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 2024-04-03.

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	5% 4 2 3 3 2 3 3 4 5% 66%	78997
	Night	7% 4 2 4 3 2 3 3 4 6% 63%	81158
Transparency	Day	21% 5% 9% 19% 25% 21%	78711
	Night	23% 5% 10% 21% 23% 18%	80966
Seeing	Day	6% 3 11% 25% 34% 22%	14516
	Night	7% 2 11% 38% 36% 6%	24221

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 8% 7% 4 6% 4 5% 7% 32%	2539
	11th to 20th	Day	9% 8% 4 9% 6% 5% 6% 6% 5% 7% 35%	2773
	21st to end	Day	15% 10% 4 7% 7% 3 5% 6% 6% 6% 31%	3146
February	1st to 10th	Day	12% 6% 3 4 5% 2 4 3 4 7% 50%	1856
	11th to 20th	Day	10% 4 2 4 4 3 3 4 6% 7% 53%	1850
	21st to end	Day	12% 6% 2 5% 4 3 4 5% 4 5% 50%	1570
March	1st to 10th	Day	10% 6% 3 5% 5% 3 4 5% 7% 7% 46%	1900
	11th to 20th	Day	9% 4 2 5% 4 3 4 5% 6% 9% 51%	2028
	21st to end	Day	8% 3 2 5% 4 2 4 4 5% 6% 58%	2470
April	1st to 10th	Day	7% 3 2 3 4 2 3 3 4 6% 62%	2185
	11th to 20th	Day	6% 3 1 4 3 2 3 4 4 6% 65%	2129
	21st to end	Day	5% 2 1 2 3 2 2 3 4 5% 71%	2160
May	1st to 10th	Day	2 2 1 2 3 1 3 3 3 5% 76%	2196
	11th to 20th	Day	2 2 1 2 3 1 2 3 3 6% 75%	2301
	21st to end	Day	2 2 1 2 3 1 3 3 3 5% 75%	2657
June	1st to 10th	Day	2 2 1 2 2 1 2 2 4 6% 77%	2520
	11th to 20th	Day	1 1 1 1 2 3 90%	2520
	21st to end	Day	1 1 1 1 1 2 1 2 3 88%	2520
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 89%	2520
	11th to 20th	Day	2 1 1 1 1 2 2 3 4 83%	2520
	21st to end	Day	1 1 1 2 2 1 2 2 4 5% 80%	2569
August	1st to 10th	Day	1 1 1 1 1 2 2 2 3 85%	2161
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	2122
	21st to end	Day	1 1 1 1 1 2 2 3 88%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	2160
	11th to 20th	Day	1 1 1 1 2 2 2 4 88%	2098
	21st to end	Day	1 1 1 1 2 1 1 1 2 4 84%	1980
October	1st to 10th	Day	4 2 1 2 2 2 2 3 4 5% 74%	1904
	11th to 20th	Day	2 2 1 2 2 2 2 3 4 4 77%	1800
	21st to end	Day	2 1 1 3 2 1 2 2 3 6% 75%	1974
November	1st to 10th	Day	6% 3 2 2 3 2 4 3 4 6% 65%	1674
	11th to 20th	Day	8% 5% 2 4 3 2 2 4 5% 5% 58%	1710
	21st to end	Day	8% 5% 3 3 4 2 4 3 5% 7% 56%	1710
December	1st to 10th	Day	9% 9% 4 6% 5% 3 3 4 5% 7% 46%	1677
	11th to 20th	Day	14% 8% 4 7% 7% 3 4 5% 6% 7% 35%	1928
	21st to end	Day	13% 13% 5% 8% 7% 3 5% 6% 5% 6% 29%	2794
January	1st to 10th	Night	13% 9% 3 7% 6% 3 4 4 4 7% 40%	4406
	11th to 20th	Night	10% 7% 4 6% 6% 3 4 4 6% 8% 42%	4189
	21st to end	Night	17% 7% 3 6% 5% 3 4 5% 5% 8% 38%	4521
February	1st to 10th	Night	13% 6% 2 4 3 2 3 3 3 7% 55%	2321
	11th to 20th	Night	10% 3 2 3 3 2 3 3 4 7% 59%	2218
	21st to end	Night	11% 4 2 4 4 2 3 5% 4 6% 55%	1884
March	1st to 10th	Night	9% 4 3 5% 4 2 4 5% 6% 9% 50%	2280
	11th to 20th	Night	8% 4 1 3 4 3 4 5% 5% 7% 54%	2147
	21st to end	Night	11% 4 2 4 4 2 3 3 3 7% 56%	2128
April	1st to 10th	Night	8% 3 2 4 4 2 4 3 4 6% 62%	1820
	11th to 20th	Night	5% 2 1 3 3 1 2 3 4 5% 70%	1779
	21st to end	Night	5% 2 1 1 2 1 2 3 4 6% 73%	1800
May	1st to 10th	Night	4 2 1 2 3 2 2 2 3 7% 72%	1760
	11th to 20th	Night	4 1 1 3 2 2 2 3 3 5% 76%	1594
	21st to end	Night	4 2 1 2 2 1 1 3 3 4 77%	1614
June	1st to 10th	Night	3 1 2 2 1 2 2 3 5% 79%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 3 88%	1440
	21st to end	Night	1 1 1 1 2 2 91%	1440
July	1st to 10th	Night	1 1 1 1 1 3 4 88%	1440
	11th to 20th	Night	1 1 2 1 3 2 2 3 84%	1440
	21st to end	Night	1 1 1 2 2 2 3 3 3 5% 77%	1745
August	1st to 10th	Night	2 2 1 1 1 1 1 1 2 3 86%	1776
	11th to 20th	Night	1 1 1 1 1 1 2 4 88%	1780
	21st to end	Night	1 1 1 1 1 2 2 3 89%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 89%	1800
	11th to 20th	Night	1 1 1 1 2 2 3 3 86%	1862
	21st to end	Night	1 1 1 1 1 1 2 1 2 4 85%	1980
October	1st to 10th	Night	3 1 1 2 2 2 2 3 3 6% 74%	2052
	11th to 20th	Night	2 2 1 2 2 1 2 3 4 5% 76%	2160
	21st to end	Night	3 2 1 2 2 1 2 2 3 3 80%	2380
November	1st to 10th	Night	5% 3 1 3 4 2 3 3 4 7% 66%	2405
	11th to 20th	Night	5% 2 1 3 3 2 3 3 5% 6% 66%	2470
	21st to end	Night	7% 4 2 4 4 2 3 4 5% 6% 58%	2470
December	1st to 10th	Night	8% 6% 2 5% 5% 2 4 4 5% 7% 52%	2499
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 5% 6% 8% 38%	3276
	21st to end	Night	11% 7% 3 8% 7% 4 5% 5% 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% 8% 14% 23% 18% 4	1900
	11th to 20th	Day	28% 6% 17% 25% 20% 4	2028
	21st to end	Day	27% 8% 15% 29% 17% 5%	2470
April	1st to 10th	Day	23% 4 13% 29% 22% 8%	2185
	11th to 20th	Day	22% 4 13% 28% 24% 10%	2132
	21st to end	Day	16% 5% 12% 30% 29% 8%	2160
May	1st to 10th	Day	12% 4 10% 25% 37% 13%	2196
	11th to 20th	Day	12% 3 8% 27% 34% 15%	2304
	21st to end	Day	14% 3 8% 30% 33% 12%	2657
June	1st to 10th	Day	11% 3 9% 20% 32% 24%	2520
	11th to 20th	Day	4 2 5% 13% 37% 39%	2520
	21st to end	Day	6% 1 2 9% 29% 53%	2520
July	1st to 10th	Day	5% 1 3 7% 23% 61%	2451
	11th to 20th	Day	7% 2 3 9% 27% 52%	2445
	21st to end	Day	9% 2 2 12% 28% 46%	2426
August	1st to 10th	Day	7% 2 3 7% 30% 51%	2146
	11th to 20th	Day	5% 1 2 10% 31% 51%	2122
	21st to end	Day	5% 2 2 8% 35% 49%	2376
September	1st to 10th	Day	6% 1 2 9% 28% 54%	2160
	11th to 20th	Day	5% 2 4 17% 42% 31%	2098
	21st to end	Day	8% 1 3 18% 44% 26%	1980
October	1st to 10th	Day	13% 3 7% 22% 27% 28%	1904
	11th to 20th	Day	12% 3 6% 23% 30% 25%	1800
	21st to end	Day	12% 4 7% 23% 31% 22%	1974
November	1st to 10th	Day	20% 5% 10% 27% 29% 10%	1674
	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	30% 7% 14% 25% 19% 5%	2280
	11th to 20th	Night	28% 7% 15% 25% 21% 4	2147
	21st to end	Night	30% 5% 13% 30% 17% 5%	2128
April	1st to 10th	Night	24% 4 13% 29% 22% 9%	1820
	11th to 20th	Night	18% 4 14% 27% 28% 10%	1779
	21st to end	Night	15% 4 11% 32% 33% 6%	1800
May	1st to 10th	Night	15% 4 9% 24% 34% 15%	1760
	11th to 20th	Night	14% 2 6% 23% 38% 16%	1595
	21st to end	Night	12% 4 10% 24% 35% 15%	1614
June	1st to 10th	Night	10% 3 8% 20% 33% 26%	1440
	11th to 20th	Night	4 2 5% 14% 34% 41%	1440
	21st to end	Night	3 1 2 7% 30% 57%	1440
July	1st to 10th	Night	4 1 2 6% 21% 65%	1400
	11th to 20th	Night	7% 2 3 9% 20% 59%	1392
	21st to end	Night	10% 4 4 9% 26% 48%	1648
August	1st to 10th	Night	8% 1 2 9% 28% 53%	1767
	11th to 20th	Night	4 1 3 9% 32% 51%	1780
	21st to end	Night	5% 1 1 7% 36% 50%	1980
September	1st to 10th	Night	6% 1 1 9% 27% 56%	1800
	11th to 20th	Night	6% 1 4 15% 40% 33%	1862
	21st to end	Night	6% 2 3 17% 40% 32%	1980
October	1st to 10th	Night	13% 3 5% 19% 28% 31%	2052
	11th to 20th	Night	13% 3 7% 22% 30% 25%	2160
	21st to end	Night	11% 3 5% 25% 34% 22%	2380
November	1st to 10th	Night	20% 4 9% 25% 28% 13%	2405
	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% 32% 24% 9%	506
April	1st to 10th	Day	10% 3 21% 27% 27% 14%	429
	11th to 20th	Day	6% 7% 21% 28% 27% 11%	412
	21st to end	Day	3 5% 20% 26% 26% 20%	420
May	1st to 10th	Day	4 5% 18% 29% 31% 14%	423
	11th to 20th	Day	3 2 17% 32% 27% 19%	448
	21st to end	Day	3 4 14% 22% 35% 21%	485
June	1st to 10th	Day	2 3 11% 25% 33% 27%	450
	11th to 20th	Day	2 1 12% 21% 38% 27%	450
	21st to end	Day	1 6% 24% 41% 27%	450
July	1st to 10th	Day	1 3 18% 46% 32%	446
	11th to 20th	Day	2 2 13% 36% 47%	444
	21st to end	Day	1 2 11% 47% 39%	487
August	1st to 10th	Day	2 8% 20% 42% 28%	447
	11th to 20th	Day	2 2 19% 42% 35%	437
	21st to end	Day	1 4 20% 43% 33%	495
September	1st to 10th	Day	2 1 4 22% 44% 28%	450
	11th to 20th	Day	1 1 7% 28% 42% 20%	450
	21st to end	Day	1 2 8% 23% 40% 26%	450
October	1st to 10th	Day	2 2 11% 26% 34% 24%	450
	11th to 20th	Day	3 3 9% 23% 37% 26%	450
	21st to end	Day	1 3 9% 25% 36% 26%	489
November	1st to 10th	Day	6% 2 9% 24% 33% 26%	270
	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	728
	11th to 20th	Night	5% 6% 21% 38% 27% 2	711
	21st to end	Night	6% 4 23% 35% 27% 5%	720
May	1st to 10th	Night	5% 5% 21% 40% 28% 3	680
	11th to 20th	Night	4 4 22% 40% 29% 1	531
	21st to end	Night	4 6% 26% 39% 25% 1	452
June	1st to 10th	Night	3 5% 20% 47% 24% 1	360
	11th to 20th	Night	4 16% 50% 28% 2	360
	21st to end	Night	1 1 16% 51% 30% 1	360
July	1st to 10th	Night	8% 50% 39% 2	350
	11th to 20th	Night	1 5% 41% 52% 1	348
	21st to end	Night	1 2 30% 57% 10%	563
August	1st to 10th	Night	3 7% 35% 50% 5%	702
	11th to 20th	Night	1 3 35% 55% 7%	712
	21st to end	Night	1 2 28% 59% 9%	792
September	1st to 10th	Night	1 1 5% 28% 58% 8%	720
	11th to 20th	Night	1 1 9% 36% 47% 6%	720
	21st to end	Night	2 1 8% 34% 48% 7%	720
October	1st to 10th	Night	4 1 9% 40% 40% 7%	720
	11th to 20th	Night	3 2 8% 36% 43% 8%	720
	21st to end	Night	4 1 7% 34% 40% 15%	792
November	1st to 10th	Night	5% 2 8% 33% 41% 11%	740
	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

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