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 2024-11-04.

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%	104883
	Night	6% 3 1 4 3 2 3 2 4 5% 67%	102879
Transparency	Day	17% 5% 8% 18% 25% 27%	104634
	Night	20% 5% 9% 19% 24% 23%	102718
Seeing	Day	6% 3 11% 25% 34% 22%	15371
	Night	7% 2 11% 39% 36% 5%	24939

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	10% 14% 4 10% 5% 6% 5% 3 5% 6% 32%	2539
	11th to 20th	Day	9% 8% 4 10% 5% 6% 6% 4 5% 7% 35%	2773
	21st to end	Day	15% 10% 4 8% 6% 5% 5% 4 6% 6% 31%	3146
February	1st to 10th	Day	11% 7% 2 5% 4 3 3 3 5% 7% 50%	1856
	11th to 20th	Day	10% 5% 2 4 3 4 3 3 6% 7% 52%	1850
	21st to end	Day	12% 7% 2 6% 4 4 4 4 4 5% 49%	1570
March	1st to 10th	Day	9% 7% 3 7% 4 5% 5% 5% 7% 7% 42%	3138
	11th to 20th	Day	5% 4 1 4 2 3 3 3 5% 8% 62%	3392
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 47%	4103
April	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 6% 62%	2280
	11th to 20th	Day	6% 3 1 4 3 3 2 3 4 6% 64%	2249
	21st to end	Day	5% 2 1 2 2 2 2 3 5% 5% 71%	2280
May	1st to 10th	Day	3 2 1 2 1 1 2 1 2 4 81%	3875
	11th to 20th	Day	1 1 1 2 1 2 2 3 4 82%	4071
	21st to end	Day	1 1 1 1 1 2 1 2 3 84%	4708
June	1st to 10th	Day	2 2 1 2 2 1 3 2 4 6% 76%	2660
	11th to 20th	Day	1 1 1 1 2 3 90%	2660
	21st to end	Day	1 1 1 1 1 1 1 1 2 2 88%	2660
July	1st to 10th	Day	1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 2 1 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4491
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	3795
	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% 5% 1 3 3 3 3 3 4 6% 64%	1701
	11th to 20th	Day	8% 6% 2 5% 3 2 3 3 5% 6% 57%	1710
	21st to end	Day	8% 6% 2 4 4 3 4 3 5% 6% 56%	1710
December	1st to 10th	Day	9% 9% 4 6% 5% 3 4 3 5% 6% 46%	1677
	11th to 20th	Day	14% 9% 4 8% 6% 4 4 4 6% 7% 34%	1928
	21st to end	Day	13% 14% 5% 10% 5% 4 5% 4 6% 6% 29%	2794
January	1st to 10th	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	4406
	11th to 20th	Night	10% 8% 3 7% 5% 4 4 3 6% 8% 41%	4189
	21st to end	Night	17% 7% 3 7% 4 4 4 4 5% 8% 38%	4521
February	1st to 10th	Night	13% 6% 2 5% 3 3 2 2 4 7% 54%	2321
	11th to 20th	Night	10% 3 2 4 3 3 3 3 4 7% 59%	2218
	21st to end	Night	11% 5% 2 4 4 3 3 4 4 6% 55%	1884
March	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 7% 8% 48%	3736
	11th to 20th	Night	5% 2 1 3 3 4 4 3 5% 7% 63%	3583
	21st to end	Night	14% 6% 3 7% 4 4 4 3 4 6% 45%	3550
April	1st to 10th	Night	7% 3 1 4 4 3 3 3 4 6% 62%	1900
	11th to 20th	Night	5% 3 1 4 2 2 2 2 4 5% 70%	1879
	21st to end	Night	5% 2 1 2 2 2 2 2 4 6% 73%	1900
May	1st to 10th	Night	4 2 1 1 2 1 1 1 2 4 81%	3098
	11th to 20th	Night	2 1 1 1 1 1 2 3 5% 82%	2834
	21st to end	Night	2 1 1 1 1 1 1 2 2 3 86%	2866
June	1st to 10th	Night	2 1 2 2 1 2 1 3 6% 78%	1520
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	1520
	21st to end	Night	1 1 1 1 1 1 2 2 91%	1520
July	1st to 10th	Night	1 1 1 2 2 93%	2540
	11th to 20th	Night	1 1 1 1 2 2 1 2 3 86%	2540
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	3067
August	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 85%	3152
	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	5% 3 1 4 3 2 3 3 4 7% 66%	2450
	11th to 20th	Night	5% 3 1 3 3 2 3 2 5% 6% 66%	2470
	21st to end	Night	7% 4 2 4 4 3 3 3 5% 6% 58%	2470
December	1st to 10th	Night	8% 6% 2 6% 5% 3 4 3 5% 6% 52%	2499
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 6% 7% 38%	3276
	21st to end	Night	11% 7% 3 9% 6% 6% 5% 4 6% 7% 36%	4862

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	45% 14% 16% 18% 7% 1	2539
	11th to 20th	Day	42% 12% 16% 16% 10% 3	2773
	21st to end	Day	46% 12% 15% 16% 9% 2	3146
February	1st to 10th	Day	33% 6% 12% 26% 18% 5%	1856
	11th to 20th	Day	29% 6% 14% 26% 19% 5%	1860
	21st to end	Day	35% 7% 14% 25% 17% 2	1570
March	1st to 10th	Day	34% 11% 15% 22% 15% 3	3138
	11th to 20th	Day	20% 6% 16% 34% 17% 7%	3392
	21st to end	Day	31% 11% 14% 26% 15% 3	4103
April	1st to 10th	Day	23% 4 13% 30% 22% 8%	2280
	11th to 20th	Day	21% 4 13% 29% 24% 10%	2252
	21st to end	Day	16% 5% 12% 30% 29% 8%	2280
May	1st to 10th	Day	9% 3 8% 24% 36% 19%	3875
	11th to 20th	Day	7% 4 6% 24% 40% 19%	4074
	21st to end	Day	8% 2 5% 25% 40% 19%	4708
June	1st to 10th	Day	11% 3 9% 20% 33% 24%	2660
	11th to 20th	Day	4 2 5% 13% 39% 39%	2660
	21st to end	Day	6% 1 2 8% 28% 55%	2660
July	1st to 10th	Day	3 1 1 4 13% 78%	4351
	11th to 20th	Day	7% 2 2 5% 17% 67%	4345
	21st to end	Day	5% 1 1 9% 30% 54%	4349
August	1st to 10th	Day	6% 1 2 5% 32% 54%	3781
	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	21% 5% 10% 26% 28% 10%	1701
	11th to 20th	Day	25% 7% 14% 24% 25% 5%	1710
	21st to end	Day	28% 5% 10% 26% 21% 9%	1710
December	1st to 10th	Day	36% 7% 11% 20% 17% 8%	1677
	11th to 20th	Day	44% 10% 12% 19% 12% 3	1928
	21st to end	Day	49% 12% 13% 17% 7% 2	2794
January	1st to 10th	Night	40% 10% 15% 25% 8% 1	4406
	11th to 20th	Night	37% 10% 21% 17% 10% 5%	4189
	21st to end	Night	42% 8% 15% 20% 12% 3	4521
February	1st to 10th	Night	33% 4 9% 28% 19% 7%	2321
	11th to 20th	Night	26% 5% 11% 29% 23% 6%	2219
	21st to end	Night	31% 5% 13% 29% 18% 4	1884
March	1st to 10th	Night	29% 11% 15% 24% 19% 3	3736
	11th to 20th	Night	19% 8% 16% 34% 17% 6%	3583
	21st to end	Night	37% 9% 13% 24% 14% 3	3550
April	1st to 10th	Night	24% 4 13% 29% 22% 8%	1900
	11th to 20th	Night	17% 4 13% 28% 28% 10%	1879
	21st to end	Night	14% 4 11% 31% 33% 6%	1900
May	1st to 10th	Night	11% 3 6% 24% 38% 18%	3098
	11th to 20th	Night	8% 2 4 23% 43% 20%	2835
	21st to end	Night	7% 2 6% 23% 42% 20%	2866
June	1st to 10th	Night	10% 3 8% 20% 33% 26%	1520
	11th to 20th	Night	4 2 4 14% 35% 41%	1520
	21st to end	Night	3 1 2 7% 28% 59%	1520
July	1st to 10th	Night	2 1 1 3 12% 81%	2500
	11th to 20th	Night	6% 1 2 5% 14% 72%	2492
	21st to end	Night	5% 2 2 7% 27% 57%	2971
August	1st to 10th	Night	8% 1 1 6% 29% 55%	3143
	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	20% 4 10% 26% 28% 13%	2450
	11th to 20th	Night	18% 6% 14% 24% 29% 9%	2470
	21st to end	Night	26% 6% 10% 22% 29% 8%	2470
December	1st to 10th	Night	31% 7% 13% 21% 17% 10%	2499
	11th to 20th	Night	41% 9% 13% 20% 14% 3	3276
	21st to end	Night	42% 12% 16% 21% 8% 2	4862

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	23% 6% 10% 27% 26% 8%	180
	11th to 20th	Day	13% 2 9% 31% 27% 17%	322
	21st to end	Day	15% 3 10% 25% 30% 16%	407
February	1st to 10th	Day	13% 3 11% 27% 26% 20%	439
	11th to 20th	Day	12% 4 14% 31% 28% 11%	455
	21st to end	Day	12% 5% 14% 32% 30% 7%	382
March	1st to 10th	Day	10% 6% 17% 28% 28% 10%	460
	11th to 20th	Day	10% 4 16% 30% 25% 14%	460
	21st to end	Day	9% 7% 19% 33% 24% 9%	506
April	1st to 10th	Day	9% 4 20% 26% 27% 14%	460
	11th to 20th	Day	6% 6% 20% 27% 29% 12%	452
	21st to end	Day	3 6% 19% 27% 26% 20%	460
May	1st to 10th	Day	4 5% 18% 30% 30% 14%	463
	11th to 20th	Day	3 2 16% 32% 27% 20%	488
	21st to end	Day	2 4 14% 23% 37% 21%	529
June	1st to 10th	Day	1 3 10% 24% 34% 28%	490
	11th to 20th	Day	1 1 13% 22% 37% 26%	490
	21st to end	Day	1 6% 24% 41% 27%	490
July	1st to 10th	Day	1 3 18% 46% 32%	486
	11th to 20th	Day	2 2 13% 39% 44%	484
	21st to end	Day	1 2 12% 47% 38%	531
August	1st to 10th	Day	2 7% 20% 41% 29%	487
	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	7% 2 9% 24% 33% 25%	279
	11th to 20th	Day	6% 4 6% 24% 38% 23%	270
	21st to end	Day	10% 9% 28% 33% 20%	270
December	1st to 10th	Day	14% 2 11% 33% 29% 12%	255
	11th to 20th	Day	11% 3 12% 25% 27% 22%	180
	21st to end	Day	23% 3 12% 28% 23% 12%	198
January	1st to 10th	Night	19% 2 10% 33% 28% 9%	760
	11th to 20th	Night	9% 2 8% 40% 35% 6%	760
	21st to end	Night	15% 2 9% 39% 30% 5%	836
February	1st to 10th	Night	14% 3 10% 36% 31% 6%	760
	11th to 20th	Night	12% 3 10% 40% 30% 6%	744
	21st to end	Night	13% 3 12% 46% 24% 3	628
March	1st to 10th	Night	10% 3 13% 47% 24% 3	760
	11th to 20th	Night	9% 3 14% 38% 31% 5%	760
	21st to end	Night	12% 4 17% 39% 26% 2	836
April	1st to 10th	Night	8% 4 22% 37% 27% 2	760
	11th to 20th	Night	5% 6% 21% 38% 28% 2	751
	21st to end	Night	5% 4 23% 35% 27% 5%	760
May	1st to 10th	Night	5% 4 21% 40% 26% 2	717
	11th to 20th	Night	4 3 21% 42% 29% 1	561
	21st to end	Night	3 5% 25% 40% 25% 1	477
June	1st to 10th	Night	3 5% 21% 46% 24% 1	380
	11th to 20th	Night	4 16% 51% 27% 2	380
	21st to end	Night	1 1 16% 51% 30% 1	380
July	1st to 10th	Night	8% 49% 41% 2	370
	11th to 20th	Night	1 5% 41% 52% 1	368
	21st to end	Night	1 3 30% 56% 10%	595
August	1st to 10th	Night	3 6% 35% 50% 5%	742
	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%	754
	11th to 20th	Night	6% 1 5% 41% 38% 9%	760
	21st to end	Night	9% 1 8% 39% 36% 7%	760
December	1st to 10th	Night	12% 1 9% 41% 33% 5%	760
	11th to 20th	Night	15% 2 7% 42% 29% 5%	760
	21st to end	Night	14% 2 10% 44% 26% 3	836

Page generated at 2024-11-01 19:47:30. Version ac07

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