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 2025-02-06.

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% 8% 3 8% 6% 5% 5% 5% 6% 8% 40%	112585
	Night	6% 9% 3 9% 6% 5% 5% 4 6% 8% 37%	114294
Transparency	Day	40% 10% 16% 20% 11% 3	112347
	Night	42% 11% 17% 19% 9% 2	114178
Seeing	Day	13% 17% 29% 25% 12% 3	15470
	Night	13% 10% 27% 31% 16% 2	24066

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% 15% 6% 12% 8% 8% 6% 5% 6% 7% 21%	3639
	11th to 20th	Day	5% 11% 4 8% 6% 5% 5% 4 8% 13% 30%	3616
	21st to end	Day	7% 14% 4 9% 7% 6% 6% 5% 7% 7% 29%	4004
February	1st to 10th	Day	5% 14% 5% 12% 8% 7% 7% 5% 6% 6% 26%	1612
	11th to 20th	Day	5% 12% 4 11% 7% 7% 6% 5% 7% 7% 29%	1665
	21st to end	Day	4 12% 5% 12% 9% 7% 8% 5% 7% 7% 24%	1551
March	1st to 10th	Day	2 10% 4 12% 9% 7% 7% 6% 8% 8% 27%	3138
	11th to 20th	Day	3 8% 2 7% 5% 5% 5% 4 6% 8% 47%	3330
	21st to end	Day	7% 13% 4 11% 6% 6% 7% 5% 7% 8% 26%	4121
April	1st to 10th	Day	8% 11% 3 9% 7% 7% 7% 6% 6% 8% 28%	2280
	11th to 20th	Day	7% 10% 3 9% 7% 7% 6% 5% 8% 8% 32%	2261
	21st to end	Day	8% 10% 3 9% 6% 6% 6% 5% 7% 9% 32%	2599
May	1st to 10th	Day	6% 9% 3 7% 5% 5% 5% 4 6% 8% 42%	4420
	11th to 20th	Day	6% 6% 3 8% 5% 6% 6% 5% 7% 9% 38%	4377
	21st to end	Day	13% 10% 3 9% 6% 6% 6% 5% 7% 7% 27%	4813
June	1st to 10th	Day	13% 8% 2 8% 7% 6% 5% 4 6% 7% 34%	2821
	11th to 20th	Day	13% 8% 2 9% 6% 5% 6% 5% 8% 8% 31%	3039
	21st to end	Day	8% 5% 2 6% 6% 5% 6% 5% 7% 8% 43%	3037
July	1st to 10th	Day	3 3 1 4 4 3 3 3 5% 5% 65%	4740
	11th to 20th	Day	2 3 1 3 2 3 3 3 4 6% 71%	4430
	21st to end	Day	3 2 1 3 3 3 3 3 6% 8% 64%	4790
August	1st to 10th	Day	2 1 1 2 3 4 5% 4 6% 7% 64%	4401
	11th to 20th	Day	3 2 1 4 3 4 4 5% 7% 8% 57%	4285
	21st to end	Day	6% 4 1 4 3 5% 5% 5% 6% 8% 54%	4221
September	1st to 10th	Day	5% 4 2 5% 4 3 3 3 5% 7% 58%	2280
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 7% 47%	2262
	21st to end	Day	8% 7% 2 6% 4 4 4 5% 6% 7% 47%	2068
October	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 4 6% 8% 51%	3180
	11th to 20th	Day	7% 7% 3 9% 5% 6% 6% 5% 5% 7% 40%	3075
	21st to end	Day	8% 10% 3 8% 5% 4 5% 4 7% 9% 37%	3101
November	1st to 10th	Day	9% 12% 4 9% 7% 7% 5% 4 7% 7% 28%	1674
	11th to 20th	Day	8% 12% 5% 11% 8% 6% 8% 5% 6% 6% 26%	1600
	21st to end	Day	6% 12% 6% 11% 8% 8% 8% 5% 7% 8% 22%	1513
December	1st to 10th	Day	3 8% 4 9% 12% 9% 7% 6% 7% 12% 22%	2290
	11th to 20th	Day	9% 13% 5% 13% 8% 7% 6% 5% 7% 8% 18%	2642
	21st to end	Day	6% 14% 5% 14% 8% 7% 7% 4 6% 8% 19%	3710
January	1st to 10th	Night	7% 18% 7% 15% 9% 7% 5% 5% 6% 6% 17%	6306
	11th to 20th	Night	4 10% 4 11% 6% 5% 5% 6% 9% 11% 29%	6282
	21st to end	Night	7% 14% 5% 10% 6% 6% 7% 5% 6% 7% 28%	6952
February	1st to 10th	Night	7% 15% 5% 15% 6% 5% 5% 4 7% 7% 24%	2678
	11th to 20th	Night	6% 13% 4 9% 6% 6% 6% 3 6% 7% 34%	2402
	21st to end	Night	5% 13% 5% 12% 8% 5% 6% 5% 7% 7% 26%	1903
March	1st to 10th	Night	3 7% 4 9% 7% 7% 8% 6% 8% 9% 32%	3736
	11th to 20th	Night	3 7% 2 7% 5% 4 4 4 6% 8% 50%	3647
	21st to end	Night	7% 12% 3 9% 6% 6% 6% 5% 7% 8% 29%	3531
April	1st to 10th	Night	6% 10% 2 7% 5% 6% 6% 4 9% 9% 37%	1900
	11th to 20th	Night	5% 8% 3 7% 6% 6% 6% 6% 7% 8% 37%	1870
	21st to end	Night	8% 9% 3 8% 6% 5% 6% 4 6% 8% 37%	1577
May	1st to 10th	Night	7% 7% 2 6% 4 4 5% 5% 6% 8% 46%	2540
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 7% 11% 46%	2516
	21st to end	Night	9% 7% 2 6% 5% 5% 6% 5% 8% 9% 40%	2762
June	1st to 10th	Night	10% 7% 2 7% 5% 4 4 5% 7% 9% 38%	1350
	11th to 20th	Night	9% 7% 2 6% 4 5% 4 6% 7% 9% 41%	1140
	21st to end	Night	4 4 2 5% 5% 4 7% 4 7% 7% 52%	1140
July	1st to 10th	Night	3 2 1 3 2 2 3 3 4 6% 72%	2217
	11th to 20th	Night	2 1 1 2 2 2 3 3 4 6% 74%	2526
	21st to end	Night	2 2 1 3 2 2 3 3 4 6% 71%	2762
August	1st to 10th	Night	2 1 1 3 3 4 4 4 6% 7% 65%	2529
	11th to 20th	Night	2 2 1 3 3 4 4 4 6% 10% 60%	2529
	21st to end	Night	5% 4 1 5% 4 3 5% 4 7% 8% 54%	3455
September	1st to 10th	Night	4 4 1 4 3 3 4 3 5% 7% 63%	1900
	11th to 20th	Night	6% 5% 2 4 4 3 4 4 6% 8% 55%	1917
	21st to end	Night	7% 6% 2 7% 4 4 4 3 7% 8% 49%	2109
October	1st to 10th	Night	6% 5% 1 4 3 4 4 5% 7% 8% 52%	3800
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 4 6% 8% 42%	3859
	21st to end	Night	9% 9% 3 7% 5% 5% 5% 4 6% 7% 40%	4457
November	1st to 10th	Night	10% 11% 4 9% 6% 7% 5% 5% 7% 7% 28%	2623
	11th to 20th	Night	6% 13% 5% 10% 8% 6% 6% 5% 7% 7% 26%	2800
	21st to end	Night	6% 15% 6% 11% 8% 6% 7% 4 7% 8% 22%	2865
December	1st to 10th	Night	4 9% 3 10% 9% 9% 7% 4 5% 11% 29%	4890
	11th to 20th	Night	7% 15% 6% 16% 10% 8% 7% 5% 6% 6% 17%	5550
	21st to end	Night	9% 16% 5% 14% 8% 7% 6% 4 5% 6% 19%	7274

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	57% 15% 15% 12% 2	3639
	11th to 20th	Day	42% 13% 23% 14% 4 3	3640
	21st to end	Day	51% 13% 19% 13% 4	4004
February	1st to 10th	Day	57% 11% 13% 12% 5% 1	1612
	11th to 20th	Day	53% 12% 17% 13% 6%	1674
	21st to end	Day	57% 13% 14% 12% 4	1551
March	1st to 10th	Day	49% 13% 14% 18% 6% 1	3138
	11th to 20th	Day	34% 10% 23% 29% 4	3330
	21st to end	Day	52% 14% 16% 10% 8%	4121
April	1st to 10th	Day	53% 10% 18% 16% 4	2280
	11th to 20th	Day	49% 10% 15% 17% 8% 1	2261
	21st to end	Day	48% 10% 16% 17% 8% 1	2599
May	1st to 10th	Day	35% 10% 16% 25% 13%	4420
	11th to 20th	Day	34% 11% 23% 25% 7%	4380
	21st to end	Day	49% 12% 16% 18% 4	4814
June	1st to 10th	Day	50% 9% 14% 15% 9% 4	2821
	11th to 20th	Day	49% 10% 15% 17% 8% 1	3039
	21st to end	Day	37% 10% 17% 21% 13% 2	3037
July	1st to 10th	Day	21% 6% 9% 19% 28% 17%	4666
	11th to 20th	Day	16% 4 8% 17% 34% 20%	4354
	21st to end	Day	17% 7% 13% 27% 27% 10%	4639
August	1st to 10th	Day	16% 7% 13% 29% 29% 6%	4384
	11th to 20th	Day	22% 8% 21% 32% 14% 3	4327
	21st to end	Day	26% 9% 22% 28% 13% 1	4220
September	1st to 10th	Day	27% 7% 17% 28% 18% 3	2280
	11th to 20th	Day	33% 10% 14% 23% 17% 3	2262
	21st to end	Day	36% 9% 21% 21% 11% 2	2068
October	1st to 10th	Day	32% 11% 20% 31% 6%	3180
	11th to 20th	Day	43% 10% 13% 29% 5%	3075
	21st to end	Day	43% 12% 19% 21% 5%	3101
November	1st to 10th	Day	54% 12% 17% 13% 4	1674
	11th to 20th	Day	57% 13% 15% 10% 4 1	1600
	21st to end	Day	57% 13% 16% 10% 4	1513
December	1st to 10th	Day	51% 19% 20% 10% 1	2290
	11th to 20th	Day	61% 14% 13% 9% 3	2644
	21st to end	Day	59% 15% 14% 11% 2	3710
January	1st to 10th	Night	64% 12% 13% 9% 1	6306
	11th to 20th	Night	43% 11% 21% 17% 6% 2	6320
	21st to end	Night	52% 13% 18% 13% 4	6952
February	1st to 10th	Night	59% 10% 14% 12% 5% 1	2678
	11th to 20th	Night	50% 9% 16% 18% 7%	2404
	21st to end	Night	54% 10% 17% 14% 5%	1903
March	1st to 10th	Night	42% 14% 18% 18% 8%	3736
	11th to 20th	Night	32% 9% 23% 31% 5%	3648
	21st to end	Night	48% 13% 16% 16% 7%	3531
April	1st to 10th	Night	41% 13% 24% 18% 5%	1900
	11th to 20th	Night	42% 11% 17% 19% 11% 1	1870
	21st to end	Night	44% 10% 16% 20% 10%	1577
May	1st to 10th	Night	33% 8% 16% 29% 13% 1	2540
	11th to 20th	Night	25% 12% 26% 28% 8%	2516
	21st to end	Night	35% 12% 23% 25% 4	2762
June	1st to 10th	Night	41% 12% 14% 20% 10% 3	1350
	11th to 20th	Night	38% 9% 20% 24% 9%	1140
	21st to end	Night	29% 10% 21% 25% 13% 3	1140
July	1st to 10th	Night	15% 4 9% 25% 33% 13%	2182
	11th to 20th	Night	12% 5% 10% 17% 35% 20%	2479
	21st to end	Night	14% 4 12% 33% 24% 12%	2674
August	1st to 10th	Night	17% 6% 10% 30% 32% 5%	2521
	11th to 20th	Night	18% 7% 26% 31% 15% 2	2553
	21st to end	Night	26% 8% 23% 27% 14% 1	3455
September	1st to 10th	Night	22% 7% 16% 30% 19% 4	1900
	11th to 20th	Night	27% 10% 14% 26% 20% 4	1917
	21st to end	Night	34% 9% 20% 24% 11% 2	2109
October	1st to 10th	Night	29% 11% 22% 28% 10%	3800
	11th to 20th	Night	41% 10% 16% 27% 7%	3859
	21st to end	Night	42% 10% 17% 26% 5%	4457
November	1st to 10th	Night	53% 12% 15% 14% 5%	2623
	11th to 20th	Night	55% 14% 14% 12% 4 1	2800
	21st to end	Night	59% 12% 16% 9% 4	2865
December	1st to 10th	Night	44% 19% 20% 14% 2	4890
	11th to 20th	Night	65% 14% 11% 8% 2	5547
	21st to end	Night	58% 15% 13% 12% 2	7274

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% 18% 29% 24% 11% 3	200
	11th to 20th	Day	16% 17% 29% 25% 12% 3	200
	21st to end	Day	15% 19% 32% 25% 8% 1	220
February	1st to 10th	Day	15% 21% 41% 18% 5%	242
	11th to 20th	Day	14% 22% 37% 20% 6% 1	365
	21st to end	Day	13% 19% 36% 21% 9% 1	373
March	1st to 10th	Day	17% 25% 30% 20% 7% 1	460
	11th to 20th	Day	12% 26% 27% 21% 12% 2	460
	21st to end	Day	18% 25% 28% 20% 8% 1	506
April	1st to 10th	Day	23% 23% 29% 15% 7% 2	460
	11th to 20th	Day	18% 24% 28% 18% 9% 3	452
	21st to end	Day	16% 18% 36% 19% 8% 2	460
May	1st to 10th	Day	13% 21% 32% 23% 9% 2	463
	11th to 20th	Day	15% 15% 28% 25% 15% 3	488
	21st to end	Day	15% 15% 27% 26% 14% 3	529
June	1st to 10th	Day	16% 15% 31% 25% 11% 1	651
	11th to 20th	Day	16% 18% 30% 23% 11% 2	869
	21st to end	Day	9% 11% 30% 29% 18% 3	867
July	1st to 10th	Day	7% 11% 32% 33% 16% 1	671
	11th to 20th	Day	6% 15% 26% 33% 17% 3	493
	21st to end	Day	5% 9% 27% 39% 15% 5%	531
August	1st to 10th	Day	5% 7% 26% 38% 18% 6%	487
	11th to 20th	Day	6% 9% 27% 30% 19% 8%	477
	21st to end	Day	8% 18% 25% 28% 15% 6%	539
September	1st to 10th	Day	10% 15% 29% 25% 15% 6%	490
	11th to 20th	Day	12% 14% 24% 29% 16% 5%	490
	21st to end	Day	17% 18% 31% 21% 11% 2	490
October	1st to 10th	Day	14% 19% 27% 23% 12% 3	490
	11th to 20th	Day	17% 20% 24% 24% 12% 2	442
	21st to end	Day	20% 16% 31% 25% 7% 2	330
November	1st to 10th	Day	22% 18% 32% 24% 5%	255
	11th to 20th	Day	20% 24% 28% 17% 10% 3	200
	21st to end	Day	16% 19% 29% 22% 12% 3	200
December	1st to 10th	Day	17% 16% 20% 25% 20% 4	200
	11th to 20th	Day	13% 18% 36% 23% 9% 2	200
	21st to end	Day	14% 15% 31% 30% 8% 3	220
January	1st to 10th	Night	18% 15% 33% 25% 9% 1	800
	11th to 20th	Night	15% 11% 34% 29% 9% 1	800
	21st to end	Night	13% 12% 32% 30% 11% 2	880
February	1st to 10th	Night	17% 14% 32% 27% 8% 2	780
	11th to 20th	Night	13% 10% 32% 33% 12% 1	744
	21st to end	Night	14% 14% 25% 29% 17% 1	628
March	1st to 10th	Night	11% 11% 32% 27% 17% 2	760
	11th to 20th	Night	10% 13% 31% 30% 14% 2	760
	21st to end	Night	12% 12% 29% 32% 15% 1	836
April	1st to 10th	Night	12% 12% 25% 33% 14% 2	760
	11th to 20th	Night	11% 12% 29% 30% 17% 1	742
	21st to end	Night	12% 11% 32% 32% 12% 1	437
May	1st to 10th	Night	11% 9% 27% 35% 17% 2	380
	11th to 20th	Night	10% 5% 19% 40% 24% 2	374
	21st to end	Night	13% 8% 22% 29% 26% 2	411
June	1st to 10th	Night	12% 9% 28% 36% 14% 1	380
	11th to 20th	Night	13% 7% 28% 32% 18% 1	380
	21st to end	Night	6% 7% 25% 39% 21% 2	380
July	1st to 10th	Night	6% 5% 20% 40% 28% 1	370
	11th to 20th	Night	4 7% 24% 39% 25% 2	368
	21st to end	Night	4 4 18% 44% 28% 2	416
August	1st to 10th	Night	4 2 17% 36% 35% 4	378
	11th to 20th	Night	5% 2 16% 37% 35% 5%	395
	21st to end	Night	6% 5% 21% 36% 29% 3	806
September	1st to 10th	Night	6% 6% 22% 36% 28% 3	760
	11th to 20th	Night	9% 6% 16% 31% 28% 9%	760
	21st to end	Night	13% 8% 21% 34% 21% 3	760
October	1st to 10th	Night	13% 7% 19% 34% 24% 3	760
	11th to 20th	Night	12% 11% 25% 33% 17% 2	760
	21st to end	Night	18% 11% 30% 26% 12% 3	836
November	1st to 10th	Night	18% 12% 32% 30% 7% 1	780
	11th to 20th	Night	18% 16% 29% 25% 12% 1	800
	21st to end	Night	17% 14% 29% 27% 10% 3	865
December	1st to 10th	Night	13% 12% 30% 31% 12% 2	1000
	11th to 20th	Night	18% 12% 35% 26% 9% 1	1000
	21st to end	Night	20% 12% 34% 26% 7%	1020

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