Sanctuary Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sanctuary Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 4 5% 4 6% 8% 39%	126732
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	122931
Transparency	Day	42% 6% 12% 23% 16% 3	130322
	Night	41% 5% 11% 23% 17% 3	127427
Seeing	Day	20% 14% 27% 22% 12% 4	16149
	Night	19% 11% 29% 29% 11% 1	25464

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	12% 6% 3 10% 5% 4 5% 4 5% 5% 42%	3597
	11th to 20th	Day	13% 10% 4 10% 6% 5% 4 3 5% 6% 34%	3843
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 5% 5% 36%	4466
February	1st to 10th	Day	15% 8% 4 7% 5% 5% 5% 4 6% 7% 35%	1800
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 4 6% 7% 38%	1832
	21st to end	Day	18% 10% 3 7% 4 4 4 3 4 6% 36%	1650
March	1st to 10th	Day	20% 4 2 5% 4 4 4 3 5% 6% 43%	4678
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 5% 7% 43%	4980
	21st to end	Day	17% 7% 2 5% 4 3 5% 4 5% 6% 42%	5666
April	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 3 6% 7% 38%	2400
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 6% 6% 42%	2374
	21st to end	Day	17% 6% 2 5% 4 4 5% 4 7% 7% 40%	2400
May	1st to 10th	Day	26% 6% 2 6% 4 4 4 4 6% 8% 31%	5871
	11th to 20th	Day	25% 8% 2 6% 4 4 5% 3 5% 8% 31%	6318
	21st to end	Day	19% 5% 2 6% 4 4 5% 4 7% 9% 37%	6949
June	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 4 8% 9% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 4 4 6% 4 7% 9% 38%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 11% 43%	2800
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 11% 40%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 6% 8% 11% 31%	6360
	21st to end	Day	10% 7% 2 6% 5% 5% 5% 4 6% 9% 40%	6591
August	1st to 10th	Day	19% 6% 2 6% 5% 6% 5% 4 7% 8% 33%	4234
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 7% 9% 41%	3961
	21st to end	Day	13% 5% 2 5% 3 3 3 4 5% 8% 49%	4312
September	1st to 10th	Day	15% 7% 2 5% 4 3 4 3 5% 6% 47%	2400
	11th to 20th	Day	13% 5% 2 4 4 3 4 3 5% 7% 50%	2400
	21st to end	Day	22% 8% 2 6% 4 4 4 3 5% 6% 37%	2131
October	1st to 10th	Day	17% 6% 2 4 4 3 3 3 6% 7% 45%	2000
	11th to 20th	Day	17% 5% 2 6% 3 4 4 3 4 7% 45%	1980
	21st to end	Day	17% 6% 2 5% 3 3 4 4 5% 7% 43%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 5% 7% 51%	1761
	11th to 20th	Day	16% 6% 2 5% 4 3 4 4 5% 7% 46%	1782
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 42%	1600
December	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 5% 6% 42%	2600
	11th to 20th	Day	21% 9% 2 5% 3 3 4 3 4 7% 38%	2986
	21st to end	Day	18% 7% 2 8% 5% 5% 5% 4 6% 7% 33%	3960
January	1st to 10th	Night	13% 7% 2 7% 5% 5% 4 4 6% 7% 41%	6349
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 42%	6026
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 7% 42%	6490
February	1st to 10th	Night	17% 8% 2 5% 5% 4 5% 4 5% 7% 37%	2600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	2453
	21st to end	Night	17% 8% 2 6% 5% 5% 4 3 5% 6% 39%	1980
March	1st to 10th	Night	18% 8% 2 6% 4 3 3 3 4 6% 43%	5147
	11th to 20th	Night	10% 6% 3 5% 4 4 4 3 4 6% 50%	4979
	21st to end	Night	15% 8% 2 6% 4 4 4 4 5% 6% 43%	5266
April	1st to 10th	Night	19% 8% 3 6% 5% 4 4 4 6% 7% 36%	2000
	11th to 20th	Night	14% 7% 2 7% 4 3 4 4 6% 7% 41%	1975
	21st to end	Night	18% 6% 2 5% 4 3 5% 3 5% 7% 44%	2000
May	1st to 10th	Night	26% 7% 3 7% 4 4 4 4 5% 8% 28%	4095
	11th to 20th	Night	24% 9% 3 7% 3 3 3 3 5% 8% 32%	3575
	21st to end	Night	21% 6% 2 6% 4 4 4 3 5% 7% 38%	3926
June	1st to 10th	Night	14% 5% 2 6% 4 3 5% 4 6% 8% 42%	1600
	11th to 20th	Night	14% 5% 2 5% 4 5% 5% 4 7% 9% 41%	1600
	21st to end	Night	11% 4 2 5% 5% 3 5% 4 7% 10% 42%	1600
July	1st to 10th	Night	8% 7% 3 7% 5% 5% 5% 5% 7% 9% 39%	3600
	11th to 20th	Night	11% 7% 3 9% 6% 6% 6% 5% 7% 10% 31%	3600
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 6% 8% 43%	4244
August	1st to 10th	Night	20% 5% 2 6% 5% 4 4 4 7% 7% 35%	2915
	11th to 20th	Night	14% 7% 2 7% 4 5% 5% 4 5% 7% 41%	3039
	21st to end	Night	13% 5% 2 5% 3 3 5% 3 6% 8% 46%	3586
September	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 5% 7% 47%	2000
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 5% 8% 50%	2000
	21st to end	Night	20% 6% 3 6% 4 4 5% 4 5% 6% 39%	2265
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 4 6% 46%	2400
	11th to 20th	Night	16% 5% 2 6% 3 2 3 3 5% 6% 48%	2377
	21st to end	Night	17% 5% 2 4 3 3 3 3 5% 7% 46%	2508
November	1st to 10th	Night	11% 5% 1 4 4 4 4 3 5% 6% 54%	2537
	11th to 20th	Night	15% 8% 3 5% 4 3 4 3 5% 6% 45%	2615
	21st to end	Night	15% 5% 2 5% 5% 5% 5% 4 5% 8% 43%	2800
December	1st to 10th	Night	13% 8% 3 6% 4 4 5% 3 6% 7% 42%	4580
	11th to 20th	Night	22% 7% 2 5% 4 3 4 3 5% 6% 39%	5208
	21st to end	Night	19% 8% 3 7% 5% 4 5% 4 6% 7% 34%	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	42% 4 11% 25% 16% 2	3598
	11th to 20th	Day	49% 4 10% 18% 15% 4	3876
	21st to end	Day	47% 6% 11% 21% 14% 1	4466
February	1st to 10th	Day	47% 6% 12% 23% 11% 1	1800
	11th to 20th	Day	45% 5% 10% 24% 13% 2	1841
	21st to end	Day	49% 5% 10% 21% 12% 3	1650
March	1st to 10th	Day	41% 4 10% 24% 15% 7%	4681
	11th to 20th	Day	39% 4 12% 27% 13% 5%	4980
	21st to end	Day	43% 4 10% 17% 21% 4	5666
April	1st to 10th	Day	45% 4 11% 19% 16% 5%	2400
	11th to 20th	Day	41% 5% 12% 22% 16% 4	2374
	21st to end	Day	41% 6% 10% 22% 17% 4	2400
May	1st to 10th	Day	50% 7% 12% 19% 10% 2	5880
	11th to 20th	Day	50% 7% 9% 22% 10% 1	6322
	21st to end	Day	42% 6% 12% 24% 14% 2	6949
June	1st to 10th	Day	38% 8% 12% 24% 16% 1	2800
	11th to 20th	Day	40% 8% 13% 25% 12% 3	2800
	21st to end	Day	35% 8% 14% 26% 16% 1	2800
July	1st to 10th	Day	38% 10% 13% 22% 16% 2	6290
	11th to 20th	Day	44% 9% 16% 21% 10% 1	6285
	21st to end	Day	39% 7% 10% 21% 22% 1	6403
August	1st to 10th	Day	47% 8% 11% 23% 10%	4221
	11th to 20th	Day	38% 6% 11% 29% 15% 1	4000
	21st to end	Day	34% 5% 10% 29% 21% 2	4312
September	1st to 10th	Day	39% 5% 11% 21% 20% 4	2400
	11th to 20th	Day	33% 4 10% 26% 23% 3	2400
	21st to end	Day	48% 5% 8% 17% 19% 3	2131
October	1st to 10th	Day	36% 4 8% 21% 27% 5%	3260
	11th to 20th	Day	29% 3 12% 28% 22% 7%	3220
	21st to end	Day	26% 4 13% 27% 28% 3	3428
November	1st to 10th	Day	34% 4 14% 24% 20% 4	1761
	11th to 20th	Day	39% 4 11% 24% 19% 2	1782
	21st to end	Day	41% 5% 12% 30% 10% 2	1600
December	1st to 10th	Day	42% 4 16% 24% 11% 3	2600
	11th to 20th	Day	45% 4 14% 29% 7% 1	2986
	21st to end	Day	46% 5% 16% 19% 12% 1	3960
January	1st to 10th	Night	39% 5% 11% 24% 17% 3	6349
	11th to 20th	Night	39% 4 14% 22% 16% 5%	6067
	21st to end	Night	43% 3 12% 24% 16% 2	6490
February	1st to 10th	Night	45% 5% 13% 25% 11% 1	2600
	11th to 20th	Night	46% 4 9% 21% 18% 3	2455
	21st to end	Night	45% 6% 10% 20% 16% 4	1980
March	1st to 10th	Night	43% 4 10% 23% 13% 8%	5157
	11th to 20th	Night	34% 5% 15% 28% 13% 5%	4980
	21st to end	Night	41% 4 8% 18% 25% 4	5266
April	1st to 10th	Night	46% 4 10% 20% 16% 4	2000
	11th to 20th	Night	40% 6% 12% 20% 20% 3	1975
	21st to end	Night	40% 5% 11% 22% 18% 5%	2000
May	1st to 10th	Night	54% 6% 9% 17% 11% 3	4102
	11th to 20th	Night	50% 8% 11% 19% 11% 1	3576
	21st to end	Night	46% 6% 10% 23% 13% 3	3927
June	1st to 10th	Night	39% 7% 10% 22% 20% 2	1600
	11th to 20th	Night	37% 8% 14% 24% 15% 2	1600
	21st to end	Night	35% 7% 14% 26% 16% 2	1600
July	1st to 10th	Night	41% 9% 14% 19% 14% 4	3560
	11th to 20th	Night	46% 8% 15% 19% 11% 1	3552
	21st to end	Night	37% 7% 12% 21% 22% 1	4118
August	1st to 10th	Night	46% 7% 17% 20% 10% 1	2904
	11th to 20th	Night	42% 6% 11% 24% 16% 1	3066
	21st to end	Night	34% 6% 11% 29% 18% 1	3586
September	1st to 10th	Night	37% 6% 11% 22% 20% 4	2000
	11th to 20th	Night	33% 6% 9% 25% 24% 4	2000
	21st to end	Night	46% 6% 8% 17% 20% 3	2265
October	1st to 10th	Night	43% 4 9% 21% 22% 2	3920
	11th to 20th	Night	27% 5% 9% 29% 25% 5%	3876
	21st to end	Night	27% 3 11% 32% 25% 3	4120
November	1st to 10th	Night	32% 5% 11% 26% 22% 4	2537
	11th to 20th	Night	39% 4 9% 25% 18% 4	2615
	21st to end	Night	39% 5% 12% 28% 15% 1	2800
December	1st to 10th	Night	41% 5% 13% 29% 11% 1	4580
	11th to 20th	Night	45% 4 13% 23% 12% 2	5208
	21st to end	Night	48% 6% 11% 19% 15% 1	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	29% 25% 28% 14% 4 1	200
	11th to 20th	Day	32% 25% 29% 11% 2	260
	21st to end	Day	24% 29% 29% 14% 3 1	330
February	1st to 10th	Day	26% 25% 32% 14% 3	300
	11th to 20th	Day	32% 24% 34% 9% 1	379
	21st to end	Day	31% 17% 32% 17% 2	414
March	1st to 10th	Day	25% 20% 33% 18% 4	500
	11th to 20th	Day	24% 26% 31% 15% 4 1	500
	21st to end	Day	25% 22% 31% 15% 5% 2	550
April	1st to 10th	Day	28% 21% 28% 16% 6% 1	500
	11th to 20th	Day	25% 20% 26% 18% 7% 3	492
	21st to end	Day	25% 17% 30% 20% 7% 2	500
May	1st to 10th	Day	26% 18% 26% 19% 8% 3	503
	11th to 20th	Day	23% 14% 30% 19% 11% 3	527
	21st to end	Day	20% 8% 26% 22% 18% 5%	573
June	1st to 10th	Day	16% 13% 27% 21% 17% 5%	530
	11th to 20th	Day	19% 11% 26% 25% 16% 4	530
	21st to end	Day	14% 8% 27% 31% 15% 6%	530
July	1st to 10th	Day	12% 4 20% 30% 24% 10%	525
	11th to 20th	Day	7% 6% 22% 32% 24% 9%	524
	21st to end	Day	11% 6% 26% 34% 18% 6%	576
August	1st to 10th	Day	13% 4 23% 29% 21% 10%	526
	11th to 20th	Day	15% 5% 20% 29% 20% 11%	519
	21st to end	Day	13% 4 20% 33% 19% 11%	583
September	1st to 10th	Day	19% 8% 18% 24% 21% 9%	530
	11th to 20th	Day	13% 4 21% 30% 22% 11%	530
	21st to end	Day	24% 14% 20% 22% 14% 5%	530
October	1st to 10th	Day	21% 8% 23% 24% 17% 6%	530
	11th to 20th	Day	19% 17% 27% 21% 14% 3	522
	21st to end	Day	18% 15% 32% 24% 9% 2	539
November	1st to 10th	Day	17% 10% 30% 29% 12% 3	395
	11th to 20th	Day	19% 19% 31% 22% 8% 1	382
	21st to end	Day	26% 20% 35% 12% 8% 1	200
December	1st to 10th	Day	26% 23% 34% 12% 5% 2	200
	11th to 20th	Day	30% 19% 32% 15% 4 1	200
	21st to end	Day	25% 20% 30% 18% 7%	220
January	1st to 10th	Night	19% 18% 40% 19% 3	800
	11th to 20th	Night	25% 16% 33% 23% 4	800
	21st to end	Night	18% 17% 40% 23% 2	880
February	1st to 10th	Night	21% 18% 34% 26% 2	800
	11th to 20th	Night	24% 21% 37% 16% 2	783
	21st to end	Night	23% 19% 36% 19% 3	660
March	1st to 10th	Night	19% 17% 37% 23% 5%	800
	11th to 20th	Night	23% 17% 34% 20% 5% 1	800
	21st to end	Night	20% 14% 38% 22% 5%	880
April	1st to 10th	Night	23% 18% 34% 21% 5%	800
	11th to 20th	Night	20% 12% 34% 26% 7%	791
	21st to end	Night	20% 13% 32% 27% 8%	800
May	1st to 10th	Night	24% 9% 29% 29% 9%	629
	11th to 20th	Night	20% 8% 31% 31% 9% 1	394
	21st to end	Night	17% 4 24% 39% 15% 1	433
June	1st to 10th	Night	16% 5% 25% 37% 19%	400
	11th to 20th	Night	14% 4 27% 42% 14%	400
	21st to end	Night	11% 2 25% 46% 16% 1	400
July	1st to 10th	Night	11% 3 19% 43% 22% 3	390
	11th to 20th	Night	9% 2 16% 44% 27% 2	388
	21st to end	Night	11% 3 18% 41% 27% 1	577
August	1st to 10th	Night	14% 1 11% 44% 27% 3	597
	11th to 20th	Night	15% 4 11% 40% 27% 3	693
	21st to end	Night	14% 4 17% 36% 26% 3	880
September	1st to 10th	Night	15% 5% 22% 33% 23% 3	800
	11th to 20th	Night	14% 3 18% 35% 26% 4	800
	21st to end	Night	23% 5% 19% 33% 20% 2	800
October	1st to 10th	Night	22% 6% 21% 29% 19% 4	800
	11th to 20th	Night	18% 10% 26% 33% 13% 1	793
	21st to end	Night	20% 10% 29% 29% 11%	836
November	1st to 10th	Night	13% 9% 33% 32% 12% 1	780
	11th to 20th	Night	18% 16% 29% 28% 8% 1	800
	21st to end	Night	17% 16% 33% 27% 7%	800
December	1st to 10th	Night	24% 14% 34% 22% 6%	800
	11th to 20th	Night	20% 14% 37% 24% 5%	800
	21st to end	Night	22% 13% 33% 29% 3	880

Page generated at 2025-10-16 01:50:25. Version ac07

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