Summerland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summerland clear sky chart from 2005-11-06 to 2026-06-01.

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	6% 7% 3 7% 5% 5% 5% 4 7% 8% 42%	136457
	Night	6% 9% 3 8% 6% 5% 5% 4 6% 8% 39%	135365
Transparency	Day	38% 10% 17% 22% 11% 3	136178
	Night	41% 11% 17% 21% 9% 2	135222
Seeing	Day	13% 17% 29% 26% 12% 3	17211
	Night	13% 10% 28% 31% 16% 2	25694

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% 5% 10% 7% 8% 6% 5% 8% 8% 22%	4723
	11th to 20th	Day	4 9% 4 7% 6% 5% 5% 3 7% 11% 39%	4716
	21st to end	Day	6% 12% 4 9% 7% 6% 6% 5% 8% 7% 31%	5186
February	1st to 10th	Day	5% 13% 5% 13% 8% 6% 7% 5% 6% 6% 27%	1738
	11th to 20th	Day	5% 11% 4 11% 7% 6% 6% 6% 7% 7% 30%	1845
	21st to end	Day	4 11% 5% 12% 8% 7% 8% 5% 8% 7% 24%	1709
March	1st to 10th	Day	2 7% 3 9% 7% 7% 6% 6% 9% 9% 36%	4618
	11th to 20th	Day	3 8% 3 8% 6% 5% 6% 4 6% 9% 41%	4880
	21st to end	Day	8% 12% 3 10% 6% 6% 6% 5% 7% 9% 28%	6042
April	1st to 10th	Day	7% 11% 3 9% 6% 6% 7% 6% 6% 9% 30%	2520
	11th to 20th	Day	6% 9% 3 8% 6% 6% 6% 5% 8% 9% 34%	2502
	21st to end	Day	7% 9% 3 9% 5% 6% 6% 4 7% 9% 35%	2873
May	1st to 10th	Day	6% 7% 2 6% 4 4 5% 4 6% 7% 48%	6460
	11th to 20th	Day	8% 8% 3 9% 5% 6% 7% 5% 7% 9% 33%	6417
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 7% 8% 38%	7057
June	1st to 10th	Day	13% 7% 2 8% 6% 6% 5% 5% 6% 7% 36%	2971
	11th to 20th	Day	13% 8% 2 9% 6% 5% 6% 5% 8% 8% 31%	3199
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 8% 42%	3197
July	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 6% 67%	6780
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 6% 71%	6330
	21st to end	Day	2 2 1 3 3 3 4 4 6% 9% 62%	6874
August	1st to 10th	Day	2 1 1 2 3 4 5% 5% 7% 7% 64%	4541
	11th to 20th	Day	3 2 1 4 3 4 4 5% 7% 8% 57%	4424
	21st to end	Day	6% 4 1 4 3 4 5% 5% 6% 8% 54%	4354
September	1st to 10th	Day	5% 4 2 5% 4 3 3 3 5% 8% 59%	2400
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 6% 7% 49%	2381
	21st to end	Day	8% 7% 2 6% 4 4 4 5% 5% 7% 48%	2177
October	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 4 6% 8% 51%	3280
	11th to 20th	Day	7% 7% 3 9% 6% 6% 6% 5% 5% 7% 40%	3172
	21st to end	Day	8% 10% 3 8% 5% 5% 5% 4 7% 9% 37%	3200
November	1st to 10th	Day	9% 12% 4 9% 7% 7% 5% 4 7% 7% 29%	1759
	11th to 20th	Day	8% 12% 5% 11% 8% 6% 7% 5% 6% 6% 25%	1680
	21st to end	Day	6% 12% 6% 11% 7% 8% 8% 5% 7% 8% 23%	1588
December	1st to 10th	Day	3 8% 4 9% 12% 9% 7% 6% 7% 12% 22%	2360
	11th to 20th	Day	8% 13% 5% 13% 9% 7% 7% 5% 7% 8% 18%	2712
	21st to end	Day	6% 14% 5% 14% 8% 7% 7% 4 6% 8% 20%	3792
January	1st to 10th	Night	6% 17% 6% 13% 8% 7% 6% 5% 6% 7% 17%	8172
	11th to 20th	Night	4 8% 3 10% 6% 5% 5% 5% 8% 10% 36%	8182
	21st to end	Night	5% 12% 4 9% 7% 6% 7% 5% 6% 7% 32%	8994
February	1st to 10th	Night	7% 13% 5% 14% 7% 5% 6% 5% 7% 7% 25%	2882
	11th to 20th	Night	6% 13% 4 9% 6% 6% 6% 3 6% 7% 34%	2662
	21st to end	Night	5% 12% 5% 12% 7% 6% 6% 5% 8% 8% 27%	2096
March	1st to 10th	Night	3 6% 3 8% 6% 5% 6% 5% 8% 10% 40%	5496
	11th to 20th	Night	3 9% 3 9% 5% 5% 5% 4 6% 8% 42%	5337
	21st to end	Night	7% 11% 4 9% 6% 7% 7% 5% 8% 9% 27%	5174
April	1st to 10th	Night	6% 9% 2 7% 5% 6% 5% 3 8% 9% 38%	2100
	11th to 20th	Night	5% 8% 3 7% 6% 6% 6% 6% 7% 9% 37%	2068
	21st to end	Night	7% 9% 3 8% 5% 5% 6% 4 6% 8% 39%	1743
May	1st to 10th	Night	5% 5% 2 4 4 3 4 5% 7% 9% 52%	3720
	11th to 20th	Night	7% 7% 3 7% 4 5% 5% 4 6% 10% 41%	3696
	21st to end	Night	8% 5% 2 5% 4 4 5% 4 7% 8% 48%	4060
June	1st to 10th	Night	10% 7% 2 7% 5% 4 4 5% 7% 9% 39%	1421
	11th to 20th	Night	9% 7% 2 7% 4 5% 4 5% 6% 9% 42%	1200
	21st to end	Night	5% 4 2 5% 5% 4 7% 4 7% 7% 52%	1200
July	1st to 10th	Night	4 2 1 2 2 2 3 2 4 6% 71%	3177
	11th to 20th	Night	2 1 1 2 2 2 3 3 4 6% 73%	3612
	21st to end	Night	3 3 1 3 3 3 3 3 4 7% 68%	3969
August	1st to 10th	Night	2 1 1 4 3 4 4 4 6% 7% 65%	2609
	11th to 20th	Night	3 3 1 3 3 4 4 4 6% 10% 59%	2610
	21st to end	Night	5% 4 1 5% 4 3 4 4 7% 8% 55%	3564
September	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 7% 63%	2000
	11th to 20th	Night	6% 5% 2 4 4 3 4 4 5% 7% 56%	2018
	21st to end	Night	7% 6% 2 7% 4 4 4 3 7% 8% 49%	2220
October	1st to 10th	Night	6% 5% 1 4 3 4 4 4 7% 8% 52%	3920
	11th to 20th	Night	11% 7% 2 7% 5% 4 4 4 6% 8% 42%	3982
	21st to end	Night	9% 9% 3 7% 5% 5% 5% 4 6% 7% 41%	4600
November	1st to 10th	Night	10% 11% 4 9% 6% 7% 5% 5% 7% 8% 28%	2758
	11th to 20th	Night	7% 13% 5% 10% 8% 6% 6% 5% 7% 7% 26%	2940
	21st to end	Night	6% 15% 6% 11% 8% 6% 7% 4 7% 8% 21%	3008
December	1st to 10th	Night	4 9% 3 10% 9% 9% 7% 4 5% 11% 29%	5040
	11th to 20th	Night	7% 14% 5% 15% 11% 8% 7% 5% 6% 6% 17%	5700
	21st to end	Night	9% 15% 5% 14% 8% 7% 6% 4 5% 6% 19%	7435

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	53% 16% 15% 13% 2	4723
	11th to 20th	Day	37% 11% 24% 20% 5% 3	4740
	21st to end	Day	48% 14% 18% 15% 5%	5187
February	1st to 10th	Day	56% 11% 13% 12% 7% 1	1738
	11th to 20th	Day	52% 12% 17% 14% 5%	1854
	21st to end	Day	56% 14% 15% 11% 5%	1709
March	1st to 10th	Day	38% 14% 19% 19% 10%	4618
	11th to 20th	Day	38% 10% 18% 29% 5%	4880
	21st to end	Day	50% 13% 16% 13% 8%	6042
April	1st to 10th	Day	50% 9% 19% 17% 4	2520
	11th to 20th	Day	46% 10% 16% 19% 8% 1	2502
	21st to end	Day	45% 9% 16% 19% 10% 1	2873
May	1st to 10th	Day	31% 10% 12% 28% 18% 2	6460
	11th to 20th	Day	41% 11% 20% 22% 6%	6420
	21st to end	Day	39% 10% 18% 26% 6%	7058
June	1st to 10th	Day	48% 8% 14% 16% 11% 4	2971
	11th to 20th	Day	47% 10% 15% 18% 9% 1	3199
	21st to end	Day	37% 10% 17% 21% 13% 2	3197
July	1st to 10th	Day	19% 6% 11% 26% 24% 14%	6706
	11th to 20th	Day	15% 5% 12% 29% 26% 14%	6254
	21st to end	Day	16% 7% 17% 32% 22% 7%	6681
August	1st to 10th	Day	16% 7% 13% 29% 28% 6%	4524
	11th to 20th	Day	22% 8% 21% 32% 14% 3	4466
	21st to end	Day	26% 8% 22% 28% 14% 2	4353
September	1st to 10th	Day	26% 7% 17% 28% 18% 3	2400
	11th to 20th	Day	32% 9% 14% 24% 17% 3	2381
	21st to end	Day	36% 9% 21% 22% 11% 2	2177
October	1st to 10th	Day	32% 11% 21% 31% 6%	3280
	11th to 20th	Day	43% 10% 13% 28% 6%	3172
	21st to end	Day	43% 12% 19% 21% 5%	3200
November	1st to 10th	Day	53% 12% 17% 13% 4	1759
	11th to 20th	Day	58% 13% 15% 10% 4 1	1680
	21st to end	Day	57% 13% 16% 10% 3	1588
December	1st to 10th	Day	51% 19% 20% 10% 1	2360
	11th to 20th	Day	61% 14% 13% 9% 3	2714
	21st to end	Day	59% 15% 14% 11% 2	3792
January	1st to 10th	Night	61% 13% 14% 10% 2	8172
	11th to 20th	Night	39% 10% 22% 22% 6% 2	8220
	21st to end	Night	48% 13% 16% 18% 4	8994
February	1st to 10th	Night	57% 10% 15% 12% 6% 1	2882
	11th to 20th	Night	50% 9% 15% 18% 7%	2664
	21st to end	Night	52% 11% 18% 14% 5%	2096
March	1st to 10th	Night	35% 12% 21% 24% 8%	5496
	11th to 20th	Night	38% 9% 19% 26% 8%	5338
	21st to end	Night	51% 13% 14% 16% 6%	5174
April	1st to 10th	Night	40% 12% 24% 19% 5%	2100
	11th to 20th	Night	41% 11% 18% 20% 10% 1	2068
	21st to end	Night	41% 9% 16% 21% 11% 1	1743
May	1st to 10th	Night	25% 10% 13% 34% 17% 1	3717
	11th to 20th	Night	34% 11% 23% 26% 6%	3696
	21st to end	Night	28% 11% 23% 32% 5% 1	4060
June	1st to 10th	Night	39% 12% 14% 20% 11% 3	1421
	11th to 20th	Night	38% 9% 19% 23% 11%	1200
	21st to end	Night	29% 9% 22% 25% 12% 3	1200
July	1st to 10th	Night	16% 4 14% 26% 28% 11%	3142
	11th to 20th	Night	12% 5% 18% 25% 26% 14%	3565
	21st to end	Night	17% 5% 14% 37% 19% 8%	3857
August	1st to 10th	Night	18% 6% 10% 30% 31% 5%	2601
	11th to 20th	Night	19% 7% 25% 31% 15% 3	2634
	21st to end	Night	25% 8% 23% 27% 15% 2	3564
September	1st to 10th	Night	22% 7% 17% 30% 20% 4	2000
	11th to 20th	Night	27% 9% 13% 27% 20% 4	2018
	21st to end	Night	33% 9% 20% 25% 11% 2	2220
October	1st to 10th	Night	29% 11% 23% 28% 10%	3920
	11th to 20th	Night	40% 9% 16% 27% 7%	3982
	21st to end	Night	41% 10% 17% 26% 5%	4600
November	1st to 10th	Night	53% 13% 15% 14% 5%	2758
	11th to 20th	Night	56% 13% 14% 12% 4 1	2940
	21st to end	Night	59% 13% 16% 9% 4	3008
December	1st to 10th	Night	45% 19% 20% 14% 2	5040
	11th to 20th	Night	65% 14% 11% 8% 2	5697
	21st to end	Night	58% 15% 13% 12% 2	7435

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	16% 19% 28% 25% 10% 3	220
	11th to 20th	Day	15% 15% 30% 25% 11% 3	220
	21st to end	Day	15% 18% 32% 25% 10% 1	242
February	1st to 10th	Day	14% 21% 38% 20% 6% 1	278
	11th to 20th	Day	13% 21% 36% 22% 7% 1	425
	21st to end	Day	15% 19% 36% 21% 9% 1	435
March	1st to 10th	Day	15% 24% 30% 21% 8% 1	540
	11th to 20th	Day	14% 26% 27% 21% 11% 2	540
	21st to end	Day	17% 24% 29% 21% 8% 2	594
April	1st to 10th	Day	21% 22% 29% 17% 9% 2	540
	11th to 20th	Day	17% 22% 28% 20% 9% 4	532
	21st to end	Day	15% 16% 33% 21% 11% 3	540
May	1st to 10th	Day	12% 20% 33% 24% 9% 2	543
	11th to 20th	Day	15% 14% 28% 24% 15% 3	568
	21st to end	Day	15% 15% 29% 26% 13% 2	617
June	1st to 10th	Day	15% 15% 31% 27% 12% 1	701
	11th to 20th	Day	15% 18% 30% 25% 11% 2	929
	21st to end	Day	9% 11% 29% 29% 19% 3	927
July	1st to 10th	Day	7% 11% 32% 32% 16% 2	721
	11th to 20th	Day	6% 14% 27% 33% 17% 4	533
	21st to end	Day	5% 9% 26% 38% 16% 6%	575
August	1st to 10th	Day	5% 7% 26% 38% 18% 6%	527
	11th to 20th	Day	6% 10% 27% 31% 19% 7%	517
	21st to end	Day	8% 17% 24% 29% 16% 6%	583
September	1st to 10th	Day	9% 14% 28% 24% 17% 8%	530
	11th to 20th	Day	11% 13% 24% 30% 16% 6%	530
	21st to end	Day	16% 18% 32% 22% 10% 3	530
October	1st to 10th	Day	14% 19% 29% 23% 13% 3	530
	11th to 20th	Day	17% 20% 26% 24% 11% 2	479
	21st to end	Day	20% 17% 31% 24% 6% 1	363
November	1st to 10th	Day	23% 17% 33% 22% 5%	280
	11th to 20th	Day	21% 22% 28% 18% 9% 2	220
	21st to end	Day	16% 21% 29% 20% 11% 3	220
December	1st to 10th	Day	18% 17% 20% 24% 18% 4	220
	11th to 20th	Day	14% 20% 34% 23% 8% 2	220
	21st to end	Day	13% 17% 31% 29% 8% 2	242
January	1st to 10th	Night	17% 16% 33% 25% 8% 1	840
	11th to 20th	Night	15% 11% 34% 29% 9% 1	840
	21st to end	Night	13% 12% 32% 31% 10% 2	924
February	1st to 10th	Night	15% 13% 33% 28% 8% 2	840
	11th to 20th	Night	12% 10% 31% 35% 11% 1	824
	21st to end	Night	13% 15% 26% 28% 16% 1	692
March	1st to 10th	Night	11% 11% 33% 27% 16% 2	840
	11th to 20th	Night	11% 13% 32% 28% 13% 1	840
	21st to end	Night	12% 12% 31% 31% 14% 1	924
April	1st to 10th	Night	12% 12% 26% 34% 14% 2	840
	11th to 20th	Night	11% 12% 30% 30% 16% 1	820
	21st to end	Night	12% 10% 32% 32% 12% 1	483
May	1st to 10th	Night	10% 9% 29% 36% 15% 1	420
	11th to 20th	Night	11% 5% 20% 39% 23% 2	414
	21st to end	Night	13% 7% 23% 31% 24% 2	455
June	1st to 10th	Night	11% 9% 28% 38% 14% 1	400
	11th to 20th	Night	12% 7% 28% 33% 18% 1	400
	21st to end	Night	6% 7% 24% 40% 21% 2	400
July	1st to 10th	Night	6% 4 20% 40% 28% 1	390
	11th to 20th	Night	4 7% 23% 40% 24% 2	388
	21st to end	Night	4 5% 17% 43% 29% 3	438
August	1st to 10th	Night	5% 3 16% 37% 35% 5%	398
	11th to 20th	Night	5% 2 15% 39% 34% 4	416
	21st to end	Night	6% 5% 20% 36% 30% 3	849
September	1st to 10th	Night	6% 6% 21% 35% 29% 4	800
	11th to 20th	Night	9% 6% 16% 32% 28% 9%	800
	21st to end	Night	13% 8% 22% 35% 20% 3	800
October	1st to 10th	Night	13% 7% 20% 34% 23% 3	800
	11th to 20th	Night	12% 11% 25% 34% 16% 2	800
	21st to end	Night	18% 11% 30% 27% 11% 3	880
November	1st to 10th	Night	18% 13% 31% 30% 7% 1	820
	11th to 20th	Night	19% 15% 29% 25% 11% 1	840
	21st to end	Night	16% 14% 29% 27% 10% 3	908
December	1st to 10th	Night	13% 13% 31% 30% 12% 2	1050
	11th to 20th	Night	18% 13% 36% 24% 8% 1	1050
	21st to end	Night	20% 13% 34% 27% 7%	1071

Page generated at 2026-05-28 22:53:25. Version ac07

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