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 2024-12-02.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	101315
	Night	13% 6% 2 6% 4 4 5% 4 6% 7% 42%	98233
Transparency	Day	40% 7% 11% 23% 15% 3	104962
	Night	39% 6% 12% 24% 16% 3	102725
Seeing	Day	20% 14% 26% 22% 13% 5%	15003
	Night	17% 12% 25% 31% 13% 1	24699

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	14% 7% 2 7% 6% 5% 4 4 4 6% 40%	2786
	11th to 20th	Day	11% 10% 4 10% 7% 5% 4 4 4 5% 37%	2840
	21st to end	Day	14% 9% 3 8% 5% 4 5% 4 5% 6% 36%	3124
February	1st to 10th	Day	10% 9% 3 7% 6% 4 5% 4 5% 6% 39%	1710
	11th to 20th	Day	14% 9% 2 7% 6% 5% 5% 4 5% 6% 39%	1663
	21st to end	Day	16% 8% 3 7% 4 4 5% 3 5% 6% 40%	1604
March	1st to 10th	Day	17% 10% 3 8% 5% 4 4 3 4 5% 35%	3435
	11th to 20th	Day	8% 6% 2 6% 4 4 3 4 5% 7% 51%	3480
	21st to end	Day	16% 8% 3 7% 5% 4 5% 4 5% 6% 38%	3815
April	1st to 10th	Day	9% 7% 2 7% 6% 5% 5% 4 6% 7% 40%	2247
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 3 5% 6% 46%	2254
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 4 6% 6% 39%	2345
May	1st to 10th	Day	19% 7% 3 8% 5% 5% 5% 4 6% 7% 30%	4082
	11th to 20th	Day	23% 7% 3 6% 5% 4 4 4 6% 7% 31%	4081
	21st to end	Day	9% 4 2 6% 5% 5% 6% 5% 7% 10% 40%	4792
June	1st to 10th	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	10% 5% 2 6% 6% 6% 6% 4 8% 11% 37%	2660
	21st to end	Day	9% 4 2 5% 5% 6% 6% 5% 7% 10% 41%	2660
July	1st to 10th	Day	5% 4 2 7% 6% 5% 6% 5% 8% 10% 42%	4351
	11th to 20th	Day	8% 5% 2 6% 5% 5% 7% 5% 8% 11% 37%	4120
	21st to end	Day	11% 8% 3 8% 5% 6% 6% 5% 6% 8% 34%	4466
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 7% 9% 31%	4104
	11th to 20th	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 9% 39%	3995
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 6% 8% 46%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 5% 6% 44%	2280
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 5% 7% 46%	2199
	21st to end	Day	21% 7% 2 5% 5% 5% 4 3 6% 6% 37%	2027
October	1st to 10th	Day	15% 7% 2 6% 3 3 4 3 6% 7% 44%	1900
	11th to 20th	Day	15% 5% 2 6% 4 4 4 3 5% 5% 47%	1900
	21st to end	Day	13% 5% 2 5% 3 4 4 3 5% 6% 48%	2090
November	1st to 10th	Day	10% 6% 2 4 4 3 3 3 5% 7% 53%	1957
	11th to 20th	Day	14% 6% 2 5% 4 4 4 4 6% 6% 46%	1698
	21st to end	Day	13% 6% 2 5% 5% 4 5% 3 5% 6% 46%	1600
December	1st to 10th	Day	15% 9% 3 5% 4 4 5% 4 6% 7% 37%	1528
	11th to 20th	Day	14% 6% 2 6% 5% 3 4 4 5% 6% 44%	1910
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 8% 34%	2772
January	1st to 10th	Night	10% 8% 3 6% 4 5% 4 4 6% 7% 43%	4141
	11th to 20th	Night	14% 9% 3 5% 3 4 4 4 5% 6% 44%	4120
	21st to end	Night	17% 9% 3 6% 4 3 4 4 5% 6% 39%	4532
February	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 6% 41%	2470
	11th to 20th	Night	15% 8% 2 6% 5% 4 5% 4 6% 6% 39%	2403
	21st to end	Night	14% 9% 2 6% 7% 5% 3 4 5% 7% 38%	1844
March	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 3 5% 6% 36%	3418
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 49%	3480
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 47%	3817
April	1st to 10th	Night	12% 8% 2 6% 5% 5% 5% 5% 6% 6% 40%	1933
	11th to 20th	Night	12% 5% 2 6% 5% 4 4 4 6% 8% 44%	1875
	21st to end	Night	15% 6% 2 5% 4 4 5% 3 4 7% 45%	1851
May	1st to 10th	Night	19% 9% 3 8% 5% 5% 4 3 6% 8% 31%	2813
	11th to 20th	Night	22% 8% 3 7% 5% 4 4 3 5% 6% 34%	2812
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 6% 10% 44%	2745
June	1st to 10th	Night	10% 5% 1 5% 5% 5% 4 4 6% 8% 46%	1520
	11th to 20th	Night	10% 6% 1 7% 4 4 5% 4 8% 9% 42%	1520
	21st to end	Night	9% 4 2 5% 4 4 5% 4 9% 10% 43%	1520
July	1st to 10th	Night	6% 4 2 6% 6% 5% 6% 5% 9% 10% 41%	2604
	11th to 20th	Night	7% 6% 3 7% 6% 5% 5% 5% 7% 9% 39%	2840
	21st to end	Night	8% 6% 3 8% 6% 5% 5% 5% 8% 10% 36%	3087
August	1st to 10th	Night	14% 5% 2 7% 6% 5% 6% 5% 8% 9% 35%	2825
	11th to 20th	Night	9% 6% 2 7% 5% 5% 6% 4 7% 10% 40%	2800
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 9% 48%	3476
September	1st to 10th	Night	12% 7% 2 6% 4 4 5% 3 6% 7% 45%	1900
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 6% 7% 49%	1976
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 5% 43%	2152
October	1st to 10th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 45%	2280
	11th to 20th	Night	13% 5% 1 5% 4 4 4 3 5% 6% 52%	2280
	21st to end	Night	13% 5% 2 4 3 4 4 4 4 6% 51%	2508
November	1st to 10th	Night	9% 6% 2 6% 4 3 4 3 5% 7% 52%	2341
	11th to 20th	Night	13% 7% 2 5% 4 3 4 4 5% 7% 47%	2695
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 5% 7% 45%	2800
December	1st to 10th	Night	13% 8% 3 8% 5% 4 4 4 5% 6% 41%	2686
	11th to 20th	Night	14% 5% 3 6% 4 4 5% 4 5% 7% 43%	3285
	21st to end	Night	15% 6% 2 7% 5% 6% 5% 4 7% 8% 35%	4884

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	42% 7% 6% 25% 16% 3	2787
	11th to 20th	Day	50% 5% 7% 18% 16% 5%	2840
	21st to end	Day	48% 5% 9% 20% 15% 2	3124
February	1st to 10th	Day	46% 5% 12% 20% 14% 3	1710
	11th to 20th	Day	46% 5% 9% 21% 17% 3	1672
	21st to end	Day	45% 4 11% 19% 16% 4	1604
March	1st to 10th	Day	50% 5% 9% 20% 13% 4	3435
	11th to 20th	Day	33% 5% 8% 28% 22% 4	3480
	21st to end	Day	46% 6% 10% 20% 15% 3	3815
April	1st to 10th	Day	40% 8% 10% 19% 16% 6%	2247
	11th to 20th	Day	38% 5% 10% 21% 20% 5%	2254
	21st to end	Day	44% 7% 11% 21% 14% 4	2345
May	1st to 10th	Day	51% 8% 10% 20% 10% 1	4120
	11th to 20th	Day	50% 8% 10% 23% 8% 1	4085
	21st to end	Day	35% 11% 14% 21% 16% 3	4806
June	1st to 10th	Day	39% 8% 12% 24% 15% 1	2660
	11th to 20th	Day	40% 9% 14% 21% 14% 2	2660
	21st to end	Day	36% 9% 15% 24% 14% 2	2660
July	1st to 10th	Day	35% 10% 18% 23% 10% 5%	4283
	11th to 20th	Day	37% 9% 16% 27% 9% 1	4051
	21st to end	Day	45% 8% 10% 22% 15% 1	4324
August	1st to 10th	Day	46% 8% 16% 22% 7%	4091
	11th to 20th	Day	38% 8% 12% 27% 14% 1	4034
	21st to end	Day	34% 7% 15% 26% 16% 1	4180
September	1st to 10th	Day	40% 7% 12% 17% 20% 4	2280
	11th to 20th	Day	37% 6% 8% 28% 18% 3	2199
	21st to end	Day	48% 6% 9% 20% 16% 2	2027
October	1st to 10th	Day	34% 5% 8% 21% 26% 6%	3160
	11th to 20th	Day	27% 4 8% 29% 22% 10%	3140
	21st to end	Day	25% 3 10% 29% 26% 6%	3428
November	1st to 10th	Day	32% 4 11% 25% 24% 4	1957
	11th to 20th	Day	37% 5% 11% 20% 23% 5%	1698
	21st to end	Day	38% 5% 11% 24% 19% 2	1600
December	1st to 10th	Day	46% 6% 12% 22% 12% 2	1528
	11th to 20th	Day	39% 5% 9% 31% 12% 4	1906
	21st to end	Day	44% 7% 13% 23% 11% 1	2772
January	1st to 10th	Night	38% 6% 10% 24% 19% 3	4141
	11th to 20th	Night	42% 5% 9% 24% 16% 4	4120
	21st to end	Night	45% 6% 8% 24% 15% 2	4532
February	1st to 10th	Night	42% 6% 12% 25% 14% 1	2470
	11th to 20th	Night	45% 5% 9% 21% 16% 4	2405
	21st to end	Night	45% 6% 11% 21% 12% 4	1844
March	1st to 10th	Night	47% 7% 9% 25% 11% 2	3418
	11th to 20th	Night	33% 5% 9% 30% 20% 2	3480
	21st to end	Night	36% 7% 10% 25% 19% 4	3817
April	1st to 10th	Night	42% 7% 10% 20% 17% 3	1933
	11th to 20th	Night	37% 7% 11% 22% 17% 5%	1875
	21st to end	Night	40% 7% 11% 26% 14% 3	1851
May	1st to 10th	Night	51% 8% 11% 18% 10% 2	2840
	11th to 20th	Night	49% 7% 12% 23% 8% 1	2813
	21st to end	Night	34% 10% 17% 27% 9% 4	2752
June	1st to 10th	Night	35% 8% 13% 28% 14% 1	1520
	11th to 20th	Night	37% 10% 14% 25% 13% 1	1520
	21st to end	Night	33% 8% 16% 24% 16% 2	1520
July	1st to 10th	Night	35% 9% 22% 18% 13% 4	2562
	11th to 20th	Night	38% 8% 19% 24% 9% 1	2786
	21st to end	Night	40% 9% 17% 21% 13%	2988
August	1st to 10th	Night	44% 8% 18% 23% 7%	2814
	11th to 20th	Night	38% 8% 16% 24% 15%	2827
	21st to end	Night	34% 7% 16% 29% 14% 1	3476
September	1st to 10th	Night	39% 7% 13% 20% 17% 4	1900
	11th to 20th	Night	34% 6% 11% 29% 18% 2	1976
	21st to end	Night	43% 6% 10% 21% 17% 2	2152
October	1st to 10th	Night	38% 4 11% 23% 21% 3	3800
	11th to 20th	Night	24% 4 12% 22% 30% 7%	3779
	21st to end	Night	23% 3 11% 31% 28% 5%	4120
November	1st to 10th	Night	32% 5% 11% 24% 22% 6%	2341
	11th to 20th	Night	37% 5% 8% 24% 21% 5%	2695
	21st to end	Night	39% 5% 11% 23% 19% 3	2800
December	1st to 10th	Night	44% 5% 11% 25% 12% 3	2686
	11th to 20th	Night	41% 4 10% 23% 18% 3	3288
	21st to end	Night	43% 8% 14% 21% 14% 1	4884

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	33% 28% 26% 9% 3 1	258
	11th to 20th	Day	33% 24% 30% 11% 2	270
	21st to end	Day	29% 29% 31% 9% 2	297
February	1st to 10th	Day	30% 28% 31% 10% 1	270
	11th to 20th	Day	41% 26% 29% 4 1	270
	21st to end	Day	37% 22% 25% 14% 2	354
March	1st to 10th	Day	28% 24% 25% 19% 4	460
	11th to 20th	Day	27% 25% 26% 16% 5% 2	460
	21st to end	Day	27% 20% 31% 16% 5% 1	506
April	1st to 10th	Day	22% 26% 26% 19% 6% 1	460
	11th to 20th	Day	21% 19% 29% 18% 9% 4	452
	21st to end	Day	25% 17% 32% 17% 8% 2	460
May	1st to 10th	Day	25% 16% 29% 16% 9% 5%	463
	11th to 20th	Day	24% 14% 29% 21% 9% 3	487
	21st to end	Day	18% 9% 28% 22% 17% 6%	529
June	1st to 10th	Day	15% 11% 27% 28% 13% 6%	490
	11th to 20th	Day	15% 9% 26% 27% 16% 7%	490
	21st to end	Day	11% 9% 32% 30% 15% 3	490
July	1st to 10th	Day	10% 4 22% 32% 24% 8%	485
	11th to 20th	Day	6% 4 23% 32% 24% 10%	484
	21st to end	Day	12% 4 22% 32% 23% 8%	532
August	1st to 10th	Day	12% 7% 22% 25% 25% 10%	486
	11th to 20th	Day	11% 5% 22% 29% 23% 10%	479
	21st to end	Day	9% 7% 19% 27% 24% 14%	539
September	1st to 10th	Day	16% 7% 19% 24% 23% 11%	490
	11th to 20th	Day	14% 4 23% 25% 24% 10%	490
	21st to end	Day	29% 9% 21% 23% 14% 4	490
October	1st to 10th	Day	19% 7% 22% 31% 17% 4	490
	11th to 20th	Day	20% 16% 24% 25% 12% 3	490
	21st to end	Day	17% 18% 31% 22% 9% 3	539
November	1st to 10th	Day	17% 14% 31% 24% 10% 4	495
	11th to 20th	Day	20% 18% 32% 22% 8% 1	288
	21st to end	Day	27% 19% 37% 14% 4	200
December	1st to 10th	Day	26% 22% 37% 10% 3 1	182
	11th to 20th	Day	26% 23% 29% 16% 6%	180
	21st to end	Day	20% 24% 32% 19% 5% 1	198
January	1st to 10th	Night	20% 20% 35% 22% 3	760
	11th to 20th	Night	24% 17% 28% 29% 3	760
	21st to end	Night	20% 18% 37% 24% 1	836
February	1st to 10th	Night	17% 21% 30% 29% 2	760
	11th to 20th	Night	27% 25% 29% 18% 1	743
	21st to end	Night	27% 16% 34% 20% 2	628
March	1st to 10th	Night	21% 18% 33% 25% 4	760
	11th to 20th	Night	24% 22% 26% 22% 5% 1	760
	21st to end	Night	20% 19% 33% 22% 6% 1	836
April	1st to 10th	Night	19% 22% 31% 23% 5%	760
	11th to 20th	Night	17% 16% 31% 26% 10% 1	751
	21st to end	Night	19% 15% 27% 31% 9%	711
May	1st to 10th	Night	20% 9% 26% 29% 15% 1	570
	11th to 20th	Night	18% 7% 27% 32% 15% 1	561
	21st to end	Night	13% 4 22% 35% 23% 3	425
June	1st to 10th	Night	11% 4 25% 38% 21% 2	380
	11th to 20th	Night	12% 6% 14% 45% 21% 2	380
	21st to end	Night	8% 3 19% 47% 21% 1	380
July	1st to 10th	Night	11% 2 13% 41% 27% 6%	413
	11th to 20th	Night	7% 1 12% 39% 37% 4	552
	21st to end	Night	9% 3 14% 42% 28% 5%	624
August	1st to 10th	Night	10% 2 12% 42% 32% 3	567
	11th to 20th	Night	10% 4 13% 42% 28% 4	566
	21st to end	Night	11% 4 13% 37% 32% 4	836
September	1st to 10th	Night	15% 4 13% 32% 31% 5%	760
	11th to 20th	Night	11% 5% 16% 34% 30% 5%	760
	21st to end	Night	23% 5% 21% 31% 20% 1	760
October	1st to 10th	Night	20% 8% 17% 30% 21% 3	760
	11th to 20th	Night	16% 12% 25% 32% 15% 1	760
	21st to end	Night	18% 13% 25% 32% 12% 1	836
November	1st to 10th	Night	12% 12% 28% 33% 13% 1	780
	11th to 20th	Night	18% 17% 26% 29% 9% 1	800
	21st to end	Night	18% 16% 29% 30% 7% 1	800
December	1st to 10th	Night	21% 17% 30% 25% 6% 1	768
	11th to 20th	Night	18% 13% 34% 31% 4	760
	21st to end	Night	19% 16% 28% 35% 2	836

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