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 2026-09-29.

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	7% 5% 2 7% 5% 5% 6% 5% 7% 11% 41%	148597
	Night	8% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	142788
Transparency	Day	36% 10% 18% 19% 12% 5%	152321
	Night	36% 9% 17% 19% 13% 6%	147136
Seeing	Day	9% 6% 19% 26% 24% 16%	18079
	Night	10% 5% 21% 33% 25% 7%	28092

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	7% 5% 2 6% 4 3 4 3 4 7% 55%	5258
	11th to 20th	Day	18% 9% 3 8% 4 4 5% 3 4 6% 35%	5280
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 50%	5997
February	1st to 10th	Day	12% 6% 1 6% 4 4 5% 4 7% 7% 45%	2100
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 5% 7% 52%	2048
	21st to end	Day	9% 6% 3 5% 4 4 4 4 5% 7% 50%	1814
March	1st to 10th	Day	7% 6% 2 7% 4 4 5% 4 7% 9% 44%	5045
	11th to 20th	Day	5% 4 2 5% 4 3 3 3 5% 8% 58%	5090
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 9% 49%	5586
April	1st to 10th	Day	7% 4 2 4 4 4 5% 4 6% 9% 51%	2310
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 4 7% 8% 47%	2312
	21st to end	Day	4 4 1 4 4 4 5% 4 8% 9% 53%	2530
May	1st to 10th	Day	3 3 1 5% 5% 5% 6% 4 7% 11% 48%	7616
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	7626
	21st to end	Day	4 3 1 5% 5% 5% 6% 5% 8% 14% 43%	8391
June	1st to 10th	Day	9% 7% 3 8% 7% 7% 8% 6% 8% 11% 27%	2730
	11th to 20th	Day	6% 5% 2 7% 8% 9% 8% 7% 8% 12% 28%	2730
	21st to end	Day	7% 6% 3 8% 8% 7% 7% 6% 8% 11% 29%	2730
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 8% 7% 11% 15% 25%	7680
	11th to 20th	Day	4 3 2 7% 6% 6% 7% 6% 10% 17% 32%	7680
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 6% 9% 15% 33%	8340
August	1st to 10th	Day	7% 8% 3 10% 7% 8% 7% 5% 8% 13% 25%	6014
	11th to 20th	Day	3 4 1 5% 6% 6% 8% 5% 8% 13% 41%	5689
	21st to end	Day	7% 6% 3 9% 7% 7% 8% 6% 9% 13% 26%	5566
September	1st to 10th	Day	7% 7% 3 11% 9% 7% 9% 6% 10% 10% 21%	2310
	11th to 20th	Day	9% 7% 3 10% 8% 7% 8% 6% 8% 10% 25%	2310
	21st to end	Day	8% 6% 3 9% 7% 7% 6% 5% 8% 9% 32%	2288
October	1st to 10th	Day	11% 6% 3 9% 7% 6% 6% 4 6% 9% 34%	2190
	11th to 20th	Day	6% 5% 2 6% 5% 4 4 4 6% 8% 52%	2000
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 5% 8% 50%	2200
November	1st to 10th	Day	8% 5% 3 5% 5% 4 4 4 5% 9% 49%	2057
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 3 5% 6% 48%	2100
	21st to end	Day	9% 6% 2 5% 4 5% 4 4 6% 8% 47%	1932
December	1st to 10th	Day	6% 4 1 5% 3 4 5% 4 6% 8% 56%	3020
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 5% 6% 9% 42%	3463
	21st to end	Day	11% 8% 3 7% 4 5% 4 3 5% 8% 43%	4565
January	1st to 10th	Night	8% 5% 2 4 4 3 3 3 5% 7% 57%	7635
	11th to 20th	Night	18% 8% 3 7% 5% 4 5% 4 5% 7% 35%	7680
	21st to end	Night	13% 6% 2 6% 4 3 4 4 6% 8% 44%	8179
February	1st to 10th	Night	12% 7% 2 5% 4 4 3 3 6% 6% 48%	2520
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 4 5% 7% 48%	2458
	21st to end	Night	8% 7% 2 5% 5% 3 4 4 6% 6% 50%	1991
March	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 5% 45%	5028
	11th to 20th	Night	6% 4 2 5% 3 3 4 4 6% 9% 54%	5090
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 50%	5588
April	1st to 10th	Night	7% 5% 2 4 4 4 4 4 6% 9% 51%	2310
	11th to 20th	Night	6% 6% 2 5% 4 3 3 3 6% 9% 53%	2254
	21st to end	Night	6% 3 2 4 4 5% 5% 3 5% 8% 56%	2100
May	1st to 10th	Night	5% 4 2 7% 4 5% 5% 5% 6% 10% 47%	5242
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 6% 9% 47%	5234
	21st to end	Night	4 4 2 6% 5% 5% 5% 4 6% 11% 48%	5761
June	1st to 10th	Night	10% 6% 2 7% 6% 6% 6% 6% 7% 10% 36%	1890
	11th to 20th	Night	6% 5% 2 7% 7% 6% 7% 7% 8% 12% 33%	1890
	21st to end	Night	6% 7% 3 9% 5% 6% 5% 5% 10% 10% 33%	1890
July	1st to 10th	Night	7% 5% 3 8% 6% 7% 7% 6% 12% 14% 24%	5278
	11th to 20th	Night	4 5% 2 7% 7% 7% 8% 7% 9% 13% 31%	5277
	21st to end	Night	6% 6% 2 8% 6% 7% 8% 6% 9% 11% 31%	5736
August	1st to 10th	Night	5% 6% 3 10% 8% 8% 7% 6% 8% 10% 29%	4135
	11th to 20th	Night	3 3 1 5% 4 4 5% 5% 9% 15% 44%	4278
	21st to end	Night	6% 5% 2 8% 6% 6% 8% 7% 10% 14% 28%	5559
September	1st to 10th	Night	9% 8% 2 9% 6% 6% 6% 5% 8% 11% 30%	2310
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 6% 8% 11% 30%	2310
	21st to end	Night	7% 7% 3 7% 5% 5% 6% 5% 6% 10% 38%	2289
October	1st to 10th	Night	10% 7% 3 6% 5% 5% 5% 4 7% 8% 40%	2207
	11th to 20th	Night	5% 4 2 5% 3 5% 4 4 6% 8% 55%	2400
	21st to end	Night	8% 5% 2 6% 4 4 4 3 6% 7% 52%	2640
November	1st to 10th	Night	8% 5% 2 6% 4 3 4 3 7% 7% 50%	2461
	11th to 20th	Night	7% 7% 2 7% 5% 4 4 3 5% 7% 49%	2520
	21st to end	Night	8% 6% 3 7% 4 5% 5% 3 6% 8% 47%	2683
December	1st to 10th	Night	6% 4 2 5% 4 3 4 3 5% 7% 56%	4380
	11th to 20th	Night	9% 7% 2 5% 4 4 4 4 4 6% 50%	4952
	21st to end	Night	13% 7% 2 7% 4 5% 5% 3 5% 7% 40%	6633

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% 12% 17% 25% 12%	5259
	11th to 20th	Day	51% 4 6% 12% 15% 12%	5280
	21st to end	Day	32% 5% 10% 25% 17% 11%	5997
February	1st to 10th	Day	37% 6% 10% 14% 21% 12%	2100
	11th to 20th	Day	32% 5% 10% 19% 23% 11%	2058
	21st to end	Day	34% 5% 10% 18% 23% 11%	1814
March	1st to 10th	Day	34% 7% 13% 18% 20% 7%	5045
	11th to 20th	Day	24% 5% 10% 22% 22% 17%	5090
	21st to end	Day	31% 5% 11% 27% 17% 8%	5586
April	1st to 10th	Day	29% 7% 13% 22% 20% 8%	2310
	11th to 20th	Day	32% 7% 12% 18% 22% 8%	2312
	21st to end	Day	23% 8% 15% 24% 23% 7%	2530
May	1st to 10th	Day	27% 8% 20% 28% 13% 3	7651
	11th to 20th	Day	33% 9% 15% 30% 12% 1	7630
	21st to end	Day	28% 12% 22% 26% 11% 1	8405
June	1st to 10th	Day	47% 14% 17% 16% 6%	2730
	11th to 20th	Day	44% 14% 21% 18% 2	2730
	21st to end	Day	44% 15% 23% 15% 3	2730
July	1st to 10th	Day	41% 16% 34% 9%	7615
	11th to 20th	Day	35% 14% 39% 11%	7611
	21st to end	Day	36% 15% 28% 21% 1	8156
August	1st to 10th	Day	48% 14% 26% 10% 2	6001
	11th to 20th	Day	30% 14% 32% 21% 3	5728
	21st to end	Day	43% 16% 26% 13% 1	5566
September	1st to 10th	Day	50% 18% 18% 10% 4	2310
	11th to 20th	Day	49% 14% 20% 12% 5%	2310
	21st to end	Day	44% 11% 14% 16% 11% 3	2288
October	1st to 10th	Day	55% 9% 12% 12% 7% 5%	3567
	11th to 20th	Day	26% 4 8% 21% 32% 9%	3240
	21st to end	Day	26% 4 8% 22% 32% 8%	3538
November	1st to 10th	Day	32% 7% 12% 19% 19% 11%	2057
	11th to 20th	Day	36% 6% 10% 18% 21% 10%	2100
	21st to end	Day	35% 6% 10% 21% 20% 8%	1932
December	1st to 10th	Day	25% 5% 16% 25% 19% 9%	3020
	11th to 20th	Day	37% 7% 11% 18% 17% 10%	3460
	21st to end	Day	40% 5% 11% 21% 19% 4	4565
January	1st to 10th	Night	27% 4 11% 21% 26% 11%	7636
	11th to 20th	Night	49% 4 8% 16% 15% 9%	7680
	21st to end	Night	38% 5% 12% 20% 14% 11%	8179
February	1st to 10th	Night	35% 6% 9% 17% 18% 14%	2520
	11th to 20th	Night	34% 7% 11% 19% 20% 10%	2459
	21st to end	Night	32% 5% 13% 18% 23% 9%	1991
March	1st to 10th	Night	40% 6% 10% 16% 19% 9%	5028
	11th to 20th	Night	25% 6% 16% 20% 17% 16%	5090
	21st to end	Night	32% 5% 11% 24% 20% 8%	5588
April	1st to 10th	Night	28% 7% 17% 23% 15% 8%	2310
	11th to 20th	Night	28% 6% 11% 26% 21% 9%	2254
	21st to end	Night	25% 6% 15% 26% 21% 7%	2100
May	1st to 10th	Night	29% 12% 21% 24% 12% 3	5269
	11th to 20th	Night	33% 7% 20% 26% 13% 1	5235
	21st to end	Night	29% 12% 21% 26% 13% 1	5771
June	1st to 10th	Night	40% 11% 23% 20% 6%	1890
	11th to 20th	Night	40% 15% 24% 19% 2	1890
	21st to end	Night	42% 14% 23% 17% 4 1	1890
July	1st to 10th	Night	44% 17% 32% 7% 1	5235
	11th to 20th	Night	39% 17% 33% 10% 1	5226
	21st to end	Night	41% 14% 25% 19% 1	5606
August	1st to 10th	Night	47% 14% 29% 9% 2	4124
	11th to 20th	Night	25% 15% 37% 19% 3	4305
	21st to end	Night	41% 19% 26% 12% 2	5559
September	1st to 10th	Night	45% 17% 21% 13% 4	2310
	11th to 20th	Night	44% 15% 20% 16% 4 1	2310
	21st to end	Night	39% 12% 18% 19% 9% 2	2289
October	1st to 10th	Night	53% 10% 12% 14% 7% 5%	3610
	11th to 20th	Night	25% 5% 9% 21% 35% 6%	3899
	21st to end	Night	28% 6% 9% 19% 31% 7%	4252
November	1st to 10th	Night	31% 8% 12% 21% 18% 11%	2461
	11th to 20th	Night	35% 6% 9% 19% 20% 10%	2520
	21st to end	Night	35% 6% 10% 22% 19% 8%	2683
December	1st to 10th	Night	27% 5% 15% 26% 14% 13%	4380
	11th to 20th	Night	34% 7% 10% 22% 16% 10%	4954
	21st to end	Night	42% 6% 12% 20% 15% 5%	6633

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	9% 12% 42% 25% 9% 2	330
	11th to 20th	Day	10% 15% 38% 25% 9% 3	330
	21st to end	Day	15% 19% 39% 22% 4	363
February	1st to 10th	Day	11% 15% 32% 25% 14% 3	330
	11th to 20th	Day	11% 11% 39% 27% 9% 3	330
	21st to end	Day	11% 12% 31% 30% 16% 1	357
March	1st to 10th	Day	8% 12% 32% 34% 14% 1	540
	11th to 20th	Day	10% 12% 30% 29% 16% 3	540
	21st to end	Day	8% 9% 30% 29% 20% 4	594
April	1st to 10th	Day	10% 11% 27% 32% 19% 3	540
	11th to 20th	Day	8% 9% 27% 30% 20% 6%	532
	21st to end	Day	6% 5% 20% 33% 25% 11%	540
May	1st to 10th	Day	4 5% 23% 33% 24% 11%	543
	11th to 20th	Day	9% 2 19% 29% 27% 14%	567
	21st to end	Day	7% 3 11% 23% 35% 22%	617
June	1st to 10th	Day	11% 3 10% 25% 31% 20%	570
	11th to 20th	Day	7% 2 10% 23% 29% 29%	570
	21st to end	Day	6% 2 6% 18% 30% 39%	570
July	1st to 10th	Day	7% 2 3 13% 28% 47%	565
	11th to 20th	Day	6% 3 15% 32% 43%	564
	21st to end	Day	9% 2 5% 15% 31% 38%	620
August	1st to 10th	Day	7% 3 7% 17% 29% 36%	566
	11th to 20th	Day	5% 1 4 14% 31% 45%	559
	21st to end	Day	10% 2 7% 17% 31% 33%	627
September	1st to 10th	Day	9% 2 7% 18% 33% 31%	570
	11th to 20th	Day	8% 3 11% 28% 30% 21%	570
	21st to end	Day	8% 4 10% 23% 30% 24%	562
October	1st to 10th	Day	15% 5% 17% 34% 23% 6%	530
	11th to 20th	Day	5% 7% 17% 32% 30% 8%	530
	21st to end	Day	8% 5% 23% 34% 25% 4	583
November	1st to 10th	Day	9% 7% 24% 30% 26% 4	535
	11th to 20th	Day	13% 5% 24% 34% 19% 4	540
	21st to end	Day	8% 10% 32% 30% 16% 3	372
December	1st to 10th	Day	8% 10% 38% 27% 15% 3	330
	11th to 20th	Day	13% 9% 34% 31% 12% 1	330
	21st to end	Day	9% 9% 28% 37% 13% 3	363
January	1st to 10th	Night	12% 9% 36% 32% 11% 1	840
	11th to 20th	Night	13% 9% 28% 36% 13%	840
	21st to end	Night	18% 9% 36% 30% 7%	924
February	1st to 10th	Night	15% 8% 30% 35% 11%	840
	11th to 20th	Night	12% 8% 32% 38% 10% 1	823
	21st to end	Night	12% 6% 34% 42% 6%	692
March	1st to 10th	Night	8% 9% 33% 38% 11%	840
	11th to 20th	Night	10% 8% 31% 37% 12% 1	840
	21st to end	Night	9% 7% 29% 42% 12% 1	924
April	1st to 10th	Night	9% 6% 30% 41% 13%	840
	11th to 20th	Night	8% 6% 30% 39% 15% 1	831
	21st to end	Night	6% 4 26% 44% 19% 1	840
May	1st to 10th	Night	7% 2 17% 49% 23% 2	635
	11th to 20th	Night	6% 2 14% 44% 30% 3	621
	21st to end	Night	8% 2 10% 35% 40% 6%	683
June	1st to 10th	Night	11% 1 10% 37% 34% 7%	630
	11th to 20th	Night	7% 2 8% 25% 37% 21%	630
	21st to end	Night	9% 1 6% 20% 43% 21%	630
July	1st to 10th	Night	8% 1 4 16% 45% 26%	615
	11th to 20th	Night	5% 1 3 14% 48% 29%	612
	21st to end	Night	6% 1 4 15% 51% 23%	690
August	1st to 10th	Night	7% 2 3 17% 46% 24%	627
	11th to 20th	Night	5% 1 3 14% 47% 29%	624
	21st to end	Night	11% 2 8% 22% 37% 20%	924
September	1st to 10th	Night	10% 3 9% 22% 38% 18%	840
	11th to 20th	Night	11% 3 11% 29% 34% 12%	840
	21st to end	Night	11% 3 11% 27% 39% 9%	833
October	1st to 10th	Night	13% 3 17% 37% 25% 4	800
	11th to 20th	Night	6% 4 14% 40% 34% 3	800
	21st to end	Night	11% 3 18% 40% 26% 2	880
November	1st to 10th	Night	10% 5% 23% 37% 23% 2	820
	11th to 20th	Night	10% 8% 22% 38% 20% 2	840
	21st to end	Night	10% 4 30% 38% 17% 1	840
December	1st to 10th	Night	10% 4 28% 37% 20% 1	840
	11th to 20th	Night	11% 7% 28% 36% 17% 1	840
	21st to end	Night	12% 7% 27% 38% 16% 1	924

Page generated at 2026-09-26 04:18:42. Version ac07

Copyright © 2026 Attilla Danko.