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 2024-11-02.

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% 6% 5% 6% 5% 7% 10% 39%	100884
	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	98004
Transparency	Day	37% 11% 17% 18% 12% 5%	100697
	Night	36% 10% 17% 19% 13% 5%	97861
Seeing	Day	9% 6% 19% 25% 24% 17%	15483
	Night	10% 5% 20% 33% 25% 7%	25532

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	12% 7% 3 7% 5% 4 5% 4 5% 6% 43%	2836
	11th to 20th	Day	19% 10% 3 7% 4 3 4 3 5% 6% 35%	2840
	21st to end	Day	10% 6% 2 6% 5% 4 4 4 6% 7% 47%	3229
February	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 7% 7% 42%	1900
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 6% 52%	1848
	21st to end	Day	9% 6% 3 5% 4 4 4 4 5% 8% 49%	1646
March	1st to 10th	Day	7% 8% 3 8% 5% 5% 6% 4 7% 10% 38%	3435
	11th to 20th	Day	5% 5% 2 6% 4 4 4 4 7% 10% 49%	3480
	21st to end	Day	10% 6% 2 6% 4 4 4 3 5% 7% 49%	3815
April	1st to 10th	Day	7% 4 2 4 4 4 5% 4 6% 9% 51%	2090
	11th to 20th	Day	8% 4 2 7% 5% 5% 5% 5% 7% 8% 44%	2090
	21st to end	Day	4 4 1 4 4 4 4 4 8% 9% 54%	2289
May	1st to 10th	Day	2 2 1 4 4 3 4 4 7% 10% 59%	4082
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 7% 9% 37%	4081
	21st to end	Day	4 3 1 5% 4 4 4 5% 7% 10% 52%	4476
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% 2 5% 5% 4 4 4 6% 9% 48%	1867
	11th to 20th	Day	9% 7% 2 6% 6% 5% 5% 3 5% 6% 45%	1900
	21st to end	Day	10% 7% 3 5% 4 5% 4 4 6% 9% 44%	1748
December	1st to 10th	Day	8% 5% 2 6% 3 5% 5% 4 6% 9% 47%	1710
	11th to 20th	Day	12% 7% 2 7% 4 4 4 4 6% 8% 43%	2153
	21st to end	Day	12% 8% 3 8% 5% 4 5% 3 5% 7% 40%	3124
January	1st to 10th	Night	13% 7% 3 6% 5% 4 4 4 6% 7% 42%	4109
	11th to 20th	Night	16% 10% 3 7% 4 3 4 4 6% 8% 34%	4120
	21st to end	Night	12% 7% 2 7% 5% 4 5% 4 6% 8% 41%	4422
February	1st to 10th	Night	12% 7% 2 5% 4 4 3 3 6% 7% 46%	2280
	11th to 20th	Night	9% 5% 2 6% 4 5% 4 4 5% 7% 49%	2218
	21st to end	Night	8% 7% 2 6% 5% 3 4 4 6% 6% 49%	1807
March	1st to 10th	Night	15% 10% 4 8% 4 4 4 3 5% 5% 39%	3418
	11th to 20th	Night	6% 4 2 5% 4 4 4 5% 8% 10% 47%	3480
	21st to end	Night	10% 7% 2 6% 4 3 3 3 5% 8% 49%	3817
April	1st to 10th	Night	6% 5% 2 4 4 4 4 4 6% 9% 52%	2090
	11th to 20th	Night	6% 6% 2 5% 4 3 3 3 6% 9% 51%	2037
	21st to end	Night	6% 3 2 4 4 4 5% 3 5% 8% 57%	1900
May	1st to 10th	Night	3 3 1 6% 4 4 4 4 5% 9% 56%	2818
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 46%	2812
	21st to end	Night	3 3 1 3 4 3 4 4 5% 8% 62%	3079
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% 3 3 4 3 7% 7% 51%	2234
	11th to 20th	Night	8% 7% 2 8% 5% 4 4 3 5% 7% 46%	2280
	21st to end	Night	8% 6% 3 7% 4 5% 5% 3 6% 8% 45%	2427
December	1st to 10th	Night	8% 5% 2 6% 5% 3 4 4 6% 8% 48%	2470
	11th to 20th	Night	11% 8% 2 5% 4 4 3 3 4 6% 49%	3042
	21st to end	Night	15% 7% 2 7% 5% 5% 5% 4 5% 7% 38%	4532

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	40% 6% 8% 16% 20% 10%	2837
	11th to 20th	Day	49% 5% 6% 14% 15% 10%	2840
	21st to end	Day	35% 8% 9% 19% 20% 9%	3229
February	1st to 10th	Day	39% 7% 10% 14% 20% 11%	1900
	11th to 20th	Day	32% 5% 10% 18% 24% 12%	1858
	21st to end	Day	34% 5% 11% 17% 23% 10%	1646
March	1st to 10th	Day	38% 10% 15% 19% 14% 5%	3435
	11th to 20th	Day	29% 7% 13% 25% 21% 6%	3480
	21st to end	Day	33% 6% 12% 27% 15% 8%	3815
April	1st to 10th	Day	29% 8% 13% 22% 20% 9%	2090
	11th to 20th	Day	35% 8% 13% 19% 20% 6%	2090
	21st to end	Day	23% 8% 15% 23% 23% 8%	2289
May	1st to 10th	Day	17% 8% 19% 32% 18% 6%	4120
	11th to 20th	Day	37% 12% 16% 23% 10% 2	4085
	21st to end	Day	23% 10% 15% 34% 17% 1	4489
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	45% 10% 10% 15% 11% 9%	2081
	11th to 20th	Day	30% 6% 11% 21% 26% 7%	1900
	21st to end	Day	32% 6% 9% 16% 25% 11%	2090
November	1st to 10th	Day	32% 7% 12% 19% 19% 11%	1867
	11th to 20th	Day	38% 6% 10% 16% 21% 9%	1900
	21st to end	Day	37% 7% 10% 20% 21% 6%	1748
December	1st to 10th	Day	32% 7% 16% 23% 17% 6%	1710
	11th to 20th	Day	38% 7% 10% 17% 17% 11%	2150
	21st to end	Day	43% 6% 10% 19% 15% 6%	3124
January	1st to 10th	Night	40% 7% 8% 16% 20% 9%	4109
	11th to 20th	Night	47% 6% 8% 18% 15% 6%	4120
	21st to end	Night	39% 8% 12% 15% 17% 10%	4422
February	1st to 10th	Night	37% 6% 10% 17% 17% 13%	2280
	11th to 20th	Night	33% 7% 11% 17% 21% 11%	2219
	21st to end	Night	33% 5% 13% 18% 21% 9%	1807
March	1st to 10th	Night	45% 8% 12% 16% 15% 5%	3418
	11th to 20th	Night	28% 8% 21% 23% 15% 5%	3480
	21st to end	Night	34% 6% 10% 22% 21% 7%	3817
April	1st to 10th	Night	27% 7% 17% 23% 16% 9%	2090
	11th to 20th	Night	29% 7% 12% 26% 19% 7%	2037
	21st to end	Night	25% 6% 15% 25% 21% 8%	1900
May	1st to 10th	Night	19% 13% 21% 28% 15% 5%	2845
	11th to 20th	Night	30% 11% 22% 25% 10% 2	2813
	21st to end	Night	18% 9% 15% 37% 20% 1	3087
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	40% 10% 13% 18% 11% 8%	2096
	11th to 20th	Night	26% 7% 11% 25% 25% 6%	2280
	21st to end	Night	30% 8% 11% 17% 25% 10%	2508
November	1st to 10th	Night	30% 8% 12% 21% 18% 11%	2234
	11th to 20th	Night	37% 7% 9% 19% 19% 10%	2280
	21st to end	Night	36% 6% 10% 21% 20% 7%	2427
December	1st to 10th	Night	31% 7% 14% 27% 14% 6%	2470
	11th to 20th	Night	36% 6% 8% 19% 19% 11%	3044
	21st to end	Night	43% 7% 13% 19% 14% 4	4532

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	11% 12% 42% 25% 8% 2	270
	11th to 20th	Day	8% 12% 39% 27% 11% 3	270
	21st to end	Day	15% 19% 39% 22% 5%	297
February	1st to 10th	Day	13% 15% 33% 22% 15% 2	270
	11th to 20th	Day	11% 11% 37% 27% 11% 4	270
	21st to end	Day	10% 10% 31% 31% 17% 1	301
March	1st to 10th	Day	7% 12% 34% 32% 14% 1	460
	11th to 20th	Day	10% 11% 27% 31% 18% 3	460
	21st to end	Day	9% 10% 29% 28% 20% 3	506
April	1st to 10th	Day	10% 11% 28% 31% 18% 3	460
	11th to 20th	Day	9% 10% 28% 30% 18% 5%	452
	21st to end	Day	6% 5% 21% 33% 23% 12%	460
May	1st to 10th	Day	3 5% 21% 32% 27% 13%	463
	11th to 20th	Day	9% 2 16% 29% 29% 15%	487
	21st to end	Day	7% 3 10% 21% 35% 23%	529
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% 5%	459
	11th to 20th	Day	15% 6% 25% 34% 17% 3	460
	21st to end	Day	10% 9% 31% 33% 16% 1	308
December	1st to 10th	Day	6% 9% 35% 29% 17% 3	270
	11th to 20th	Day	16% 9% 32% 30% 13% 1	270
	21st to end	Day	9% 9% 28% 38% 13% 3	297
January	1st to 10th	Night	13% 9% 36% 31% 11% 1	760
	11th to 20th	Night	13% 9% 27% 37% 14% 1	760
	21st to end	Night	17% 10% 36% 30% 7%	836
February	1st to 10th	Night	16% 7% 31% 35% 11%	760
	11th to 20th	Night	11% 7% 32% 38% 10% 1	743
	21st to end	Night	12% 6% 33% 42% 6%	628
March	1st to 10th	Night	7% 9% 34% 38% 11%	760
	11th to 20th	Night	11% 8% 29% 38% 13% 1	760
	21st to end	Night	9% 7% 30% 41% 13% 1	836
April	1st to 10th	Night	8% 6% 31% 41% 13%	760
	11th to 20th	Night	9% 7% 31% 38% 14% 2	751
	21st to end	Night	7% 4 25% 44% 19% 1	760
May	1st to 10th	Night	6% 2 17% 49% 24% 2	575
	11th to 20th	Night	6% 2 14% 43% 31% 3	561
	21st to end	Night	7% 1 9% 36% 40% 6%	617
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% 24% 36% 23% 2	745
	11th to 20th	Night	11% 7% 23% 38% 20% 2	760
	21st to end	Night	11% 4 30% 39% 15% 1	760
December	1st to 10th	Night	10% 4 27% 36% 22% 1	760
	11th to 20th	Night	12% 7% 27% 35% 18% 1	760
	21st to end	Night	12% 7% 25% 39% 16% 1	836

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