Stanley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanley clear sky chart from 2005-11-06 to 2025-10-04.

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	3 9% 4 9% 5% 4 4 3 5% 6% 47%	134487
	Night	2 9% 4 9% 5% 4 4 4 5% 7% 48%	132185
Transparency	Day	37% 5% 9% 20% 19% 10%	129231
	Night	36% 6% 11% 22% 18% 8%	128454
Seeing	Day	14% 20% 32% 22% 10% 3	15768
	Night	9% 10% 26% 35% 17% 2	25308

By Month

Percentage Cloud Cover SW (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% 8% 20% 7% 6% 5% 5% 6% 7% 25%	3639
	11th to 20th	Day	1 10% 4 16% 6% 5% 4 3 4 6% 42%	3640
	21st to end	Day	2 15% 5% 9% 4 4 4 3 4 5% 45%	4004
February	1st to 10th	Day	2 16% 7% 14% 7% 5% 5% 3 5% 7% 30%	1769
	11th to 20th	Day	1 13% 8% 17% 7% 5% 5% 4 5% 5% 29%	1920
	21st to end	Day	1 17% 7% 17% 8% 6% 5% 3 6% 5% 24%	1650
March	1st to 10th	Day	1 10% 6% 15% 6% 5% 4 4 6% 8% 35%	4518
	11th to 20th	Day	1 14% 6% 16% 5% 4 3 3 3 4 40%	4929
	21st to end	Day	2 20% 8% 15% 8% 7% 6% 5% 5% 5% 20%	5563
April	1st to 10th	Day	3 19% 7% 12% 7% 5% 5% 4 5% 7% 28%	2400
	11th to 20th	Day	2 17% 5% 11% 5% 5% 4 3 5% 6% 36%	2485
	21st to end	Day	6% 14% 5% 11% 6% 5% 5% 4 5% 6% 33%	2600
May	1st to 10th	Day	4 14% 4 8% 4 5% 4 3 5% 5% 43%	5880
	11th to 20th	Day	5% 12% 3 9% 5% 5% 5% 4 5% 6% 39%	5883
	21st to end	Day	4 10% 3 6% 4 4 4 3 5% 6% 49%	6902
June	1st to 10th	Day	11% 10% 2 7% 6% 5% 5% 5% 7% 7% 36%	2825
	11th to 20th	Day	8% 9% 3 6% 5% 4 5% 4 5% 7% 46%	2998
	21st to end	Day	3 4 1 5% 4 4 3 3 4 5% 62%	2800
July	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 5% 71%	6319
	11th to 20th	Day	2 2 1 3 2 3 3 2 4 5% 74%	6320
	21st to end	Day	2 2 1 4 3 5% 5% 4 6% 7% 58%	6880
August	1st to 10th	Day	2 3 2 5% 5% 5% 5% 4 7% 8% 54%	5836
	11th to 20th	Day	1 1 1 3 3 3 3 3 4 6% 72%	5798
	21st to end	Day	5% 4 2 5% 3 4 4 3 4 5% 60%	6248
September	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 6% 65%	2365
	11th to 20th	Day	6% 5% 2 5% 4 4 4 4 5% 6% 54%	2200
	21st to end	Day	6% 7% 1 4 2 3 4 2 4 7% 60%	2200
October	1st to 10th	Day	5% 6% 2 4 4 3 4 2 3 5% 63%	3524
	11th to 20th	Day	4 8% 2 5% 4 3 4 4 4 7% 54%	3045
	21st to end	Day	2 8% 4 9% 5% 5% 5% 3 5% 8% 45%	3101
November	1st to 10th	Day	4 14% 4 10% 5% 5% 4 3 6% 7% 39%	1764
	11th to 20th	Day	3 14% 4 12% 6% 4 4 4 6% 7% 36%	1800
	21st to end	Day	1 11% 5% 9% 6% 4 5% 4 6% 8% 40%	1733
December	1st to 10th	Day	1 6% 3 12% 5% 4 3 3 3 5% 53%	2328
	11th to 20th	Day	1 13% 7% 16% 8% 8% 7% 4 6% 7% 22%	2645
	21st to end	Day	1 13% 5% 16% 7% 5% 5% 4 6% 6% 31%	3976
January	1st to 10th	Night	1 12% 8% 15% 6% 5% 4 4 6% 8% 29%	6306
	11th to 20th	Night	2 11% 6% 15% 6% 4 3 3 4 6% 40%	6320
	21st to end	Night	2 13% 5% 9% 5% 4 4 3 5% 6% 47%	6952
February	1st to 10th	Night	1 15% 7% 11% 6% 6% 6% 4 7% 8% 28%	2630
	11th to 20th	Night	1 13% 6% 14% 7% 5% 4 4 6% 7% 34%	2367
	21st to end	Night	1 13% 4 17% 8% 7% 6% 3 5% 7% 28%	1980
March	1st to 10th	Night	10% 6% 12% 6% 5% 5% 4 5% 8% 41%	5376
	11th to 20th	Night	1 12% 5% 12% 5% 4 4 4 6% 8% 38%	5070
	21st to end	Night	1 11% 8% 15% 6% 5% 5% 4 6% 7% 31%	5407
April	1st to 10th	Night	2 12% 6% 9% 5% 4 6% 5% 6% 7% 37%	2000
	11th to 20th	Night	3 12% 5% 9% 5% 4 5% 5% 6% 6% 41%	1861
	21st to end	Night	4 11% 4 8% 4 3 4 4 6% 9% 42%	1800
May	1st to 10th	Night	3 13% 4 6% 4 3 3 4 5% 7% 48%	4100
	11th to 20th	Night	4 12% 2 6% 4 5% 5% 5% 6% 7% 44%	4012
	21st to end	Night	3 6% 2 5% 4 4 4 3 5% 7% 58%	3972
June	1st to 10th	Night	7% 6% 2 6% 4 4 5% 4 8% 9% 45%	1560
	11th to 20th	Night	8% 7% 2 4 4 4 4 3 5% 6% 54%	1400
	21st to end	Night	2 3 1 3 3 4 3 3 5% 7% 65%	1588
July	1st to 10th	Night	1 1 1 2 1 2 3 3 4 5% 77%	3640
	11th to 20th	Night	1 1 1 1 2 2 2 4 5% 81%	3640
	21st to end	Night	1 2 1 3 3 3 4 3 6% 10% 64%	3970
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 7% 63%	3960
	11th to 20th	Night	1 1 1 1 2 2 2 4 6% 78%	4051
	21st to end	Night	4 3 1 4 3 3 3 3 4 5% 65%	4735
September	1st to 10th	Night	3 3 1 2 2 3 2 2 5% 6% 71%	2035
	11th to 20th	Night	4 5% 1 5% 3 3 4 3 5% 6% 60%	2200
	21st to end	Night	5% 5% 1 4 2 2 2 3 4 6% 66%	2200
October	1st to 10th	Night	3 5% 2 3 2 3 3 3 4 6% 66%	3523
	11th to 20th	Night	4 9% 3 5% 4 4 4 3 4 5% 57%	3880
	21st to end	Night	3 9% 4 7% 4 5% 5% 5% 7% 8% 43%	4457
November	1st to 10th	Night	4 12% 5% 9% 5% 4 4 4 6% 8% 39%	2535
	11th to 20th	Night	3 13% 4 9% 4 5% 5% 5% 7% 9% 37%	2600
	21st to end	Night	2 8% 4 9% 6% 5% 7% 4 5% 7% 42%	2643
December	1st to 10th	Night	1 8% 4 10% 5% 3 4 3 5% 7% 51%	4848
	11th to 20th	Night	2 13% 6% 14% 8% 7% 5% 5% 7% 8% 26%	5546
	21st to end	Night	1 13% 6% 16% 8% 6% 6% 4 6% 7% 27%	7021

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	56% 8% 10% 16% 9% 1	3639
	11th to 20th	Day	42% 5% 11% 24% 14% 3	3640
	21st to end	Day	42% 5% 10% 20% 18% 7%	4004
February	1st to 10th	Day	53% 9% 12% 18% 6% 2	1769
	11th to 20th	Day	57% 6% 11% 15% 7% 4	1930
	21st to end	Day	61% 6% 8% 13% 9% 2	1650
March	1st to 10th	Day	47% 4 9% 17% 17% 5%	4518
	11th to 20th	Day	49% 3 10% 26% 12%	4929
	21st to end	Day	63% 7% 11% 16% 3	5563
April	1st to 10th	Day	57% 7% 12% 15% 8% 1	2400
	11th to 20th	Day	48% 6% 14% 19% 11% 1	2488
	21st to end	Day	51% 7% 13% 21% 8%	2600
May	1st to 10th	Day	41% 6% 10% 24% 17% 1	5880
	11th to 20th	Day	42% 4 11% 24% 14% 5%	5886
	21st to end	Day	35% 5% 9% 28% 21% 3	6904
June	1st to 10th	Day	45% 8% 12% 24% 11% 1	2825
	11th to 20th	Day	38% 5% 9% 22% 19% 7%	2998
	21st to end	Day	24% 4 6% 21% 35% 9%	2800
July	1st to 10th	Day	15% 2 4 14% 24% 41%	6251
	11th to 20th	Day	14% 1 4 10% 27% 45%	6245
	21st to end	Day	22% 3 6% 16% 32% 20%	6681
August	1st to 10th	Day	28% 3 7% 12% 28% 21%	4251
	11th to 20th	Day	16% 3 6% 17% 39% 19%	4197
	21st to end	Day	21% 3 5% 20% 34% 17%	4503
September	1st to 10th	Day	21% 4 5% 21% 30% 18%	2365
	11th to 20th	Day	30% 5% 9% 24% 23% 10%	2200
	21st to end	Day	26% 4 10% 26% 28% 7%	2200
October	1st to 10th	Day	26% 3 6% 20% 36% 8%	3524
	11th to 20th	Day	31% 3 13% 25% 24% 3	3045
	21st to end	Day	38% 5% 9% 24% 22% 2	3101
November	1st to 10th	Day	46% 7% 12% 20% 13% 2	1764
	11th to 20th	Day	47% 7% 13% 22% 9% 2	1800
	21st to end	Day	42% 7% 12% 20% 13% 6%	1733
December	1st to 10th	Day	35% 4 18% 27% 15% 3	2328
	11th to 20th	Day	58% 8% 13% 15% 6% 1	2644
	21st to end	Day	51% 6% 11% 19% 10% 3	3976
January	1st to 10th	Night	52% 6% 9% 23% 10% 1	6306
	11th to 20th	Night	46% 4 10% 22% 15% 3	6320
	21st to end	Night	38% 7% 8% 21% 18% 9%	6952
February	1st to 10th	Night	53% 8% 13% 17% 8% 2	2630
	11th to 20th	Night	48% 8% 15% 17% 9% 3	2368
	21st to end	Night	56% 7% 8% 15% 9% 4	1980
March	1st to 10th	Night	42% 7% 9% 19% 17% 7%	5376
	11th to 20th	Night	42% 6% 12% 28% 12% 1	5070
	21st to end	Night	48% 10% 16% 20% 6%	5407
April	1st to 10th	Night	45% 8% 13% 23% 10% 1	2000
	11th to 20th	Night	41% 8% 14% 24% 12% 1	1861
	21st to end	Night	40% 7% 18% 27% 9%	1800
May	1st to 10th	Night	34% 6% 11% 31% 17% 2	4097
	11th to 20th	Night	35% 6% 13% 27% 16% 2	4013
	21st to end	Night	26% 6% 10% 35% 22%	3972
June	1st to 10th	Night	33% 11% 17% 28% 11%	1560
	11th to 20th	Night	31% 5% 11% 25% 20% 7%	1400
	21st to end	Night	18% 5% 8% 24% 37% 9%	1588
July	1st to 10th	Night	11% 2 4 15% 26% 42%	3600
	11th to 20th	Night	8% 1 3 13% 29% 45%	3592
	21st to end	Night	17% 2 4 24% 31% 22%	3857
August	1st to 10th	Night	25% 2 6% 15% 33% 19%	2882
	11th to 20th	Night	9% 3 4 20% 44% 20%	2928
	21st to end	Night	15% 3 5% 23% 33% 20%	3406
September	1st to 10th	Night	15% 3 7% 22% 34% 18%	2035
	11th to 20th	Night	23% 5% 10% 26% 24% 12%	2200
	21st to end	Night	21% 3 9% 31% 29% 8%	2200
October	1st to 10th	Night	20% 5% 8% 22% 36% 10%	3523
	11th to 20th	Night	30% 3 13% 24% 24% 5%	3880
	21st to end	Night	36% 6% 13% 25% 18% 2	4457
November	1st to 10th	Night	42% 7% 13% 20% 15% 3	2535
	11th to 20th	Night	44% 9% 14% 22% 11% 1	2600
	21st to end	Night	41% 7% 11% 19% 15% 7%	2643
December	1st to 10th	Night	33% 5% 22% 26% 12% 3	4848
	11th to 20th	Night	53% 8% 14% 18% 7% 1	5547
	21st to end	Night	53% 6% 11% 16% 10% 4	7021

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% 20% 29% 27% 6% 3	200
	11th to 20th	Day	12% 24% 30% 19% 10% 6%	200
	21st to end	Day	13% 24% 29% 20% 10% 4	220
February	1st to 10th	Day	22% 28% 32% 11% 6%	285
	11th to 20th	Day	18% 26% 32% 16% 6% 1	465
	21st to end	Day	18% 23% 36% 19% 3 1	414
March	1st to 10th	Day	15% 30% 36% 14% 4 1	500
	11th to 20th	Day	14% 25% 35% 18% 8% 1	500
	21st to end	Day	20% 25% 33% 18% 4	550
April	1st to 10th	Day	22% 22% 30% 17% 7% 1	500
	11th to 20th	Day	20% 27% 31% 15% 6% 2	492
	21st to end	Day	18% 23% 31% 20% 6% 2	500
May	1st to 10th	Day	14% 20% 33% 21% 10% 1	503
	11th to 20th	Day	20% 23% 28% 18% 9% 2	528
	21st to end	Day	18% 17% 27% 21% 12% 4	573
June	1st to 10th	Day	14% 21% 30% 23% 10% 3	555
	11th to 20th	Day	18% 19% 31% 19% 11% 2	728
	21st to end	Day	8% 19% 36% 25% 9% 3	530
July	1st to 10th	Day	5% 18% 39% 25% 9% 4	526
	11th to 20th	Day	5% 16% 42% 24% 11% 2	524
	21st to end	Day	6% 15% 34% 30% 12% 4	575
August	1st to 10th	Day	7% 14% 39% 25% 13% 2	527
	11th to 20th	Day	3 14% 37% 28% 14% 4	517
	21st to end	Day	7% 18% 32% 27% 12% 4	583
September	1st to 10th	Day	7% 14% 31% 25% 17% 5%	530
	11th to 20th	Day	13% 14% 30% 28% 11% 4	530
	21st to end	Day	11% 15% 32% 25% 14% 3	530
October	1st to 10th	Day	17% 16% 29% 24% 13% 2	503
	11th to 20th	Day	12% 20% 26% 20% 15% 6%	317
	21st to end	Day	14% 22% 32% 23% 7% 2	330
November	1st to 10th	Day	19% 20% 27% 21% 12% 1	300
	11th to 20th	Day	13% 21% 32% 23% 9% 2	300
	21st to end	Day	10% 18% 34% 25% 11% 2	300
December	1st to 10th	Day	14% 15% 27% 22% 14% 9%	213
	11th to 20th	Day	19% 26% 33% 14% 9% 1	200
	21st to end	Day	19% 20% 29% 22% 9% 2	220
January	1st to 10th	Night	10% 15% 31% 31% 11% 1	800
	11th to 20th	Night	10% 12% 32% 29% 14% 3	800
	21st to end	Night	6% 11% 36% 32% 13% 2	880
February	1st to 10th	Night	11% 18% 37% 25% 9% 2	800
	11th to 20th	Night	9% 15% 36% 28% 12% 1	784
	21st to end	Night	10% 13% 36% 30% 8% 2	660
March	1st to 10th	Night	9% 15% 35% 31% 9% 1	800
	11th to 20th	Night	11% 14% 28% 33% 12% 2	800
	21st to end	Night	10% 13% 32% 34% 10% 1	880
April	1st to 10th	Night	10% 12% 28% 34% 14% 3	800
	11th to 20th	Night	10% 14% 33% 30% 11% 2	673
	21st to end	Night	10% 11% 26% 38% 13% 2	600
May	1st to 10th	Night	10% 9% 24% 36% 20% 3	600
	11th to 20th	Night	9% 8% 21% 35% 23% 3	569
	21st to end	Night	11% 6% 20% 32% 28% 3	433
June	1st to 10th	Night	12% 6% 17% 40% 22% 3	400
	11th to 20th	Night	11% 6% 23% 42% 17% 2	400
	21st to end	Night	4 2 16% 44% 33% 2	400
July	1st to 10th	Night	2 1 10% 48% 35% 3	390
	11th to 20th	Night	2 1 12% 58% 27% 1	388
	21st to end	Night	2 2 10% 48% 35% 4	438
August	1st to 10th	Night	4 2 15% 47% 30% 2	562
	11th to 20th	Night	2 3 12% 47% 35% 2	594
	21st to end	Night	4 3 16% 45% 29% 3	756
September	1st to 10th	Night	5% 2 14% 41% 34% 4	800
	11th to 20th	Night	7% 3 14% 38% 33% 5%	800
	21st to end	Night	8% 4 19% 37% 25% 7%	800
October	1st to 10th	Night	10% 6% 23% 36% 23% 3	772
	11th to 20th	Night	11% 11% 21% 36% 17% 4	760
	21st to end	Night	11% 9% 30% 35% 11% 4	836
November	1st to 10th	Night	14% 11% 28% 32% 14% 1	780
	11th to 20th	Night	13% 13% 35% 31% 8% 1	800
	21st to end	Night	9% 13% 33% 29% 12% 4	843
December	1st to 10th	Night	10% 14% 32% 33% 10% 1	1000
	11th to 20th	Night	13% 15% 32% 30% 8% 1	1000
	21st to end	Night	12% 19% 34% 24% 9% 2	910

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