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 2024-04-10.

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% 3 5% 5% 3 5% 5% 6% 8% 39%	83177
	Night	16% 6% 3 5% 5% 3 4 4 5% 7% 41%	85596
Transparency	Day	42% 6% 12% 23% 14% 2	82890
	Night	42% 6% 12% 23% 15% 2	85418
Seeing	Day	20% 14% 27% 22% 12% 4	14086
	Night	19% 11% 29% 29% 11% 1	23602

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	14% 7% 3 6% 5% 3 4 4 5% 6% 42%	2517
	11th to 20th	Day	14% 11% 4 8% 7% 3 5% 4 5% 7% 30%	2712
	21st to end	Day	20% 10% 4 7% 6% 3 5% 4 4 5% 31%	3124
February	1st to 10th	Day	15% 8% 4 6% 5% 3 6% 5% 5% 7% 36%	1710
	11th to 20th	Day	15% 7% 4 5% 6% 3 5% 5% 5% 7% 39%	1738
	21st to end	Day	19% 9% 4 6% 5% 4 4 4 4 5% 36%	1570
March	1st to 10th	Day	25% 5% 2 5% 5% 3 5% 4 5% 6% 36%	3262
	11th to 20th	Day	10% 5% 2 5% 6% 4 5% 5% 6% 8% 43%	3480
	21st to end	Day	21% 7% 2 5% 4 2 3 3 4 5% 43%	3960
April	1st to 10th	Day	16% 7% 3 5% 6% 3 5% 5% 5% 7% 39%	2270
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 4 5% 7% 43%	2134
	21st to end	Day	18% 5% 2 4 5% 3 5% 5% 6% 7% 39%	2160
May	1st to 10th	Day	18% 6% 2 6% 5% 3 5% 5% 6% 8% 36%	2323
	11th to 20th	Day	20% 8% 3 4 5% 2 5% 5% 5% 7% 36%	2478
	21st to end	Day	15% 6% 2 5% 4 4 5% 5% 7% 8% 39%	2725
June	1st to 10th	Day	13% 5% 2 5% 5% 3 5% 6% 7% 10% 39%	2520
	11th to 20th	Day	13% 6% 3 5% 5% 3 6% 6% 6% 9% 38%	2520
	21st to end	Day	11% 4 2 4 6% 3 5% 5% 6% 11% 43%	2520
July	1st to 10th	Day	10% 4 2 6% 6% 4 6% 6% 8% 11% 38%	2520
	11th to 20th	Day	7% 5% 2 6% 5% 4 5% 6% 7% 10% 43%	2520
	21st to end	Day	10% 6% 3 6% 7% 4 6% 6% 7% 8% 38%	2616
August	1st to 10th	Day	11% 5% 3 5% 6% 5% 6% 7% 7% 9% 37%	2327
	11th to 20th	Day	12% 6% 2 6% 5% 3 5% 5% 7% 9% 40%	2194
	21st to end	Day	13% 5% 2 4 4 3 4 6% 6% 9% 42%	2376
September	1st to 10th	Day	16% 7% 2 4 4 2 4 4 5% 6% 44%	2160
	11th to 20th	Day	13% 4 2 4 4 3 4 3 5% 8% 49%	2160
	21st to end	Day	20% 6% 2 5% 4 3 4 4 5% 6% 40%	1919
October	1st to 10th	Day	18% 7% 2 3 4 3 4 4 6% 7% 43%	1800
	11th to 20th	Day	18% 5% 2 5% 5% 3 4 4 4 7% 44%	1800
	21st to end	Day	18% 6% 3 4 4 2 4 4 5% 8% 42%	1980
November	1st to 10th	Day	14% 6% 2 4 3 3 3 3 4 7% 50%	1671
	11th to 20th	Day	16% 6% 2 4 4 2 3 4 5% 7% 45%	1693
	21st to end	Day	17% 4 3 5% 4 3 5% 5% 5% 7% 42%	1520
December	1st to 10th	Day	19% 8% 3 6% 5% 3 5% 5% 5% 6% 35%	1520
	11th to 20th	Day	20% 6% 2 4 5% 3 5% 5% 4 8% 37%	1906
	21st to end	Day	18% 7% 3 7% 6% 4 5% 6% 6% 7% 31%	2772
January	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 6% 8% 39%	4429
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 4 5% 7% 38%	4245
	21st to end	Night	23% 8% 3 6% 5% 3 3 3 4 6% 35%	4532
February	1st to 10th	Night	17% 8% 3 5% 5% 3 5% 5% 5% 8% 38%	2470
	11th to 20th	Night	17% 8% 3 5% 5% 3 5% 4 5% 6% 40%	2327
	21st to end	Night	18% 7% 3 5% 6% 3 4 4 5% 6% 39%	1884
March	1st to 10th	Night	22% 9% 2 6% 5% 3 3 3 4 6% 37%	3577
	11th to 20th	Night	11% 6% 2 5% 5% 3 5% 4 4 6% 48%	3479
	21st to end	Night	15% 8% 3 5% 4 3 4 4 4 5% 46%	3672
April	1st to 10th	Night	18% 7% 3 5% 5% 3 4 5% 5% 8% 37%	1900
	11th to 20th	Night	14% 6% 3 6% 5% 2 4 4 6% 8% 42%	1775
	21st to end	Night	19% 5% 3 4 4 2 4 4 4 7% 43%	1800
May	1st to 10th	Night	19% 6% 3 6% 5% 3 4 4 5% 7% 37%	1648
	11th to 20th	Night	20% 6% 2 6% 5% 3 5% 4 5% 8% 37%	1415
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 6% 8% 41%	1550
June	1st to 10th	Night	15% 4 2 5% 5% 3 4 5% 6% 8% 43%	1440
	11th to 20th	Night	13% 5% 2 4 5% 4 5% 5% 7% 10% 40%	1440
	21st to end	Night	11% 3 3 4 5% 3 5% 5% 7% 11% 43%	1440
July	1st to 10th	Night	9% 6% 3 6% 6% 4 6% 6% 8% 8% 37%	1440
	11th to 20th	Night	6% 3 2 5% 5% 3 6% 6% 7% 11% 45%	1440
	21st to end	Night	10% 5% 3 5% 6% 3 6% 7% 6% 10% 41%	1699
August	1st to 10th	Night	10% 5% 2 5% 6% 3 5% 6% 7% 8% 42%	1611
	11th to 20th	Night	11% 6% 2 6% 7% 4 5% 6% 5% 7% 40%	1687
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 6% 9% 42%	1980
September	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 46%	1800
	11th to 20th	Night	13% 4 2 3 3 3 4 4 4 8% 51%	1800
	21st to end	Night	18% 5% 2 5% 4 3 5% 4 5% 6% 42%	2037
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 3 4 6% 44%	2160
	11th to 20th	Night	17% 5% 3 5% 4 2 3 3 5% 6% 46%	2160
	21st to end	Night	18% 5% 2 4 4 2 3 4 5% 8% 45%	2376
November	1st to 10th	Night	11% 5% 2 4 4 3 4 4 4 6% 53%	2407
	11th to 20th	Night	15% 7% 3 4 4 2 4 3 5% 7% 45%	2484
	21st to end	Night	15% 5% 2 4 5% 3 5% 5% 5% 8% 43%	2660
December	1st to 10th	Night	19% 7% 3 5% 4 3 5% 5% 5% 7% 36%	2660
	11th to 20th	Night	18% 6% 2 5% 5% 3 4 5% 5% 6% 40%	3288
	21st to end	Night	18% 7% 3 5% 6% 3 6% 5% 6% 8% 32%	4884

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% 5% 14% 26% 11% 3	2518
	11th to 20th	Day	52% 5% 12% 17% 12% 2	2712
	21st to end	Day	53% 6% 11% 21% 8% 1	3124
February	1st to 10th	Day	46% 6% 13% 23% 11% 1	1710
	11th to 20th	Day	44% 5% 11% 24% 14% 2	1747
	21st to end	Day	50% 4 10% 21% 11% 4	1570
March	1st to 10th	Day	47% 5% 11% 26% 9% 2	3265
	11th to 20th	Day	37% 5% 12% 27% 15% 3	3480
	21st to end	Day	45% 4 9% 20% 19% 2	3960
April	1st to 10th	Day	43% 4 12% 20% 16% 5%	2270
	11th to 20th	Day	40% 5% 12% 22% 16% 5%	2134
	21st to end	Day	43% 6% 10% 20% 17% 4	2160
May	1st to 10th	Day	45% 7% 9% 20% 15% 3	2323
	11th to 20th	Day	47% 7% 10% 21% 12% 3	2482
	21st to end	Day	41% 8% 14% 22% 12% 2	2725
June	1st to 10th	Day	39% 9% 12% 23% 15% 1	2520
	11th to 20th	Day	41% 8% 13% 24% 12% 3	2520
	21st to end	Day	35% 8% 14% 26% 15% 1	2520
July	1st to 10th	Day	39% 8% 12% 23% 15% 2	2450
	11th to 20th	Day	33% 8% 14% 26% 17% 2	2445
	21st to end	Day	41% 9% 11% 21% 16% 3	2470
August	1st to 10th	Day	40% 9% 13% 23% 15% 1	2314
	11th to 20th	Day	39% 7% 14% 21% 17% 2	2194
	21st to end	Day	38% 7% 12% 23% 16% 3	2376
September	1st to 10th	Day	41% 5% 11% 21% 18% 4	2160
	11th to 20th	Day	33% 5% 11% 25% 22% 4	2160
	21st to end	Day	44% 6% 8% 18% 21% 3	1919
October	1st to 10th	Day	40% 5% 12% 24% 17% 2	1800
	11th to 20th	Day	42% 4 11% 24% 18% 2	1800
	21st to end	Day	41% 5% 11% 25% 17% 2	1980
November	1st to 10th	Day	35% 4 14% 24% 19% 4	1671
	11th to 20th	Day	39% 4 11% 24% 19% 2	1693
	21st to end	Day	41% 5% 11% 31% 10% 2	1520
December	1st to 10th	Day	48% 6% 15% 19% 11% 1	1520
	11th to 20th	Day	44% 5% 15% 28% 8% 1	1906
	21st to end	Day	49% 7% 15% 19% 9% 1	2772
January	1st to 10th	Night	43% 5% 12% 26% 11% 3	4429
	11th to 20th	Night	45% 4 15% 23% 12% 2	4245
	21st to end	Night	50% 4 10% 26% 9% 1	4532
February	1st to 10th	Night	45% 5% 13% 25% 11% 1	2470
	11th to 20th	Night	46% 4 9% 21% 18% 2	2329
	21st to end	Night	45% 6% 10% 20% 15% 4	1884
March	1st to 10th	Night	48% 5% 9% 23% 13% 1	3587
	11th to 20th	Night	34% 6% 14% 29% 16% 2	3480
	21st to end	Night	41% 4 8% 20% 23% 5%	3672
April	1st to 10th	Night	45% 4 10% 21% 16% 4	1900
	11th to 20th	Night	39% 6% 11% 21% 20% 3	1775
	21st to end	Night	42% 5% 11% 21% 16% 6%	1800
May	1st to 10th	Night	45% 8% 10% 19% 15% 4	1648
	11th to 20th	Night	47% 7% 11% 15% 18% 2	1416
	21st to end	Night	41% 8% 14% 20% 15% 3	1551
June	1st to 10th	Night	39% 7% 11% 22% 19% 2	1440
	11th to 20th	Night	38% 9% 14% 22% 15% 2	1440
	21st to end	Night	34% 7% 14% 27% 16% 2	1440
July	1st to 10th	Night	42% 10% 13% 19% 13% 4	1400
	11th to 20th	Night	32% 9% 15% 25% 16% 2	1392
	21st to end	Night	35% 10% 17% 20% 17% 2	1605
August	1st to 10th	Night	36% 9% 16% 24% 15% 1	1600
	11th to 20th	Night	41% 8% 14% 18% 17% 2	1687
	21st to end	Night	36% 8% 14% 21% 18% 3	1980
September	1st to 10th	Night	39% 6% 11% 22% 19% 4	1800
	11th to 20th	Night	32% 6% 9% 25% 25% 4	1800
	21st to end	Night	42% 6% 8% 18% 22% 3	2037
October	1st to 10th	Night	44% 4 11% 24% 16% 2	2160
	11th to 20th	Night	37% 6% 9% 29% 17% 2	2160
	21st to end	Night	38% 5% 10% 27% 19% 2	2376
November	1st to 10th	Night	32% 5% 11% 26% 20% 4	2407
	11th to 20th	Night	39% 4 10% 25% 18% 4	2484
	21st to end	Night	38% 5% 12% 28% 15% 1	2660
December	1st to 10th	Night	47% 6% 12% 22% 13% 1	2660
	11th to 20th	Night	43% 5% 14% 22% 14% 3	3288
	21st to end	Night	46% 8% 14% 22% 9% 1	4884

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	28% 23% 29% 16% 4 1	180
	11th to 20th	Day	31% 26% 29% 12% 3	234
	21st to end	Day	24% 29% 29% 14% 3 1	297
February	1st to 10th	Day	25% 24% 33% 14% 4	270
	11th to 20th	Day	29% 23% 37% 10% 1	345
	21st to end	Day	33% 17% 30% 18% 3	382
March	1st to 10th	Day	24% 20% 32% 19% 4	460
	11th to 20th	Day	24% 26% 30% 15% 3 1	460
	21st to end	Day	25% 22% 31% 16% 5% 2	506
April	1st to 10th	Day	26% 21% 28% 17% 7% 1	457
	11th to 20th	Day	23% 19% 25% 20% 8% 3	412
	21st to end	Day	26% 17% 31% 18% 6% 2	420
May	1st to 10th	Day	25% 19% 26% 19% 8% 4	423
	11th to 20th	Day	22% 14% 32% 19% 11% 3	447
	21st to end	Day	19% 6% 26% 24% 19% 6%	485
June	1st to 10th	Day	17% 13% 26% 22% 17% 5%	450
	11th to 20th	Day	18% 12% 26% 25% 16% 4	450
	21st to end	Day	14% 9% 26% 31% 15% 6%	450
July	1st to 10th	Day	13% 4 20% 29% 25% 10%	445
	11th to 20th	Day	6% 5% 21% 33% 27% 9%	444
	21st to end	Day	11% 6% 26% 32% 19% 6%	488
August	1st to 10th	Day	13% 4 22% 28% 23% 10%	446
	11th to 20th	Day	14% 5% 19% 30% 21% 11%	439
	21st to end	Day	13% 4 20% 33% 18% 12%	495
September	1st to 10th	Day	21% 7% 18% 23% 21% 11%	450
	11th to 20th	Day	14% 4 20% 28% 21% 12%	450
	21st to end	Day	22% 13% 21% 23% 15% 6%	450
October	1st to 10th	Day	22% 8% 24% 23% 18% 5%	450
	11th to 20th	Day	20% 15% 26% 22% 15% 2	450
	21st to end	Day	19% 15% 33% 23% 8% 2	495
November	1st to 10th	Day	18% 10% 28% 29% 13% 3	365
	11th to 20th	Day	20% 18% 30% 22% 9% 1	353
	21st to end	Day	22% 19% 38% 12% 8% 1	180
December	1st to 10th	Day	26% 22% 34% 12% 5% 2	180
	11th to 20th	Day	29% 20% 31% 16% 4 1	180
	21st to end	Day	25% 21% 28% 18% 8% 1	198
January	1st to 10th	Night	18% 17% 41% 20% 3	760
	11th to 20th	Night	25% 15% 33% 23% 4	760
	21st to end	Night	17% 17% 40% 23% 2	836
February	1st to 10th	Night	21% 18% 33% 26% 2	760
	11th to 20th	Night	23% 21% 38% 16% 2	743
	21st to end	Night	23% 19% 36% 19% 3	628
March	1st to 10th	Night	19% 16% 37% 23% 5%	760
	11th to 20th	Night	23% 17% 34% 20% 5% 1	760
	21st to end	Night	21% 14% 37% 22% 6%	836
April	1st to 10th	Night	22% 18% 34% 22% 5%	760
	11th to 20th	Night	19% 11% 34% 27% 8%	711
	21st to end	Night	22% 12% 32% 27% 8%	720
May	1st to 10th	Night	23% 8% 29% 30% 10% 1	568
	11th to 20th	Night	19% 8% 31% 32% 10% 1	354
	21st to end	Night	16% 4 23% 39% 16% 2	389
June	1st to 10th	Night	16% 4 24% 36% 19%	360
	11th to 20th	Night	13% 4 27% 41% 15%	360
	21st to end	Night	10% 3 24% 48% 15% 1	360
July	1st to 10th	Night	11% 2 18% 43% 22% 3	350
	11th to 20th	Night	8% 2 15% 45% 28% 2	348
	21st to end	Night	10% 3 18% 41% 26% 2	519
August	1st to 10th	Night	13% 1 11% 45% 27% 3	537
	11th to 20th	Night	15% 4 10% 39% 28% 3	623
	21st to end	Night	14% 4 17% 36% 27% 3	792
September	1st to 10th	Night	15% 4 21% 32% 23% 4	720
	11th to 20th	Night	15% 3 19% 34% 25% 5%	720
	21st to end	Night	20% 4 20% 33% 21% 2	720
October	1st to 10th	Night	23% 5% 21% 30% 18% 4	720
	11th to 20th	Night	19% 9% 26% 33% 13%	720
	21st to end	Night	21% 11% 30% 28% 10%	792
November	1st to 10th	Night	13% 9% 32% 32% 13% 1	740
	11th to 20th	Night	18% 16% 29% 28% 8% 1	760
	21st to end	Night	17% 15% 33% 28% 8%	760
December	1st to 10th	Night	24% 14% 33% 22% 6%	760
	11th to 20th	Night	19% 14% 38% 25% 5%	760
	21st to end	Night	21% 13% 34% 30% 2	836

Page generated at 2024-04-07 09:28:11. Version ac07

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