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 2024-01-14.

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 5% 3 5% 5% 3 5% 5% 6% 7% 53%	74731
	Night	3 4 2 4 4 3 4 5% 6% 7% 58%	75039
Transparency	Day	28% 7% 11% 19% 24% 12%	74450
	Night	23% 7% 11% 20% 25% 13%	74842
Seeing	Day	14% 21% 30% 23% 10% 2	14039
	Night	9% 17% 31% 28% 14% 2	24259

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	1 2 2 4 3 2 3 5% 7% 9% 61%	1616
	11th to 20th	Day	2 6% 3 4 3 2 2 2 4 6% 65%	1647
	21st to end	Day	2 6% 2 6% 6% 3 4 4 4 5% 59%	1782
February	1st to 10th	Day	4 3 7% 5% 3 4 4 7% 7% 55%	1667
	11th to 20th	Day	1 6% 3 5% 6% 2 4 4 6% 7% 56%	1749
	21st to end	Day	1 8% 4 5% 4 3 4 5% 4 6% 56%	1480
March	1st to 10th	Day	2 7% 3 5% 6% 3 4 5% 6% 7% 53%	1849
	11th to 20th	Day	3 10% 4 7% 4 2 4 3 4 6% 52%	1980
	21st to end	Day	3 7% 3 5% 4 3 4 5% 5% 7% 53%	2272
April	1st to 10th	Day	2 5% 1 4 4 3 5% 4 7% 8% 57%	2160
	11th to 20th	Day	4 8% 3 6% 5% 3 3 4 3 6% 56%	2131
	21st to end	Day	4 7% 3 4 5% 2 4 4 5% 8% 54%	2160
May	1st to 10th	Day	5% 5% 3 4 5% 3 4 4 4 6% 55%	2160
	11th to 20th	Day	7% 7% 3 4 5% 3 6% 5% 4 6% 50%	2245
	21st to end	Day	4 4 2 5% 4 2 5% 4 6% 6% 57%	2724
June	1st to 10th	Day	3 3 2 4 4 2 5% 5% 7% 8% 56%	2520
	11th to 20th	Day	4 1 1 3 4 3 4 5% 5% 7% 63%	2520
	21st to end	Day	7% 4 2 5% 5% 4 6% 6% 7% 7% 48%	2520
July	1st to 10th	Day	7% 5% 3 7% 8% 4 8% 8% 8% 7% 36%	2520
	11th to 20th	Day	6% 5% 3 7% 6% 4 7% 8% 9% 10% 35%	2386
	21st to end	Day	10% 6% 3 9% 9% 6% 10% 8% 7% 8% 25%	2545
August	1st to 10th	Day	8% 5% 4 7% 8% 5% 7% 7% 7% 9% 32%	2247
	11th to 20th	Day	8% 6% 3 7% 8% 5% 7% 7% 7% 8% 35%	2123
	21st to end	Day	7% 7% 3 6% 7% 4 6% 7% 7% 7% 38%	2376
September	1st to 10th	Day	9% 6% 3 7% 6% 4 5% 6% 6% 7% 41%	2160
	11th to 20th	Day	7% 6% 2 5% 5% 4 5% 4 5% 6% 50%	2160
	21st to end	Day	4 3 2 5% 4 3 3 5% 5% 6% 61%	1976
October	1st to 10th	Day	10% 6% 3 5% 6% 3 3 4 4 5% 52%	1800
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 73%	1800
	21st to end	Day	4 5% 1 3 3 2 2 3 4 4 68%	1980
November	1st to 10th	Day	2 3 1 3 3 2 2 5% 4 7% 68%	1755
	11th to 20th	Day	1 3 2 3 3 2 4 4 5% 7% 65%	1710
	21st to end	Day	4 5% 2 5% 4 2 4 4 5% 6% 60%	1710
December	1st to 10th	Day	2 3 2 4 3 2 4 5% 6% 7% 61%	1581
	11th to 20th	Day	2 5% 3 4 6% 4 5% 5% 7% 7% 53%	1926
	21st to end	Day	1 6% 3 4 4 2 4 4 4 6% 63%	2794
January	1st to 10th	Night	4 1 4 3 3 3 4 5% 8% 64%	2565
	11th to 20th	Night	2 4 1 4 5% 2 3 4 4 6% 64%	2385
	21st to end	Night	2 5% 2 4 3 2 4 5% 7% 7% 58%	2574
February	1st to 10th	Night	6% 2 6% 3 2 3 4 6% 7% 61%	2290
	11th to 20th	Night	2 6% 3 5% 4 3 4 4 5% 8% 56%	2098
	21st to end	Night	2 6% 3 4 5% 3 3 3 5% 6% 60%	1776
March	1st to 10th	Night	2 5% 2 4 6% 2 4 5% 6% 8% 56%	2106
	11th to 20th	Night	3 6% 2 5% 5% 3 4 5% 5% 7% 56%	1980
	21st to end	Night	3 5% 2 5% 4 3 3 3 4 7% 60%	2083
April	1st to 10th	Night	1 3 1 3 4 3 4 4 5% 10% 62%	1800
	11th to 20th	Night	3 6% 2 5% 4 3 4 3 4 7% 59%	1777
	21st to end	Night	3 4 1 4 4 3 4 3 6% 6% 62%	1800
May	1st to 10th	Night	3 5% 1 3 4 2 4 5% 5% 7% 61%	1800
	11th to 20th	Night	6% 5% 3 5% 4 4 4 5% 5% 8% 50%	1642
	21st to end	Night	4 2 1 3 4 3 5% 4 5% 6% 61%	1552
June	1st to 10th	Night	2 3 2 5% 5% 4 6% 6% 8% 9% 51%	1440
	11th to 20th	Night	2 2 1 3 4 2 4 4 4 7% 66%	1440
	21st to end	Night	6% 4 2 3 5% 4 6% 6% 7% 8% 50%	1440
July	1st to 10th	Night	5% 3 3 6% 7% 5% 6% 7% 7% 9% 42%	1440
	11th to 20th	Night	3 4 2 6% 4 4 6% 8% 8% 12% 42%	1570
	21st to end	Night	8% 5% 3 6% 6% 4 8% 8% 9% 11% 33%	1773
August	1st to 10th	Night	4 3 3 5% 6% 4 8% 8% 9% 12% 39%	1690
	11th to 20th	Night	6% 4 2 7% 7% 4 7% 7% 9% 11% 36%	1778
	21st to end	Night	5% 4 2 4 5% 3 6% 7% 9% 13% 43%	1980
September	1st to 10th	Night	6% 3 3 4 5% 3 5% 5% 8% 8% 50%	1800
	11th to 20th	Night	6% 4 3 4 4 3 4 6% 5% 6% 54%	1800
	21st to end	Night	5% 4 2 3 4 3 3 4 5% 5% 64%	1980
October	1st to 10th	Night	8% 4 2 4 4 3 3 4 4 5% 60%	2160
	11th to 20th	Night	3 3 1 2 2 1 2 3 4 4 74%	2160
	21st to end	Night	4 3 1 3 3 2 3 4 5% 6% 67%	2376
November	1st to 10th	Night	2 2 1 2 3 2 3 3 4 6% 71%	2304
	11th to 20th	Night	1 2 1 2 2 3 4 4 5% 7% 70%	2470
	21st to end	Night	3 5% 2 4 4 3 4 3 6% 6% 60%	2470
December	1st to 10th	Night	2 4 2 4 3 3 3 5% 6% 7% 62%	2596
	11th to 20th	Night	2 5% 2 4 5% 4 5% 5% 8% 8% 52%	3271
	21st to end	Night	2 5% 3 5% 4 2 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	17% 8% 10% 25% 30% 10%	1617
	11th to 20th	Day	20% 5% 10% 25% 28% 11%	1647
	21st to end	Day	27% 4 7% 18% 31% 13%	1782
February	1st to 10th	Day	27% 7% 10% 18% 24% 13%	1667
	11th to 20th	Day	28% 6% 8% 23% 21% 14%	1759
	21st to end	Day	27% 5% 7% 15% 28% 18%	1480
March	1st to 10th	Day	25% 8% 9% 19% 25% 13%	1849
	11th to 20th	Day	31% 5% 9% 17% 29% 10%	1980
	21st to end	Day	28% 6% 10% 21% 27% 8%	2272
April	1st to 10th	Day	24% 5% 9% 22% 26% 14%	2160
	11th to 20th	Day	28% 6% 5% 18% 30% 12%	2134
	21st to end	Day	27% 6% 9% 22% 29% 8%	2160
May	1st to 10th	Day	28% 4 8% 19% 29% 12%	2160
	11th to 20th	Day	34% 6% 10% 17% 25% 8%	2249
	21st to end	Day	24% 6% 10% 17% 24% 19%	2724
June	1st to 10th	Day	22% 7% 13% 20% 23% 15%	2520
	11th to 20th	Day	19% 5% 8% 15% 25% 28%	2520
	21st to end	Day	30% 7% 11% 17% 20% 14%	2520
July	1st to 10th	Day	39% 11% 14% 12% 15% 9%	2451
	11th to 20th	Day	35% 11% 19% 20% 9% 6%	2314
	21st to end	Day	48% 13% 19% 15% 4 1	2404
August	1st to 10th	Day	42% 9% 17% 16% 10% 6%	2232
	11th to 20th	Day	40% 10% 15% 17% 14% 4	2123
	21st to end	Day	38% 10% 12% 22% 15% 3	2376
September	1st to 10th	Day	38% 7% 13% 18% 17% 6%	2160
	11th to 20th	Day	31% 6% 10% 18% 22% 12%	2160
	21st to end	Day	22% 5% 8% 23% 33% 10%	1976
October	1st to 10th	Day	31% 6% 7% 18% 23% 14%	1800
	11th to 20th	Day	17% 3 5% 14% 38% 24%	1800
	21st to end	Day	20% 3 8% 19% 36% 13%	1980
November	1st to 10th	Day	15% 4 9% 19% 36% 16%	1755
	11th to 20th	Day	17% 5% 8% 24% 33% 13%	1710
	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	17% 7% 11% 25% 28% 12%	2565
	11th to 20th	Night	20% 7% 8% 26% 25% 14%	2385
	21st to end	Night	23% 5% 9% 20% 31% 11%	2574
February	1st to 10th	Night	23% 6% 11% 21% 23% 15%	2290
	11th to 20th	Night	27% 5% 10% 24% 21% 13%	2098
	21st to end	Night	23% 6% 7% 15% 31% 17%	1776
March	1st to 10th	Night	23% 7% 10% 22% 23% 14%	2106
	11th to 20th	Night	25% 7% 12% 19% 29% 8%	1980
	21st to end	Night	22% 8% 10% 21% 32% 8%	2083
April	1st to 10th	Night	20% 6% 10% 24% 29% 12%	1800
	11th to 20th	Night	25% 5% 9% 20% 29% 13%	1777
	21st to end	Night	21% 9% 10% 23% 30% 8%	1800
May	1st to 10th	Night	21% 6% 10% 20% 31% 12%	1800
	11th to 20th	Night	30% 9% 12% 19% 21% 9%	1643
	21st to end	Night	20% 8% 13% 17% 25% 17%	1552
June	1st to 10th	Night	25% 9% 16% 18% 18% 15%	1440
	11th to 20th	Night	17% 5% 9% 16% 24% 29%	1440
	21st to end	Night	29% 8% 12% 16% 19% 15%	1440
July	1st to 10th	Night	34% 10% 17% 15% 11% 12%	1400
	11th to 20th	Night	26% 15% 23% 20% 10% 7%	1519
	21st to end	Night	36% 15% 29% 14% 5% 1	1674
August	1st to 10th	Night	30% 15% 22% 14% 11% 7%	1681
	11th to 20th	Night	34% 15% 18% 18% 11% 4	1778
	21st to end	Night	26% 13% 19% 23% 15% 3	1980
September	1st to 10th	Night	28% 8% 18% 23% 16% 7%	1800
	11th to 20th	Night	26% 9% 11% 16% 25% 13%	1800
	21st to end	Night	21% 7% 9% 23% 30% 11%	1980
October	1st to 10th	Night	26% 6% 9% 21% 23% 15%	2160
	11th to 20th	Night	14% 4 6% 17% 36% 23%	2160
	21st to end	Night	17% 5% 9% 22% 36% 13%	2376
November	1st to 10th	Night	13% 4 8% 24% 35% 15%	2304
	11th to 20th	Night	14% 5% 8% 25% 35% 14%	2470
	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	25% 28% 28% 16% 4	249
	21st to end	Day	13% 35% 33% 14% 4 1	264
February	1st to 10th	Day	17% 33% 34% 14% 3	240
	11th to 20th	Day	18% 40% 28% 13% 2	240
	21st to end	Day	38% 29% 24% 8% 1	198
March	1st to 10th	Day	23% 30% 35% 11% 1	289
	11th to 20th	Day	27% 29% 30% 12% 2 1	420
	21st to end	Day	28% 37% 27% 6% 2	462
April	1st to 10th	Day	21% 35% 34% 7% 2	420
	11th to 20th	Day	28% 38% 25% 8% 1	412
	21st to end	Day	23% 40% 28% 8% 1	420
May	1st to 10th	Day	16% 36% 38% 7% 2 1	423
	11th to 20th	Day	13% 23% 37% 18% 7% 1	535
	21st to end	Day	11% 23% 39% 22% 6%	679
June	1st to 10th	Day	6% 12% 36% 33% 12% 1	630
	11th to 20th	Day	8% 15% 34% 27% 14% 1	630
	21st to end	Day	8% 12% 29% 33% 14% 3	630
July	1st to 10th	Day	7% 3 24% 37% 24% 5%	621
	11th to 20th	Day	8% 2 15% 42% 26% 6%	487
	21st to end	Day	9% 1 14% 44% 27% 6%	487
August	1st to 10th	Day	6% 4 24% 45% 18% 3	447
	11th to 20th	Day	8% 5% 29% 40% 17% 2	437
	21st to end	Day	6% 5% 29% 39% 18% 2	495
September	1st to 10th	Day	9% 5% 31% 38% 14% 2	450
	11th to 20th	Day	8% 11% 39% 30% 11% 2	450
	21st to end	Day	5% 22% 32% 29% 9% 4	270
October	1st to 10th	Day	15% 25% 27% 21% 10% 2	270
	11th to 20th	Day	12% 26% 33% 20% 6% 3	270
	21st to end	Day	20% 20% 32% 19% 7% 2	297
November	1st to 10th	Day	17% 29% 32% 19% 3	270
	11th to 20th	Day	12% 25% 35% 20% 7%	270
	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	11% 22% 36% 20% 8% 2	734
	21st to end	Night	8% 25% 38% 18% 9% 2	792
February	1st to 10th	Night	9% 23% 42% 20% 5% 1	720
	11th to 20th	Night	11% 29% 36% 18% 5%	704
	21st to end	Night	15% 30% 40% 14% 1	592
March	1st to 10th	Night	13% 26% 35% 20% 5% 1	720
	11th to 20th	Night	11% 21% 32% 26% 10%	720
	21st to end	Night	14% 23% 39% 20% 4	697
April	1st to 10th	Night	12% 27% 39% 20% 3	540
	11th to 20th	Night	14% 26% 31% 24% 5%	533
	21st to end	Night	12% 25% 34% 23% 6%	540
May	1st to 10th	Night	7% 18% 39% 25% 11%	540
	11th to 20th	Night	9% 17% 31% 27% 14% 2	531
	21st to end	Night	9% 18% 31% 33% 8%	584
June	1st to 10th	Night	4 6% 34% 38% 17%	540
	11th to 20th	Night	2 11% 36% 37% 13% 1	540
	21st to end	Night	7% 5% 25% 38% 23% 1	540
July	1st to 10th	Night	5% 2 14% 42% 33% 4	525
	11th to 20th	Night	3 1 10% 43% 37% 7%	522
	21st to end	Night	8% 2 9% 31% 43% 8%	591
August	1st to 10th	Night	5% 1 10% 38% 37% 9%	537
	11th to 20th	Night	6% 3 12% 44% 30% 5%	534
	21st to end	Night	5% 2 16% 39% 33% 6%	594
September	1st to 10th	Night	6% 1 20% 43% 26% 3	540
	11th to 20th	Night	6% 4 20% 44% 25% 3	540
	21st to end	Night	7% 9% 24% 42% 18% 1	540
October	1st to 10th	Night	8% 12% 30% 33% 15% 1	720
	11th to 20th	Night	7% 14% 36% 27% 13% 3	720
	21st to end	Night	9% 16% 29% 26% 15% 4	792
November	1st to 10th	Night	9% 16% 33% 30% 12% 2	824
	11th to 20th	Night	7% 18% 37% 27% 10% 2	950
	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

Page generated at 2024-01-11 02:22:38. Version ac07

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