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 2024-11-05.

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 10% 4 9% 5% 4 4 3 5% 6% 46%	104822
	Night	2 9% 4 9% 5% 4 4 4 5% 7% 46%	102853
Transparency	Day	38% 5% 9% 19% 18% 10%	104565
	Night	36% 7% 11% 21% 18% 8%	102700
Seeing	Day	14% 20% 33% 21% 9% 2	14585
	Night	9% 10% 26% 35% 17% 3	24165

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	11% 6% 22% 7% 6% 6% 5% 6% 6% 24%	2539
	11th to 20th	Day	2 14% 6% 16% 7% 5% 4 3 5% 6% 33%	2540
	21st to end	Day	2 21% 7% 11% 5% 4 4 3 5% 6% 31%	2794
February	1st to 10th	Day	2 15% 6% 13% 6% 5% 5% 3 6% 7% 31%	1680
	11th to 20th	Day	1 13% 8% 17% 7% 5% 5% 4 5% 5% 29%	1821
	21st to end	Day	1 18% 7% 17% 9% 6% 5% 4 6% 6% 23%	1570
March	1st to 10th	Day	1 10% 7% 18% 7% 5% 4 4 6% 7% 30%	3138
	11th to 20th	Day	1 12% 5% 11% 4 4 3 3 4 5% 49%	3445
	21st to end	Day	2 22% 8% 15% 8% 7% 6% 5% 5% 5% 16%	3876
April	1st to 10th	Day	3 19% 7% 12% 6% 5% 5% 4 5% 6% 27%	2280
	11th to 20th	Day	3 17% 5% 11% 5% 5% 4 3 4 6% 35%	2359
	21st to end	Day	6% 15% 5% 11% 6% 5% 5% 4 5% 6% 33%	2470
May	1st to 10th	Day	5% 18% 6% 10% 5% 5% 5% 3 5% 4 33%	4110
	11th to 20th	Day	5% 7% 3 7% 5% 5% 5% 4 5% 7% 48%	4100
	21st to end	Day	6% 15% 4 7% 5% 5% 5% 4 5% 6% 38%	4813
June	1st to 10th	Day	11% 10% 2 7% 6% 5% 5% 5% 7% 7% 35%	2684
	11th to 20th	Day	9% 9% 3 6% 5% 4 5% 4 4 6% 45%	2848
	21st to end	Day	3 4 1 5% 4 4 3 3 4 5% 62%	2660
July	1st to 10th	Day	1 1 1 3 2 3 3 3 4 5% 75%	4420
	11th to 20th	Day	2 2 1 4 2 3 3 3 5% 6% 70%	4420
	21st to end	Day	3 3 1 4 3 4 4 3 5% 6% 64%	4790
August	1st to 10th	Day	3 4 2 6% 5% 5% 5% 4 6% 7% 54%	4136
	11th to 20th	Day	2 1 1 3 3 4 4 4 5% 7% 67%	4028
	21st to end	Day	3 3 2 4 3 3 4 2 4 4 68%	4366
September	1st to 10th	Day	4 4 1 4 3 3 3 3 4 5% 67%	2247
	11th to 20th	Day	6% 5% 2 4 4 4 4 4 5% 6% 54%	2090
	21st to end	Day	6% 7% 1 4 2 3 4 2 4 7% 60%	2090
October	1st to 10th	Day	5% 6% 2 4 4 3 4 2 3 5% 64%	3490
	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% 38%	1710
	11th to 20th	Day	3 14% 4 11% 6% 4 4 4 6% 7% 38%	1710
	21st to end	Day	1 11% 5% 9% 6% 4 5% 4 6% 9% 39%	1647
December	1st to 10th	Day	2 11% 5% 14% 7% 5% 4 4 4 7% 37%	1354
	11th to 20th	Day	2 14% 5% 14% 7% 7% 6% 4 7% 8% 27%	1685
	21st to end	Day	2 12% 5% 13% 7% 5% 4 4 6% 6% 35%	2766
January	1st to 10th	Night	1 14% 7% 17% 7% 5% 4 4 5% 8% 28%	4406
	11th to 20th	Night	3 15% 7% 15% 7% 4 4 3 4 6% 31%	4420
	21st to end	Night	2 18% 7% 12% 6% 5% 5% 3 5% 6% 31%	4862
February	1st to 10th	Night	1 14% 7% 11% 6% 6% 5% 4 7% 8% 29%	2499
	11th to 20th	Night	1 13% 6% 13% 7% 5% 4 4 6% 7% 34%	2246
	21st to end	Night	1 13% 4 17% 8% 7% 6% 3 5% 7% 27%	1884
March	1st to 10th	Night	1 11% 6% 12% 6% 5% 5% 4 5% 9% 36%	3736
	11th to 20th	Night	1 11% 5% 10% 4 3 4 4 5% 6% 48%	3534
	21st to end	Night	1 12% 8% 14% 6% 5% 6% 5% 7% 8% 27%	3772
April	1st to 10th	Night	3 12% 6% 9% 5% 4 6% 5% 6% 7% 36%	1900
	11th to 20th	Night	3 13% 5% 9% 5% 4 5% 5% 6% 6% 40%	1767
	21st to end	Night	4 11% 4 8% 4 4 4 4 6% 9% 42%	1710
May	1st to 10th	Night	4 15% 4 8% 4 3 4 4 5% 8% 41%	2860
	11th to 20th	Night	3 7% 2 4 4 4 4 4 5% 8% 56%	2786
	21st to end	Night	4 8% 3 6% 5% 4 5% 4 5% 7% 47%	2762
June	1st to 10th	Night	8% 7% 2 6% 4 4 5% 5% 8% 9% 43%	1482
	11th to 20th	Night	8% 7% 2 4 4 3 4 3 5% 6% 53%	1330
	21st to end	Night	2 3 1 3 3 4 3 3 5% 7% 65%	1508
July	1st to 10th	Night	1 1 2 1 2 2 3 4 5% 79%	2540
	11th to 20th	Night	1 1 1 2 2 2 3 3 5% 6% 76%	2540
	21st to end	Night	1 2 1 3 3 3 3 3 5% 7% 69%	2761
August	1st to 10th	Night	2 3 1 5% 4 5% 5% 4 5% 7% 59%	2802
	11th to 20th	Night	1 1 1 2 1 2 2 2 4 7% 77%	2811
	21st to end	Night	2 1 1 3 2 3 3 3 4 5% 72%	3302
September	1st to 10th	Night	3 3 1 2 2 2 2 2 4 6% 72%	1933
	11th to 20th	Night	4 5% 1 5% 3 3 3 3 5% 6% 61%	2090
	21st to end	Night	5% 5% 1 4 2 2 2 3 4 7% 66%	2090
October	1st to 10th	Night	3 5% 2 3 2 3 3 3 4 6% 67%	3490
	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	5% 12% 6% 9% 5% 4 4 4 6% 8% 38%	2469
	11th to 20th	Night	3 13% 4 9% 4 5% 5% 5% 6% 9% 38%	2470
	21st to end	Night	2 8% 4 9% 6% 5% 7% 4 5% 7% 42%	2510
December	1st to 10th	Night	1 12% 6% 12% 7% 5% 5% 4 6% 8% 35%	2821
	11th to 20th	Night	2 14% 5% 13% 7% 5% 5% 5% 6% 7% 30%	3506
	21st to end	Night	1 11% 6% 13% 7% 6% 5% 4 6% 8% 34%	4917

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	58% 9% 11% 16% 5% 1	2539
	11th to 20th	Day	51% 7% 11% 15% 12% 4	2540
	21st to end	Day	54% 6% 13% 19% 7% 2	2794
February	1st to 10th	Day	52% 9% 13% 19% 6% 2	1680
	11th to 20th	Day	57% 7% 11% 15% 7% 3	1831
	21st to end	Day	62% 6% 8% 13% 9% 2	1570
March	1st to 10th	Day	53% 6% 10% 12% 19% 1	3138
	11th to 20th	Day	41% 5% 11% 30% 14%	3445
	21st to end	Day	67% 8% 11% 11% 3	3876
April	1st to 10th	Day	59% 7% 12% 14% 7% 1	2280
	11th to 20th	Day	49% 6% 14% 18% 11% 1	2362
	21st to end	Day	51% 7% 13% 21% 8%	2470
May	1st to 10th	Day	52% 6% 11% 17% 12% 1	4110
	11th to 20th	Day	34% 4 12% 29% 13% 7%	4103
	21st to end	Day	45% 6% 10% 27% 9% 2	4814
June	1st to 10th	Day	47% 8% 12% 23% 10%	2684
	11th to 20th	Day	39% 5% 9% 21% 18% 7%	2848
	21st to end	Day	24% 4 7% 21% 34% 9%	2660
July	1st to 10th	Day	13% 3 4 13% 20% 48%	4351
	11th to 20th	Day	17% 2 4 9% 22% 47%	4345
	21st to end	Day	19% 2 6% 11% 33% 28%	4639
August	1st to 10th	Day	29% 3 7% 11% 28% 22%	4120
	11th to 20th	Day	16% 3 6% 17% 39% 18%	4067
	21st to end	Day	20% 3 5% 20% 35% 17%	4365
September	1st to 10th	Day	21% 4 5% 20% 31% 19%	2247
	11th to 20th	Day	30% 5% 9% 23% 22% 10%	2090
	21st to end	Day	26% 4 10% 26% 28% 7%	2090
October	1st to 10th	Day	26% 3 6% 20% 36% 8%	3490
	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% 19% 14% 3	1710
	11th to 20th	Day	46% 8% 13% 23% 9% 2	1710
	21st to end	Day	42% 7% 12% 19% 13% 7%	1647
December	1st to 10th	Day	49% 6% 14% 17% 11% 4	1354
	11th to 20th	Day	54% 9% 15% 13% 9% 1	1684
	21st to end	Day	47% 7% 8% 19% 13% 5%	2766
January	1st to 10th	Night	53% 8% 9% 21% 8% 1	4406
	11th to 20th	Night	55% 5% 10% 16% 11% 3	4420
	21st to end	Night	51% 10% 10% 19% 7% 3	4862
February	1st to 10th	Night	52% 8% 13% 18% 8% 2	2499
	11th to 20th	Night	48% 9% 14% 17% 9% 3	2247
	21st to end	Night	57% 8% 8% 15% 9% 3	1884
March	1st to 10th	Night	45% 9% 9% 14% 21% 2	3736
	11th to 20th	Night	36% 7% 13% 30% 12% 1	3534
	21st to end	Night	51% 12% 15% 18% 4	3772
April	1st to 10th	Night	46% 8% 13% 22% 9% 1	1900
	11th to 20th	Night	42% 8% 15% 22% 12% 1	1767
	21st to end	Night	39% 7% 18% 27% 9%	1710
May	1st to 10th	Night	40% 7% 12% 27% 12% 2	2860
	11th to 20th	Night	25% 7% 14% 34% 17% 2	2787
	21st to end	Night	34% 8% 14% 34% 11%	2762
June	1st to 10th	Night	35% 11% 17% 27% 10%	1482
	11th to 20th	Night	32% 5% 12% 25% 20% 7%	1330
	21st to end	Night	18% 5% 8% 25% 36% 9%	1508
July	1st to 10th	Night	9% 2 5% 12% 25% 47%	2500
	11th to 20th	Night	11% 2 4 12% 22% 50%	2492
	21st to end	Night	16% 2 5% 14% 32% 31%	2674
August	1st to 10th	Night	25% 2 6% 13% 34% 20%	2794
	11th to 20th	Night	9% 3 4 20% 44% 20%	2838
	21st to end	Night	14% 3 6% 23% 34% 20%	3302
September	1st to 10th	Night	16% 3 6% 21% 36% 19%	1933
	11th to 20th	Night	23% 5% 11% 25% 23% 12%	2090
	21st to end	Night	21% 3 9% 32% 27% 9%	2090
October	1st to 10th	Night	19% 4 8% 22% 36% 10%	3490
	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	43% 8% 13% 19% 14% 3	2469
	11th to 20th	Night	43% 9% 14% 22% 11% 1	2470
	21st to end	Night	41% 8% 12% 18% 15% 7%	2510
December	1st to 10th	Night	46% 8% 17% 15% 9% 5%	2821
	11th to 20th	Night	51% 9% 13% 16% 10% 1	3507
	21st to end	Night	47% 7% 10% 18% 12% 6%	4917

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	17% 18% 28% 27% 7% 3	180
	11th to 20th	Day	13% 25% 28% 17% 11% 6%	180
	21st to end	Day	14% 25% 29% 19% 10% 3	198
February	1st to 10th	Day	20% 30% 32% 12% 5%	256
	11th to 20th	Day	19% 27% 31% 15% 6% 1	426
	21st to end	Day	19% 24% 35% 18% 3 1	382
March	1st to 10th	Day	17% 31% 36% 13% 3	460
	11th to 20th	Day	13% 24% 35% 18% 8% 1	460
	21st to end	Day	20% 25% 33% 18% 4	506
April	1st to 10th	Day	23% 23% 30% 17% 6% 1	460
	11th to 20th	Day	21% 27% 31% 15% 6% 1	452
	21st to end	Day	19% 24% 30% 20% 5% 2	460
May	1st to 10th	Day	14% 22% 33% 21% 9% 1	463
	11th to 20th	Day	19% 22% 29% 19% 9% 2	488
	21st to end	Day	19% 18% 28% 21% 11% 4	529
June	1st to 10th	Day	15% 21% 29% 23% 9% 2	514
	11th to 20th	Day	19% 19% 31% 19% 11% 2	678
	21st to end	Day	9% 19% 36% 25% 8% 3	490
July	1st to 10th	Day	5% 18% 40% 24% 9% 4	486
	11th to 20th	Day	5% 17% 43% 24% 10% 2	484
	21st to end	Day	6% 16% 35% 29% 11% 3	531
August	1st to 10th	Day	6% 15% 40% 24% 13% 2	487
	11th to 20th	Day	4 14% 36% 29% 14% 4	477
	21st to end	Day	7% 19% 32% 26% 12% 4	539
September	1st to 10th	Day	8% 15% 33% 24% 17% 4	490
	11th to 20th	Day	14% 14% 31% 27% 10% 3	490
	21st to end	Day	11% 15% 33% 25% 14% 2	490
October	1st to 10th	Day	16% 16% 29% 24% 13% 2	490
	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% 21% 28% 20% 11% 1	282
	11th to 20th	Day	13% 20% 31% 24% 10% 3	270
	21st to end	Day	10% 17% 36% 25% 11% 1	270
December	1st to 10th	Day	16% 16% 28% 21% 13% 6%	192
	11th to 20th	Day	19% 24% 34% 13% 8% 1	180
	21st to end	Day	17% 20% 30% 22% 10% 2	198
January	1st to 10th	Night	9% 16% 30% 32% 12% 1	760
	11th to 20th	Night	10% 13% 31% 30% 14% 3	760
	21st to end	Night	6% 12% 35% 32% 13% 2	836
February	1st to 10th	Night	11% 17% 37% 25% 9% 2	760
	11th to 20th	Night	9% 15% 35% 28% 12% 1	744
	21st to end	Night	9% 13% 37% 30% 8% 3	628
March	1st to 10th	Night	9% 16% 34% 30% 9% 1	760
	11th to 20th	Night	11% 13% 28% 34% 12% 2	760
	21st to end	Night	10% 13% 32% 34% 11% 1	836
April	1st to 10th	Night	10% 13% 28% 33% 13% 3	760
	11th to 20th	Night	11% 14% 33% 29% 11% 2	639
	21st to end	Night	10% 11% 27% 37% 13% 2	570
May	1st to 10th	Night	9% 9% 24% 36% 19% 3	570
	11th to 20th	Night	9% 7% 21% 37% 24% 3	540
	21st to end	Night	12% 6% 20% 31% 27% 4	411
June	1st to 10th	Night	12% 6% 17% 39% 22% 3	380
	11th to 20th	Night	12% 6% 22% 41% 18% 2	380
	21st to end	Night	4 1 16% 43% 33% 2	380
July	1st to 10th	Night	2 1 10% 48% 36% 4	370
	11th to 20th	Night	2 1 12% 58% 27% 1	368
	21st to end	Night	2 1 10% 48% 34% 4	416
August	1st to 10th	Night	4 2 14% 46% 31% 2	534
	11th to 20th	Night	2 3 11% 46% 36% 2	564
	21st to end	Night	3 3 16% 45% 29% 3	718
September	1st to 10th	Night	5% 2 14% 42% 33% 4	760
	11th to 20th	Night	7% 4 14% 38% 32% 6%	760
	21st to end	Night	8% 4 19% 37% 25% 7%	760
October	1st to 10th	Night	9% 5% 23% 37% 23% 3	760
	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	15% 11% 28% 32% 14% 1	760
	11th to 20th	Night	13% 13% 34% 32% 8% 1	760
	21st to end	Night	9% 13% 32% 29% 13% 4	800
December	1st to 10th	Night	10% 14% 32% 33% 9% 1	950
	11th to 20th	Night	13% 15% 31% 31% 9% 1	950
	21st to end	Night	11% 18% 34% 25% 10% 2	865

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