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 2025-03-27.

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% 10% 4 9% 6% 5% 5% 4 6% 7% 32%	112395
	Night	10% 11% 4 10% 6% 5% 5% 4 6% 7% 30%	115650
Transparency	Day	49% 7% 14% 22% 7%	112136
	Night	52% 6% 13% 21% 8%	115556
Seeing	Day	15% 10% 31% 28% 13% 3	16688
	Night	16% 7% 31% 33% 13% 1	26832

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	5% 15% 8% 16% 7% 6% 8% 7% 5% 8% 16%	3179
	11th to 20th	Day	4 12% 6% 16% 10% 11% 9% 8% 9% 6% 10%	3159
	21st to end	Day	5% 19% 9% 18% 8% 7% 7% 4 6% 5% 12%	3884
February	1st to 10th	Day	8% 13% 6% 11% 8% 7% 9% 6% 7% 7% 18%	1750
	11th to 20th	Day	7% 10% 4 10% 8% 9% 6% 7% 8% 8% 24%	1754
	21st to end	Day	6% 12% 6% 10% 9% 8% 6% 5% 5% 7% 24%	1520
March	1st to 10th	Day	9% 12% 4 11% 10% 5% 5% 5% 5% 6% 26%	3355
	11th to 20th	Day	7% 10% 6% 15% 11% 7% 6% 5% 5% 6% 23%	3588
	21st to end	Day	9% 15% 6% 12% 6% 4 4 3 4 6% 32%	3880
April	1st to 10th	Day	11% 11% 4 9% 6% 3 4 4 5% 6% 36%	2308
	11th to 20th	Day	13% 10% 4 7% 5% 5% 4 4 6% 7% 35%	2442
	21st to end	Day	14% 9% 3 7% 5% 4 5% 4 6% 7% 36%	2470
May	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 7% 38%	4334
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 3 5% 7% 32%	4550
	21st to end	Day	21% 9% 3 7% 4 4 4 3 5% 6% 34%	5173
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 5% 8% 35%	2850
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 4 5% 8% 34%	2850
	21st to end	Day	12% 7% 2 7% 6% 5% 6% 5% 7% 8% 33%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 6% 6% 5% 7% 9% 40%	4750
	11th to 20th	Day	8% 5% 2 4 3 4 4 4 6% 9% 50%	4750
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	5150
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 4 4 6% 9% 44%	4248
	11th to 20th	Day	11% 5% 2 6% 4 5% 4 4 6% 8% 43%	4041
	21st to end	Day	15% 6% 2 6% 4 3 5% 4 5% 9% 42%	4532
September	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 6% 7% 46%	2353
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 6% 9% 35%	2114
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 40%	2090
October	1st to 10th	Day	13% 10% 3 8% 4 4 4 3 5% 7% 39%	3165
	11th to 20th	Day	11% 10% 4 7% 4 4 5% 3 6% 7% 38%	3140
	21st to end	Day	16% 13% 4 8% 5% 4 5% 4 6% 9% 28%	3061
November	1st to 10th	Day	15% 18% 4 10% 5% 6% 4 4 5% 5% 25%	1567
	11th to 20th	Day	12% 17% 5% 15% 8% 5% 4 3 5% 5% 22%	1600
	21st to end	Day	13% 14% 6% 16% 11% 5% 3 4 4 5% 19%	1511
December	1st to 10th	Day	12% 15% 7% 18% 11% 10% 4 4 4 5% 10%	2290
	11th to 20th	Day	9% 14% 6% 14% 6% 4 5% 5% 8% 9% 20%	2639
	21st to end	Day	8% 14% 8% 18% 6% 7% 6% 5% 6% 6% 15%	3498
January	1st to 10th	Night	4 13% 7% 17% 8% 9% 8% 6% 7% 7% 14%	6787
	11th to 20th	Night	5% 15% 6% 13% 10% 11% 10% 9% 8% 5% 10%	6759
	21st to end	Night	7% 19% 7% 14% 7% 6% 6% 4 7% 6% 17%	7089
February	1st to 10th	Night	7% 13% 5% 11% 7% 6% 7% 5% 6% 7% 25%	2648
	11th to 20th	Night	6% 9% 4 8% 6% 7% 7% 6% 7% 8% 32%	2533
	21st to end	Night	6% 11% 5% 11% 8% 7% 6% 4 6% 6% 30%	2129
March	1st to 10th	Night	9% 12% 4 12% 7% 5% 6% 4 5% 7% 28%	3724
	11th to 20th	Night	5% 10% 4 9% 5% 5% 4 4 6% 8% 41%	3590
	21st to end	Night	11% 14% 4 9% 5% 4 4 3 6% 6% 33%	3889
April	1st to 10th	Night	11% 9% 3 5% 5% 4 5% 4 6% 7% 40%	1869
	11th to 20th	Night	14% 10% 4 8% 4 4 4 3 4 6% 38%	1689
	21st to end	Night	11% 10% 4 7% 5% 5% 6% 3 5% 5% 38%	1710
May	1st to 10th	Night	19% 9% 3 6% 4 3 4 3 5% 7% 37%	2634
	11th to 20th	Night	17% 7% 2 5% 4 5% 3 3 6% 7% 40%	2341
	21st to end	Night	18% 10% 2 4 4 3 3 3 6% 5% 40%	2414
June	1st to 10th	Night	12% 6% 2 6% 5% 4 5% 4 7% 10% 39%	1330
	11th to 20th	Night	16% 6% 2 5% 4 4 4 4 5% 9% 41%	1330
	21st to end	Night	12% 6% 2 5% 4 5% 4 4 6% 8% 44%	1330
July	1st to 10th	Night	9% 4 1 4 4 4 5% 4 6% 9% 50%	2220
	11th to 20th	Night	5% 3 1 4 3 4 4 4 6% 10% 54%	2220
	21st to end	Night	5% 3 2 4 4 4 5% 4 7% 10% 53%	2414
August	1st to 10th	Night	6% 4 2 5% 4 5% 5% 4 8% 10% 47%	2688
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 6% 9% 50%	2798
	21st to end	Night	11% 5% 2 5% 5% 4 5% 4 5% 9% 45%	3135
September	1st to 10th	Night	11% 4 2 6% 4 4 4 3 5% 7% 51%	1826
	11th to 20th	Night	16% 7% 2 7% 3 4 5% 3 4 8% 42%	2063
	21st to end	Night	17% 6% 3 6% 4 4 4 3 5% 7% 40%	2090
October	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 5% 8% 45%	3763
	11th to 20th	Night	12% 10% 3 8% 4 4 4 4 5% 6% 39%	3779
	21st to end	Night	11% 12% 4 9% 5% 5% 5% 3 5% 6% 35%	4495
November	1st to 10th	Night	14% 15% 5% 9% 5% 5% 5% 4 5% 5% 28%	2731
	11th to 20th	Night	12% 18% 6% 12% 7% 5% 4 3 4 6% 23%	2800
	21st to end	Night	12% 16% 7% 15% 9% 6% 5% 3 4 5% 18%	2885
December	1st to 10th	Night	9% 19% 7% 19% 10% 6% 5% 3 4 4 12%	4900
	11th to 20th	Night	8% 13% 6% 14% 7% 5% 5% 7% 7% 7% 20%	5568
	21st to end	Night	8% 12% 7% 20% 8% 6% 6% 5% 6% 5% 17%	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	66% 6% 8% 13% 7%	3179
	11th to 20th	Day	68% 5% 9% 14% 4	3180
	21st to end	Day	72% 5% 10% 11% 3	3884
February	1st to 10th	Day	60% 5% 13% 13% 8% 1	1750
	11th to 20th	Day	53% 6% 15% 20% 7%	1763
	21st to end	Day	58% 6% 13% 14% 9% 1	1520
March	1st to 10th	Day	55% 6% 15% 17% 6%	3355
	11th to 20th	Day	55% 10% 13% 15% 6%	3590
	21st to end	Day	53% 6% 12% 22% 7% 1	3880
April	1st to 10th	Day	48% 6% 11% 23% 12% 1	2308
	11th to 20th	Day	48% 6% 11% 22% 12% 2	2442
	21st to end	Day	47% 6% 14% 23% 9% 1	2470
May	1st to 10th	Day	44% 7% 12% 25% 11% 1	4334
	11th to 20th	Day	49% 7% 10% 21% 11% 1	4554
	21st to end	Day	50% 7% 10% 23% 10% 1	5173
June	1st to 10th	Day	46% 7% 14% 22% 11% 1	2850
	11th to 20th	Day	47% 7% 14% 21% 10% 1	2850
	21st to end	Day	45% 9% 17% 21% 7% 1	2850
July	1st to 10th	Day	38% 9% 20% 26% 7%	4676
	11th to 20th	Day	29% 7% 20% 37% 7%	4669
	21st to end	Day	32% 7% 19% 34% 8%	4988
August	1st to 10th	Day	36% 7% 17% 31% 9%	4231
	11th to 20th	Day	36% 7% 21% 30% 5%	4080
	21st to end	Day	40% 6% 21% 28% 5%	4531
September	1st to 10th	Day	36% 8% 17% 29% 9% 1	2353
	11th to 20th	Day	45% 8% 19% 24% 4	2114
	21st to end	Day	45% 6% 13% 30% 7%	2090
October	1st to 10th	Day	45% 5% 15% 28% 6%	3165
	11th to 20th	Day	44% 5% 13% 25% 13%	3140
	21st to end	Day	52% 6% 18% 20% 4	3061
November	1st to 10th	Day	61% 7% 13% 14% 4 1	1567
	11th to 20th	Day	63% 7% 14% 11% 4	1600
	21st to end	Day	67% 6% 13% 11% 2	1511
December	1st to 10th	Day	76% 7% 8% 6% 2	2290
	11th to 20th	Day	59% 5% 14% 16% 7%	2640
	21st to end	Day	65% 7% 11% 14% 3	3498
January	1st to 10th	Night	66% 6% 10% 11% 8%	6787
	11th to 20th	Night	70% 6% 7% 14% 3	6800
	21st to end	Night	65% 7% 10% 13% 5%	7089
February	1st to 10th	Night	55% 6% 13% 17% 9%	2648
	11th to 20th	Night	46% 6% 17% 22% 9%	2534
	21st to end	Night	52% 7% 11% 19% 10%	2129
March	1st to 10th	Night	53% 7% 12% 18% 10% 1	3724
	11th to 20th	Night	38% 9% 17% 24% 11% 1	3590
	21st to end	Night	50% 7% 10% 22% 10% 1	3889
April	1st to 10th	Night	41% 6% 11% 23% 16% 2	1869
	11th to 20th	Night	47% 5% 8% 21% 16% 2	1689
	21st to end	Night	47% 5% 10% 24% 12% 1	1710
May	1st to 10th	Night	45% 7% 10% 24% 12%	2634
	11th to 20th	Night	41% 8% 12% 28% 9% 3	2341
	21st to end	Night	44% 6% 11% 26% 13% 1	2414
June	1st to 10th	Night	38% 9% 17% 23% 12% 1	1330
	11th to 20th	Night	40% 8% 14% 22% 14% 2	1330
	21st to end	Night	37% 8% 19% 26% 9% 1	1330
July	1st to 10th	Night	29% 8% 21% 34% 7% 1	2185
	11th to 20th	Night	23% 7% 23% 38% 7% 1	2178
	21st to end	Night	26% 6% 23% 36% 8% 1	2337
August	1st to 10th	Night	31% 5% 16% 35% 11%	2681
	11th to 20th	Night	29% 7% 25% 31% 7%	2825
	21st to end	Night	35% 6% 21% 31% 7%	3135
September	1st to 10th	Night	32% 6% 18% 31% 12%	1826
	11th to 20th	Night	42% 7% 16% 25% 9% 1	2063
	21st to end	Night	43% 5% 14% 29% 8% 1	2090
October	1st to 10th	Night	40% 4 12% 32% 11%	3763
	11th to 20th	Night	45% 5% 14% 28% 8%	3779
	21st to end	Night	51% 5% 13% 26% 5%	4495
November	1st to 10th	Night	58% 6% 15% 15% 5% 1	2731
	11th to 20th	Night	64% 6% 13% 12% 4	2800
	21st to end	Night	69% 6% 12% 9% 3	2885
December	1st to 10th	Night	74% 5% 11% 8% 2	4900
	11th to 20th	Night	61% 6% 11% 13% 9%	5566
	21st to end	Night	66% 6% 9% 14% 4	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	12% 7% 34% 29% 14% 4	300
	11th to 20th	Day	10% 8% 28% 37% 15% 1	300
	21st to end	Day	14% 9% 33% 28% 12% 3	330
February	1st to 10th	Day	15% 10% 32% 31% 9% 2	300
	11th to 20th	Day	11% 10% 33% 28% 15% 3	300
	21st to end	Day	10% 8% 39% 32% 11%	249
March	1st to 10th	Day	9% 11% 35% 29% 14% 1	300
	11th to 20th	Day	12% 11% 32% 28% 14% 4	300
	21st to end	Day	13% 10% 31% 28% 16% 2	315
April	1st to 10th	Day	13% 9% 29% 35% 11% 3	488
	11th to 20th	Day	17% 13% 31% 26% 13% 1	640
	21st to end	Day	17% 10% 35% 26% 10% 2	650
May	1st to 10th	Day	18% 12% 32% 24% 11% 2	653
	11th to 20th	Day	21% 11% 26% 27% 13% 3	675
	21st to end	Day	22% 10% 24% 27% 14% 2	734
June	1st to 10th	Day	17% 13% 28% 24% 13% 5%	680
	11th to 20th	Day	18% 12% 33% 26% 9% 3	680
	21st to end	Day	13% 8% 26% 31% 16% 5%	680
July	1st to 10th	Day	8% 8% 30% 27% 18% 8%	671
	11th to 20th	Day	10% 9% 30% 28% 18% 4	668
	21st to end	Day	8% 6% 29% 29% 19% 8%	739
August	1st to 10th	Day	7% 9% 29% 30% 18% 6%	676
	11th to 20th	Day	11% 6% 25% 32% 18% 8%	665
	21st to end	Day	15% 9% 30% 27% 17% 3	748
September	1st to 10th	Day	11% 8% 29% 35% 15% 2	563
	11th to 20th	Day	17% 11% 38% 22% 9% 2	324
	21st to end	Day	21% 8% 36% 27% 8% 1	300
October	1st to 10th	Day	18% 15% 28% 27% 8% 4	300
	11th to 20th	Day	17% 10% 34% 30% 8% 1	300
	21st to end	Day	27% 12% 29% 22% 9% 2	330
November	1st to 10th	Day	22% 11% 29% 27% 11% 1	300
	11th to 20th	Day	17% 14% 36% 26% 6% 1	300
	21st to end	Day	17% 12% 37% 29% 5%	300
December	1st to 10th	Day	16% 7% 32% 29% 13% 3	300
	11th to 20th	Day	17% 7% 40% 27% 9% 1	300
	21st to end	Day	16% 6% 28% 31% 14% 5%	330
January	1st to 10th	Night	11% 8% 33% 31% 15% 2	1000
	11th to 20th	Night	16% 5% 32% 35% 11% 1	1000
	21st to end	Night	14% 8% 34% 32% 12% 1	1100
February	1st to 10th	Night	13% 6% 31% 36% 13% 1	1000
	11th to 20th	Night	11% 6% 31% 35% 15% 3	979
	21st to end	Night	11% 7% 35% 33% 13% 1	809
March	1st to 10th	Night	13% 7% 33% 32% 14% 1	722
	11th to 20th	Night	12% 7% 40% 29% 12% 1	600
	21st to end	Night	18% 7% 29% 29% 14% 3	647
April	1st to 10th	Night	15% 6% 29% 37% 11% 1	570
	11th to 20th	Night	18% 7% 26% 34% 13% 1	563
	21st to end	Night	16% 8% 31% 34% 11%	570
May	1st to 10th	Night	20% 6% 34% 30% 9%	570
	11th to 20th	Night	18% 7% 30% 32% 12% 1	561
	21st to end	Night	20% 7% 32% 31% 10%	617
June	1st to 10th	Night	15% 6% 29% 33% 17%	570
	11th to 20th	Night	18% 6% 31% 32% 13% 1	570
	21st to end	Night	14% 6% 20% 38% 20% 1	570
July	1st to 10th	Night	7% 4 25% 41% 22% 1	555
	11th to 20th	Night	8% 4 27% 41% 19% 1	552
	21st to end	Night	9% 3 23% 40% 22% 3	624
August	1st to 10th	Night	8% 4 22% 44% 21% 1	567
	11th to 20th	Night	9% 5% 23% 37% 23% 3	564
	21st to end	Night	15% 4 24% 38% 18% 1	627
September	1st to 10th	Night	12% 6% 29% 35% 16% 2	570
	11th to 20th	Night	18% 6% 27% 37% 12%	570
	21st to end	Night	20% 5% 32% 32% 11% 1	570
October	1st to 10th	Night	23% 7% 30% 31% 8%	736
	11th to 20th	Night	21% 6% 33% 29% 9% 1	760
	21st to end	Night	23% 9% 31% 29% 8% 1	1044
November	1st to 10th	Night	20% 8% 37% 25% 10% 1	975
	11th to 20th	Night	21% 9% 32% 28% 7% 2	1000
	21st to end	Night	19% 10% 39% 24% 7% 1	1000
December	1st to 10th	Night	13% 8% 32% 35% 11% 2	1000
	11th to 20th	Night	16% 7% 38% 30% 8% 1	1000
	21st to end	Night	18% 6% 29% 31% 14% 1	1100

Page generated at 2025-03-24 08:04:28. Version ac07

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