Shadowlands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shadowlands Observatory 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	12% 6% 2 6% 4 4 4 3 5% 7% 47%	109102
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	109353
Transparency	Day	36% 5% 9% 25% 20% 5%	112752
	Night	33% 4 9% 24% 23% 7%	113843
Seeing	Day	14% 8% 21% 26% 22% 7%	15671
	Night	14% 7% 23% 36% 18% 2	25635

By Month

Percentage Cloud Cover SE (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	16% 10% 3 6% 3 3 3 2 3 4 48%	4077
	11th to 20th	Day	8% 7% 2 6% 4 4 3 3 5% 5% 54%	4080
	21st to end	Day	21% 8% 2 4 3 3 2 2 4 4 47%	4488
February	1st to 10th	Day	19% 9% 3 6% 3 2 3 2 4 4 45%	1755
	11th to 20th	Day	17% 9% 2 6% 4 3 3 3 5% 5% 44%	1686
	21st to end	Day	19% 9% 3 6% 4 4 3 3 4 4 40%	1727
March	1st to 10th	Day	18% 8% 3 5% 4 3 3 2 3 5% 46%	3436
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 6% 47%	3480
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 6% 41%	3814
April	1st to 10th	Day	13% 6% 3 7% 4 4 4 3 4 6% 46%	2194
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 3 4 5% 45%	2254
	21st to end	Day	17% 8% 2 7% 5% 4 4 3 5% 5% 40%	2280
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 37%	4005
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 6% 8% 42%	4082
	21st to end	Day	15% 7% 3 7% 5% 4 6% 3 6% 7% 37%	4477
June	1st to 10th	Day	8% 6% 3 6% 4 5% 4 4 7% 9% 43%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 7% 8% 46%	2470
	21st to end	Day	4 3 2 4 4 5% 6% 5% 8% 9% 52%	2470
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 5% 7% 11% 43%	4120
	11th to 20th	Day	4 3 1 5% 3 4 4 5% 7% 8% 56%	4120
	21st to end	Day	7% 6% 3 7% 6% 6% 6% 6% 8% 10% 36%	4465
August	1st to 10th	Day	4 2 1 4 4 3 5% 4 6% 8% 59%	4104
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 45%	3930
	21st to end	Day	4 3 1 5% 4 4 5% 4 7% 8% 55%	4180
September	1st to 10th	Day	11% 5% 2 4 4 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	12% 4 2 4 3 4 5% 4 5% 9% 47%	2129
	21st to end	Day	8% 4 1 4 4 4 5% 4 5% 6% 56%	2057
October	1st to 10th	Day	8% 4 1 5% 4 3 4 3 4 7% 57%	1900
	11th to 20th	Day	13% 4 2 4 3 2 3 3 3 5% 59%	1900
	21st to end	Day	17% 5% 2 5% 3 2 3 3 4 5% 50%	2090
November	1st to 10th	Day	13% 5% 1 4 3 3 3 3 4 6% 55%	1958
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 6% 56%	2000
	21st to end	Day	16% 7% 3 5% 3 2 3 2 4 5% 50%	1805
December	1st to 10th	Day	16% 7% 2 5% 3 2 3 2 5% 5% 50%	2940
	11th to 20th	Day	18% 8% 3 6% 3 3 3 3 4 6% 44%	3391
	21st to end	Day	20% 13% 4 7% 3 3 3 3 4 5% 36%	4488
January	1st to 10th	Night	15% 7% 3 6% 4 3 3 3 4 5% 47%	5888
	11th to 20th	Night	9% 5% 2 4 3 2 3 3 4 5% 62%	5900
	21st to end	Night	22% 7% 2 4 3 2 2 2 3 5% 48%	6490
February	1st to 10th	Night	18% 8% 2 6% 4 3 2 2 5% 5% 45%	2536
	11th to 20th	Night	16% 7% 3 5% 3 2 3 3 5% 5% 47%	2389
	21st to end	Night	17% 6% 2 6% 4 3 3 3 4 6% 47%	1732
March	1st to 10th	Night	21% 6% 2 5% 4 3 2 2 4 6% 44%	3419
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	3480
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 7% 44%	3817
April	1st to 10th	Night	12% 5% 3 6% 3 3 3 3 4 7% 49%	1984
	11th to 20th	Night	12% 6% 2 6% 3 4 3 3 5% 6% 50%	1875
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 45%	1900
May	1st to 10th	Night	14% 7% 2 6% 4 4 3 3 5% 7% 43%	2894
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 3 6% 8% 43%	2823
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 42%	3090
June	1st to 10th	Night	4 4 1 4 4 3 4 4 7% 9% 56%	1710
	11th to 20th	Night	4 3 1 3 3 3 3 4 6% 11% 60%	1710
	21st to end	Night	3 2 1 3 3 3 4 3 5% 9% 64%	1710
July	1st to 10th	Night	8% 4 2 6% 4 4 4 3 6% 9% 49%	2850
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 9% 61%	2850
	21st to end	Night	5% 5% 2 7% 4 5% 5% 5% 7% 9% 45%	3098
August	1st to 10th	Night	3 2 1 4 3 4 4 4 5% 8% 60%	2836
	11th to 20th	Night	5% 4 2 5% 3 4 4 4 5% 9% 56%	2891
	21st to end	Night	4 3 1 5% 4 4 4 3 6% 8% 58%	3476
September	1st to 10th	Night	8% 4 1 4 4 3 4 2 5% 8% 58%	1900
	11th to 20th	Night	9% 4 1 6% 4 3 3 4 6% 8% 51%	2046
	21st to end	Night	7% 4 2 5% 3 3 4 3 5% 7% 57%	2103
October	1st to 10th	Night	6% 3 1 4 2 3 4 3 5% 7% 61%	2280
	11th to 20th	Night	12% 4 2 3 3 3 3 2 3 5% 61%	2280
	21st to end	Night	18% 4 2 5% 2 2 2 2 3 4 54%	2508
November	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 57%	2340
	11th to 20th	Night	12% 5% 1 3 3 3 2 3 5% 6% 58%	2400
	21st to end	Night	17% 5% 2 4 3 3 3 2 3 6% 52%	2592
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 3 5% 52%	4250
	11th to 20th	Night	19% 7% 2 5% 3 3 3 2 4 4 48%	4816
	21st to end	Night	21% 9% 3 6% 4 3 3 2 3 4 40%	6490

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	42% 2 8% 25% 20% 2	4078
	11th to 20th	Day	33% 4 9% 25% 26% 4	4080
	21st to end	Day	42% 3 4 19% 24% 7%	4488
February	1st to 10th	Day	44% 3 10% 21% 18% 3	1755
	11th to 20th	Day	44% 3 7% 22% 20% 4	1695
	21st to end	Day	48% 4 6% 16% 19% 8%	1727
March	1st to 10th	Day	41% 5% 7% 18% 18% 11%	3436
	11th to 20th	Day	38% 5% 10% 23% 19% 4	3480
	21st to end	Day	45% 4 8% 26% 15% 1	3814
April	1st to 10th	Day	40% 4 9% 21% 20% 6%	2194
	11th to 20th	Day	41% 4 9% 22% 18% 5%	2254
	21st to end	Day	46% 6% 7% 22% 18% 2	2280
May	1st to 10th	Day	46% 6% 8% 22% 16% 2	4042
	11th to 20th	Day	38% 7% 11% 26% 17% 1	4086
	21st to end	Day	45% 8% 11% 20% 15% 1	4489
June	1st to 10th	Day	36% 8% 13% 26% 15% 2	2470
	11th to 20th	Day	33% 7% 15% 25% 19% 2	2470
	21st to end	Day	26% 8% 13% 29% 21% 4	2470
July	1st to 10th	Day	33% 7% 17% 28% 13% 2	4056
	11th to 20th	Day	24% 7% 9% 36% 22% 2	4051
	21st to end	Day	39% 8% 16% 26% 10% 1	4325
August	1st to 10th	Day	22% 6% 13% 30% 23% 6%	4090
	11th to 20th	Day	34% 7% 15% 30% 14% 1	3969
	21st to end	Day	24% 8% 13% 37% 15% 2	4179
September	1st to 10th	Day	32% 6% 9% 25% 19% 9%	2280
	11th to 20th	Day	33% 6% 10% 24% 21% 5%	2129
	21st to end	Day	29% 6% 9% 21% 28% 8%	2057
October	1st to 10th	Day	16% 3 5% 13% 37% 25%	3160
	11th to 20th	Day	20% 2 4 17% 40% 18%	3140
	21st to end	Day	32% 4 7% 27% 24% 6%	3428
November	1st to 10th	Day	32% 4 8% 25% 24% 7%	1958
	11th to 20th	Day	31% 3 6% 24% 26% 10%	2000
	21st to end	Day	37% 3 7% 20% 26% 6%	1805
December	1st to 10th	Day	38% 3 6% 30% 21% 2	2940
	11th to 20th	Day	43% 3 6% 22% 23% 4	3389
	21st to end	Day	51% 4 6% 22% 13% 3	4488
January	1st to 10th	Night	41% 3 9% 26% 19% 3	5888
	11th to 20th	Night	26% 3 10% 22% 33% 6%	5900
	21st to end	Night	42% 2 7% 16% 27% 6%	6490
February	1st to 10th	Night	42% 4 8% 25% 17% 5%	2536
	11th to 20th	Night	39% 3 7% 23% 21% 6%	2390
	21st to end	Night	40% 3 7% 19% 21% 9%	1732
March	1st to 10th	Night	43% 5% 8% 16% 18% 11%	3419
	11th to 20th	Night	36% 4 8% 26% 19% 6%	3480
	21st to end	Night	40% 5% 10% 25% 17% 2	3817
April	1st to 10th	Night	35% 4 10% 21% 21% 9%	1984
	11th to 20th	Night	35% 5% 8% 22% 20% 10%	1875
	21st to end	Night	38% 6% 10% 22% 23% 2	1900
May	1st to 10th	Night	38% 8% 11% 24% 15% 4	2922
	11th to 20th	Night	38% 8% 12% 24% 16% 1	2824
	21st to end	Night	38% 8% 14% 23% 14% 3	3099
June	1st to 10th	Night	23% 8% 15% 31% 20% 3	1710
	11th to 20th	Night	19% 7% 14% 34% 24% 3	1710
	21st to end	Night	18% 5% 13% 31% 27% 6%	1710
July	1st to 10th	Night	30% 7% 17% 26% 17% 3	2805
	11th to 20th	Night	18% 6% 10% 38% 25% 2	2796
	21st to end	Night	33% 7% 17% 28% 15% 1	2999
August	1st to 10th	Night	21% 6% 12% 33% 25% 4	2826
	11th to 20th	Night	25% 5% 19% 33% 17% 2	2918
	21st to end	Night	23% 5% 12% 33% 25% 3	3476
September	1st to 10th	Night	26% 5% 8% 27% 24% 10%	1900
	11th to 20th	Night	29% 7% 11% 23% 24% 6%	2046
	21st to end	Night	26% 5% 8% 23% 30% 8%	2103
October	1st to 10th	Night	14% 3 8% 13% 32% 30%	3800
	11th to 20th	Night	18% 2 4 15% 39% 22%	3779
	21st to end	Night	28% 2 5% 26% 29% 10%	4120
November	1st to 10th	Night	31% 4 8% 22% 25% 10%	2340
	11th to 20th	Night	28% 3 6% 24% 27% 11%	2400
	21st to end	Night	35% 3 8% 20% 27% 6%	2592
December	1st to 10th	Night	37% 3 7% 31% 18% 3	4250
	11th to 20th	Night	41% 2 5% 22% 21% 8%	4817
	21st to end	Night	49% 3 8% 22% 14% 4	6490

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	27% 13% 29% 23% 7% 1	300
	11th to 20th	Day	23% 16% 36% 19% 5%	300
	21st to end	Day	22% 18% 33% 18% 7% 1	330
February	1st to 10th	Day	26% 14% 35% 18% 5% 1	285
	11th to 20th	Day	27% 19% 33% 16% 4	270
	21st to end	Day	36% 21% 28% 12% 3	225
March	1st to 10th	Day	21% 19% 30% 23% 6% 1	270
	11th to 20th	Day	27% 17% 36% 17% 3	270
	21st to end	Day	29% 19% 33% 13% 5%	297
April	1st to 10th	Day	22% 17% 33% 19% 8% 1	374
	11th to 20th	Day	21% 14% 32% 25% 8%	452
	21st to end	Day	18% 18% 30% 23% 9% 2	460
May	1st to 10th	Day	17% 12% 31% 30% 10%	601
	11th to 20th	Day	15% 7% 25% 30% 20% 3	675
	21st to end	Day	12% 6% 27% 30% 19% 6%	734
June	1st to 10th	Day	9% 4 16% 28% 33% 10%	680
	11th to 20th	Day	6% 3 14% 36% 31% 10%	680
	21st to end	Day	4 3 14% 37% 33% 10%	680
July	1st to 10th	Day	7% 2 10% 28% 39% 14%	671
	11th to 20th	Day	4 1 8% 31% 37% 18%	668
	21st to end	Day	6% 2 8% 27% 38% 20%	739
August	1st to 10th	Day	7% 2 8% 32% 39% 11%	676
	11th to 20th	Day	12% 2 8% 31% 36% 11%	606
	21st to end	Day	6% 1 6% 23% 42% 22%	539
September	1st to 10th	Day	9% 1 8% 25% 42% 15%	490
	11th to 20th	Day	11% 3 11% 29% 35% 11%	339
	21st to end	Day	11% 4 18% 29% 24% 14%	300
October	1st to 10th	Day	4 4 20% 35% 30% 7%	300
	11th to 20th	Day	13% 10% 27% 31% 17% 2	300
	21st to end	Day	26% 14% 25% 22% 11% 2	330
November	1st to 10th	Day	21% 7% 32% 20% 18% 3	300
	11th to 20th	Day	15% 13% 33% 28% 9% 2	300
	21st to end	Day	22% 17% 22% 24% 14% 1	300
December	1st to 10th	Day	19% 16% 34% 18% 10% 3	300
	11th to 20th	Day	22% 16% 32% 21% 9%	300
	21st to end	Day	26% 16% 34% 19% 5% 1	330
January	1st to 10th	Night	20% 9% 33% 32% 6%	1000
	11th to 20th	Night	18% 12% 31% 33% 6%	1000
	21st to end	Night	18% 8% 35% 34% 5%	1100
February	1st to 10th	Night	22% 11% 30% 33% 4	976
	11th to 20th	Night	21% 9% 37% 30% 4	915
	21st to end	Night	21% 13% 37% 28% 1	476
March	1st to 10th	Night	20% 9% 30% 36% 5%	570
	11th to 20th	Night	20% 11% 32% 31% 6%	570
	21st to end	Night	19% 10% 33% 30% 7%	627
April	1st to 10th	Night	16% 10% 34% 33% 6%	570
	11th to 20th	Night	14% 10% 34% 33% 9%	563
	21st to end	Night	19% 6% 35% 30% 10%	570
May	1st to 10th	Night	14% 5% 28% 42% 11% 1	570
	11th to 20th	Night	14% 6% 23% 38% 17% 2	561
	21st to end	Night	10% 3 22% 37% 25% 3	617
June	1st to 10th	Night	6% 3 10% 39% 40% 2	570
	11th to 20th	Night	4 1 11% 49% 33% 2	570
	21st to end	Night	4 1 11% 38% 41% 6%	570
July	1st to 10th	Night	6% 1 7% 37% 43% 6%	555
	11th to 20th	Night	3 1 7% 38% 43% 9%	552
	21st to end	Night	5% 4 35% 46% 9%	624
August	1st to 10th	Night	3 1 7% 41% 43% 5%	567
	11th to 20th	Night	8% 1 9% 35% 42% 5%	564
	21st to end	Night	6% 2 4 36% 46% 7%	627
September	1st to 10th	Night	9% 1 6% 36% 44% 4	570
	11th to 20th	Night	12% 1 9% 38% 36% 4	570
	21st to end	Night	8% 2 14% 38% 33% 5%	583
October	1st to 10th	Night	8% 3 14% 40% 32% 3	760
	11th to 20th	Night	14% 5% 24% 41% 16% 1	760
	21st to end	Night	19% 8% 22% 34% 16% 1	836
November	1st to 10th	Night	13% 6% 25% 41% 14% 1	780
	11th to 20th	Night	13% 10% 28% 38% 12%	800
	21st to end	Night	19% 10% 26% 33% 11%	992
December	1st to 10th	Night	20% 8% 29% 35% 8%	1000
	11th to 20th	Night	22% 11% 24% 38% 5%	1000
	21st to end	Night	20% 12% 29% 33% 6%	1100

Page generated at 2025-02-03 02:05:51. Version ac07

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