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 2025-05-11.

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%	113659
	Night	17% 10% 4 8% 5% 5% 4 4 5% 6% 32%	115263
Transparency	Day	48% 7% 13% 20% 11% 1	117185
	Night	50% 7% 12% 18% 11% 1	119900
Seeing	Day	22% 16% 30% 21% 9% 2	15335
	Night	21% 12% 31% 28% 8% 1	24490

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% 14% 7% 6% 6% 4 5% 5% 23%	3597
	11th to 20th	Day	11% 13% 6% 14% 6% 5% 5% 4 5% 4 25%	3570
	21st to end	Day	12% 12% 4 11% 6% 5% 5% 4 6% 6% 28%	4006
February	1st to 10th	Day	13% 14% 4 9% 7% 6% 5% 4 6% 6% 26%	1800
	11th to 20th	Day	10% 13% 6% 11% 7% 5% 5% 3 6% 6% 29%	1907
	21st to end	Day	18% 12% 3 8% 6% 5% 4 3 4 6% 30%	1650
March	1st to 10th	Day	19% 13% 4 12% 5% 4 3 3 4 4 28%	4476
	11th to 20th	Day	15% 11% 3 8% 4 4 5% 4 6% 6% 34%	4694
	21st to end	Day	20% 11% 3 7% 4 5% 4 3 4 5% 33%	5970
April	1st to 10th	Day	20% 11% 3 7% 5% 4 3 3 5% 5% 35%	2400
	11th to 20th	Day	18% 9% 3 7% 6% 4 4 3 5% 6% 36%	2374
	21st to end	Day	21% 9% 3 8% 5% 6% 4 3 6% 6% 31%	2519
May	1st to 10th	Day	24% 9% 3 8% 5% 4 4 3 5% 6% 28%	4361
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 5% 7% 33%	4398
	21st to end	Day	13% 5% 2 6% 5% 4 4 5% 7% 8% 41%	4837
June	1st to 10th	Day	17% 7% 2 7% 6% 6% 6% 5% 7% 8% 30%	2660
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 5% 6% 8% 34%	2660
	21st to end	Day	17% 7% 2 8% 6% 5% 5% 4 7% 8% 30%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 11% 35%	4440
	21st to end	Day	13% 6% 2 6% 5% 6% 6% 5% 7% 8% 35%	4814
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	4423
	11th to 20th	Day	15% 6% 3 7% 5% 6% 6% 5% 7% 10% 30%	3789
	21st to end	Day	12% 5% 2 5% 5% 5% 6% 5% 7% 9% 38%	4180
September	1st to 10th	Day	13% 5% 2 4 4 4 5% 4 7% 9% 43%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2190
	21st to end	Day	16% 7% 2 5% 4 5% 4 4 5% 8% 40%	2090
October	1st to 10th	Day	21% 9% 3 7% 5% 4 4 4 4 5% 33%	1986
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 6% 8% 36%	1887
	21st to end	Day	23% 10% 3 8% 5% 4 5% 3 4 7% 28%	1881
November	1st to 10th	Day	13% 9% 3 7% 5% 4 4 4 6% 9% 35%	1761
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 6% 7% 34%	1800
	21st to end	Day	17% 11% 3 8% 5% 4 5% 3 6% 7% 31%	1613
December	1st to 10th	Day	17% 16% 7% 13% 6% 4 5% 3 5% 5% 20%	2600
	11th to 20th	Day	21% 13% 5% 8% 5% 4 4 3 4 5% 27%	2986
	21st to end	Day	21% 16% 5% 9% 4 3 3 3 4 6% 26%	3960
January	1st to 10th	Night	13% 12% 5% 12% 6% 5% 4 3 6% 6% 27%	6349
	11th to 20th	Night	9% 14% 7% 15% 7% 7% 6% 4 5% 6% 20%	6315
	21st to end	Night	14% 12% 4 11% 6% 6% 5% 4 5% 6% 27%	6932
February	1st to 10th	Night	12% 13% 5% 12% 7% 6% 5% 4 5% 6% 25%	2600
	11th to 20th	Night	10% 11% 4 10% 8% 5% 5% 4 5% 6% 31%	2378
	21st to end	Night	14% 12% 4 8% 6% 5% 5% 4 6% 6% 31%	1980
March	1st to 10th	Night	21% 11% 4 8% 5% 4 5% 4 5% 5% 28%	5364
	11th to 20th	Night	16% 12% 3 6% 5% 4 4 3 5% 6% 36%	5261
	21st to end	Night	21% 11% 3 9% 5% 4 4 3 4 5% 31%	4962
April	1st to 10th	Night	20% 9% 3 8% 5% 5% 4 4 5% 6% 34%	2000
	11th to 20th	Night	20% 9% 3 8% 4 4 4 3 5% 6% 35%	1975
	21st to end	Night	19% 9% 3 7% 4 4 4 4 5% 7% 36%	1880
May	1st to 10th	Night	27% 9% 3 8% 4 4 4 3 5% 6% 27%	2828
	11th to 20th	Night	21% 10% 4 9% 5% 4 4 3 4 5% 30%	2501
	21st to end	Night	14% 6% 2 6% 4 4 5% 5% 7% 8% 39%	2738
June	1st to 10th	Night	19% 6% 2 6% 4 3 5% 5% 7% 7% 36%	1520
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 4 6% 8% 39%	1520
	21st to end	Night	17% 6% 2 6% 6% 5% 5% 4 5% 7% 37%	1520
July	1st to 10th	Night	12% 6% 2 7% 6% 6% 5% 5% 6% 8% 37%	2520
	11th to 20th	Night	15% 6% 2 6% 4 5% 5% 3 5% 8% 42%	2520
	21st to end	Night	13% 7% 2 6% 4 4 5% 4 5% 7% 41%	2739
August	1st to 10th	Night	18% 7% 3 7% 5% 4 5% 3 5% 7% 36%	2506
	11th to 20th	Night	15% 7% 3 8% 5% 5% 4 4 6% 7% 38%	2993
	21st to end	Night	9% 5% 2 5% 5% 5% 6% 5% 6% 9% 43%	3476
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 6% 7% 47%	1900
	11th to 20th	Night	13% 5% 1 5% 3 3 4 2 4 6% 52%	1990
	21st to end	Night	16% 6% 2 4 4 3 5% 4 5% 6% 44%	2090
October	1st to 10th	Night	20% 9% 3 8% 5% 4 5% 4 5% 5% 33%	2192
	11th to 20th	Night	17% 7% 2 7% 5% 5% 5% 3 6% 6% 37%	2291
	21st to end	Night	22% 10% 3 7% 5% 4 4 3 5% 6% 29%	2717
November	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 5% 6% 39%	2537
	11th to 20th	Night	18% 8% 3 7% 5% 4 3 3 6% 7% 36%	2600
	21st to end	Night	17% 11% 3 8% 5% 4 4 4 6% 7% 32%	2785
December	1st to 10th	Night	17% 16% 6% 11% 5% 6% 5% 4 5% 6% 20%	4580
	11th to 20th	Night	23% 14% 3 7% 5% 3 4 2 5% 5% 29%	5208
	21st to end	Night	23% 15% 5% 9% 4 4 3 3 4 5% 25%	6996

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	59% 8% 11% 18% 4 1	3598
	11th to 20th	Day	59% 6% 10% 13% 8% 3	3600
	21st to end	Day	54% 7% 12% 14% 12% 2	4006
February	1st to 10th	Day	55% 9% 13% 16% 6% 1	1800
	11th to 20th	Day	55% 6% 12% 16% 10% 1	1916
	21st to end	Day	55% 5% 11% 14% 13% 2	1650
March	1st to 10th	Day	60% 5% 9% 12% 9% 6%	4479
	11th to 20th	Day	48% 8% 18% 15% 8% 4	4694
	21st to end	Day	52% 5% 6% 15% 18% 4	5970
April	1st to 10th	Day	51% 5% 11% 19% 12% 3	2400
	11th to 20th	Day	49% 6% 10% 18% 15% 2	2374
	21st to end	Day	52% 7% 10% 16% 12% 3	2519
May	1st to 10th	Day	55% 8% 10% 18% 9% 1	4375
	11th to 20th	Day	48% 9% 14% 19% 9% 1	4402
	21st to end	Day	36% 9% 14% 27% 13% 1	4837
June	1st to 10th	Day	49% 9% 15% 16% 10% 1	2660
	11th to 20th	Day	45% 8% 15% 18% 13% 1	2660
	21st to end	Day	49% 9% 12% 18% 11% 1	2660
July	1st to 10th	Day	40% 9% 15% 23% 13%	4370
	11th to 20th	Day	39% 9% 17% 28% 8%	4365
	21st to end	Day	43% 9% 16% 23% 9%	4661
August	1st to 10th	Day	44% 8% 16% 19% 12% 1	4409
	11th to 20th	Day	45% 9% 17% 24% 5%	3826
	21st to end	Day	37% 8% 18% 25% 11% 1	4180
September	1st to 10th	Day	36% 9% 13% 29% 13% 1	2280
	11th to 20th	Day	34% 5% 13% 29% 18% 1	2190
	21st to end	Day	40% 8% 15% 24% 12% 1	2090
October	1st to 10th	Day	44% 7% 10% 25% 13% 1	3298
	11th to 20th	Day	39% 6% 14% 25% 15% 1	3115
	21st to end	Day	41% 6% 14% 31% 9%	3081
November	1st to 10th	Day	45% 8% 15% 21% 11%	1761
	11th to 20th	Day	47% 7% 13% 17% 14% 2	1800
	21st to end	Day	52% 8% 17% 16% 7% 1	1613
December	1st to 10th	Day	62% 8% 12% 14% 4	2600
	11th to 20th	Day	56% 7% 11% 16% 8% 1	2986
	21st to end	Day	61% 4 8% 14% 12% 1	3960
January	1st to 10th	Night	55% 8% 13% 14% 9%	6349
	11th to 20th	Night	62% 7% 10% 12% 8% 1	6360
	21st to end	Night	56% 6% 11% 11% 13% 3	6932
February	1st to 10th	Night	58% 9% 11% 15% 7% 1	2600
	11th to 20th	Night	52% 7% 12% 18% 10% 1	2380
	21st to end	Night	51% 7% 13% 19% 8% 2	1980
March	1st to 10th	Night	55% 5% 9% 14% 13% 3	5374
	11th to 20th	Night	48% 6% 18% 15% 10% 2	5262
	21st to end	Night	54% 5% 7% 16% 17% 2	4962
April	1st to 10th	Night	51% 5% 11% 19% 13% 2	2000
	11th to 20th	Night	50% 5% 10% 20% 13% 1	1975
	21st to end	Night	48% 6% 11% 18% 14% 3	1880
May	1st to 10th	Night	56% 8% 10% 15% 10% 1	2834
	11th to 20th	Night	54% 8% 11% 17% 9% 1	2502
	21st to end	Night	39% 9% 14% 21% 15% 2	2739
June	1st to 10th	Night	45% 8% 14% 19% 12% 1	1520
	11th to 20th	Night	43% 6% 16% 20% 13% 1	1520
	21st to end	Night	45% 8% 13% 22% 11% 1	1520
July	1st to 10th	Night	42% 8% 16% 20% 13% 1	2480
	11th to 20th	Night	40% 8% 15% 24% 13%	2472
	21st to end	Night	41% 7% 12% 28% 10% 1	2651
August	1st to 10th	Night	46% 8% 15% 21% 10%	2496
	11th to 20th	Night	43% 7% 18% 23% 8%	3021
	21st to end	Night	33% 9% 19% 27% 11% 1	3476
September	1st to 10th	Night	36% 7% 15% 28% 13% 1	1900
	11th to 20th	Night	34% 5% 13% 30% 17% 1	1990
	21st to end	Night	38% 7% 13% 25% 15% 2	2090
October	1st to 10th	Night	49% 7% 11% 22% 11%	3660
	11th to 20th	Night	37% 6% 14% 25% 16% 2	3802
	21st to end	Night	46% 6% 12% 27% 9% 1	4467
November	1st to 10th	Night	46% 6% 13% 21% 12% 2	2537
	11th to 20th	Night	46% 7% 13% 19% 13% 2	2600
	21st to end	Night	50% 8% 16% 17% 8%	2785
December	1st to 10th	Night	61% 9% 14% 14% 3	4580
	11th to 20th	Night	58% 6% 10% 14% 12% 1	5208
	21st to end	Night	61% 6% 8% 14% 9% 2	6996

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% 5%	200
	11th to 20th	Day	28% 27% 28% 15% 2 1	200
	21st to end	Day	20% 27% 33% 16% 4	230
February	1st to 10th	Day	29% 32% 29% 9% 1	300
	11th to 20th	Day	26% 27% 32% 13% 2	455
	21st to end	Day	28% 19% 34% 16% 3	414
March	1st to 10th	Day	28% 16% 34% 19% 4	500
	11th to 20th	Day	28% 23% 31% 15% 3 1	500
	21st to end	Day	30% 22% 26% 15% 5% 1	550
April	1st to 10th	Day	34% 20% 25% 15% 7%	500
	11th to 20th	Day	33% 22% 26% 12% 6% 2	492
	21st to end	Day	29% 19% 33% 12% 6% 1	500
May	1st to 10th	Day	31% 18% 27% 15% 8% 1	503
	11th to 20th	Day	19% 15% 35% 23% 6% 2	487
	21st to end	Day	18% 14% 32% 24% 11% 2	529
June	1st to 10th	Day	18% 17% 32% 22% 10% 1	490
	11th to 20th	Day	18% 14% 33% 24% 9% 2	490
	21st to end	Day	20% 13% 35% 22% 8% 2	490
July	1st to 10th	Day	15% 9% 31% 28% 15% 2	485
	11th to 20th	Day	13% 10% 32% 29% 14% 3	484
	21st to end	Day	14% 8% 36% 30% 11% 2	532
August	1st to 10th	Day	13% 8% 29% 30% 15% 4	486
	11th to 20th	Day	17% 7% 27% 29% 16% 5%	479
	21st to end	Day	11% 13% 29% 27% 16% 4	539
September	1st to 10th	Day	14% 9% 28% 27% 15% 7%	490
	11th to 20th	Day	13% 9% 28% 23% 18% 9%	490
	21st to end	Day	18% 9% 31% 23% 11% 7%	490
October	1st to 10th	Day	24% 15% 25% 20% 12% 4	490
	11th to 20th	Day	19% 16% 30% 24% 9% 2	483
	21st to end	Day	31% 14% 28% 22% 6%	429
November	1st to 10th	Day	24% 17% 27% 20% 11% 1	395
	11th to 20th	Day	23% 20% 32% 18% 7% 1	400
	21st to end	Day	27% 17% 34% 18% 3	213
December	1st to 10th	Day	28% 21% 30% 17% 6%	200
	11th to 20th	Day	31% 23% 34% 12% 1	200
	21st to end	Day	28% 20% 33% 15% 3	220
January	1st to 10th	Night	21% 16% 37% 24% 3	800
	11th to 20th	Night	23% 17% 34% 24% 3	800
	21st to end	Night	16% 16% 37% 27% 4	880
February	1st to 10th	Night	19% 17% 41% 21% 2	800
	11th to 20th	Night	18% 17% 41% 21% 2	783
	21st to end	Night	22% 16% 40% 20% 3	660
March	1st to 10th	Night	22% 16% 33% 26% 3	800
	11th to 20th	Night	22% 17% 33% 24% 4	800
	21st to end	Night	22% 18% 31% 22% 6%	880
April	1st to 10th	Night	26% 14% 32% 25% 4	800
	11th to 20th	Night	25% 17% 31% 19% 7%	791
	21st to end	Night	24% 15% 31% 25% 5%	680
May	1st to 10th	Night	28% 14% 25% 24% 8%	543
	11th to 20th	Night	20% 8% 36% 29% 6%	376
	21st to end	Night	16% 7% 28% 36% 12%	411
June	1st to 10th	Night	19% 5% 29% 34% 13%	380
	11th to 20th	Night	18% 8% 26% 37% 10%	380
	21st to end	Night	17% 4 29% 38% 11% 1	380
July	1st to 10th	Night	14% 4 21% 44% 16% 2	370
	11th to 20th	Night	13% 4 22% 42% 18% 1	368
	21st to end	Night	11% 4 23% 46% 15% 1	416
August	1st to 10th	Night	12% 2 21% 44% 21% 1	378
	11th to 20th	Night	16% 3 20% 39% 20% 3	682
	21st to end	Night	14% 5% 21% 38% 21% 1	836
September	1st to 10th	Night	15% 6% 23% 37% 17% 2	760
	11th to 20th	Night	16% 6% 22% 31% 21% 4	760
	21st to end	Night	18% 7% 27% 29% 18% 2	760
October	1st to 10th	Night	23% 11% 23% 28% 14% 1	760
	11th to 20th	Night	20% 10% 29% 31% 10% 1	760
	21st to end	Night	28% 10% 30% 27% 5%	836
November	1st to 10th	Night	19% 13% 32% 28% 7%	780
	11th to 20th	Night	22% 13% 31% 26% 7% 1	800
	21st to end	Night	22% 14% 34% 24% 6%	800
December	1st to 10th	Night	27% 13% 35% 21% 4	800
	11th to 20th	Night	26% 12% 34% 25% 4	800
	21st to end	Night	28% 13% 35% 21% 3	880

Page generated at 2025-05-08 09:34:00. Version ac07

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