Phil's Observatory at Sanctuary
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Phil's Observatory at Sanctuary clear sky chart from 2005-11-06 to 2025-05-26.

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	8% 6% 2 7% 5% 5% 5% 5% 7% 10% 41%	114387
	Night	9% 6% 2 6% 5% 4 5% 4 6% 9% 44%	114870
Transparency	Day	36% 9% 16% 19% 14% 6%	118155
	Night	35% 9% 15% 19% 15% 6%	119241
Seeing	Day	9% 6% 19% 26% 24% 17%	16209
	Night	10% 5% 21% 33% 25% 7%	26328

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	8% 5% 2 5% 4 3 3 3 4 7% 54%	4056
	11th to 20th	Day	19% 10% 3 8% 4 4 4 3 4 6% 34%	4060
	21st to end	Day	12% 5% 2 5% 4 3 3 3 6% 8% 48%	4627
February	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 7% 43%	2000
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 5% 6% 52%	1948
	21st to end	Day	9% 6% 3 5% 4 4 4 4 5% 8% 49%	1730
March	1st to 10th	Day	8% 6% 2 7% 4 4 5% 4 7% 9% 43%	4935
	11th to 20th	Day	5% 4 2 5% 4 3 3 3 5% 8% 58%	4980
	21st to end	Day	9% 6% 2 6% 4 4 4 4 6% 9% 49%	5465
April	1st to 10th	Day	6% 4 2 4 4 4 5% 4 6% 9% 52%	2200
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 5% 7% 8% 47%	2201
	21st to end	Day	4 4 1 4 4 4 4 4 8% 9% 54%	2410
May	1st to 10th	Day	2 2 1 4 4 3 4 4 7% 10% 58%	4212
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 7% 9% 38%	4211
	21st to end	Day	4 3 1 5% 4 4 4 5% 7% 10% 52%	4541
June	1st to 10th	Day	9% 7% 3 8% 7% 7% 8% 6% 8% 10% 26%	2470
	11th to 20th	Day	6% 6% 2 8% 8% 9% 8% 7% 8% 11% 28%	2470
	21st to end	Day	8% 7% 3 8% 8% 7% 7% 7% 8% 10% 28%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 8% 8% 7% 10% 14% 23%	4120
	11th to 20th	Day	5% 5% 2 8% 7% 7% 8% 7% 10% 15% 27%	4120
	21st to end	Day	6% 6% 2 8% 7% 7% 8% 7% 10% 16% 24%	4466
August	1st to 10th	Day	6% 7% 3 8% 6% 7% 7% 6% 8% 13% 29%	4104
	11th to 20th	Day	5% 5% 2 7% 6% 7% 7% 5% 8% 12% 36%	3856
	21st to end	Day	8% 6% 3 9% 7% 7% 9% 6% 8% 10% 26%	3828
September	1st to 10th	Day	7% 7% 3 11% 9% 7% 8% 6% 10% 9% 22%	2090
	11th to 20th	Day	9% 7% 3 10% 8% 7% 8% 6% 8% 9% 25%	2090
	21st to end	Day	8% 7% 3 9% 7% 7% 6% 5% 8% 9% 32%	2090
October	1st to 10th	Day	11% 6% 3 9% 7% 5% 6% 4 6% 9% 35%	2081
	11th to 20th	Day	6% 5% 2 6% 5% 4 4 4 6% 8% 50%	1900
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 5% 8% 49%	2090
November	1st to 10th	Day	8% 5% 3 6% 5% 4 4 4 6% 9% 48%	1957
	11th to 20th	Day	9% 7% 2 6% 6% 5% 5% 3 5% 7% 46%	2000
	21st to end	Day	10% 6% 2 5% 4 5% 4 4 6% 8% 46%	1840
December	1st to 10th	Day	5% 4 1 5% 3 4 4 4 6% 8% 56%	2930
	11th to 20th	Day	10% 7% 2 7% 4 5% 5% 5% 6% 9% 42%	3373
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 3 5% 8% 43%	4466
January	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 7% 55%	5889
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 4 5% 8% 31%	5900
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 8% 42%	6324
February	1st to 10th	Night	12% 7% 2 6% 4 4 3 3 6% 7% 46%	2400
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 4 5% 7% 48%	2338
	21st to end	Night	8% 7% 2 6% 5% 3 4 4 6% 6% 49%	1899
March	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 5% 45%	4918
	11th to 20th	Night	6% 4 2 5% 3 3 3 4 6% 9% 55%	4980
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 50%	5467
April	1st to 10th	Night	6% 5% 2 4 4 3 4 4 6% 9% 52%	2200
	11th to 20th	Night	6% 6% 2 5% 4 3 3 3 6% 8% 53%	2145
	21st to end	Night	6% 3 2 4 4 5% 5% 3 5% 8% 57%	2000
May	1st to 10th	Night	3 3 1 6% 4 4 4 4 5% 9% 55%	2908
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 46%	2902
	21st to end	Night	3 3 1 3 4 3 4 4 5% 8% 63%	3130
June	1st to 10th	Night	10% 5% 2 6% 5% 6% 6% 6% 7% 10% 36%	1710
	11th to 20th	Night	6% 5% 2 6% 7% 6% 6% 7% 8% 13% 34%	1710
	21st to end	Night	7% 8% 3 9% 5% 6% 5% 6% 10% 10% 32%	1710
July	1st to 10th	Night	8% 7% 3 10% 7% 7% 7% 6% 8% 12% 26%	2840
	11th to 20th	Night	4 6% 2 8% 8% 8% 9% 7% 10% 12% 27%	2840
	21st to end	Night	8% 8% 3 11% 8% 8% 8% 6% 9% 11% 22%	3087
August	1st to 10th	Night	6% 5% 2 9% 6% 7% 6% 5% 8% 11% 35%	2825
	11th to 20th	Night	3 4 2 5% 5% 5% 6% 6% 9% 15% 40%	2908
	21st to end	Night	7% 6% 3 8% 6% 6% 6% 6% 8% 12% 32%	3828
September	1st to 10th	Night	8% 8% 2 9% 6% 6% 6% 5% 8% 11% 32%	2090
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 6% 8% 11% 29%	2090
	21st to end	Night	8% 7% 3 7% 6% 5% 7% 5% 6% 10% 37%	2090
October	1st to 10th	Night	10% 7% 3 6% 5% 5% 5% 4 7% 9% 39%	2096
	11th to 20th	Night	5% 4 2 5% 3 5% 4 4 6% 8% 54%	2280
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 7% 51%	2508
November	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 7% 7% 50%	2341
	11th to 20th	Night	8% 7% 2 8% 5% 5% 4 3 5% 7% 46%	2400
	21st to end	Night	8% 6% 3 6% 4 5% 5% 3 6% 8% 47%	2555
December	1st to 10th	Night	6% 4 2 5% 4 3 4 3 5% 7% 58%	4250
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 4 6% 50%	4822
	21st to end	Night	14% 7% 2 7% 4 5% 5% 3 5% 7% 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	30% 5% 9% 14% 28% 15%	4057
	11th to 20th	Day	52% 4 5% 14% 17% 8%	4060
	21st to end	Day	33% 6% 9% 26% 18% 8%	4627
February	1st to 10th	Day	38% 7% 10% 14% 20% 11%	2000
	11th to 20th	Day	32% 5% 9% 18% 23% 12%	1958
	21st to end	Day	34% 5% 11% 18% 22% 10%	1730
March	1st to 10th	Day	35% 7% 13% 17% 20% 8%	4935
	11th to 20th	Day	24% 5% 9% 22% 22% 18%	4980
	21st to end	Day	31% 5% 11% 26% 17% 9%	5465
April	1st to 10th	Day	28% 7% 13% 22% 21% 9%	2200
	11th to 20th	Day	33% 7% 12% 18% 21% 9%	2201
	21st to end	Day	23% 8% 15% 24% 23% 8%	2410
May	1st to 10th	Day	17% 8% 20% 32% 17% 5%	4250
	11th to 20th	Day	36% 12% 16% 24% 10% 2	4215
	21st to end	Day	23% 9% 15% 34% 17% 1	4554
June	1st to 10th	Day	47% 14% 17% 16% 5%	2470
	11th to 20th	Day	45% 14% 20% 19% 3	2470
	21st to end	Day	46% 15% 22% 14% 4	2470
July	1st to 10th	Day	44% 16% 29% 10% 1	4055
	11th to 20th	Day	42% 17% 35% 6%	4051
	21st to end	Day	42% 20% 28% 9% 1	4324
August	1st to 10th	Day	42% 14% 29% 12% 3	4091
	11th to 20th	Day	35% 14% 25% 22% 4	3895
	21st to end	Day	46% 15% 22% 15% 2	3828
September	1st to 10th	Day	50% 18% 17% 10% 4	2090
	11th to 20th	Day	50% 13% 20% 11% 5%	2090
	21st to end	Day	44% 11% 14% 16% 11% 3	2090
October	1st to 10th	Day	55% 9% 12% 11% 7% 5%	3458
	11th to 20th	Day	26% 4 8% 21% 32% 8%	3140
	21st to end	Day	27% 4 8% 21% 32% 7%	3428
November	1st to 10th	Day	33% 7% 12% 19% 19% 11%	1957
	11th to 20th	Day	37% 6% 10% 17% 21% 9%	2000
	21st to end	Day	35% 6% 10% 20% 21% 8%	1840
December	1st to 10th	Day	25% 5% 16% 25% 20% 10%	2930
	11th to 20th	Day	37% 8% 11% 18% 16% 10%	3370
	21st to end	Day	40% 6% 11% 21% 18% 4	4466
January	1st to 10th	Night	28% 5% 7% 16% 30% 14%	5889
	11th to 20th	Night	52% 5% 8% 16% 16% 4	5900
	21st to end	Night	39% 6% 10% 21% 15% 9%	6324
February	1st to 10th	Night	37% 6% 10% 17% 17% 13%	2400
	11th to 20th	Night	34% 7% 11% 18% 20% 11%	2339
	21st to end	Night	33% 5% 13% 18% 22% 9%	1899
March	1st to 10th	Night	40% 6% 10% 16% 19% 9%	4918
	11th to 20th	Night	25% 6% 15% 20% 18% 17%	4980
	21st to end	Night	32% 5% 11% 24% 20% 8%	5467
April	1st to 10th	Night	27% 7% 17% 24% 16% 9%	2200
	11th to 20th	Night	28% 6% 12% 26% 20% 8%	2145
	21st to end	Night	25% 6% 15% 26% 21% 7%	2000
May	1st to 10th	Night	19% 13% 21% 27% 15% 4	2935
	11th to 20th	Night	30% 11% 22% 26% 10% 2	2903
	21st to end	Night	18% 9% 15% 37% 20% 1	3138
June	1st to 10th	Night	39% 11% 23% 21% 6%	1710
	11th to 20th	Night	37% 15% 24% 21% 2	1710
	21st to end	Night	42% 14% 23% 16% 4 1	1710
July	1st to 10th	Night	47% 15% 27% 9% 1	2795
	11th to 20th	Night	44% 18% 30% 7% 1	2786
	21st to end	Night	50% 16% 25% 9% 1	2988
August	1st to 10th	Night	39% 14% 33% 12% 2	2814
	11th to 20th	Night	29% 16% 29% 21% 5%	2935
	21st to end	Night	41% 15% 26% 16% 2	3828
September	1st to 10th	Night	43% 17% 22% 14% 5%	2090
	11th to 20th	Night	44% 16% 21% 15% 3 1	2090
	21st to end	Night	39% 12% 18% 19% 10% 2	2090
October	1st to 10th	Night	54% 10% 12% 13% 7% 5%	3499
	11th to 20th	Night	25% 5% 9% 21% 34% 5%	3779
	21st to end	Night	28% 6% 10% 19% 31% 7%	4120
November	1st to 10th	Night	31% 8% 12% 21% 18% 10%	2341
	11th to 20th	Night	36% 6% 10% 19% 19% 10%	2400
	21st to end	Night	35% 6% 10% 21% 20% 8%	2555
December	1st to 10th	Night	26% 5% 14% 26% 15% 14%	4250
	11th to 20th	Night	35% 7% 10% 22% 16% 10%	4824
	21st to end	Night	42% 7% 12% 20% 14% 5%	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	10% 13% 44% 24% 7% 2	300
	11th to 20th	Day	9% 15% 38% 25% 10% 3	300
	21st to end	Day	15% 19% 39% 22% 5%	330
February	1st to 10th	Day	12% 15% 31% 25% 15% 3	300
	11th to 20th	Day	12% 11% 38% 26% 10% 3	300
	21st to end	Day	11% 10% 31% 30% 17% 1	329
March	1st to 10th	Day	8% 13% 34% 32% 13% 1	500
	11th to 20th	Day	10% 12% 28% 31% 17% 3	500
	21st to end	Day	9% 10% 30% 28% 20% 3	550
April	1st to 10th	Day	9% 11% 28% 32% 18% 2	500
	11th to 20th	Day	8% 9% 28% 31% 19% 5%	492
	21st to end	Day	6% 5% 20% 33% 26% 11%	500
May	1st to 10th	Day	4 5% 22% 32% 26% 12%	503
	11th to 20th	Day	9% 2 17% 30% 28% 15%	527
	21st to end	Day	7% 3 11% 21% 35% 23%	549
June	1st to 10th	Day	12% 2 8% 25% 31% 22%	490
	11th to 20th	Day	7% 2 10% 23% 28% 30%	490
	21st to end	Day	6% 2 6% 17% 31% 39%	490
July	1st to 10th	Day	7% 2 4 12% 28% 48%	485
	11th to 20th	Day	6% 3 11% 31% 48%	484
	21st to end	Day	10% 2 5% 14% 30% 40%	532
August	1st to 10th	Day	8% 3 7% 16% 30% 37%	486
	11th to 20th	Day	5% 1 5% 14% 29% 46%	479
	21st to end	Day	11% 1 7% 17% 30% 34%	539
September	1st to 10th	Day	9% 2 5% 17% 33% 34%	490
	11th to 20th	Day	9% 3 9% 26% 31% 22%	490
	21st to end	Day	9% 3 9% 24% 31% 24%	490
October	1st to 10th	Day	15% 5% 16% 32% 24% 7%	490
	11th to 20th	Day	5% 7% 18% 32% 29% 9%	490
	21st to end	Day	8% 5% 22% 34% 26% 5%	539
November	1st to 10th	Day	9% 6% 24% 30% 26% 4	495
	11th to 20th	Day	14% 5% 25% 35% 17% 4	500
	21st to end	Day	9% 11% 32% 30% 16% 2	340
December	1st to 10th	Day	6% 9% 38% 28% 16% 3	300
	11th to 20th	Day	15% 9% 33% 30% 12% 1	300
	21st to end	Day	10% 9% 29% 37% 12% 2	330
January	1st to 10th	Night	12% 9% 37% 31% 11% 1	800
	11th to 20th	Night	13% 9% 27% 36% 14% 1	800
	21st to end	Night	18% 9% 36% 30% 7%	880
February	1st to 10th	Night	16% 7% 30% 35% 12%	800
	11th to 20th	Night	11% 8% 32% 38% 10% 1	783
	21st to end	Night	12% 6% 34% 42% 6%	660
March	1st to 10th	Night	8% 9% 34% 37% 11%	800
	11th to 20th	Night	11% 9% 30% 38% 13% 1	800
	21st to end	Night	9% 7% 30% 42% 12% 1	880
April	1st to 10th	Night	9% 6% 31% 41% 13%	800
	11th to 20th	Night	8% 7% 31% 38% 15% 2	791
	21st to end	Night	7% 4 26% 44% 20% 1	800
May	1st to 10th	Night	7% 2 17% 50% 24% 2	605
	11th to 20th	Night	6% 2 14% 43% 31% 3	591
	21st to end	Night	7% 1 9% 37% 40% 6%	634
June	1st to 10th	Night	11% 1 9% 36% 35% 8%	570
	11th to 20th	Night	7% 2 7% 26% 37% 21%	570
	21st to end	Night	9% 1 6% 21% 43% 20%	570
July	1st to 10th	Night	8% 1 4 15% 45% 26%	555
	11th to 20th	Night	5% 1 3 13% 48% 30%	552
	21st to end	Night	7% 1 3 14% 50% 24%	624
August	1st to 10th	Night	6% 2 3 17% 47% 25%	567
	11th to 20th	Night	5% 1 3 15% 46% 30%	564
	21st to end	Night	11% 2 8% 22% 37% 20%	836
September	1st to 10th	Night	10% 2 8% 23% 38% 19%	760
	11th to 20th	Night	12% 3 10% 28% 34% 12%	760
	21st to end	Night	12% 2 11% 28% 38% 9%	760
October	1st to 10th	Night	14% 3 17% 36% 26% 4	760
	11th to 20th	Night	5% 3 15% 40% 33% 3	760
	21st to end	Night	11% 3 18% 40% 26% 2	836
November	1st to 10th	Night	10% 5% 23% 37% 23% 2	780
	11th to 20th	Night	10% 7% 23% 38% 20% 2	800
	21st to end	Night	11% 4 30% 38% 16% 1	800
December	1st to 10th	Night	10% 4 28% 37% 21% 1	800
	11th to 20th	Night	11% 7% 28% 35% 18% 1	800
	21st to end	Night	12% 7% 27% 38% 15% 1	880

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