Clegg Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clegg Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	130954
	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	130188
Transparency	Day	40% 7% 12% 23% 16% 4	134557
	Night	38% 6% 12% 24% 17% 3	134655
Seeing	Day	20% 15% 27% 22% 13% 5%	16564
	Night	17% 12% 26% 31% 13% 1	26358

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	12% 6% 3 8% 5% 5% 4 3 4 6% 44%	5181
	11th to 20th	Day	10% 8% 3 7% 5% 4 4 3 4 5% 47%	5280
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 4 6% 6% 41%	5780
February	1st to 10th	Day	10% 9% 3 7% 6% 4 5% 4 5% 6% 40%	1890
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 3 4 6% 38%	1843
	21st to end	Day	15% 8% 3 7% 4 4 5% 3 5% 6% 40%	1769
March	1st to 10th	Day	15% 7% 3 7% 4 4 5% 4 4 5% 42%	4957
	11th to 20th	Day	9% 6% 2 6% 4 4 3 4 5% 7% 51%	4980
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 5% 6% 6% 39%	5465
April	1st to 10th	Day	11% 7% 2 7% 6% 5% 5% 4 6% 7% 40%	2365
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 3 5% 7% 46%	2374
	21st to end	Day	16% 6% 3 6% 5% 5% 5% 4 6% 6% 40%	2469
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 6% 5% 7% 9% 29%	5862
	11th to 20th	Day	22% 6% 3 6% 5% 4 5% 4 7% 8% 29%	5861
	21st to end	Day	15% 5% 2 6% 5% 5% 6% 5% 7% 9% 36%	6889
June	1st to 10th	Day	11% 5% 2 5% 5% 5% 6% 5% 7% 9% 40%	2800
	11th to 20th	Day	10% 5% 2 6% 6% 6% 6% 4 8% 10% 37%	2800
	21st to end	Day	9% 4 2 5% 5% 6% 6% 5% 7% 10% 42%	2800
July	1st to 10th	Day	6% 4 2 7% 5% 5% 6% 4 8% 11% 42%	6229
	11th to 20th	Day	7% 6% 3 8% 6% 6% 8% 6% 9% 11% 30%	5900
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	6424
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 7% 9% 31%	4234
	11th to 20th	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 9% 39%	4124
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 6% 8% 46%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 5% 6% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 5% 7% 46%	2315
	21st to end	Day	21% 8% 2 6% 5% 5% 4 3 6% 6% 36%	2133
October	1st to 10th	Day	14% 7% 2 5% 3 3 4 3 6% 7% 45%	2000
	11th to 20th	Day	15% 5% 2 6% 4 4 4 3 5% 5% 47%	2000
	21st to end	Day	13% 5% 2 5% 4 4 4 3 6% 7% 47%	2200
November	1st to 10th	Day	9% 6% 2 4 4 3 3 3 5% 7% 53%	2057
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 6% 6% 46%	1783
	21st to end	Day	13% 6% 2 5% 5% 4 5% 3 5% 6% 46%	1680
December	1st to 10th	Day	11% 6% 2 5% 3 4 5% 4 6% 6% 49%	2680
	11th to 20th	Day	17% 8% 2 6% 4 4 4 4 4 6% 41%	3070
	21st to end	Day	16% 7% 3 6% 5% 5% 5% 4 6% 8% 35%	4048
January	1st to 10th	Night	8% 7% 3 6% 4 5% 6% 5% 6% 7% 43%	7694
	11th to 20th	Night	9% 8% 3 6% 4 4 4 3 5% 6% 49%	7680
	21st to end	Night	11% 7% 2 5% 3 4 5% 4 5% 6% 48%	8401
February	1st to 10th	Night	11% 7% 3 7% 5% 4 5% 4 6% 6% 41%	2730
	11th to 20th	Night	15% 8% 2 6% 5% 4 5% 4 6% 6% 39%	2663
	21st to end	Night	13% 9% 2 6% 6% 5% 4 4 5% 7% 40%	2031
March	1st to 10th	Night	15% 6% 3 6% 4 4 4 3 4 6% 47%	4941
	11th to 20th	Night	8% 4 2 5% 4 4 4 3 6% 8% 51%	4980
	21st to end	Night	10% 5% 2 7% 5% 5% 5% 4 5% 7% 45%	5467
April	1st to 10th	Night	13% 8% 3 6% 5% 5% 5% 5% 6% 5% 40%	2035
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 5% 6% 8% 43%	1975
	21st to end	Night	15% 6% 2 5% 4 4 5% 3 4 7% 45%	1948
May	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 4 6% 8% 30%	4033
	11th to 20th	Night	22% 7% 2 6% 5% 4 4 4 6% 7% 32%	4032
	21st to end	Night	15% 6% 3 7% 5% 5% 5% 5% 6% 8% 35%	3947
June	1st to 10th	Night	10% 5% 1 6% 6% 5% 4 4 6% 8% 46%	1600
	11th to 20th	Night	11% 7% 1 7% 4 4 5% 4 8% 9% 41%	1600
	21st to end	Night	9% 4 2 5% 4 4 5% 5% 9% 10% 43%	1600
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 5% 5% 9% 10% 40%	3724
	11th to 20th	Night	7% 7% 3 10% 8% 6% 7% 5% 7% 10% 30%	4057
	21st to end	Night	6% 5% 3 7% 6% 5% 6% 5% 9% 12% 36%	4427
August	1st to 10th	Night	14% 5% 2 7% 5% 5% 6% 5% 7% 9% 35%	2915
	11th to 20th	Night	9% 6% 2 7% 5% 5% 6% 4 8% 10% 39%	2890
	21st to end	Night	11% 5% 2 5% 4 5% 4 3 5% 9% 47%	3586
September	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 5% 7% 47%	2000
	11th to 20th	Night	12% 5% 3 5% 4 3 3 3 6% 7% 49%	2080
	21st to end	Night	19% 7% 2 6% 4 4 4 3 5% 5% 41%	2266
October	1st to 10th	Night	16% 6% 2 6% 3 3 4 3 5% 6% 47%	2400
	11th to 20th	Night	13% 5% 1 4 4 4 4 3 5% 6% 51%	2400
	21st to end	Night	13% 5% 2 4 4 4 4 4 4 6% 50%	2640
November	1st to 10th	Night	9% 6% 2 6% 4 3 4 3 5% 6% 52%	2461
	11th to 20th	Night	13% 7% 2 5% 4 3 4 4 5% 7% 47%	2830
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 5% 7% 45%	2940
December	1st to 10th	Night	8% 5% 2 5% 4 3 4 4 6% 8% 51%	4720
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 5% 7% 39%	5345
	21st to end	Night	14% 7% 3 7% 4 6% 5% 5% 7% 8% 34%	7150

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	40% 5% 8% 22% 19% 5%	5182
	11th to 20th	Day	39% 5% 6% 19% 20% 11%	5280
	21st to end	Day	40% 5% 10% 28% 14% 4	5780
February	1st to 10th	Day	44% 5% 12% 21% 15% 3	1890
	11th to 20th	Day	47% 5% 9% 20% 17% 2	1852
	21st to end	Day	45% 4 11% 21% 16% 4	1769
March	1st to 10th	Day	43% 4 8% 21% 15% 9%	4957
	11th to 20th	Day	32% 5% 9% 26% 21% 6%	4980
	21st to end	Day	44% 6% 10% 17% 18% 6%	5465
April	1st to 10th	Day	42% 8% 10% 19% 16% 6%	2365
	11th to 20th	Day	38% 5% 10% 21% 20% 5%	2374
	21st to end	Day	43% 6% 11% 22% 15% 3	2469
May	1st to 10th	Day	47% 8% 14% 19% 10% 2	5897
	11th to 20th	Day	50% 7% 11% 23% 9% 1	5865
	21st to end	Day	41% 10% 13% 20% 14% 2	6904
June	1st to 10th	Day	39% 8% 12% 24% 16% 1	2800
	11th to 20th	Day	41% 9% 14% 21% 14% 2	2800
	21st to end	Day	36% 9% 15% 24% 14% 2	2800
July	1st to 10th	Day	35% 10% 17% 22% 13% 4	6161
	11th to 20th	Day	42% 10% 18% 22% 7% 1	5831
	21st to end	Day	39% 7% 12% 23% 17% 1	6240
August	1st to 10th	Day	46% 8% 16% 23% 8%	4221
	11th to 20th	Day	38% 8% 12% 27% 14% 1	4163
	21st to end	Day	34% 7% 15% 27% 16% 1	4312
September	1st to 10th	Day	39% 7% 12% 18% 20% 4	2400
	11th to 20th	Day	37% 6% 8% 28% 18% 3	2315
	21st to end	Day	49% 6% 9% 19% 16% 2	2133
October	1st to 10th	Day	33% 5% 8% 21% 27% 6%	3260
	11th to 20th	Day	28% 4 8% 29% 22% 10%	3240
	21st to end	Day	25% 3 10% 29% 26% 6%	3538
November	1st to 10th	Day	31% 4 11% 25% 24% 4	2057
	11th to 20th	Day	37% 5% 10% 20% 23% 5%	1783
	21st to end	Day	39% 5% 11% 25% 18% 2	1680
December	1st to 10th	Day	35% 5% 13% 25% 17% 5%	2680
	11th to 20th	Day	45% 4 11% 25% 12% 3	3066
	21st to end	Day	44% 6% 15% 19% 15% 1	4048
January	1st to 10th	Night	37% 6% 10% 23% 21% 3	7695
	11th to 20th	Night	35% 5% 8% 23% 22% 8%	7680
	21st to end	Night	36% 5% 8% 29% 19% 3	8401
February	1st to 10th	Night	41% 6% 12% 24% 15% 1	2730
	11th to 20th	Night	45% 5% 9% 21% 15% 4	2665
	21st to end	Night	44% 5% 12% 21% 14% 5%	2031
March	1st to 10th	Night	38% 5% 8% 26% 15% 8%	4941
	11th to 20th	Night	30% 5% 11% 30% 19% 5%	4980
	21st to end	Night	37% 7% 11% 22% 18% 5%	5467
April	1st to 10th	Night	43% 7% 10% 20% 17% 3	2035
	11th to 20th	Night	38% 7% 11% 22% 17% 5%	1975
	21st to end	Night	40% 7% 10% 26% 14% 3	1948
May	1st to 10th	Night	50% 7% 14% 16% 9% 3	4060
	11th to 20th	Night	49% 6% 12% 23% 9%	4033
	21st to end	Night	44% 8% 16% 23% 6% 3	3954
June	1st to 10th	Night	36% 8% 13% 27% 15% 1	1600
	11th to 20th	Night	38% 10% 14% 24% 13% 1	1600
	21st to end	Night	33% 8% 16% 24% 17% 2	1600
July	1st to 10th	Night	37% 10% 20% 19% 12% 2	3684
	11th to 20th	Night	47% 10% 18% 17% 7% 1	4006
	21st to end	Night	36% 9% 18% 23% 15%	4297
August	1st to 10th	Night	44% 8% 18% 23% 7%	2904
	11th to 20th	Night	37% 8% 16% 24% 15%	2917
	21st to end	Night	34% 7% 15% 29% 14% 1	3586
September	1st to 10th	Night	38% 7% 12% 21% 18% 4	2000
	11th to 20th	Night	34% 6% 11% 29% 19% 2	2080
	21st to end	Night	45% 7% 10% 20% 17% 2	2266
October	1st to 10th	Night	37% 4 11% 23% 22% 3	3920
	11th to 20th	Night	25% 4 12% 22% 30% 7%	3899
	21st to end	Night	23% 3 11% 31% 28% 4	4252
November	1st to 10th	Night	32% 5% 11% 24% 22% 5%	2461
	11th to 20th	Night	37% 5% 9% 24% 21% 5%	2830
	21st to end	Night	39% 5% 11% 23% 19% 3	2940
December	1st to 10th	Night	31% 5% 15% 29% 19% 2	4720
	11th to 20th	Night	44% 4 12% 19% 17% 4	5348
	21st to end	Night	43% 7% 14% 20% 15% 1	7150

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	30% 30% 26% 10% 3 1	316
	11th to 20th	Day	31% 27% 31% 9% 2	330
	21st to end	Day	27% 28% 34% 9% 2	363
February	1st to 10th	Day	28% 29% 31% 10% 2	330
	11th to 20th	Day	42% 25% 28% 5% 1	330
	21st to end	Day	34% 21% 28% 16% 2	416
March	1st to 10th	Day	28% 25% 24% 20% 4	508
	11th to 20th	Day	26% 25% 25% 17% 5% 1	500
	21st to end	Day	26% 21% 31% 17% 5% 1	550
April	1st to 10th	Day	24% 25% 26% 18% 6% 1	500
	11th to 20th	Day	21% 20% 30% 17% 9% 3	492
	21st to end	Day	23% 16% 31% 19% 9% 2	500
May	1st to 10th	Day	24% 16% 30% 17% 9% 4	503
	11th to 20th	Day	24% 15% 28% 21% 10% 3	527
	21st to end	Day	20% 9% 29% 21% 16% 6%	573
June	1st to 10th	Day	16% 11% 26% 29% 13% 6%	530
	11th to 20th	Day	16% 9% 25% 27% 16% 7%	530
	21st to end	Day	10% 8% 30% 29% 17% 5%	530
July	1st to 10th	Day	10% 4 22% 31% 25% 8%	525
	11th to 20th	Day	6% 4 23% 32% 24% 11%	524
	21st to end	Day	11% 4 20% 32% 24% 9%	576
August	1st to 10th	Day	12% 7% 21% 25% 24% 10%	526
	11th to 20th	Day	12% 5% 20% 29% 24% 10%	519
	21st to end	Day	9% 7% 21% 26% 24% 13%	583
September	1st to 10th	Day	15% 7% 19% 25% 23% 11%	530
	11th to 20th	Day	14% 4 22% 26% 24% 10%	530
	21st to end	Day	29% 9% 21% 23% 14% 3	530
October	1st to 10th	Day	18% 7% 21% 30% 19% 5%	530
	11th to 20th	Day	19% 16% 25% 25% 12% 2	530
	21st to end	Day	17% 19% 31% 21% 9% 3	583
November	1st to 10th	Day	16% 16% 32% 23% 9% 3	535
	11th to 20th	Day	21% 18% 32% 21% 7% 1	313
	21st to end	Day	28% 18% 35% 16% 4	220
December	1st to 10th	Day	25% 21% 40% 10% 3 1	220
	11th to 20th	Day	25% 20% 31% 17% 6%	220
	21st to end	Day	21% 22% 33% 18% 5%	242
January	1st to 10th	Night	19% 20% 35% 22% 3	840
	11th to 20th	Night	22% 17% 30% 28% 2	840
	21st to end	Night	19% 18% 39% 23% 1	924
February	1st to 10th	Night	17% 20% 31% 29% 2	840
	11th to 20th	Night	27% 23% 31% 18% 1	823
	21st to end	Night	25% 16% 35% 21% 2	692
March	1st to 10th	Night	20% 17% 34% 25% 4	809
	11th to 20th	Night	23% 22% 27% 22% 6% 1	800
	21st to end	Night	20% 18% 34% 22% 5% 1	880
April	1st to 10th	Night	19% 22% 31% 23% 5%	800
	11th to 20th	Night	17% 17% 31% 26% 9% 1	791
	21st to end	Night	18% 14% 27% 32% 9%	748
May	1st to 10th	Night	21% 8% 26% 29% 14% 1	600
	11th to 20th	Night	19% 6% 27% 32% 14% 1	591
	21st to end	Night	14% 4 21% 36% 22% 3	448
June	1st to 10th	Night	11% 5% 24% 39% 20% 2	400
	11th to 20th	Night	13% 6% 13% 46% 20% 2	400
	21st to end	Night	7% 4 19% 46% 22% 2	400
July	1st to 10th	Night	10% 2 13% 42% 27% 6%	436
	11th to 20th	Night	7% 1 11% 38% 37% 5%	582
	21st to end	Night	8% 3 13% 41% 29% 5%	657
August	1st to 10th	Night	11% 2 11% 42% 31% 3	597
	11th to 20th	Night	10% 4 12% 42% 29% 4	596
	21st to end	Night	10% 5% 13% 37% 31% 4	880
September	1st to 10th	Night	15% 4 13% 33% 30% 4	800
	11th to 20th	Night	12% 5% 15% 35% 29% 5%	800
	21st to end	Night	23% 5% 20% 32% 19% 1	800
October	1st to 10th	Night	20% 8% 17% 30% 22% 3	800
	11th to 20th	Night	16% 12% 25% 33% 14% 1	800
	21st to end	Night	17% 13% 24% 33% 12% 1	880
November	1st to 10th	Night	12% 12% 29% 33% 12% 1	820
	11th to 20th	Night	18% 18% 27% 28% 8% 1	840
	21st to end	Night	18% 16% 30% 30% 7% 1	840
December	1st to 10th	Night	20% 16% 31% 26% 6% 1	840
	11th to 20th	Night	18% 13% 34% 30% 4	840
	21st to end	Night	18% 16% 29% 35% 2	924

Page generated at 2026-02-28 01:40:23. Version ac07

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