Bolton Landing
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bolton Landing clear sky chart from 2005-11-06 to 2026-09-14.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	149277
	Night	16% 10% 4 8% 5% 5% 4 4 5% 6% 32%	141805
Transparency	Day	48% 7% 13% 20% 10% 1	152757
	Night	50% 7% 13% 19% 11% 1	146413
Seeing	Day	22% 17% 31% 20% 8% 2	17112
	Night	21% 12% 31% 28% 8% 1	26058

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	10% 13% 7% 15% 8% 6% 6% 4 5% 4 22%	4661
	11th to 20th	Day	10% 15% 7% 15% 7% 6% 5% 4 5% 4 23%	4650
	21st to end	Day	10% 11% 4 11% 7% 5% 5% 5% 7% 6% 29%	5184
February	1st to 10th	Day	12% 13% 4 9% 7% 6% 5% 4 6% 6% 27%	1890
	11th to 20th	Day	11% 13% 6% 11% 6% 5% 5% 4 5% 6% 29%	2005
	21st to end	Day	17% 13% 4 9% 6% 5% 4 3 4 5% 30%	1730
March	1st to 10th	Day	19% 13% 4 12% 5% 4 3 3 4 4 28%	4576
	11th to 20th	Day	15% 11% 3 8% 5% 4 5% 4 6% 6% 33%	4797
	21st to end	Day	20% 11% 3 8% 4 5% 5% 3 4 5% 33%	6102
April	1st to 10th	Day	20% 11% 3 7% 5% 4 3 3 5% 5% 34%	2520
	11th to 20th	Day	18% 9% 3 7% 6% 4 4 3 5% 6% 35%	2494
	21st to end	Day	20% 9% 3 7% 5% 6% 4 3 6% 6% 31%	2645
May	1st to 10th	Day	28% 10% 3 8% 5% 5% 4 3 5% 6% 23%	7870
	11th to 20th	Day	16% 8% 3 9% 5% 5% 5% 4 6% 7% 32%	8221
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 6% 7% 31%	9060
June	1st to 10th	Day	16% 7% 2 7% 6% 6% 6% 5% 7% 8% 31%	2940
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 5% 7% 8% 33%	2940
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 5% 7% 7% 32%	2940
July	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 5% 7% 10% 35%	8280
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 7% 11% 41%	8280
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 38%	8993
August	1st to 10th	Day	14% 7% 3 7% 5% 6% 6% 4 7% 7% 34%	6483
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	5591
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 36%	6080
September	1st to 10th	Day	13% 5% 2 4 4 4 5% 4 7% 8% 44%	2520
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 50%	2341
	21st to end	Day	17% 7% 2 5% 4 5% 4 4 5% 9% 40%	2200
October	1st to 10th	Day	20% 9% 3 7% 5% 4 4 4 4 6% 35%	2090
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 6% 8% 36%	1986
	21st to end	Day	23% 11% 3 7% 5% 4 5% 3 4 7% 29%	1980
November	1st to 10th	Day	14% 9% 3 8% 5% 4 4 4 6% 8% 35%	1851
	11th to 20th	Day	16% 10% 4 7% 5% 5% 4 4 6% 7% 34%	1890
	21st to end	Day	17% 12% 3 8% 5% 4 5% 3 6% 7% 30%	1693
December	1st to 10th	Day	17% 16% 7% 12% 6% 4 5% 3 5% 5% 20%	2680
	11th to 20th	Day	21% 13% 5% 8% 5% 5% 4 3 4 6% 27%	3066
	21st to end	Day	21% 16% 5% 9% 4 4 4 3 4 6% 25%	4048
January	1st to 10th	Night	12% 14% 6% 13% 6% 5% 4 3 5% 6% 25%	8234
	11th to 20th	Night	9% 15% 7% 15% 8% 7% 5% 3 5% 6% 19%	8235
	21st to end	Night	11% 11% 4 10% 7% 6% 6% 4 6% 7% 28%	8979
February	1st to 10th	Night	12% 13% 5% 11% 7% 6% 5% 4 5% 6% 26%	2730
	11th to 20th	Night	10% 11% 4 10% 8% 5% 5% 4 5% 7% 30%	2500
	21st to end	Night	14% 12% 4 8% 6% 5% 5% 4 5% 6% 31%	2076
March	1st to 10th	Night	21% 11% 4 8% 5% 4 5% 4 5% 5% 29%	5484
	11th to 20th	Night	16% 12% 3 6% 5% 4 4 3 5% 6% 35%	5377
	21st to end	Night	21% 11% 3 9% 5% 4 4 3 4 5% 31%	5072
April	1st to 10th	Night	20% 8% 3 8% 5% 5% 3 4 5% 5% 34%	2100
	11th to 20th	Night	20% 9% 3 8% 4 4 4 3 5% 6% 35%	2075
	21st to end	Night	19% 9% 3 7% 4 3 3 3 4 7% 37%	1974
May	1st to 10th	Night	29% 10% 3 8% 4 4 4 4 5% 6% 25%	5083
	11th to 20th	Night	18% 10% 4 9% 5% 5% 4 3 5% 7% 31%	4639
	21st to end	Night	19% 7% 2 6% 4 5% 5% 5% 6% 8% 33%	5114
June	1st to 10th	Night	19% 6% 2 6% 3 3 4 4 6% 7% 38%	1680
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 4 5% 8% 38%	1680
	21st to end	Night	17% 6% 2 6% 5% 5% 5% 4 5% 7% 38%	1680
July	1st to 10th	Night	12% 7% 3 7% 5% 5% 4 5% 6% 9% 37%	4680
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 3 5% 7% 44%	4680
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 8% 44%	5084
August	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	3666
	11th to 20th	Night	16% 6% 2 7% 5% 5% 5% 4 5% 7% 38%	4393
	21st to end	Night	9% 6% 2 6% 5% 5% 6% 5% 6% 9% 40%	5045
September	1st to 10th	Night	14% 5% 3 5% 3 4 4 3 5% 7% 46%	2100
	11th to 20th	Night	12% 5% 1 5% 3 3 4 2 4 6% 53%	2126
	21st to end	Night	17% 6% 2 4 4 3 5% 5% 5% 6% 45%	2200
October	1st to 10th	Night	20% 8% 2 7% 5% 4 5% 4 5% 6% 35%	2308
	11th to 20th	Night	17% 7% 2 7% 5% 5% 5% 3 6% 6% 37%	2412
	21st to end	Night	22% 10% 3 7% 5% 4 4 3 5% 6% 30%	2860
November	1st to 10th	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 38%	2667
	11th to 20th	Night	18% 8% 3 8% 5% 4 4 3 6% 7% 36%	2730
	21st to end	Night	16% 11% 4 8% 5% 4 4 4 5% 7% 31%	2924
December	1st to 10th	Night	16% 16% 5% 11% 5% 6% 5% 4 5% 6% 21%	4720
	11th to 20th	Night	23% 14% 3 8% 5% 3 4 2 5% 5% 29%	5348
	21st to end	Night	23% 15% 5% 9% 4 4 3 3 4 5% 25%	7150

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	61% 8% 11% 16% 4	4662
	11th to 20th	Day	62% 6% 9% 12% 10% 3	4680
	21st to end	Day	50% 7% 12% 19% 11% 1	5184
February	1st to 10th	Day	55% 8% 12% 17% 7% 1	1890
	11th to 20th	Day	54% 7% 11% 16% 10% 1	2014
	21st to end	Day	55% 5% 11% 14% 13% 2	1730
March	1st to 10th	Day	59% 5% 9% 12% 9% 6%	4579
	11th to 20th	Day	48% 8% 17% 15% 8% 4	4797
	21st to end	Day	52% 5% 6% 15% 18% 4	6102
April	1st to 10th	Day	51% 5% 11% 19% 12% 3	2520
	11th to 20th	Day	49% 6% 10% 18% 15% 2	2494
	21st to end	Day	51% 7% 10% 17% 12% 3	2645
May	1st to 10th	Day	60% 7% 9% 15% 9% 1	7882
	11th to 20th	Day	48% 9% 13% 20% 9% 1	8225
	21st to end	Day	48% 8% 11% 23% 9% 1	9061
June	1st to 10th	Day	48% 9% 14% 18% 10% 1	2940
	11th to 20th	Day	45% 8% 15% 18% 13% 1	2940
	21st to end	Day	49% 8% 13% 18% 11% 1	2940
July	1st to 10th	Day	41% 8% 17% 24% 10%	8210
	11th to 20th	Day	34% 8% 16% 31% 12%	8205
	21st to end	Day	42% 8% 14% 28% 9%	8795
August	1st to 10th	Day	47% 9% 15% 16% 12% 1	6469
	11th to 20th	Day	41% 9% 20% 26% 5%	5628
	21st to end	Day	38% 9% 19% 25% 9%	6080
September	1st to 10th	Day	35% 8% 13% 30% 13% 1	2520
	11th to 20th	Day	33% 5% 13% 29% 19% 1	2341
	21st to end	Day	40% 8% 15% 24% 12% 1	2200
October	1st to 10th	Day	43% 7% 10% 26% 13% 1	3402
	11th to 20th	Day	39% 6% 14% 25% 15% 1	3214
	21st to end	Day	41% 6% 13% 31% 9%	3180
November	1st to 10th	Day	45% 8% 15% 21% 11%	1851
	11th to 20th	Day	48% 6% 13% 18% 14% 2	1890
	21st to end	Day	53% 7% 16% 16% 7% 1	1693
December	1st to 10th	Day	62% 8% 12% 14% 4	2680
	11th to 20th	Day	56% 7% 11% 17% 8% 1	3066
	21st to end	Day	61% 4 8% 14% 11% 1	4048
January	1st to 10th	Night	58% 7% 13% 13% 9% 1	8234
	11th to 20th	Night	64% 6% 9% 10% 9% 2	8280
	21st to end	Night	52% 6% 11% 15% 13% 3	8979
February	1st to 10th	Night	56% 9% 11% 16% 7% 1	2730
	11th to 20th	Night	52% 7% 12% 18% 10% 1	2502
	21st to end	Night	51% 7% 13% 18% 9% 2	2076
March	1st to 10th	Night	55% 5% 9% 14% 13% 3	5494
	11th to 20th	Night	48% 6% 18% 15% 10% 2	5378
	21st to end	Night	54% 5% 7% 15% 17% 2	5072
April	1st to 10th	Night	51% 5% 11% 18% 13% 2	2100
	11th to 20th	Night	50% 5% 10% 20% 13% 1	2075
	21st to end	Night	47% 5% 10% 18% 16% 3	1974
May	1st to 10th	Night	59% 7% 10% 13% 10% 1	5089
	11th to 20th	Night	52% 7% 11% 20% 9% 1	4640
	21st to end	Night	46% 7% 13% 22% 10% 1	5115
June	1st to 10th	Night	44% 8% 13% 21% 13% 2	1680
	11th to 20th	Night	45% 6% 15% 20% 12% 1	1680
	21st to end	Night	45% 8% 13% 23% 10% 1	1680
July	1st to 10th	Night	42% 7% 18% 21% 11%	4640
	11th to 20th	Night	38% 7% 13% 25% 17% 1	4632
	21st to end	Night	37% 8% 15% 30% 10% 1	4967
August	1st to 10th	Night	48% 7% 15% 18% 10% 2	3656
	11th to 20th	Night	44% 7% 18% 24% 7%	4421
	21st to end	Night	36% 10% 20% 24% 10%	5045
September	1st to 10th	Night	37% 7% 14% 28% 13% 1	2100
	11th to 20th	Night	33% 5% 13% 30% 18% 1	2126
	21st to end	Night	38% 8% 13% 25% 15% 2	2200
October	1st to 10th	Night	48% 6% 11% 22% 12%	3776
	11th to 20th	Night	37% 6% 14% 25% 16% 1	3923
	21st to end	Night	46% 6% 12% 27% 9% 1	4610
November	1st to 10th	Night	46% 6% 13% 21% 12% 2	2667
	11th to 20th	Night	46% 7% 13% 20% 12% 2	2730
	21st to end	Night	51% 8% 16% 17% 8%	2924
December	1st to 10th	Night	60% 8% 14% 14% 3	4720
	11th to 20th	Night	58% 6% 10% 14% 12% 1	5348
	21st to end	Night	61% 6% 8% 14% 9% 2	7150

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	20% 30% 26% 20% 4	220
	11th to 20th	Day	28% 29% 27% 14% 2 1	220
	21st to end	Day	21% 27% 32% 15% 4	253
February	1st to 10th	Day	27% 30% 30% 11% 2	330
	11th to 20th	Day	25% 27% 33% 13% 2	493
	21st to end	Day	28% 21% 33% 16% 2	446
March	1st to 10th	Day	28% 17% 34% 18% 3	540
	11th to 20th	Day	30% 23% 29% 15% 2 1	540
	21st to end	Day	31% 23% 26% 14% 5% 1	594
April	1st to 10th	Day	36% 19% 24% 14% 6%	540
	11th to 20th	Day	32% 23% 27% 11% 6% 2	532
	21st to end	Day	29% 19% 32% 12% 6% 1	540
May	1st to 10th	Day	31% 20% 27% 14% 7% 1	543
	11th to 20th	Day	20% 17% 34% 21% 6% 2	567
	21st to end	Day	21% 16% 31% 22% 9% 1	617
June	1st to 10th	Day	18% 17% 33% 22% 9% 1	570
	11th to 20th	Day	19% 15% 33% 23% 8% 2	570
	21st to end	Day	19% 13% 36% 22% 7% 2	570
July	1st to 10th	Day	13% 10% 35% 26% 14% 2	565
	11th to 20th	Day	13% 11% 32% 28% 13% 2	564
	21st to end	Day	15% 8% 35% 29% 11% 2	620
August	1st to 10th	Day	12% 8% 28% 32% 16% 4	566
	11th to 20th	Day	16% 8% 28% 28% 15% 5%	559
	21st to end	Day	11% 13% 32% 26% 15% 4	627
September	1st to 10th	Day	14% 9% 29% 27% 14% 6%	570
	11th to 20th	Day	13% 9% 28% 24% 17% 9%	542
	21st to end	Day	19% 9% 32% 22% 11% 7%	530
October	1st to 10th	Day	23% 15% 25% 22% 12% 4	530
	11th to 20th	Day	20% 16% 31% 23% 9% 2	522
	21st to end	Day	30% 13% 29% 21% 5%	462
November	1st to 10th	Day	26% 18% 27% 19% 10%	425
	11th to 20th	Day	22% 20% 32% 17% 7% 1	430
	21st to end	Day	29% 19% 32% 17% 3	233
December	1st to 10th	Day	27% 22% 30% 15% 5%	220
	11th to 20th	Day	31% 25% 32% 11% 1	220
	21st to end	Day	28% 22% 33% 13% 3	242
January	1st to 10th	Night	21% 16% 37% 24% 2	840
	11th to 20th	Night	24% 17% 33% 23% 3	840
	21st to end	Night	16% 17% 36% 26% 4	924
February	1st to 10th	Night	18% 16% 41% 23% 2	840
	11th to 20th	Night	18% 17% 42% 22% 2	823
	21st to end	Night	21% 16% 40% 20% 2	692
March	1st to 10th	Night	22% 16% 34% 25% 3	840
	11th to 20th	Night	23% 18% 32% 23% 4	840
	21st to end	Night	23% 19% 31% 21% 6%	924
April	1st to 10th	Night	26% 15% 31% 24% 3	840
	11th to 20th	Night	26% 17% 31% 19% 7%	831
	21st to end	Night	24% 16% 32% 24% 5%	714
May	1st to 10th	Night	28% 14% 26% 24% 8%	570
	11th to 20th	Night	20% 9% 37% 28% 5%	414
	21st to end	Night	17% 8% 30% 34% 11%	455
June	1st to 10th	Night	19% 5% 29% 35% 13%	420
	11th to 20th	Night	19% 8% 28% 36% 10%	420
	21st to end	Night	17% 5% 29% 38% 11% 1	420
July	1st to 10th	Night	14% 4 21% 44% 15% 1	410
	11th to 20th	Night	13% 5% 24% 41% 16% 1	408
	21st to end	Night	11% 4 24% 45% 15% 1	460
August	1st to 10th	Night	12% 2 22% 44% 20% 1	418
	11th to 20th	Night	16% 4 20% 39% 19% 2	754
	21st to end	Night	14% 5% 22% 38% 20% 1	924
September	1st to 10th	Night	16% 6% 23% 37% 16% 2	840
	11th to 20th	Night	15% 6% 23% 32% 21% 3	813
	21st to end	Night	18% 7% 27% 29% 18% 2	800
October	1st to 10th	Night	23% 11% 23% 28% 14% 1	800
	11th to 20th	Night	20% 10% 29% 30% 10% 1	800
	21st to end	Night	27% 11% 30% 27% 5%	880
November	1st to 10th	Night	21% 13% 32% 27% 7%	820
	11th to 20th	Night	23% 13% 30% 26% 6% 1	840
	21st to end	Night	23% 14% 33% 24% 6%	840
December	1st to 10th	Night	26% 13% 35% 21% 4	840
	11th to 20th	Night	26% 13% 34% 24% 4	840
	21st to end	Night	29% 14% 34% 21% 2	924

Page generated at 2026-09-11 04:11:57. Version ac07

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