Cowles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cowles clear sky chart from 2005-11-06 to 2024-11-12.

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 6% 2 6% 4 4 5% 4 6% 7% 52%	104905
	Night	3 5% 2 5% 4 4 4 4 6% 7% 56%	103006
Transparency	Day	26% 7% 11% 19% 24% 12%	104661
	Night	23% 8% 12% 20% 25% 12%	102840
Seeing	Day	15% 21% 30% 23% 9% 2	15204
	Night	10% 17% 31% 28% 13% 2	25287

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	3 3 9% 2 3 3 3 6% 8% 58%	2701
	11th to 20th	Day	1 4 2 7% 3 3 2 2 6% 6% 64%	2860
	21st to end	Day	2 9% 3 7% 5% 4 4 4 6% 6% 49%	3146
February	1st to 10th	Day	1 5% 3 8% 5% 4 4 3 7% 6% 54%	1759
	11th to 20th	Day	1 6% 3 6% 5% 3 4 3 6% 6% 56%	1849
	21st to end	Day	1 8% 4 5% 4 4 4 4 4 6% 56%	1570
March	1st to 10th	Day	1 6% 4 5% 3 3 3 3 6% 8% 59%	3221
	11th to 20th	Day	3 15% 4 8% 5% 4 4 3 6% 6% 42%	3490
	21st to end	Day	1 9% 4 6% 5% 4 4 3 5% 6% 54%	3996
April	1st to 10th	Day	2 6% 2 5% 4 5% 3 3 6% 7% 56%	2280
	11th to 20th	Day	3 8% 2 6% 4 3 3 3 4 6% 57%	2251
	21st to end	Day	3 8% 2 5% 4 3 4 3 5% 7% 55%	2280
May	1st to 10th	Day	3 5% 2 4 5% 3 3 3 4 5% 63%	3800
	11th to 20th	Day	7% 8% 2 5% 4 4 4 4 5% 6% 51%	3995
	21st to end	Day	2 3 1 4 3 2 3 3 5% 6% 69%	4825
June	1st to 10th	Day	3 3 2 5% 4 4 5% 5% 7% 8% 56%	2660
	11th to 20th	Day	3 2 1 4 4 3 4 4 5% 7% 63%	2660
	21st to end	Day	7% 4 2 6% 5% 6% 5% 5% 7% 7% 46%	2660
July	1st to 10th	Day	4 4 2 6% 5% 5% 7% 6% 8% 8% 46%	4430
	11th to 20th	Day	4 4 2 7% 6% 7% 9% 9% 11% 10% 32%	4184
	21st to end	Day	8% 7% 3 10% 8% 8% 8% 6% 8% 8% 27%	4454
August	1st to 10th	Day	5% 4 2 8% 7% 9% 8% 7% 9% 9% 31%	3946
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 9% 45%	3726
	21st to end	Day	5% 6% 2 8% 6% 7% 8% 5% 8% 9% 36%	4180
September	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 6% 7% 42%	2280
	11th to 20th	Day	7% 6% 2 6% 4 5% 4 4 5% 6% 50%	2280
	21st to end	Day	4 3 2 5% 4 4 4 4 4 6% 60%	2085
October	1st to 10th	Day	6% 4 2 5% 3 3 3 4 6% 6% 58%	3170
	11th to 20th	Day	10% 6% 2 4 2 3 3 3 4 6% 58%	3150
	21st to end	Day	3 3 1 3 2 2 2 1 3 3 76%	3438
November	1st to 10th	Day	2 4 1 4 2 2 3 4 5% 7% 66%	1849
	11th to 20th	Day	1 4 2 3 3 3 4 3 5% 7% 65%	1719
	21st to end	Day	4 6% 1 5% 3 3 3 4 5% 6% 60%	1710
December	1st to 10th	Day	2 4 2 4 3 3 4 4 6% 7% 61%	1581
	11th to 20th	Day	2 5% 2 5% 5% 5% 4 5% 7% 7% 53%	1926
	21st to end	Day	1 6% 2 4 4 3 4 3 5% 6% 62%	2794
January	1st to 10th	Night	4 1 9% 5% 4 4 4 6% 6% 56%	4283
	11th to 20th	Night	1 3 2 6% 4 4 4 4 9% 7% 57%	4110
	21st to end	Night	1 5% 2 5% 4 4 5% 5% 8% 9% 52%	4521
February	1st to 10th	Night	1 7% 2 7% 3 3 3 3 6% 7% 59%	2418
	11th to 20th	Night	1 6% 2 6% 3 4 4 3 5% 8% 57%	2218
	21st to end	Night	2 6% 2 5% 4 3 4 2 5% 6% 60%	1884
March	1st to 10th	Night	1 5% 2 4 4 3 3 3 5% 6% 63%	3641
	11th to 20th	Night	3 13% 3 6% 4 4 3 3 5% 6% 49%	3490
	21st to end	Night	2 6% 3 7% 4 5% 4 3 4 6% 57%	3650
April	1st to 10th	Night	1 4 2 4 4 4 3 3 5% 9% 61%	1900
	11th to 20th	Night	3 6% 2 5% 3 4 3 3 4 7% 60%	1877
	21st to end	Night	3 4 1 4 4 4 4 2 6% 6% 62%	1900
May	1st to 10th	Night	2 6% 2 5% 4 3 4 3 5% 6% 61%	3170
	11th to 20th	Night	6% 7% 2 6% 4 4 5% 4 6% 8% 50%	2889
	21st to end	Night	3 2 1 3 3 3 3 2 4 5% 69%	2762
June	1st to 10th	Night	2 3 2 5% 5% 5% 5% 5% 7% 8% 51%	1520
	11th to 20th	Night	2 2 1 4 3 3 4 4 4 6% 66%	1520
	21st to end	Night	7% 5% 2 3 5% 5% 6% 5% 6% 8% 48%	1520
July	1st to 10th	Night	4 5% 3 7% 6% 7% 6% 6% 8% 7% 42%	2540
	11th to 20th	Night	3 4 2 6% 3 5% 6% 6% 10% 15% 41%	2782
	21st to end	Night	7% 5% 2 6% 5% 4 7% 5% 9% 10% 41%	3109
August	1st to 10th	Night	3 5% 2 7% 6% 7% 9% 7% 10% 14% 31%	2991
	11th to 20th	Night	3 3 1 6% 4 5% 7% 5% 9% 12% 44%	3113
	21st to end	Night	3 4 1 6% 6% 7% 9% 7% 11% 13% 31%	3487
September	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 8% 8% 51%	1900
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 5% 6% 55%	1900
	21st to end	Night	5% 4 2 4 3 3 3 3 5% 5% 64%	2090
October	1st to 10th	Night	4 3 1 3 2 2 2 2 3 4 74%	3800
	11th to 20th	Night	9% 6% 1 2 2 2 2 3 4 5% 64%	3778
	21st to end	Night	3 2 1 2 2 1 2 2 4 6% 75%	4120
November	1st to 10th	Night	2 3 1 3 3 3 3 3 4 6% 70%	2429
	11th to 20th	Night	1 2 1 2 2 4 3 3 5% 6% 71%	2484
	21st to end	Night	3 6% 1 4 4 4 3 3 6% 6% 60%	2470
December	1st to 10th	Night	2 5% 2 4 3 3 3 4 6% 8% 61%	2596
	11th to 20th	Night	2 5% 2 5% 5% 5% 5% 5% 7% 8% 52%	3271
	21st to end	Night	2 6% 2 6% 3 3 3 3 5% 5% 62%	4873

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	21% 6% 11% 23% 31% 8%	2702
	11th to 20th	Day	16% 8% 13% 29% 27% 7%	2860
	21st to end	Day	28% 9% 9% 18% 27% 9%	3146
February	1st to 10th	Day	28% 7% 11% 17% 24% 13%	1759
	11th to 20th	Day	27% 6% 8% 22% 22% 14%	1859
	21st to end	Day	27% 5% 7% 15% 28% 18%	1570
March	1st to 10th	Day	20% 5% 8% 19% 36% 12%	3221
	11th to 20th	Day	31% 8% 14% 17% 25% 6%	3490
	21st to end	Day	24% 7% 12% 27% 24% 6%	3996
April	1st to 10th	Day	25% 5% 10% 21% 26% 14%	2280
	11th to 20th	Day	27% 6% 6% 18% 30% 14%	2254
	21st to end	Day	26% 6% 9% 22% 30% 8%	2280
May	1st to 10th	Day	19% 5% 10% 14% 38% 14%	3800
	11th to 20th	Day	29% 5% 12% 18% 27% 10%	3999
	21st to end	Day	15% 4 9% 13% 32% 28%	4825
June	1st to 10th	Day	21% 7% 13% 21% 23% 15%	2660
	11th to 20th	Day	19% 5% 8% 14% 25% 29%	2660
	21st to end	Day	32% 7% 11% 17% 19% 14%	2660
July	1st to 10th	Day	28% 10% 13% 13% 16% 19%	4361
	11th to 20th	Day	33% 14% 19% 25% 6% 3	4112
	21st to end	Day	44% 12% 17% 17% 7% 2	4314
August	1st to 10th	Day	40% 14% 20% 15% 9% 3	3932
	11th to 20th	Day	29% 9% 17% 23% 14% 7%	3762
	21st to end	Day	38% 11% 16% 21% 12% 2	4179
September	1st to 10th	Day	37% 7% 13% 19% 18% 7%	2280
	11th to 20th	Day	31% 6% 10% 18% 23% 12%	2280
	21st to end	Day	22% 5% 8% 22% 33% 10%	2085
October	1st to 10th	Day	22% 5% 8% 16% 33% 17%	3170
	11th to 20th	Day	27% 5% 5% 18% 30% 14%	3150
	21st to end	Day	14% 2 5% 14% 40% 25%	3438
November	1st to 10th	Day	16% 4 9% 19% 36% 16%	1849
	11th to 20th	Day	17% 5% 8% 24% 33% 13%	1719
	21st to end	Day	24% 4 7% 17% 29% 19%	1710
December	1st to 10th	Day	20% 7% 12% 26% 24% 11%	1581
	11th to 20th	Day	26% 8% 11% 15% 28% 13%	1924
	21st to end	Day	20% 5% 11% 16% 29% 19%	2794
January	1st to 10th	Night	23% 9% 12% 20% 26% 10%	4283
	11th to 20th	Night	16% 11% 14% 29% 21% 9%	4110
	21st to end	Night	24% 9% 10% 21% 26% 10%	4521
February	1st to 10th	Night	24% 6% 12% 20% 23% 15%	2418
	11th to 20th	Night	26% 5% 9% 24% 22% 14%	2218
	21st to end	Night	22% 7% 7% 15% 32% 17%	1884
March	1st to 10th	Night	18% 5% 11% 16% 38% 12%	3641
	11th to 20th	Night	30% 7% 12% 23% 23% 5%	3490
	21st to end	Night	21% 8% 12% 28% 26% 5%	3650
April	1st to 10th	Night	20% 6% 10% 23% 29% 13%	1900
	11th to 20th	Night	24% 5% 9% 19% 29% 14%	1877
	21st to end	Night	20% 9% 11% 23% 30% 7%	1900
May	1st to 10th	Night	17% 8% 13% 13% 36% 12%	3170
	11th to 20th	Night	27% 10% 12% 22% 18% 10%	2890
	21st to end	Night	14% 6% 10% 10% 30% 29%	2762
June	1st to 10th	Night	25% 8% 15% 18% 18% 15%	1520
	11th to 20th	Night	17% 5% 9% 15% 24% 29%	1520
	21st to end	Night	31% 8% 12% 16% 19% 15%	1520
July	1st to 10th	Night	33% 9% 15% 14% 10% 19%	2500
	11th to 20th	Night	25% 12% 28% 26% 5% 4	2731
	21st to end	Night	32% 11% 24% 25% 8%	3011
August	1st to 10th	Night	36% 14% 21% 15% 9% 4	2982
	11th to 20th	Night	27% 12% 21% 20% 15% 6%	3143
	21st to end	Night	34% 14% 19% 21% 9% 2	3487
September	1st to 10th	Night	27% 8% 17% 24% 17% 7%	1900
	11th to 20th	Night	26% 8% 11% 17% 26% 13%	1900
	21st to end	Night	21% 6% 9% 23% 31% 10%	2090
October	1st to 10th	Night	16% 4 6% 21% 38% 15%	3800
	11th to 20th	Night	23% 4 7% 21% 30% 16%	3778
	21st to end	Night	11% 3 6% 18% 38% 24%	4120
November	1st to 10th	Night	14% 4 9% 24% 34% 16%	2429
	11th to 20th	Night	14% 5% 8% 25% 35% 15%	2484
	21st to end	Night	24% 5% 6% 19% 27% 19%	2470
December	1st to 10th	Night	20% 9% 8% 28% 24% 11%	2596
	11th to 20th	Night	25% 7% 12% 16% 26% 14%	3272
	21st to end	Night	21% 7% 10% 17% 26% 19%	4873

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	19% 33% 27% 16% 5%	270
	11th to 20th	Day	28% 26% 27% 15% 3	270
	21st to end	Day	11% 33% 32% 16% 6% 2	297
February	1st to 10th	Day	20% 32% 33% 13% 3	270
	11th to 20th	Day	17% 41% 27% 13% 1	270
	21st to end	Day	39% 29% 24% 8% 1	225
March	1st to 10th	Day	24% 29% 34% 12% 1	322
	11th to 20th	Day	27% 29% 30% 12% 2 1	460
	21st to end	Day	29% 36% 28% 6% 2	506
April	1st to 10th	Day	22% 35% 33% 7% 2	460
	11th to 20th	Day	28% 38% 26% 7% 1	452
	21st to end	Day	23% 39% 30% 8% 1	460
May	1st to 10th	Day	18% 34% 38% 7% 2 1	463
	11th to 20th	Day	14% 23% 37% 19% 6% 1	581
	21st to end	Day	12% 23% 39% 21% 5%	734
June	1st to 10th	Day	6% 12% 35% 34% 12% 1	680
	11th to 20th	Day	9% 17% 34% 26% 13% 1	680
	21st to end	Day	9% 13% 30% 32% 14% 3	680
July	1st to 10th	Day	7% 6% 25% 36% 22% 4	671
	11th to 20th	Day	7% 2 15% 43% 27% 6%	529
	21st to end	Day	9% 3 15% 43% 25% 6%	531
August	1st to 10th	Day	6% 5% 24% 44% 18% 3	487
	11th to 20th	Day	8% 5% 29% 39% 17% 2	477
	21st to end	Day	6% 6% 31% 37% 17% 2	539
September	1st to 10th	Day	10% 6% 31% 37% 14% 2	490
	11th to 20th	Day	8% 14% 37% 29% 10% 2	490
	21st to end	Day	5% 21% 33% 28% 9% 4	300
October	1st to 10th	Day	14% 24% 28% 23% 10% 2	300
	11th to 20th	Day	15% 26% 32% 19% 6% 2	300
	21st to end	Day	22% 20% 32% 18% 7% 2	330
November	1st to 10th	Day	20% 29% 30% 18% 3	300
	11th to 20th	Day	12% 25% 35% 20% 7%	273
	21st to end	Day	16% 33% 29% 15% 7%	270
December	1st to 10th	Day	21% 28% 30% 16% 6%	270
	11th to 20th	Day	18% 32% 31% 16% 3	270
	21st to end	Day	30% 32% 27% 10% 2	297
January	1st to 10th	Night	10% 23% 34% 22% 9% 2	868
	11th to 20th	Night	12% 23% 36% 20% 8% 2	760
	21st to end	Night	8% 24% 38% 20% 9% 2	836
February	1st to 10th	Night	11% 23% 41% 19% 5% 1	760
	11th to 20th	Night	11% 28% 38% 18% 5%	744
	21st to end	Night	16% 30% 40% 14% 1	628
March	1st to 10th	Night	13% 26% 36% 19% 5% 1	760
	11th to 20th	Night	12% 20% 33% 26% 9%	760
	21st to end	Night	15% 24% 39% 19% 4	735
April	1st to 10th	Night	13% 27% 38% 19% 2	570
	11th to 20th	Night	14% 27% 31% 23% 5%	563
	21st to end	Night	11% 25% 35% 23% 5%	570
May	1st to 10th	Night	9% 18% 39% 24% 10%	570
	11th to 20th	Night	10% 17% 31% 28% 13% 1	561
	21st to end	Night	10% 18% 31% 33% 8%	617
June	1st to 10th	Night	4 7% 34% 39% 16%	570
	11th to 20th	Night	3 11% 36% 37% 12% 1	570
	21st to end	Night	8% 6% 25% 37% 22% 1	570
July	1st to 10th	Night	5% 4 15% 41% 32% 4	555
	11th to 20th	Night	3 1 10% 43% 37% 6%	552
	21st to end	Night	8% 1 11% 32% 40% 8%	624
August	1st to 10th	Night	5% 1 10% 38% 36% 9%	567
	11th to 20th	Night	6% 3 13% 45% 29% 4	564
	21st to end	Night	5% 2 16% 39% 32% 6%	627
September	1st to 10th	Night	6% 1 21% 42% 26% 3	570
	11th to 20th	Night	6% 4 22% 42% 24% 2	570
	21st to end	Night	6% 9% 25% 42% 17% 1	570
October	1st to 10th	Night	8% 11% 31% 34% 14% 1	760
	11th to 20th	Night	8% 13% 35% 27% 13% 3	760
	21st to end	Night	11% 17% 29% 25% 14% 4	836
November	1st to 10th	Night	11% 16% 32% 28% 11% 2	869
	11th to 20th	Night	7% 18% 37% 27% 10% 2	956
	21st to end	Night	12% 20% 32% 23% 10% 2	950
December	1st to 10th	Night	12% 23% 35% 22% 6% 1	950
	11th to 20th	Night	13% 25% 33% 21% 7% 1	950
	21st to end	Night	15% 24% 39% 18% 4	1045

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