Arthur Purcell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arthur Purcell Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	4 3 1 3 2 2 3 2 4 5% 70%	129292
	Night	6% 3 1 4 3 3 3 2 4 5% 67%	127696
Transparency	Day	18% 4 8% 18% 26% 26%	129000
	Night	20% 4 9% 20% 25% 21%	127503
Seeing	Day	6% 3 11% 26% 34% 22%	16342
	Night	7% 2 11% 39% 35% 5%	25889

By Month

Percentage Cloud Cover SW (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% 11% 3 8% 5% 6% 5% 3 6% 6% 39%	3639
	11th to 20th	Day	6% 6% 3 7% 4 5% 4 3 4 5% 52%	3985
	21st to end	Day	11% 8% 3 7% 5% 4 4 4 5% 7% 43%	4510
February	1st to 10th	Day	11% 7% 2 5% 4 3 3 3 5% 7% 49%	1954
	11th to 20th	Day	10% 5% 2 5% 3 4 3 4 6% 8% 51%	1950
	21st to end	Day	11% 6% 2 6% 3 4 4 4 5% 6% 49%	1650
March	1st to 10th	Day	12% 7% 2 6% 4 4 5% 4 7% 8% 42%	4518
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 8% 53%	4863
	21st to end	Day	9% 4 2 6% 3 3 5% 4 6% 8% 51%	5881
April	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 6% 62%	2400
	11th to 20th	Day	6% 3 1 4 3 3 2 3 4 6% 64%	2369
	21st to end	Day	4 3 1 3 3 2 2 3 5% 5% 70%	2400
May	1st to 10th	Day	3 2 1 2 2 1 2 1 2 4 79%	5541
	11th to 20th	Day	1 1 1 2 2 1 2 2 3 4 82%	5841
	21st to end	Day	1 1 1 1 1 2 1 2 3 86%	6745
June	1st to 10th	Day	2 1 1 2 2 1 2 2 4 6% 77%	2800
	11th to 20th	Day	1 1 1 1 2 3 90%	2800
	21st to end	Day	1 1 1 1 1 1 1 1 2 2 88%	2800
July	1st to 10th	Day	1 1 1 95%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 1 1 1 1 2 3 90%	6451
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	3844
	11th to 20th	Day	1 1 1 1 1 1 2 93%	3724
	21st to end	Day	1 1 1 1 1 1 2 92%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	2280
	11th to 20th	Day	1 1 1 1 2 1 2 4 87%	2214
	21st to end	Day	1 1 1 2 1 1 1 2 4 85%	2090
October	1st to 10th	Day	2 1 1 1 1 2 2 4 5% 80%	3349
	11th to 20th	Day	1 2 1 3 1 2 2 2 3 4 77%	3161
	21st to end	Day	2 2 1 3 2 2 3 2 4 7% 71%	3429
November	1st to 10th	Day	5% 4 1 2 3 3 3 2 4 6% 65%	1764
	11th to 20th	Day	8% 6% 2 5% 3 2 3 3 5% 6% 58%	1800
	21st to end	Day	9% 7% 2 4 4 3 4 3 5% 6% 54%	1800
December	1st to 10th	Day	6% 7% 3 8% 7% 6% 5% 4 5% 7% 43%	2889
	11th to 20th	Day	11% 8% 4 8% 6% 4 4 5% 7% 8% 34%	3028
	21st to end	Day	13% 14% 5% 10% 5% 4 5% 4 6% 6% 27%	4004
January	1st to 10th	Night	10% 8% 2 7% 5% 4 4 3 6% 7% 45%	6306
	11th to 20th	Night	7% 5% 2 5% 3 3 3 3 4 6% 58%	5985
	21st to end	Night	13% 6% 3 6% 4 3 4 3 4 7% 48%	6468
February	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 7% 54%	2443
	11th to 20th	Night	11% 3 2 4 2 3 3 3 4 7% 58%	2338
	21st to end	Night	11% 5% 2 4 4 3 3 3 3 6% 57%	1980
March	1st to 10th	Night	9% 6% 3 6% 4 3 4 4 6% 8% 47%	5376
	11th to 20th	Night	8% 3 1 5% 4 4 4 4 5% 7% 56%	5132
	21st to end	Night	11% 6% 3 6% 4 4 4 3 4 6% 49%	5094
April	1st to 10th	Night	8% 3 1 4 4 3 3 3 4 6% 63%	2000
	11th to 20th	Night	5% 3 1 4 2 2 2 3 4 5% 69%	1979
	21st to end	Night	5% 2 1 2 3 2 2 2 4 6% 72%	2000
May	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 77%	4450
	11th to 20th	Night	3 1 2 1 1 1 2 3 5% 82%	4074
	21st to end	Night	1 1 1 1 1 1 1 2 90%	4118
June	1st to 10th	Night	2 1 2 2 1 2 1 3 6% 79%	1600
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	1600
	21st to end	Night	1 1 1 1 1 1 2 2 92%	1600
July	1st to 10th	Night	1 1 1 2 95%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	3640
	21st to end	Night	1 1 1 1 1 1 2 3 88%	4416
August	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 86%	3191
	11th to 20th	Night	1 1 1 1 1 1 2 93%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 92%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 89%	1900
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 86%	1966
	21st to end	Night	1 1 1 1 1 1 2 1 2 4 85%	2090
October	1st to 10th	Night	2 1 1 1 1 1 1 2 2 4 84%	3614
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 4 81%	3777
	21st to end	Night	3 2 1 2 1 1 2 2 2 4 79%	4137
November	1st to 10th	Night	4 3 1 4 3 2 3 3 4 7% 66%	2535
	11th to 20th	Night	5% 3 1 3 3 3 3 2 5% 6% 66%	2600
	21st to end	Night	8% 4 2 5% 4 3 3 4 5% 6% 57%	2600
December	1st to 10th	Night	5% 4 2 5% 5% 6% 5% 4 6% 7% 51%	4295
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	5176
	21st to end	Night	10% 8% 3 9% 6% 6% 6% 4 7% 8% 32%	6952

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	38% 11% 19% 17% 13% 2	3639
	11th to 20th	Day	29% 9% 11% 15% 21% 14%	3985
	21st to end	Day	37% 9% 12% 14% 24% 4	4510
February	1st to 10th	Day	33% 6% 12% 26% 18% 4	1954
	11th to 20th	Day	30% 6% 14% 26% 18% 5%	1960
	21st to end	Day	34% 7% 13% 26% 16% 3	1650
March	1st to 10th	Day	35% 9% 13% 20% 19% 4	4518
	11th to 20th	Day	29% 5% 16% 33% 12% 5%	4863
	21st to end	Day	29% 9% 13% 31% 15% 3	5881
April	1st to 10th	Day	23% 4 13% 30% 23% 7%	2400
	11th to 20th	Day	21% 4 12% 29% 24% 9%	2372
	21st to end	Day	17% 5% 12% 31% 28% 8%	2400
May	1st to 10th	Day	12% 3 8% 21% 38% 18%	5541
	11th to 20th	Day	8% 3 5% 26% 42% 16%	5844
	21st to end	Day	7% 2 4 22% 47% 18%	6746
June	1st to 10th	Day	11% 3 9% 20% 34% 24%	2800
	11th to 20th	Day	3 2 4 12% 38% 41%	2800
	21st to end	Day	6% 1 2 8% 28% 56%	2800
July	1st to 10th	Day	2 1 4 15% 77%	6251
	11th to 20th	Day	5% 1 1 4 19% 70%	6245
	21st to end	Day	4 1 1 12% 36% 46%	6264
August	1st to 10th	Day	6% 1 2 5% 32% 54%	3830
	11th to 20th	Day	3 1 1 6% 24% 66%	3760
	21st to end	Day	3 1 2 9% 29% 56%	4179
September	1st to 10th	Day	5% 1 2 9% 29% 54%	2280
	11th to 20th	Day	5% 2 3 17% 42% 30%	2214
	21st to end	Day	7% 1 3 17% 43% 28%	2090
October	1st to 10th	Day	9% 2 4 13% 24% 48%	3349
	11th to 20th	Day	12% 3 5% 24% 36% 20%	3161
	21st to end	Day	15% 3 4 20% 37% 21%	3429
November	1st to 10th	Day	20% 5% 10% 27% 29% 10%	1764
	11th to 20th	Day	24% 7% 14% 24% 25% 5%	1800
	21st to end	Day	30% 5% 9% 25% 21% 9%	1800
December	1st to 10th	Day	36% 7% 10% 24% 18% 5%	2889
	11th to 20th	Day	43% 8% 11% 21% 14% 2	3028
	21st to end	Day	52% 9% 14% 17% 6% 1	4004
January	1st to 10th	Night	34% 8% 18% 25% 13% 1	6306
	11th to 20th	Night	26% 7% 15% 15% 28% 10%	5985
	21st to end	Night	36% 6% 11% 20% 22% 5%	6468
February	1st to 10th	Night	34% 4 9% 28% 20% 6%	2443
	11th to 20th	Night	27% 5% 12% 29% 22% 6%	2339
	21st to end	Night	30% 5% 13% 30% 18% 5%	1980
March	1st to 10th	Night	32% 8% 12% 23% 22% 3	5376
	11th to 20th	Night	27% 6% 16% 33% 13% 4	5132
	21st to end	Night	35% 7% 15% 26% 15% 3	5094
April	1st to 10th	Night	23% 4 13% 30% 22% 8%	2000
	11th to 20th	Night	18% 4 13% 28% 28% 9%	1979
	21st to end	Night	16% 4 11% 31% 33% 6%	2000
May	1st to 10th	Night	13% 2 7% 24% 38% 16%	4447
	11th to 20th	Night	9% 1 4 25% 46% 15%	4075
	21st to end	Night	5% 2 4 19% 50% 20%	4118
June	1st to 10th	Night	9% 3 8% 20% 35% 25%	1600
	11th to 20th	Night	4 2 4 13% 35% 42%	1600
	21st to end	Night	3 1 2 6% 28% 60%	1600
July	1st to 10th	Night	2 1 1 2 14% 81%	3600
	11th to 20th	Night	4 1 1 4 15% 74%	3592
	21st to end	Night	4 2 2 10% 36% 46%	4291
August	1st to 10th	Night	7% 1 1 6% 29% 55%	3182
	11th to 20th	Night	2 1 2 5% 23% 67%	3156
	21st to end	Night	3 1 1 7% 26% 61%	3498
September	1st to 10th	Night	6% 1 1 8% 27% 57%	1900
	11th to 20th	Night	7% 1 4 15% 40% 33%	1966
	21st to end	Night	6% 2 2 17% 40% 33%	2090
October	1st to 10th	Night	8% 2 3 11% 25% 51%	3614
	11th to 20th	Night	9% 2 4 22% 42% 21%	3777
	21st to end	Night	12% 2 3 21% 40% 22%	4137
November	1st to 10th	Night	19% 4 10% 26% 28% 13%	2535
	11th to 20th	Night	18% 5% 14% 25% 29% 8%	2600
	21st to end	Night	27% 5% 10% 22% 28% 8%	2600
December	1st to 10th	Night	28% 7% 13% 28% 18% 6%	4295
	11th to 20th	Night	39% 8% 16% 20% 16% 2	5176
	21st to end	Night	45% 9% 17% 21% 6% 1	6952

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	22% 5% 9% 31% 25% 9%	200
	11th to 20th	Day	12% 2 10% 33% 26% 17%	350
	21st to end	Day	15% 3 9% 26% 31% 17%	440
February	1st to 10th	Day	13% 3 12% 28% 26% 18%	477
	11th to 20th	Day	12% 4 14% 31% 28% 10%	495
	21st to end	Day	11% 5% 14% 32% 30% 7%	414
March	1st to 10th	Day	11% 6% 17% 28% 28% 10%	500
	11th to 20th	Day	11% 4 16% 32% 24% 13%	500
	21st to end	Day	9% 7% 19% 32% 23% 10%	550
April	1st to 10th	Day	9% 3 19% 27% 27% 14%	500
	11th to 20th	Day	6% 6% 20% 29% 28% 12%	492
	21st to end	Day	3 6% 18% 27% 26% 20%	500
May	1st to 10th	Day	4 5% 18% 30% 30% 14%	503
	11th to 20th	Day	3 2 18% 33% 26% 19%	528
	21st to end	Day	2 4 14% 25% 36% 20%	573
June	1st to 10th	Day	1 2 10% 25% 34% 27%	530
	11th to 20th	Day	1 2 13% 25% 36% 24%	530
	21st to end	Day	1 7% 25% 40% 27%	530
July	1st to 10th	Day	1 4 20% 45% 30%	526
	11th to 20th	Day	2 2 13% 39% 44%	524
	21st to end	Day	1 2 13% 47% 37%	575
August	1st to 10th	Day	2 8% 21% 41% 28%	504
	11th to 20th	Day	1 3 21% 41% 34%	477
	21st to end	Day	1 4 20% 42% 32%	539
September	1st to 10th	Day	1 1 4 22% 43% 29%	490
	11th to 20th	Day	1 1 8% 29% 40% 20%	490
	21st to end	Day	1 2 8% 23% 39% 26%	490
October	1st to 10th	Day	2 2 10% 26% 34% 25%	490
	11th to 20th	Day	2 3 9% 23% 37% 26%	490
	21st to end	Day	2 3 9% 24% 37% 26%	532
November	1st to 10th	Day	5% 2 9% 24% 32% 27%	300
	11th to 20th	Day	6% 4 7% 27% 37% 21%	300
	21st to end	Day	12% 1 9% 28% 31% 18%	300
December	1st to 10th	Day	13% 1 10% 31% 29% 16%	283
	11th to 20th	Day	11% 4 11% 25% 27% 23%	200
	21st to end	Day	21% 3 14% 28% 23% 10%	220
January	1st to 10th	Night	18% 2 10% 34% 28% 8%	800
	11th to 20th	Night	8% 2 8% 42% 35% 5%	800
	21st to end	Night	15% 2 9% 40% 30% 5%	880
February	1st to 10th	Night	14% 3 10% 38% 29% 6%	800
	11th to 20th	Night	12% 3 11% 39% 29% 6%	784
	21st to end	Night	12% 3 12% 47% 24% 3	660
March	1st to 10th	Night	10% 3 14% 47% 24% 3	800
	11th to 20th	Night	10% 4 15% 38% 29% 5%	800
	21st to end	Night	12% 4 17% 39% 26% 2	880
April	1st to 10th	Night	8% 4 22% 38% 26% 3	800
	11th to 20th	Night	5% 6% 21% 39% 27% 2	791
	21st to end	Night	6% 4 23% 36% 26% 5%	800
May	1st to 10th	Night	5% 4 21% 41% 26% 2	755
	11th to 20th	Night	4 4 22% 41% 28% 1	591
	21st to end	Night	3 5% 26% 41% 24% 1	502
June	1st to 10th	Night	3 5% 20% 48% 25% 1	400
	11th to 20th	Night	4 16% 53% 26% 2	400
	21st to end	Night	1 1 16% 51% 30% 1	400
July	1st to 10th	Night	8% 51% 39% 2	390
	11th to 20th	Night	1 5% 41% 52% 1	388
	21st to end	Night	1 3 31% 56% 9%	627
August	1st to 10th	Night	3 7% 36% 49% 5%	757
	11th to 20th	Night	1 3 37% 53% 7%	752
	21st to end	Night	1 3 29% 59% 9%	836
September	1st to 10th	Night	1 1 5% 29% 58% 7%	760
	11th to 20th	Night	1 1 10% 37% 45% 6%	760
	21st to end	Night	2 1 8% 35% 47% 7%	760
October	1st to 10th	Night	3 1 9% 40% 40% 7%	760
	11th to 20th	Night	3 2 8% 36% 43% 8%	760
	21st to end	Night	4 1 7% 35% 40% 14%	836
November	1st to 10th	Night	5% 2 8% 33% 41% 11%	780
	11th to 20th	Night	6% 1 6% 41% 37% 8%	800
	21st to end	Night	10% 1 8% 40% 35% 7%	800
December	1st to 10th	Night	12% 1 9% 40% 35% 5%	800
	11th to 20th	Night	15% 2 8% 42% 29% 5%	800
	21st to end	Night	14% 2 10% 45% 25% 3	880

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