Donnelly
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Donnelly clear sky chart from 2005-11-06 to 2024-03-01.

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% 11% 4 7% 6% 3 4 4 5% 6% 41%	78250
	Night	8% 12% 4 7% 5% 3 4 4 5% 6% 41%	81151
Transparency	Day	43% 6% 10% 19% 15% 7%	77954
	Night	43% 6% 11% 21% 14% 5%	80968
Seeing	Day	14% 17% 29% 25% 12% 3	13400
	Night	15% 7% 19% 36% 21% 3	22997

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	6% 16% 11% 15% 7% 4 5% 5% 5% 6% 21%	2539
	11th to 20th	Day	9% 18% 8% 13% 7% 3 4 4 3 5% 27%	2540
	21st to end	Day	11% 23% 6% 7% 6% 3 4 4 5% 8% 25%	2794
February	1st to 10th	Day	9% 18% 7% 13% 7% 3 4 5% 3 5% 27%	1611
	11th to 20th	Day	9% 18% 8% 9% 7% 3 5% 5% 5% 5% 26%	1850
	21st to end	Day	8% 20% 8% 14% 7% 3 4 4 5% 5% 21%	1570
March	1st to 10th	Day	10% 17% 8% 13% 7% 4 5% 5% 6% 4 21%	1801
	11th to 20th	Day	10% 17% 6% 9% 7% 3 5% 4 4 7% 27%	1902
	21st to end	Day	10% 20% 7% 12% 7% 4 5% 4 4 6% 21%	2275
April	1st to 10th	Day	9% 17% 6% 10% 8% 3 5% 5% 5% 7% 24%	2160
	11th to 20th	Day	8% 14% 5% 9% 6% 4 5% 4 5% 6% 35%	2193
	21st to end	Day	10% 14% 4 8% 6% 3 5% 5% 5% 6% 34%	2340
May	1st to 10th	Day	6% 9% 5% 7% 6% 3 5% 6% 6% 7% 40%	2340
	11th to 20th	Day	10% 9% 4 7% 6% 4 6% 5% 6% 9% 35%	2413
	21st to end	Day	11% 11% 5% 8% 8% 5% 5% 5% 6% 7% 28%	2723
June	1st to 10th	Day	12% 9% 3 7% 8% 4 6% 6% 6% 8% 30%	2520
	11th to 20th	Day	12% 8% 3 7% 6% 3 5% 4 5% 6% 42%	2520
	21st to end	Day	4 4 2 4 4 2 3 3 4 6% 63%	2520
July	1st to 10th	Day	2 2 1 3 3 2 4 3 5% 7% 67%	2520
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 5% 75%	2520
	21st to end	Day	2 2 1 3 3 2 3 3 4 5% 71%	2741
August	1st to 10th	Day	4 3 1 4 4 3 3 4 5% 7% 62%	2440
	11th to 20th	Day	2 2 1 3 3 2 3 4 4 6% 71%	2299
	21st to end	Day	4 2 1 3 3 2 3 4 4 6% 68%	2426
September	1st to 10th	Day	3 3 1 3 3 2 4 3 5% 6% 69%	2160
	11th to 20th	Day	6% 4 2 4 4 3 5% 5% 4 6% 57%	1996
	21st to end	Day	6% 5% 2 4 4 3 4 5% 4 6% 57%	1980
October	1st to 10th	Day	10% 8% 4 5% 4 2 5% 4 5% 5% 47%	1980
	11th to 20th	Day	11% 9% 2 6% 4 2 3 3 4 5% 50%	1714
	21st to end	Day	10% 12% 3 8% 5% 3 5% 5% 5% 6% 39%	1782
November	1st to 10th	Day	14% 14% 4 8% 5% 3 4 4 5% 8% 32%	1674
	11th to 20th	Day	13% 13% 4 8% 7% 3 4 4 5% 6% 32%	1710
	21st to end	Day	10% 13% 6% 7% 6% 3 4 5% 6% 5% 34%	1632
December	1st to 10th	Day	11% 18% 6% 9% 5% 3 3 3 5% 7% 30%	1410
	11th to 20th	Day	10% 21% 7% 10% 6% 4 4 4 5% 6% 21%	1861
	21st to end	Day	10% 13% 6% 10% 7% 3 5% 4 5% 6% 31%	2794
January	1st to 10th	Night	8% 17% 7% 13% 8% 4 4 5% 5% 6% 25%	4406
	11th to 20th	Night	10% 18% 7% 13% 7% 3 4 3 4 6% 26%	4420
	21st to end	Night	9% 26% 6% 8% 5% 3 4 3 3 4 27%	4862
February	1st to 10th	Night	10% 18% 7% 9% 7% 3 5% 5% 4 5% 28%	2566
	11th to 20th	Night	8% 17% 5% 11% 6% 4 4 3 5% 6% 31%	2218
	21st to end	Night	9% 16% 8% 12% 8% 4 5% 4 4 5% 25%	1884
March	1st to 10th	Night	10% 15% 7% 8% 6% 3 4 4 4 6% 31%	2165
	11th to 20th	Night	8% 13% 6% 8% 5% 3 6% 5% 5% 7% 32%	2057
	21st to end	Night	10% 14% 4 7% 7% 4 6% 5% 7% 7% 29%	2080
April	1st to 10th	Night	10% 11% 4 8% 5% 4 6% 5% 5% 7% 36%	1800
	11th to 20th	Night	10% 10% 4 8% 5% 3 5% 4 5% 6% 39%	1714
	21st to end	Night	7% 8% 3 7% 6% 3 4 4 6% 7% 46%	1620
May	1st to 10th	Night	6% 7% 3 5% 4 2 5% 5% 6% 8% 49%	1620
	11th to 20th	Night	6% 7% 3 5% 6% 3 6% 6% 8% 8% 42%	1477
	21st to end	Night	10% 8% 3 7% 6% 4 5% 5% 7% 8% 37%	1552
June	1st to 10th	Night	12% 6% 2 5% 5% 4 4 6% 7% 9% 39%	1440
	11th to 20th	Night	10% 5% 2 4 4 2 4 3 6% 8% 52%	1440
	21st to end	Night	4 3 1 4 3 2 4 5% 4 6% 63%	1440
July	1st to 10th	Night	2 1 1 3 2 2 4 4 5% 8% 69%	1440
	11th to 20th	Night	1 1 1 2 2 1 3 3 5% 6% 75%	1440
	21st to end	Night	2 1 1 2 3 2 3 3 4 6% 73%	1576
August	1st to 10th	Night	3 3 2 3 4 3 3 4 4 7% 65%	1498
	11th to 20th	Night	1 1 1 1 3 1 3 3 5% 6% 75%	1602
	21st to end	Night	3 2 1 3 3 2 3 4 4 6% 69%	1926
September	1st to 10th	Night	3 3 1 2 3 2 2 3 4 6% 71%	1800
	11th to 20th	Night	6% 3 1 3 4 2 4 5% 5% 6% 60%	1962
	21st to end	Night	7% 4 2 3 3 2 3 4 4 6% 62%	1980
October	1st to 10th	Night	7% 6% 1 5% 5% 3 5% 5% 6% 6% 52%	1980
	11th to 20th	Night	9% 7% 3 5% 5% 3 3 5% 4 8% 49%	2244
	21st to end	Night	10% 9% 3 5% 5% 3 5% 5% 6% 7% 42%	2574
November	1st to 10th	Night	13% 11% 4 7% 5% 3 3 5% 5% 7% 36%	2405
	11th to 20th	Night	15% 13% 3 6% 6% 3 4 4 4 7% 35%	2470
	21st to end	Night	10% 13% 4 7% 6% 3 5% 4 4 6% 39%	2546
December	1st to 10th	Night	10% 14% 6% 7% 6% 3 4 4 6% 6% 33%	2737
	11th to 20th	Night	10% 18% 6% 10% 7% 4 5% 5% 5% 5% 25%	3348
	21st to end	Night	8% 14% 5% 9% 6% 3 5% 4 5% 7% 32%	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	62% 10% 12% 14% 2 1	2539
	11th to 20th	Day	60% 6% 8% 15% 9% 2	2540
	21st to end	Day	59% 8% 15% 13% 4 1	2794
February	1st to 10th	Day	60% 7% 10% 17% 4 1	1611
	11th to 20th	Day	59% 8% 10% 15% 7%	1860
	21st to end	Day	64% 6% 9% 12% 7% 2	1570
March	1st to 10th	Day	62% 7% 11% 13% 7% 1	1801
	11th to 20th	Day	58% 5% 11% 20% 7%	1902
	21st to end	Day	64% 6% 11% 15% 4	2275
April	1st to 10th	Day	57% 8% 12% 16% 6% 1	2160
	11th to 20th	Day	50% 5% 12% 20% 12% 2	2196
	21st to end	Day	49% 8% 13% 21% 10%	2340
May	1st to 10th	Day	40% 7% 11% 27% 13% 1	2340
	11th to 20th	Day	45% 7% 12% 25% 11% 1	2416
	21st to end	Day	52% 9% 12% 19% 9% 1	2724
June	1st to 10th	Day	49% 8% 11% 22% 9%	2520
	11th to 20th	Day	41% 6% 8% 22% 18% 5%	2520
	21st to end	Day	23% 4 7% 23% 33% 10%	2520
July	1st to 10th	Day	16% 5% 5% 19% 30% 25%	2451
	11th to 20th	Day	13% 3 4 13% 33% 35%	2445
	21st to end	Day	15% 4 3 13% 30% 36%	2589
August	1st to 10th	Day	21% 6% 6% 20% 27% 21%	2423
	11th to 20th	Day	14% 4 6% 22% 34% 20%	2299
	21st to end	Day	17% 5% 5% 22% 34% 17%	2426
September	1st to 10th	Day	17% 4 6% 21% 30% 22%	2160
	11th to 20th	Day	27% 5% 7% 23% 27% 11%	1996
	21st to end	Day	28% 4 8% 29% 24% 6%	1980
October	1st to 10th	Day	38% 5% 10% 25% 18% 4	1980
	11th to 20th	Day	37% 4 11% 30% 17% 2	1714
	21st to end	Day	45% 8% 11% 21% 15% 1	1782
November	1st to 10th	Day	52% 6% 12% 20% 9% 1	1674
	11th to 20th	Day	52% 6% 15% 18% 8% 1	1710
	21st to end	Day	51% 6% 13% 17% 12% 2	1632
December	1st to 10th	Day	55% 6% 15% 14% 8% 2	1410
	11th to 20th	Day	62% 8% 11% 12% 6%	1861
	21st to end	Day	51% 8% 12% 19% 9% 2	2794
January	1st to 10th	Night	58% 8% 13% 17% 3 1	4406
	11th to 20th	Night	60% 6% 8% 16% 9% 1	4420
	21st to end	Night	60% 7% 12% 15% 5% 1	4862
February	1st to 10th	Night	58% 7% 13% 16% 5% 1	2566
	11th to 20th	Night	54% 7% 12% 17% 8% 1	2219
	21st to end	Night	61% 6% 9% 15% 7% 2	1884
March	1st to 10th	Night	55% 5% 14% 18% 8% 1	2165
	11th to 20th	Night	50% 8% 13% 21% 8% 1	2057
	21st to end	Night	51% 10% 17% 17% 6%	2080
April	1st to 10th	Night	47% 7% 15% 22% 10% 1	1800
	11th to 20th	Night	44% 5% 12% 24% 13% 2	1714
	21st to end	Night	37% 8% 15% 27% 13%	1620
May	1st to 10th	Night	30% 7% 13% 33% 17%	1620
	11th to 20th	Night	35% 9% 16% 29% 12%	1477
	21st to end	Night	42% 7% 16% 25% 9%	1552
June	1st to 10th	Night	39% 8% 17% 28% 8%	1440
	11th to 20th	Night	30% 6% 12% 29% 18% 5%	1440
	21st to end	Night	22% 4 9% 26% 30% 10%	1440
July	1st to 10th	Night	14% 4 7% 21% 35% 20%	1400
	11th to 20th	Night	11% 3 5% 21% 28% 32%	1392
	21st to end	Night	13% 3 4 12% 32% 35%	1488
August	1st to 10th	Night	20% 4 5% 22% 29% 20%	1490
	11th to 20th	Night	12% 4 6% 22% 36% 21%	1602
	21st to end	Night	17% 4 7% 20% 35% 18%	1926
September	1st to 10th	Night	17% 3 6% 23% 29% 22%	1800
	11th to 20th	Night	23% 4 9% 23% 26% 14%	1962
	21st to end	Night	23% 4 7% 31% 25% 9%	1980
October	1st to 10th	Night	30% 7% 10% 27% 21% 5%	1980
	11th to 20th	Night	34% 5% 12% 28% 19% 3	2244
	21st to end	Night	39% 7% 13% 23% 16% 2	2574
November	1st to 10th	Night	47% 7% 12% 22% 11% 2	2405
	11th to 20th	Night	50% 6% 13% 22% 9%	2470
	21st to end	Night	47% 6% 11% 20% 13% 3	2546
December	1st to 10th	Night	51% 7% 14% 16% 10% 2	2737
	11th to 20th	Night	60% 7% 10% 17% 5% 1	3348
	21st to end	Night	50% 7% 15% 17% 9% 3	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	21% 11% 31% 24% 11% 3	180
	11th to 20th	Day	21% 18% 21% 20% 12% 8%	180
	21st to end	Day	20% 15% 28% 21% 11% 5%	198
February	1st to 10th	Day	23% 20% 28% 18% 9% 2	233
	11th to 20th	Day	20% 22% 32% 18% 8%	455
	21st to end	Day	23% 24% 26% 19% 7% 1	382
March	1st to 10th	Day	21% 23% 38% 13% 4 1	421
	11th to 20th	Day	20% 20% 31% 21% 9%	420
	21st to end	Day	22% 23% 30% 18% 7%	462
April	1st to 10th	Day	23% 20% 26% 19% 9% 2	420
	11th to 20th	Day	20% 29% 25% 17% 7% 1	412
	21st to end	Day	19% 20% 31% 20% 8% 2	420
May	1st to 10th	Day	11% 21% 30% 27% 10% 1	423
	11th to 20th	Day	19% 17% 26% 24% 10% 3	448
	21st to end	Day	16% 15% 21% 29% 14% 5%	485
June	1st to 10th	Day	16% 19% 23% 24% 14% 3	450
	11th to 20th	Day	19% 21% 29% 19% 10% 1	450
	21st to end	Day	5% 14% 32% 33% 15% 2	450
July	1st to 10th	Day	4 13% 37% 31% 13% 2	446
	11th to 20th	Day	4 14% 39% 29% 12% 2	444
	21st to end	Day	4 9% 37% 32% 16% 2	487
August	1st to 10th	Day	6% 10% 34% 33% 13% 4	447
	11th to 20th	Day	2 8% 28% 39% 18% 4	437
	21st to end	Day	5% 13% 31% 32% 15% 4	495
September	1st to 10th	Day	6% 13% 29% 32% 18% 4	450
	11th to 20th	Day	9% 12% 28% 32% 14% 4	450
	21st to end	Day	9% 14% 32% 29% 14% 2	450
October	1st to 10th	Day	14% 16% 28% 23% 16% 2	450
	11th to 20th	Day	15% 15% 26% 23% 16% 6%	328
	21st to end	Day	18% 20% 23% 28% 9% 3	297
November	1st to 10th	Day	25% 14% 20% 29% 11% 1	270
	11th to 20th	Day	15% 18% 23% 30% 11% 3	270
	21st to end	Day	16% 12% 25% 28% 16% 3	232
December	1st to 10th	Day	23% 18% 19% 18% 16% 6%	180
	11th to 20th	Day	27% 19% 24% 16% 10% 3	180
	21st to end	Day	20% 16% 26% 25% 9% 4	198
January	1st to 10th	Night	23% 9% 21% 30% 16% 2	760
	11th to 20th	Night	22% 8% 21% 29% 17% 3	760
	21st to end	Night	17% 9% 22% 30% 19% 3	836
February	1st to 10th	Night	22% 8% 24% 33% 10% 2	760
	11th to 20th	Night	20% 9% 23% 32% 15% 2	744
	21st to end	Night	19% 12% 22% 32% 12% 2	628
March	1st to 10th	Night	18% 13% 23% 30% 13% 2	722
	11th to 20th	Night	17% 9% 21% 35% 16% 2	720
	21st to end	Night	19% 11% 21% 36% 12% 1	792
April	1st to 10th	Night	15% 11% 21% 31% 19% 3	720
	11th to 20th	Night	15% 13% 27% 28% 15% 2	646
	21st to end	Night	11% 9% 23% 38% 18% 2	540
May	1st to 10th	Night	9% 8% 22% 38% 21% 1	540
	11th to 20th	Night	9% 6% 20% 36% 25% 4	415
	21st to end	Night	12% 6% 16% 34% 28% 3	389
June	1st to 10th	Night	14% 6% 10% 41% 26% 2	360
	11th to 20th	Night	13% 5% 16% 44% 22% 2	360
	21st to end	Night	5% 2 12% 45% 35% 1	360
July	1st to 10th	Night	1 2 9% 52% 33% 2	350
	11th to 20th	Night	1 1 11% 60% 26% 1	348
	21st to end	Night	2 10% 52% 35% 1	394
August	1st to 10th	Night	4 2 9% 46% 35% 3	424
	11th to 20th	Night	1 2 9% 46% 37% 4	534
	21st to end	Night	4 2 12% 42% 36% 4	738
September	1st to 10th	Night	5% 2 12% 44% 34% 2	720
	11th to 20th	Night	8% 2 11% 39% 34% 6%	720
	21st to end	Night	10% 3 15% 36% 30% 6%	720
October	1st to 10th	Night	10% 6% 17% 38% 28% 2	720
	11th to 20th	Night	14% 6% 16% 36% 22% 5%	720
	21st to end	Night	16% 6% 21% 35% 18% 4	792
November	1st to 10th	Night	20% 7% 18% 39% 15% 2	740
	11th to 20th	Night	24% 8% 22% 30% 15% 2	760
	21st to end	Night	17% 8% 23% 30% 18% 4	760
December	1st to 10th	Night	18% 9% 22% 33% 16% 2	837
	11th to 20th	Night	24% 11% 19% 30% 14% 2	832
	21st to end	Night	24% 11% 23% 27% 13% 1	836

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