1	1838
	11th to 20th	Day	52% 6% 14% 20% 7%	1853
	21st to end	Day	58% 6% 12% 14% 9% 1	1594
March	1st to 10th	Day	52% 6% 16% 14% 10% 2	4777
	11th to 20th	Day	55% 8% 13% 16% 8%	5090
	21st to end	Day	53% 5% 10% 22% 10% 1	5585
April	1st to 10th	Day	48% 5% 11% 23% 12% 1	2551
	11th to 20th	Day	48% 5% 11% 22% 12% 1	2702
	21st to end	Day	48% 5% 13% 23% 9% 1	2730
May	1st to 10th	Day	39% 5% 10% 28% 17% 1	8063
	11th to 20th	Day	50% 5% 8% 24% 11% 1	8507
	21st to end	Day	38% 5% 8% 26% 20% 3	9683
June	1st to 10th	Day	46% 6% 14% 23% 11% 1	3150
	11th to 20th	Day	46% 7% 14% 22% 10% 1	3150
	21st to end	Day	45% 9% 17% 21% 7%	3150
July	1st to 10th	Day	37% 7% 20% 29% 6%	8776
	11th to 20th	Day	37% 6% 16% 35% 6%	8769
	21st to end	Day	33% 6% 18% 35% 8%	9398
August	1st to 10th	Day	36% 7% 17% 31% 9%	4500
	11th to 20th	Day	36% 7% 21% 30% 5%	4263
	21st to end	Day	39% 7% 21% 29% 5%	4674
September	1st to 10th	Day	37% 8% 17% 29% 9% 1	2477
	11th to 20th	Day	46% 8% 18% 23% 4	2225
	21st to end	Day	43% 6% 13% 31% 7%	2200
October	1st to 10th	Day	45% 5% 15% 28% 7%	3265
	11th to 20th	Day	45% 5% 13% 24% 13%	3240
	21st to end	Day	52% 6% 18% 20% 4	3159
November	1st to 10th	Day	61% 7% 13% 15% 4 1	1647
	11th to 20th	Day	62% 7% 15% 12% 4	1680
	21st to end	Day	67% 6% 13% 11% 2	1586
December	1st to 10th	Day	76% 7% 8% 6% 2	2360
	11th to 20th	Day	60% 4 13% 16% 7%	2710
	21st to end	Day	65% 7% 11% 14% 3	3575
January	1st to 10th	Night	66% 5% 10% 12% 7%	8800
	11th to 20th	Night	70% 5% 7% 14% 4	8850
	21st to end	Night	59% 6% 10% 18% 7%	9169
February	1st to 10th	Night	56% 6% 13% 16% 8%	2780
	11th to 20th	Night	46% 6% 17% 22% 8%	2664
	21st to end	Night	53% 7% 11% 19% 10%	2232
March	1st to 10th	Night	53% 6% 12% 14% 10% 4	5308
	11th to 20th	Night	43% 8% 14% 23% 12% 1	5090
	21st to end	Night	52% 5% 9% 22% 12% 1	5588
April	1st to 10th	Night	42% 6% 11% 24% 15% 2	2066
	11th to 20th	Night	47% 5% 9% 22% 15% 2	1869
	21st to end	Night	47% 5% 9% 25% 12% 1	1890
May	1st to 10th	Night	35% 6% 9% 33% 17%	4883
	11th to 20th	Night	41% 6% 10% 29% 13% 1	4375
	21st to end	Night	35% 5% 9% 25% 24% 3	4526
June	1st to 10th	Night	38% 9% 16% 24% 12% 1	1470
	11th to 20th	Night	39% 8% 14% 24% 14% 1	1470
	21st to end	Night	38% 8% 19% 26% 8% 1	1470
July	1st to 10th	Night	32% 7% 20% 32% 6% 2	4105
	11th to 20th	Night	31% 6% 18% 37% 7% 1	4098
	21st to end	Night	29% 6% 20% 33% 12%	4399
August	1st to 10th	Night	31% 5% 16% 36% 11%	2851
	11th to 20th	Night	30% 7% 25% 31% 7%	2951
	21st to end	Night	35% 6% 21% 31% 7%	3234
September	1st to 10th	Night	32% 6% 18% 32% 12%	1922
	11th to 20th	Night	43% 7% 16% 24% 9% 1	2172
	21st to end	Night	41% 5% 14% 30% 9% 1	2200
October	1st to 10th	Night	40% 4 12% 32% 11%	3882
	11th to 20th	Night	45% 5% 14% 27% 8%	3899
	21st to end	Night	51% 5% 13% 26% 5%	4639
November	1st to 10th	Night	58% 6% 14% 15% 6% 1	2871
	11th to 20th	Night	63% 6% 14% 13% 5%	2940
	21st to end	Night	69% 6% 12% 9% 3	3029
December	1st to 10th	Night	74% 5% 11% 8% 2	5050
	11th to 20th	Night	61% 6% 11% 13% 9%	5716
	21st to end	Night	66% 6% 9% 14% 4	7645

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	12% 9% 34% 28% 13% 3	330
	11th to 20th	Day	11% 9% 30% 35% 14% 1	330
	21st to end	Day	13% 9% 35% 28% 12% 3	363
February	1st to 10th	Day	15% 12% 32% 31% 8% 2	330
	11th to 20th	Day	11% 10% 32% 29% 14% 3	330
	21st to end	Day	11% 8% 40% 30% 11%	273
March	1st to 10th	Day	12% 13% 35% 27% 13% 1	330
	11th to 20th	Day	12% 12% 33% 27% 12% 3	330
	21st to end	Day	14% 12% 32% 26% 13% 2	363
April	1st to 10th	Day	13% 10% 31% 33% 10% 3	571
	11th to 20th	Day	17% 14% 31% 25% 12% 1	740
	21st to end	Day	18% 10% 34% 26% 10% 2	750
May	1st to 10th	Day	18% 14% 32% 25% 10% 2	753
	11th to 20th	Day	22% 11% 28% 25% 11% 2	775
	21st to end	Day	20% 10% 26% 28% 14% 2	844
June	1st to 10th	Day	17% 13% 29% 24% 12% 5%	780
	11th to 20th	Day	17% 12% 34% 27% 9% 3	780
	21st to end	Day	15% 10% 27% 29% 15% 4	780
July	1st to 10th	Day	8% 9% 31% 27% 17% 8%	771
	11th to 20th	Day	12% 10% 31% 27% 16% 4	768
	21st to end	Day	8% 7% 31% 28% 18% 7%	849
August	1st to 10th	Day	7% 10% 29% 30% 17% 6%	776
	11th to 20th	Day	12% 6% 26% 31% 17% 7%	736
	21st to end	Day	14% 9% 29% 28% 17% 3	803
September	1st to 10th	Day	12% 9% 29% 33% 15% 2	607
	11th to 20th	Day	18% 12% 38% 22% 8% 2	355
	21st to end	Day	19% 7% 38% 27% 7% 2	330
October	1st to 10th	Day	17% 14% 31% 27% 7% 3	330
	11th to 20th	Day	21% 10% 32% 29% 8% 1	330
	21st to end	Day	28% 12% 28% 23% 8% 2	363
November	1st to 10th	Day	23% 12% 29% 25% 10% 1	330
	11th to 20th	Day	17% 15% 36% 25% 7% 1	330
	21st to end	Day	15% 12% 38% 29% 5%	330
December	1st to 10th	Day	17% 9% 32% 27% 12% 3	330
	11th to 20th	Day	17% 9% 40% 25% 8% 1	330
	21st to end	Day	16% 7% 29% 30% 13% 5%	363
January	1st to 10th	Night	11% 9% 34% 31% 14% 2	1050
	11th to 20th	Night	16% 6% 32% 34% 10% 1	1050
	21st to end	Night	13% 8% 35% 32% 12% 1	1155
February	1st to 10th	Night	13% 7% 31% 35% 13% 1	1050
	11th to 20th	Night	11% 6% 31% 35% 14% 3	1029
	21st to end	Night	11% 8% 35% 33% 12% 1	848
March	1st to 10th	Night	13% 8% 33% 32% 13% 1	758
	11th to 20th	Night	11% 7% 40% 29% 11% 1	630
	21st to end	Night	18% 8% 30% 28% 13% 3	693
April	1st to 10th	Night	16% 7% 28% 36% 11% 1	630
	11th to 20th	Night	17% 8% 27% 35% 12% 1	623
	21st to end	Night	16% 8% 31% 33% 11%	630
May	1st to 10th	Night	19% 6% 35% 31% 8% 1	630
	11th to 20th	Night	19% 7% 32% 32% 11% 1	621
	21st to end	Night	19% 7% 31% 33% 10%	683
June	1st to 10th	Night	15% 7% 31% 32% 16%	630
	11th to 20th	Night	17% 6% 33% 33% 12% 1	630
	21st to end	Night	15% 7% 21% 37% 19% 1	630
July	1st to 10th	Night	7% 5% 25% 40% 22% 1	615
	11th to 20th	Night	9% 4 28% 41% 18% 1	612
	21st to end	Night	8% 3 24% 42% 20% 3	690
August	1st to 10th	Night	8% 4 23% 44% 20% 1	627
	11th to 20th	Night	9% 5% 23% 37% 23% 3	606
	21st to end	Night	15% 4 25% 38% 18% 1	660
September	1st to 10th	Night	12% 7% 29% 35% 16% 2	600
	11th to 20th	Night	19% 6% 27% 37% 12%	600
	21st to end	Night	19% 5% 32% 31% 12% 1	600
October	1st to 10th	Night	23% 8% 31% 31% 8%	775
	11th to 20th	Night	23% 6% 33% 29% 9% 1	800
	21st to end	Night	24% 8% 31% 28% 8% 1	1099
November	1st to 10th	Night	20% 9% 37% 25% 9% 1	1025
	11th to 20th	Night	21% 9% 33% 27% 8% 2	1050
	21st to end	Night	19% 10% 40% 24% 7% 1	1050
December	1st to 10th	Night	12% 8% 34% 34% 11% 2	1050
	11th to 20th	Night	16% 8% 38% 30% 8% 1	1050
	21st to end	Night	18% 7% 30% 30% 13% 1	1155

Page generated at 2026-08-12 02:37:16. Version ac07

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