Coopers Hawk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coopers Hawk Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	11% 6% 3 5% 6% 3 5% 6% 6% 8% 41%	74927
	Night	11% 5% 2 5% 5% 3 5% 5% 6% 8% 44%	74808
Transparency	Day	38% 9% 14% 18% 16% 6%	74650
	Night	36% 8% 13% 19% 17% 7%	74604
Seeing	Day	12% 8% 22% 28% 21% 9%	14290
	Night	13% 7% 22% 32% 22% 4	24422

By Month

Percentage Cloud Cover SE (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	11% 5% 3 5% 5% 3 6% 6% 5% 7% 45%	1706
	11th to 20th	Day	12% 6% 3 5% 5% 2 5% 5% 5% 8% 46%	1638
	21st to end	Day	17% 6% 4 5% 4 2 2 4 4 6% 46%	1782
February	1st to 10th	Day	15% 7% 3 5% 5% 3 5% 4 6% 7% 40%	1673
	11th to 20th	Day	18% 5% 2 4 4 2 3 3 4 8% 47%	1748
	21st to end	Day	14% 7% 4 5% 4 2 3 4 5% 7% 45%	1562
March	1st to 10th	Day	12% 4 2 4 4 2 3 4 5% 6% 53%	1980
	11th to 20th	Day	12% 6% 2 5% 5% 2 4 5% 5% 7% 48%	1980
	21st to end	Day	12% 5% 3 5% 5% 3 5% 4 5% 8% 46%	2178
April	1st to 10th	Day	9% 4 2 4 5% 3 5% 6% 5% 7% 51%	1980
	11th to 20th	Day	12% 4 3 4 5% 3 3 3 5% 7% 49%	2093
	21st to end	Day	6% 4 2 3 4 3 4 6% 7% 8% 54%	2204
May	1st to 10th	Day	5% 4 2 4 5% 4 5% 6% 6% 9% 49%	2340
	11th to 20th	Day	9% 5% 2 5% 6% 4 6% 5% 7% 9% 42%	2301
	21st to end	Day	9% 5% 3 7% 6% 4 7% 6% 6% 9% 36%	2531
June	1st to 10th	Day	8% 6% 4 7% 8% 5% 7% 9% 8% 10% 30%	2340
	11th to 20th	Day	7% 4 3 6% 7% 5% 8% 8% 8% 11% 35%	2340
	21st to end	Day	6% 5% 3 7% 7% 5% 8% 7% 8% 12% 31%	2340
July	1st to 10th	Day	8% 6% 3 6% 8% 4 8% 8% 9% 12% 29%	2340
	11th to 20th	Day	8% 5% 2 8% 8% 5% 8% 9% 8% 12% 27%	2340
	21st to end	Day	6% 5% 3 7% 8% 5% 9% 8% 9% 12% 28%	2547
August	1st to 10th	Day	9% 6% 3 6% 8% 5% 8% 8% 9% 12% 27%	2327
	11th to 20th	Day	7% 6% 4 8% 8% 5% 7% 10% 9% 12% 24%	2292
	21st to end	Day	14% 8% 3 8% 8% 5% 6% 7% 7% 9% 24%	2337
September	1st to 10th	Day	9% 6% 3 7% 9% 6% 6% 7% 7% 9% 30%	1980
	11th to 20th	Day	13% 6% 3 6% 7% 4 6% 6% 7% 7% 34%	1980
	21st to end	Day	14% 6% 3 6% 6% 4 6% 5% 5% 8% 37%	1980
October	1st to 10th	Day	11% 6% 3 6% 5% 3 5% 4 4 6% 47%	1887
	11th to 20th	Day	9% 5% 2 3 3 2 3 4 4 7% 58%	1800
	21st to end	Day	11% 5% 2 4 4 3 4 3 4 6% 54%	1980
November	1st to 10th	Day	11% 4 2 4 4 2 4 4 5% 7% 53%	1857
	11th to 20th	Day	13% 7% 2 4 4 3 3 4 4 7% 51%	1867
	21st to end	Day	14% 6% 2 4 4 2 3 4 5% 7% 49%	1710
December	1st to 10th	Day	12% 6% 3 5% 5% 3 4 5% 6% 7% 44%	1710
	11th to 20th	Day	16% 4 3 4 4 2 3 3 4 6% 51%	2153
	21st to end	Day	17% 9% 3 5% 4 3 3 4 4 5% 43%	3124
January	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 4 5% 7% 46%	2459
	11th to 20th	Night	12% 7% 2 5% 5% 3 4 4 5% 7% 46%	2367
	21st to end	Night	16% 7% 3 5% 4 2 4 4 3 6% 45%	2574
February	1st to 10th	Night	17% 5% 3 5% 5% 3 4 4 5% 6% 42%	2285
	11th to 20th	Night	16% 7% 3 4 4 2 3 3 5% 6% 46%	2098
	21st to end	Night	13% 6% 3 5% 5% 3 4 5% 5% 5% 45%	1692
March	1st to 10th	Night	10% 5% 2 4 4 2 4 4 5% 7% 52%	1980
	11th to 20th	Night	12% 6% 2 4 4 3 4 4 6% 8% 46%	1980
	21st to end	Night	10% 5% 2 5% 5% 3 4 4 5% 7% 49%	2178
April	1st to 10th	Night	11% 5% 2 4 4 3 4 4 5% 7% 50%	1980
	11th to 20th	Night	12% 3 1 4 4 3 3 4 5% 7% 53%	1816
	21st to end	Night	7% 4 2 3 4 3 4 4 6% 9% 54%	1769
May	1st to 10th	Night	6% 4 2 3 4 2 5% 4 6% 8% 55%	1620
	11th to 20th	Night	7% 5% 2 5% 4 3 5% 5% 6% 9% 49%	1592
	21st to end	Night	8% 6% 2 5% 5% 3 6% 7% 7% 10% 42%	1744
June	1st to 10th	Night	9% 6% 2 5% 7% 3 6% 6% 8% 12% 35%	1620
	11th to 20th	Night	7% 5% 3 6% 5% 4 6% 6% 8% 12% 38%	1620
	21st to end	Night	6% 4 2 6% 5% 4 7% 8% 10% 14% 35%	1620
July	1st to 10th	Night	7% 5% 3 6% 7% 3 6% 7% 9% 12% 36%	1620
	11th to 20th	Night	7% 5% 2 6% 6% 4 6% 6% 9% 14% 34%	1620
	21st to end	Night	5% 4 3 6% 7% 4 7% 8% 8% 11% 37%	1771
August	1st to 10th	Night	6% 5% 3 7% 7% 4 6% 8% 9% 13% 33%	1611
	11th to 20th	Night	8% 5% 3 6% 6% 5% 8% 8% 9% 13% 29%	1598
	21st to end	Night	11% 7% 2 6% 7% 4 6% 7% 8% 10% 32%	2004
September	1st to 10th	Night	9% 6% 3 6% 6% 4 6% 7% 8% 9% 36%	1980
	11th to 20th	Night	9% 6% 2 7% 6% 4 5% 6% 6% 8% 40%	1980
	21st to end	Night	12% 7% 4 6% 7% 3 5% 5% 6% 7% 37%	1980
October	1st to 10th	Night	13% 6% 2 4 4 3 4 4 4 6% 49%	2071
	11th to 20th	Night	10% 5% 2 4 4 2 4 4 4 5% 56%	2160
	21st to end	Night	9% 4 2 5% 5% 3 4 4 4 6% 54%	2376
November	1st to 10th	Night	11% 4 2 4 3 2 3 4 5% 7% 54%	2221
	11th to 20th	Night	14% 6% 3 5% 3 2 4 4 4 6% 50%	2308
	21st to end	Night	12% 6% 2 5% 5% 3 5% 4 5% 6% 47%	2470
December	1st to 10th	Night	13% 5% 3 5% 6% 3 4 5% 5% 8% 42%	2470
	11th to 20th	Night	18% 5% 2 4 4 2 3 3 4 6% 48%	3042
	21st to end	Night	18% 7% 2 5% 5% 3 5% 5% 4 6% 40%	4532

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	36% 7% 10% 20% 21% 6%	1707
	11th to 20th	Day	36% 5% 10% 20% 22% 7%	1638
	21st to end	Day	41% 3 7% 18% 23% 8%	1782
February	1st to 10th	Day	41% 6% 10% 18% 17% 7%	1673
	11th to 20th	Day	38% 4 9% 16% 25% 8%	1758
	21st to end	Day	39% 5% 9% 18% 19% 10%	1562
March	1st to 10th	Day	32% 5% 10% 21% 20% 13%	1980
	11th to 20th	Day	36% 6% 14% 19% 17% 8%	1980
	21st to end	Day	38% 6% 11% 21% 17% 8%	2178
April	1st to 10th	Day	31% 6% 13% 22% 19% 10%	1980
	11th to 20th	Day	34% 5% 10% 21% 20% 10%	2093
	21st to end	Day	24% 7% 13% 29% 21% 6%	2204
May	1st to 10th	Day	28% 8% 14% 22% 21% 6%	2340
	11th to 20th	Day	36% 10% 17% 21% 14% 2	2305
	21st to end	Day	40% 11% 14% 20% 15% 1	2531
June	1st to 10th	Day	43% 12% 17% 16% 10% 1	2340
	11th to 20th	Day	36% 11% 21% 23% 8%	2340
	21st to end	Day	40% 14% 22% 15% 8% 1	2340
July	1st to 10th	Day	38% 13% 27% 15% 5% 2	2275
	11th to 20th	Day	43% 15% 24% 14% 4	2271
	21st to end	Day	40% 16% 23% 17% 3 1	2405
August	1st to 10th	Day	43% 16% 21% 16% 4	2314
	11th to 20th	Day	43% 18% 22% 13% 4	2292
	21st to end	Day	51% 12% 18% 13% 6%	2337
September	1st to 10th	Day	44% 12% 16% 14% 12% 3	1980
	11th to 20th	Day	44% 11% 13% 16% 12% 4	1980
	21st to end	Day	43% 9% 13% 16% 16% 4	1980
October	1st to 10th	Day	37% 6% 9% 18% 21% 9%	1887
	11th to 20th	Day	27% 4 10% 23% 27% 8%	1800
	21st to end	Day	31% 5% 8% 17% 25% 14%	1980
November	1st to 10th	Day	30% 6% 10% 16% 24% 14%	1857
	11th to 20th	Day	35% 5% 7% 17% 22% 12%	1867
	21st to end	Day	35% 5% 9% 22% 20% 9%	1710
December	1st to 10th	Day	38% 8% 12% 18% 15% 8%	1710
	11th to 20th	Day	35% 4 7% 21% 27% 6%	2150
	21st to end	Day	43% 5% 6% 18% 23% 4	3124
January	1st to 10th	Night	37% 7% 9% 18% 21% 8%	2459
	11th to 20th	Night	38% 6% 8% 16% 24% 9%	2367
	21st to end	Night	42% 4 7% 16% 22% 10%	2574
February	1st to 10th	Night	42% 4 9% 18% 18% 8%	2285
	11th to 20th	Night	38% 6% 9% 15% 21% 11%	2099
	21st to end	Night	40% 6% 10% 18% 15% 12%	1692
March	1st to 10th	Night	31% 6% 11% 18% 22% 12%	1980
	11th to 20th	Night	34% 8% 15% 18% 15% 10%	1980
	21st to end	Night	34% 6% 14% 20% 18% 8%	2178
April	1st to 10th	Night	33% 6% 12% 19% 20% 10%	1980
	11th to 20th	Night	31% 6% 9% 23% 20% 11%	1816
	21st to end	Night	27% 6% 12% 26% 23% 6%	1769
May	1st to 10th	Night	26% 7% 15% 23% 22% 7%	1620
	11th to 20th	Night	30% 10% 20% 22% 15% 2	1593
	21st to end	Night	35% 10% 14% 26% 13% 2	1745
June	1st to 10th	Night	38% 11% 22% 19% 9%	1620
	11th to 20th	Night	35% 13% 22% 24% 7%	1620
	21st to end	Night	33% 18% 25% 18% 5% 1	1620
July	1st to 10th	Night	35% 14% 24% 20% 5% 2	1575
	11th to 20th	Night	37% 15% 28% 16% 4	1566
	21st to end	Night	34% 14% 28% 21% 3	1672
August	1st to 10th	Night	38% 16% 24% 18% 4	1600
	11th to 20th	Night	42% 15% 23% 17% 2	1598
	21st to end	Night	44% 14% 22% 15% 5%	2004
September	1st to 10th	Night	40% 14% 18% 16% 10% 2	1980
	11th to 20th	Night	39% 12% 17% 17% 12% 3	1980
	21st to end	Night	43% 9% 14% 17% 15% 3	1980
October	1st to 10th	Night	36% 6% 10% 21% 20% 8%	2071
	11th to 20th	Night	29% 6% 9% 20% 25% 10%	2160
	21st to end	Night	31% 6% 7% 17% 24% 15%	2376
November	1st to 10th	Night	30% 5% 10% 16% 24% 14%	2221
	11th to 20th	Night	36% 5% 7% 18% 22% 12%	2308
	21st to end	Night	37% 5% 9% 19% 21% 9%	2470
December	1st to 10th	Night	39% 7% 14% 16% 17% 6%	2470
	11th to 20th	Night	38% 5% 8% 16% 26% 7%	3044
	21st to end	Night	44% 6% 7% 18% 20% 4	4532

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	12% 19% 43% 22% 3 1	270
	11th to 20th	Day	10% 17% 39% 21% 11% 2	246
	21st to end	Day	22% 20% 38% 18% 2	264
February	1st to 10th	Day	17% 18% 37% 21% 7%	240
	11th to 20th	Day	25% 16% 34% 23% 2	240
	21st to end	Day	15% 16% 39% 24% 6%	280
March	1st to 10th	Day	12% 16% 34% 24% 12% 1	420
	11th to 20th	Day	12% 13% 43% 21% 8% 3	420
	21st to end	Day	13% 14% 38% 24% 10% 1	462
April	1st to 10th	Day	14% 17% 30% 27% 11% 2	420
	11th to 20th	Day	12% 11% 31% 26% 13% 6%	412
	21st to end	Day	6% 13% 25% 34% 15% 7%	420
May	1st to 10th	Day	4 13% 29% 32% 15% 6%	423
	11th to 20th	Day	10% 4 23% 30% 23% 10%	447
	21st to end	Day	14% 5% 12% 31% 26% 14%	485
June	1st to 10th	Day	10% 2 16% 33% 26% 13%	450
	11th to 20th	Day	8% 3 13% 31% 27% 19%	450
	21st to end	Day	6% 3 12% 34% 31% 14%	450
July	1st to 10th	Day	8% 7% 26% 38% 21%	445
	11th to 20th	Day	7% 2 5% 24% 38% 23%	444
	21st to end	Day	5% 2 8% 34% 35% 16%	488
August	1st to 10th	Day	11% 3 8% 27% 35% 17%	446
	11th to 20th	Day	6% 3 5% 29% 37% 20%	439
	21st to end	Day	16% 2 7% 26% 29% 20%	495
September	1st to 10th	Day	10% 3 9% 27% 34% 17%	450
	11th to 20th	Day	16% 4 13% 29% 31% 8%	450
	21st to end	Day	15% 1 12% 30% 32% 10%	450
October	1st to 10th	Day	14% 4 19% 33% 19% 10%	450
	11th to 20th	Day	9% 5% 19% 34% 23% 9%	450
	21st to end	Day	16% 8% 20% 29% 22% 5%	495
November	1st to 10th	Day	15% 5% 24% 32% 19% 5%	455
	11th to 20th	Day	19% 8% 33% 26% 13% 1	427
	21st to end	Day	13% 13% 27% 34% 12% 1	270
December	1st to 10th	Day	17% 12% 31% 28% 11% 1	270
	11th to 20th	Day	19% 12% 32% 26% 9% 3	270
	21st to end	Day	24% 13% 28% 22% 13% 1	297
January	1st to 10th	Night	15% 15% 34% 29% 6%	760
	11th to 20th	Night	14% 14% 31% 30% 10% 1	729
	21st to end	Night	20% 14% 37% 25% 4	792
February	1st to 10th	Night	19% 11% 32% 31% 7%	720
	11th to 20th	Night	20% 14% 33% 29% 4	703
	21st to end	Night	16% 11% 35% 32% 6%	592
March	1st to 10th	Night	14% 11% 33% 34% 8%	720
	11th to 20th	Night	14% 11% 33% 32% 10%	720
	21st to end	Night	12% 14% 31% 32% 11% 1	792
April	1st to 10th	Night	12% 11% 33% 32% 11%	720
	11th to 20th	Night	12% 9% 30% 32% 16% 2	711
	21st to end	Night	8% 8% 29% 37% 17% 2	689
May	1st to 10th	Night	7% 6% 25% 39% 22% 1	540
	11th to 20th	Night	8% 3 21% 37% 26% 5%	531
	21st to end	Night	11% 2 12% 32% 35% 8%	584
June	1st to 10th	Night	12% 1 11% 39% 31% 6%	540
	11th to 20th	Night	8% 2 9% 35% 33% 13%	540
	21st to end	Night	7% 1 10% 37% 36% 9%	540
July	1st to 10th	Night	9% 2 8% 29% 43% 11%	525
	11th to 20th	Night	10% 4 26% 46% 13%	522
	21st to end	Night	5% 1 6% 37% 43% 7%	591
August	1st to 10th	Night	7% 1 6% 30% 45% 11%	537
	11th to 20th	Night	10% 1 7% 25% 46% 11%	534
	21st to end	Night	13% 2 9% 28% 36% 12%	782
September	1st to 10th	Night	11% 3 9% 27% 42% 8%	720
	11th to 20th	Night	12% 2 10% 36% 33% 7%	720
	21st to end	Night	17% 2 13% 31% 31% 6%	720
October	1st to 10th	Night	15% 3 19% 31% 24% 6%	720
	11th to 20th	Night	13% 4 21% 32% 26% 5%	720
	21st to end	Night	11% 6% 24% 31% 25% 4	792
November	1st to 10th	Night	13% 7% 21% 34% 22% 3	740
	11th to 20th	Night	16% 9% 27% 33% 14% 1	760
	21st to end	Night	14% 9% 27% 32% 17% 1	760
December	1st to 10th	Night	17% 10% 26% 31% 14% 1	760
	11th to 20th	Night	19% 9% 28% 28% 15% 1	760
	21st to end	Night	23% 10% 28% 28% 11%	836

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