The Clear Nights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Clear Nights Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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	13% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	79128
	Night	13% 5% 2 5% 5% 3 4 5% 5% 8% 45%	80770
Transparency	Day	39% 7% 11% 19% 19% 6%	78851
	Night	37% 6% 10% 18% 20% 7%	80566
Seeing	Day	15% 10% 22% 24% 20% 10%	14047
	Night	15% 9% 22% 29% 22% 4	24193

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	17% 7% 3 4 5% 2 4 5% 6% 6% 41%	2836
	11th to 20th	Day	11% 6% 2 3 3 2 3 3 5% 7% 54%	2840
	21st to end	Day	25% 8% 3 4 4 2 2 3 3 5% 41%	3251
February	1st to 10th	Day	16% 7% 3 6% 6% 3 4 5% 4 7% 39%	1900
	11th to 20th	Day	19% 7% 2 5% 4 2 5% 4 4 5% 43%	1848
	21st to end	Day	18% 7% 3 6% 5% 3 4 4 5% 4 41%	1570
March	1st to 10th	Day	15% 5% 2 6% 4 2 3 4 4 7% 47%	1978
	11th to 20th	Day	17% 6% 2 5% 4 3 5% 5% 5% 7% 41%	2090
	21st to end	Day	15% 6% 3 5% 5% 3 4 5% 6% 7% 43%	2200
April	1st to 10th	Day	11% 6% 3 5% 5% 3 4 5% 4 6% 48%	1988
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 4 5% 7% 48%	2134
	21st to end	Day	8% 4 2 5% 4 3 5% 4 5% 7% 53%	2309
May	1st to 10th	Day	8% 4 2 4 5% 4 4 4 5% 8% 50%	2340
	11th to 20th	Day	10% 6% 3 5% 6% 4 6% 6% 6% 9% 39%	2301
	21st to end	Day	10% 5% 2 5% 5% 3 5% 6% 7% 10% 42%	2531
June	1st to 10th	Day	11% 6% 2 7% 6% 4 7% 7% 7% 9% 34%	2340
	11th to 20th	Day	6% 3 2 5% 6% 4 7% 7% 7% 11% 42%	2340
	21st to end	Day	6% 5% 3 5% 6% 4 8% 7% 7% 10% 38%	2340
July	1st to 10th	Day	9% 5% 3 7% 7% 5% 7% 8% 10% 11% 27%	2340
	11th to 20th	Day	8% 5% 3 6% 7% 5% 6% 9% 9% 11% 30%	2340
	21st to end	Day	6% 5% 3 6% 7% 5% 8% 8% 10% 12% 31%	2547
August	1st to 10th	Day	8% 4 2 7% 8% 5% 9% 9% 10% 12% 27%	2327
	11th to 20th	Day	12% 5% 3 6% 8% 5% 8% 7% 7% 9% 31%	2169
	21st to end	Day	14% 6% 3 7% 7% 4 7% 7% 6% 9% 30%	2358
September	1st to 10th	Day	12% 5% 3 6% 6% 3 6% 6% 8% 7% 38%	1980
	11th to 20th	Day	15% 6% 2 5% 4 3 5% 6% 7% 7% 41%	1980
	21st to end	Day	14% 6% 2 5% 4 3 4 4 4 5% 48%	1980
October	1st to 10th	Day	13% 5% 1 4 4 2 3 4 4 6% 55%	1980
	11th to 20th	Day	13% 4 1 3 4 2 3 3 5% 7% 55%	1871
	21st to end	Day	12% 4 1 4 4 3 4 4 3 5% 55%	1980
November	1st to 10th	Day	13% 4 2 4 4 2 4 3 4 5% 55%	1733
	11th to 20th	Day	14% 4 2 3 4 2 4 4 4 6% 54%	1710
	21st to end	Day	14% 5% 2 5% 5% 3 4 4 5% 7% 46%	1710
December	1st to 10th	Day	17% 7% 3 6% 6% 3 4 4 5% 7% 40%	1710
	11th to 20th	Day	18% 7% 2 5% 5% 3 3 3 4 5% 45%	2153
	21st to end	Day	21% 7% 3 5% 4 2 3 4 5% 7% 39%	3124
January	1st to 10th	Night	18% 5% 2 5% 5% 4 4 4 5% 6% 42%	4109
	11th to 20th	Night	10% 5% 2 5% 5% 3 4 5% 5% 7% 48%	4120
	21st to end	Night	22% 10% 3 6% 5% 2 3 3 3 4 40%	4403
February	1st to 10th	Night	14% 6% 3 5% 6% 3 4 4 4 7% 43%	2280
	11th to 20th	Night	19% 6% 3 5% 4 2 4 3 4 6% 44%	2218
	21st to end	Night	17% 6% 3 6% 4 3 4 4 4 6% 43%	1884
March	1st to 10th	Night	13% 7% 3 4 5% 3 4 4 4 5% 50%	2200
	11th to 20th	Night	15% 5% 2 4 4 2 4 5% 6% 7% 46%	2090
	21st to end	Night	13% 5% 2 4 4 2 4 5% 5% 8% 48%	2201
April	1st to 10th	Night	11% 4 2 4 3 3 5% 5% 6% 8% 50%	1980
	11th to 20th	Night	13% 5% 2 4 4 2 3 4 6% 8% 50%	1781
	21st to end	Night	7% 5% 2 4 4 3 4 4 6% 8% 55%	1648
May	1st to 10th	Night	8% 4 1 4 5% 4 5% 5% 5% 9% 50%	1620
	11th to 20th	Night	10% 3 2 6% 6% 4 6% 6% 6% 9% 42%	1592
	21st to end	Night	10% 5% 2 4 6% 3 5% 6% 5% 9% 45%	1744
June	1st to 10th	Night	6% 5% 3 6% 6% 4 6% 6% 8% 11% 38%	1620
	11th to 20th	Night	4 4 2 5% 5% 3 5% 5% 7% 12% 47%	1620
	21st to end	Night	5% 4 2 5% 6% 4 5% 7% 9% 12% 40%	1620
July	1st to 10th	Night	8% 5% 3 6% 7% 5% 6% 6% 10% 11% 34%	1620
	11th to 20th	Night	6% 5% 2 5% 5% 3 7% 9% 10% 14% 34%	1620
	21st to end	Night	5% 4 2 5% 6% 4 7% 8% 10% 13% 36%	1771
August	1st to 10th	Night	6% 4 2 6% 7% 4 8% 7% 8% 11% 36%	1611
	11th to 20th	Night	10% 3 3 5% 7% 5% 6% 6% 9% 11% 36%	1728
	21st to end	Night	12% 7% 3 5% 6% 4 5% 6% 7% 9% 36%	1980
September	1st to 10th	Night	9% 5% 3 5% 6% 3 6% 6% 7% 11% 39%	1980
	11th to 20th	Night	12% 5% 3 6% 5% 3 5% 5% 6% 6% 45%	1980
	21st to end	Night	11% 5% 3 5% 5% 3 4 5% 5% 8% 47%	1980
October	1st to 10th	Night	12% 4 2 4 4 3 4 3 5% 6% 54%	1980
	11th to 20th	Night	12% 4 2 4 3 3 3 4 4 5% 56%	2088
	21st to end	Night	11% 4 1 3 4 3 3 4 4 6% 56%	2376
November	1st to 10th	Night	12% 5% 2 3 3 2 3 3 5% 5% 57%	2342
	11th to 20th	Night	13% 6% 2 4 4 1 3 3 4 5% 55%	2470
	21st to end	Night	14% 5% 2 5% 5% 3 4 3 5% 6% 49%	2470
December	1st to 10th	Night	16% 6% 2 4 5% 2 4 4 5% 7% 44%	2470
	11th to 20th	Night	17% 6% 3 4 3 2 3 3 4 6% 49%	3042
	21st to end	Night	19% 8% 4 6% 5% 3 5% 4 5% 7% 35%	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	41% 4 8% 17% 22% 7%	2837
	11th to 20th	Day	31% 3 5% 24% 26% 10%	2840
	21st to end	Day	47% 2 5% 15% 22% 9%	3251
February	1st to 10th	Day	45% 5% 8% 16% 20% 6%	1900
	11th to 20th	Day	44% 4 6% 18% 22% 7%	1858
	21st to end	Day	46% 5% 7% 14% 20% 8%	1570
March	1st to 10th	Day	37% 5% 8% 17% 22% 11%	1978
	11th to 20th	Day	42% 6% 10% 17% 18% 7%	2090
	21st to end	Day	40% 6% 9% 21% 18% 5%	2200
April	1st to 10th	Day	37% 5% 9% 21% 20% 8%	1988
	11th to 20th	Day	35% 4 8% 20% 23% 10%	2134
	21st to end	Day	30% 7% 9% 24% 26% 4	2309
May	1st to 10th	Day	32% 6% 12% 22% 24% 4	2340
	11th to 20th	Day	40% 8% 15% 20% 14% 2	2305
	21st to end	Day	34% 9% 13% 24% 18% 2	2531
June	1st to 10th	Day	43% 11% 14% 19% 11% 2	2340
	11th to 20th	Day	33% 10% 18% 24% 13% 1	2340
	21st to end	Day	37% 12% 17% 19% 12% 3	2340
July	1st to 10th	Day	43% 14% 19% 15% 8% 2	2275
	11th to 20th	Day	42% 14% 17% 17% 8% 2	2271
	21st to end	Day	39% 14% 18% 20% 8% 1	2405
August	1st to 10th	Day	42% 15% 19% 20% 4	2314
	11th to 20th	Day	47% 11% 16% 16% 9% 2	2169
	21st to end	Day	49% 10% 14% 16% 9% 2	2358
September	1st to 10th	Day	42% 9% 14% 16% 15% 4	1980
	11th to 20th	Day	39% 9% 12% 16% 21% 3	1980
	21st to end	Day	38% 5% 10% 21% 21% 6%	1980
October	1st to 10th	Day	31% 5% 10% 18% 27% 8%	1980
	11th to 20th	Day	29% 4 8% 25% 26% 7%	1871
	21st to end	Day	32% 4 6% 14% 32% 12%	1980
November	1st to 10th	Day	32% 4 7% 15% 29% 13%	1733
	11th to 20th	Day	33% 3 7% 19% 26% 12%	1710
	21st to end	Day	37% 5% 8% 17% 25% 7%	1710
December	1st to 10th	Day	45% 6% 7% 17% 18% 6%	1710
	11th to 20th	Day	43% 3 6% 15% 24% 8%	2150
	21st to end	Day	44% 4 9% 17% 22% 4	3124
January	1st to 10th	Night	42% 4 6% 16% 24% 8%	4109
	11th to 20th	Night	34% 4 8% 23% 23% 8%	4120
	21st to end	Night	49% 3 5% 11% 24% 7%	4403
February	1st to 10th	Night	42% 4 7% 18% 19% 10%	2280
	11th to 20th	Night	43% 4 5% 17% 22% 10%	2219
	21st to end	Night	42% 5% 8% 16% 18% 12%	1884
March	1st to 10th	Night	37% 4 7% 19% 20% 13%	2200
	11th to 20th	Night	37% 6% 12% 16% 22% 8%	2090
	21st to end	Night	35% 5% 10% 22% 21% 7%	2201
April	1st to 10th	Night	33% 7% 9% 21% 21% 10%	1980
	11th to 20th	Night	31% 4 7% 22% 24% 11%	1781
	21st to end	Night	27% 5% 12% 24% 24% 7%	1648
May	1st to 10th	Night	29% 8% 12% 23% 22% 6%	1620
	11th to 20th	Night	38% 8% 16% 18% 17% 3	1593
	21st to end	Night	34% 10% 11% 23% 19% 3	1745
June	1st to 10th	Night	36% 10% 18% 21% 14%	1620
	11th to 20th	Night	30% 10% 18% 30% 11% 2	1620
	21st to end	Night	33% 10% 22% 19% 14% 2	1620
July	1st to 10th	Night	38% 14% 19% 18% 8% 3	1575
	11th to 20th	Night	33% 16% 26% 14% 10% 2	1566
	21st to end	Night	32% 15% 23% 20% 7% 2	1672
August	1st to 10th	Night	38% 14% 20% 22% 5% 1	1600
	11th to 20th	Night	38% 12% 19% 18% 11% 1	1728
	21st to end	Night	41% 11% 17% 18% 11% 2	1980
September	1st to 10th	Night	37% 10% 16% 17% 16% 4	1980
	11th to 20th	Night	38% 9% 10% 19% 19% 5%	1980
	21st to end	Night	36% 8% 10% 17% 21% 8%	1980
October	1st to 10th	Night	32% 5% 10% 19% 25% 9%	1980
	11th to 20th	Night	30% 4 7% 20% 29% 10%	2088
	21st to end	Night	30% 3 6% 17% 29% 15%	2376
November	1st to 10th	Night	31% 4 8% 16% 28% 14%	2342
	11th to 20th	Night	33% 4 5% 17% 26% 14%	2470
	21st to end	Night	37% 5% 6% 18% 25% 9%	2470
December	1st to 10th	Night	39% 5% 9% 19% 21% 7%	2470
	11th to 20th	Night	38% 3 7% 15% 29% 7%	3044
	21st to end	Night	49% 4 8% 18% 18% 3	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	23% 24% 38% 14% 1	270
	11th to 20th	Day	23% 23% 30% 18% 6% 1	270
	21st to end	Day	23% 26% 36% 10% 4 1	297
February	1st to 10th	Day	22% 23% 35% 16% 4	270
	11th to 20th	Day	36% 15% 33% 14% 1	270
	21st to end	Day	26% 21% 38% 14%	225
March	1st to 10th	Day	21% 19% 31% 22% 6% 1	348
	11th to 20th	Day	22% 14% 33% 22% 8%	460
	21st to end	Day	17% 19% 33% 23% 7% 1	470
April	1st to 10th	Day	17% 16% 29% 25% 11% 1	420
	11th to 20th	Day	13% 18% 27% 24% 15% 2	412
	21st to end	Day	13% 12% 25% 26% 19% 4	420
May	1st to 10th	Day	10% 13% 28% 27% 14% 7%	423
	11th to 20th	Day	12% 6% 23% 24% 23% 11%	447
	21st to end	Day	14% 4 15% 28% 26% 13%	485
June	1st to 10th	Day	12% 2 16% 25% 32% 13%	450
	11th to 20th	Day	7% 3 15% 26% 25% 24%	450
	21st to end	Day	9% 3 16% 30% 24% 19%	450
July	1st to 10th	Day	9% 2 11% 21% 34% 24%	445
	11th to 20th	Day	8% 1 8% 21% 35% 26%	444
	21st to end	Day	5% 2 9% 20% 40% 24%	488
August	1st to 10th	Day	11% 3 10% 28% 26% 23%	446
	11th to 20th	Day	10% 2 7% 22% 37% 22%	439
	21st to end	Day	16% 2 8% 25% 32% 16%	495
September	1st to 10th	Day	12% 3 10% 25% 32% 18%	450
	11th to 20th	Day	17% 2 12% 27% 30% 12%	450
	21st to end	Day	12% 2 18% 30% 28% 10%	450
October	1st to 10th	Day	17% 6% 18% 28% 22% 9%	450
	11th to 20th	Day	14% 8% 24% 26% 21% 7%	450
	21st to end	Day	15% 11% 23% 28% 18% 5%	495
November	1st to 10th	Day	21% 7% 26% 27% 15% 4	331
	11th to 20th	Day	17% 17% 39% 20% 6% 1	270
	21st to end	Day	14% 15% 34% 24% 11% 2	270
December	1st to 10th	Day	25% 17% 29% 20% 8%	270
	11th to 20th	Day	27% 19% 28% 20% 6%	270
	21st to end	Day	28% 17% 32% 17% 5% 1	297
January	1st to 10th	Night	23% 18% 30% 23% 6% 1	760
	11th to 20th	Night	19% 17% 29% 28% 8%	760
	21st to end	Night	22% 15% 35% 24% 4	836
February	1st to 10th	Night	19% 14% 28% 33% 6%	760
	11th to 20th	Night	23% 15% 32% 27% 2	743
	21st to end	Night	20% 16% 35% 26% 3	628
March	1st to 10th	Night	16% 14% 31% 32% 7%	760
	11th to 20th	Night	19% 12% 31% 27% 10%	760
	21st to end	Night	16% 14% 29% 29% 11%	801
April	1st to 10th	Night	14% 13% 29% 30% 13%	720
	11th to 20th	Night	15% 12% 24% 29% 19% 1	539
	21st to end	Night	8% 11% 23% 34% 23% 1	540
May	1st to 10th	Night	8% 8% 26% 36% 21% 1	540
	11th to 20th	Night	10% 4 21% 32% 27% 6%	531
	21st to end	Night	13% 2 12% 31% 35% 7%	584
June	1st to 10th	Night	8% 1 10% 37% 39% 5%	540
	11th to 20th	Night	5% 1 13% 30% 37% 14%	540
	21st to end	Night	7% 1 9% 34% 40% 10%	540
July	1st to 10th	Night	9% 1 8% 24% 43% 15%	525
	11th to 20th	Night	7% 1 4 24% 49% 15%	522
	21st to end	Night	6% 1 8% 26% 48% 12%	591
August	1st to 10th	Night	8% 1 6% 26% 46% 14%	537
	11th to 20th	Night	10% 1 5% 25% 48% 10%	534
	21st to end	Night	16% 2 9% 24% 38% 11%	594
September	1st to 10th	Night	9% 2 10% 26% 42% 12%	720
	11th to 20th	Night	13% 2 13% 28% 40% 5%	720
	21st to end	Night	14% 2 14% 31% 36% 4	720
October	1st to 10th	Night	15% 3 17% 31% 27% 7%	720
	11th to 20th	Night	13% 5% 22% 32% 25% 3	720
	21st to end	Night	13% 8% 22% 32% 22% 2	792
November	1st to 10th	Night	14% 8% 23% 33% 21% 1	740
	11th to 20th	Night	15% 13% 26% 31% 13% 2	760
	21st to end	Night	15% 13% 26% 32% 13% 1	760
December	1st to 10th	Night	19% 13% 28% 29% 11% 1	760
	11th to 20th	Night	21% 10% 29% 30% 9% 1	760
	21st to end	Night	26% 14% 25% 28% 7%	836

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