Lund Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lund Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	3 5% 2 5% 3 3 3 3 5% 6% 62%	135374
	Night	3 6% 2 5% 3 3 3 3 5% 6% 60%	131754
Transparency	Day	23% 4 6% 18% 26% 23%	130242
	Night	25% 4 7% 19% 27% 19%	127876
Seeing	Day	10% 16% 31% 27% 13% 3	16223
	Night	7% 11% 26% 33% 19% 4	25853

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	3 6% 7% 10% 4 3 3 3 5% 9% 48%	3639
	11th to 20th	Day	3 9% 2 5% 3 3 3 3 4 6% 60%	3846
	21st to end	Day	3 13% 4 5% 3 3 2 3 4 6% 54%	4510
February	1st to 10th	Day	4 10% 4 6% 5% 4 3 3 5% 7% 48%	1935
	11th to 20th	Day	5% 9% 3 5% 5% 4 3 3 6% 7% 50%	1950
	21st to end	Day	3 9% 4 6% 4 4 2 3 5% 5% 55%	1650
March	1st to 10th	Day	3 14% 4 7% 4 3 4 3 5% 8% 46%	4518
	11th to 20th	Day	2 9% 3 8% 4 4 5% 3 6% 8% 49%	4988
	21st to end	Day	3 12% 4 6% 4 4 5% 4 7% 9% 42%	5486
April	1st to 10th	Day	3 7% 2 7% 3 4 4 4 5% 7% 52%	2382
	11th to 20th	Day	4 7% 2 5% 4 3 4 3 6% 7% 55%	2389
	21st to end	Day	6% 6% 2 4 3 3 4 3 5% 7% 57%	2600
May	1st to 10th	Day	3 5% 2 5% 3 4 4 3 5% 7% 59%	5880
	11th to 20th	Day	4 6% 2 5% 3 3 4 3 4 6% 59%	5841
	21st to end	Day	4 4 2 4 4 4 4 4 6% 7% 58%	6423
June	1st to 10th	Day	3 3 2 4 4 4 4 3 5% 7% 61%	2640
	11th to 20th	Day	3 3 1 4 3 3 3 3 3 5% 70%	2800
	21st to end	Day	3 2 2 3 2 2 3 3 3 5% 72%	2800
July	1st to 10th	Day	2 2 1 3 2 2 2 2 3 3 79%	6319
	11th to 20th	Day	3 3 1 3 3 3 4 3 5% 6% 66%	6193
	21st to end	Day	2 2 1 3 3 3 4 3 5% 5% 68%	6401
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	5779
	11th to 20th	Day	1 1 2 2 1 2 2 3 3 83%	5696
	21st to end	Day	2 3 1 4 3 3 4 4 5% 6% 65%	5984
September	1st to 10th	Day	2 2 1 4 2 3 3 4 5% 5% 69%	2320
	11th to 20th	Day	2 2 1 3 2 3 3 3 4 4 73%	2200
	21st to end	Day	2 3 1 3 3 3 3 2 3 4 74%	2200
October	1st to 10th	Day	3 3 1 3 2 2 2 3 4 6% 73%	3600
	11th to 20th	Day	2 3 1 3 2 2 2 3 3 5% 74%	3367
	21st to end	Day	2 3 2 4 2 2 2 2 3 6% 73%	3145
November	1st to 10th	Day	3 5% 2 5% 3 3 3 3 6% 8% 58%	1764
	11th to 20th	Day	3 5% 1 4 3 2 2 2 5% 7% 66%	1800
	21st to end	Day	3 7% 2 5% 4 4 4 3 4 6% 58%	1800
December	1st to 10th	Day	2 5% 1 4 2 3 3 2 4 6% 69%	2950
	11th to 20th	Day	4 9% 3 6% 4 4 4 4 5% 10% 47%	3409
	21st to end	Day	3 9% 3 7% 5% 4 4 4 6% 7% 48%	4170
January	1st to 10th	Night	2 7% 3 9% 5% 3 3 3 5% 7% 52%	6306
	11th to 20th	Night	3 8% 3 5% 3 3 3 3 4 4 60%	6104
	21st to end	Night	4 10% 4 6% 3 2 3 2 4 6% 56%	6468
February	1st to 10th	Night	4 9% 3 6% 4 4 4 3 5% 6% 53%	2460
	11th to 20th	Night	4 9% 2 6% 3 3 3 4 5% 8% 53%	2338
	21st to end	Night	3 7% 3 6% 4 4 4 4 6% 6% 53%	1980
March	1st to 10th	Night	3 14% 4 7% 4 3 4 3 4 5% 50%	5376
	11th to 20th	Night	3 8% 2 8% 4 4 5% 4 6% 7% 49%	5011
	21st to end	Night	4 8% 3 6% 4 3 4 3 6% 7% 52%	5489
April	1st to 10th	Night	6% 7% 2 5% 3 3 3 4 6% 6% 55%	2014
	11th to 20th	Night	4 6% 2 5% 4 3 3 4 5% 7% 58%	1959
	21st to end	Night	6% 8% 1 4 3 3 4 3 5% 5% 58%	1800
May	1st to 10th	Night	4 7% 2 5% 4 4 3 3 5% 6% 58%	4100
	11th to 20th	Night	5% 4 2 4 3 3 4 3 5% 6% 61%	4074
	21st to end	Night	5% 5% 2 4 3 3 4 3 5% 7% 59%	4474
June	1st to 10th	Night	3 3 1 5% 3 3 3 4 5% 7% 62%	1754
	11th to 20th	Night	3 4 1 4 3 3 2 2 4 6% 69%	1600
	21st to end	Night	3 2 1 3 2 3 3 3 4 6% 70%	1600
July	1st to 10th	Night	1 2 1 3 2 2 3 2 3 4 78%	3640
	11th to 20th	Night	5% 4 2 6% 5% 5% 5% 5% 7% 7% 50%	3770
	21st to end	Night	2 3 2 5% 5% 5% 5% 3 5% 5% 61%	4472
August	1st to 10th	Night	2 2 1 4 3 3 3 3 4 6% 69%	4033
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 3 81%	4140
	21st to end	Night	2 2 1 3 4 4 4 3 5% 5% 66%	5015
September	1st to 10th	Night	3 2 1 4 3 3 3 4 4 5% 68%	2080
	11th to 20th	Night	2 3 1 4 3 3 3 3 4 5% 70%	2200
	21st to end	Night	3 2 1 3 2 2 2 2 3 4 78%	2200
October	1st to 10th	Night	3 3 1 3 2 2 3 2 5% 4 73%	3600
	11th to 20th	Night	4 5% 2 3 2 3 2 2 3 5% 69%	3741
	21st to end	Night	2 4 1 3 3 2 3 3 4 5% 70%	4411
November	1st to 10th	Night	3 5% 2 4 2 3 3 3 5% 7% 63%	2535
	11th to 20th	Night	2 4 2 4 3 2 3 4 5% 6% 66%	2600
	21st to end	Night	4 6% 2 6% 3 3 3 3 5% 6% 58%	2600
December	1st to 10th	Night	2 4 1 3 3 3 3 3 4 6% 66%	4240
	11th to 20th	Night	3 7% 2 6% 4 3 4 4 5% 9% 54%	4811
	21st to end	Night	3 8% 3 8% 5% 4 5% 4 7% 9% 43%	6759

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	35% 4 18% 22% 17% 4	3639
	11th to 20th	Day	28% 1 13% 16% 26% 16%	3846
	21st to end	Day	33% 4 10% 19% 24% 11%	4510
February	1st to 10th	Day	36% 4 11% 23% 18% 8%	1935
	11th to 20th	Day	34% 4 10% 22% 23% 7%	1960
	21st to end	Day	32% 4 9% 25% 23% 7%	1650
March	1st to 10th	Day	37% 3 5% 25% 18% 11%	4518
	11th to 20th	Day	32% 4 8% 30% 21% 4	4988
	21st to end	Day	37% 5% 12% 25% 17% 4	5486
April	1st to 10th	Day	31% 5% 9% 25% 20% 10%	2382
	11th to 20th	Day	29% 5% 8% 23% 26% 9%	2392
	21st to end	Day	27% 5% 5% 28% 30% 5%	2600
May	1st to 10th	Day	22% 6% 3 30% 30% 8%	5880
	11th to 20th	Day	24% 6% 3 17% 41% 8%	5844
	21st to end	Day	22% 7% 3 17% 42% 10%	6424
June	1st to 10th	Day	23% 4 4 16% 35% 18%	2640
	11th to 20th	Day	18% 3 2 11% 31% 35%	2800
	21st to end	Day	15% 4 1 7% 26% 47%	2800
July	1st to 10th	Day	13% 2 2 3 12% 68%	6251
	11th to 20th	Day	18% 4 4 6% 19% 50%	6118
	21st to end	Day	16% 4 4 9% 18% 49%	6217
August	1st to 10th	Day	15% 2 2 12% 31% 36%	4208
	11th to 20th	Day	13% 3 2 8% 24% 51%	4119
	21st to end	Day	14% 3 2 8% 24% 49%	4311
September	1st to 10th	Day	17% 4 4 10% 29% 37%	2320
	11th to 20th	Day	15% 3 3 15% 31% 32%	2200
	21st to end	Day	16% 3 2 14% 36% 30%	2200
October	1st to 10th	Day	14% 3 3 9% 36% 35%	3600
	11th to 20th	Day	15% 3 2 17% 36% 26%	3367
	21st to end	Day	14% 4 3 25% 43% 12%	3145
November	1st to 10th	Day	24% 4 10% 25% 24% 13%	1764
	11th to 20th	Day	20% 3 9% 28% 32% 8%	1800
	21st to end	Day	29% 4 7% 23% 25% 12%	1800
December	1st to 10th	Day	19% 3 7% 23% 31% 17%	2950
	11th to 20th	Day	34% 5% 12% 22% 23% 5%	3408
	21st to end	Day	35% 4 13% 25% 19% 4	4170
January	1st to 10th	Night	31% 4 17% 22% 22% 4	6306
	11th to 20th	Night	29% 2 13% 17% 27% 12%	6104
	21st to end	Night	32% 2 9% 17% 28% 11%	6468
February	1st to 10th	Night	33% 5% 10% 27% 20% 7%	2460
	11th to 20th	Night	29% 5% 10% 22% 26% 9%	2339
	21st to end	Night	32% 6% 10% 22% 22% 9%	1980
March	1st to 10th	Night	38% 3 6% 18% 22% 13%	5376
	11th to 20th	Night	33% 6% 8% 28% 22% 3	5011
	21st to end	Night	31% 4 12% 26% 25% 3	5489
April	1st to 10th	Night	29% 4 8% 27% 22% 10%	2014
	11th to 20th	Night	26% 4 7% 26% 28% 8%	1959
	21st to end	Night	29% 3 5% 27% 29% 6%	1800
May	1st to 10th	Night	27% 4 4 26% 30% 7%	4097
	11th to 20th	Night	24% 4 3 20% 40% 9%	4075
	21st to end	Night	24% 5% 3 16% 40% 12%	4474
June	1st to 10th	Night	22% 3 5% 18% 34% 18%	1754
	11th to 20th	Night	18% 2 3 12% 29% 35%	1600
	21st to end	Night	16% 2 2 10% 26% 44%	1600
July	1st to 10th	Night	13% 2 1 4 12% 68%	3600
	11th to 20th	Night	29% 4 4 9% 17% 37%	3722
	21st to end	Night	24% 4 4 8% 11% 48%	4344
August	1st to 10th	Night	23% 3 4 11% 27% 32%	2928
	11th to 20th	Night	12% 2 3 6% 25% 52%	2991
	21st to end	Night	13% 3 2 10% 23% 49%	3608
September	1st to 10th	Night	18% 4 4 11% 26% 38%	2080
	11th to 20th	Night	17% 3 4 14% 29% 33%	2200
	21st to end	Night	14% 2 3 13% 35% 33%	2200
October	1st to 10th	Night	15% 3 4 10% 35% 34%	3600
	11th to 20th	Night	20% 3 2 13% 38% 24%	3741
	21st to end	Night	17% 3 3 23% 40% 13%	4411
November	1st to 10th	Night	21% 3 10% 26% 26% 13%	2535
	11th to 20th	Night	20% 4 9% 25% 33% 10%	2600
	21st to end	Night	27% 3 6% 24% 30% 11%	2600
December	1st to 10th	Night	19% 4 8% 23% 31% 15%	4240
	11th to 20th	Night	27% 5% 16% 20% 26% 6%	4811
	21st to end	Night	36% 5% 11% 26% 17% 5%	6759

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	20% 20% 27% 21% 11% 2	200
	11th to 20th	Day	13% 20% 30% 20% 12% 5%	245
	21st to end	Day	12% 13% 25% 31% 14% 5%	330
February	1st to 10th	Day	16% 25% 28% 16% 13% 3	435
	11th to 20th	Day	16% 19% 32% 20% 9% 3	495
	21st to end	Day	13% 20% 38% 21% 7%	414
March	1st to 10th	Day	18% 18% 34% 22% 7% 1	500
	11th to 20th	Day	13% 22% 37% 17% 9% 2	500
	21st to end	Day	17% 28% 29% 19% 6% 2	550
April	1st to 10th	Day	12% 24% 32% 20% 10% 1	500
	11th to 20th	Day	15% 28% 33% 18% 5%	492
	21st to end	Day	14% 22% 34% 19% 10% 1	500
May	1st to 10th	Day	14% 23% 34% 18% 8% 2	503
	11th to 20th	Day	14% 20% 31% 23% 10% 2	528
	21st to end	Day	12% 16% 29% 27% 12% 4	573
June	1st to 10th	Day	8% 13% 30% 30% 15% 5%	530
	11th to 20th	Day	8% 20% 33% 25% 11% 3	530
	21st to end	Day	4 13% 32% 33% 15% 2	530
July	1st to 10th	Day	4 10% 38% 32% 13% 4	526
	11th to 20th	Day	4 4 30% 41% 16% 4	524
	21st to end	Day	4 7% 28% 38% 18% 6%	575
August	1st to 10th	Day	5% 10% 33% 34% 16% 2	527
	11th to 20th	Day	4 9% 30% 36% 16% 5%	517
	21st to end	Day	4 7% 31% 36% 17% 6%	583
September	1st to 10th	Day	4 9% 28% 33% 18% 8%	530
	11th to 20th	Day	5% 18% 28% 25% 17% 5%	530
	21st to end	Day	5% 15% 25% 32% 18% 4	530
October	1st to 10th	Day	8% 17% 28% 24% 18% 4	530
	11th to 20th	Day	10% 17% 25% 25% 18% 5%	409
	21st to end	Day	11% 12% 32% 26% 17% 2	330
November	1st to 10th	Day	12% 17% 24% 27% 17% 3	300
	11th to 20th	Day	7% 18% 31% 24% 15% 5%	300
	21st to end	Day	12% 17% 22% 27% 18% 4	300
December	1st to 10th	Day	9% 16% 28% 27% 14% 6%	300
	11th to 20th	Day	9% 13% 30% 33% 11% 3	300
	21st to end	Day	16% 21% 28% 23% 7% 4	257
January	1st to 10th	Night	10% 12% 27% 27% 18% 7%	800
	11th to 20th	Night	8% 10% 27% 34% 16% 6%	800
	21st to end	Night	9% 11% 26% 31% 18% 5%	880
February	1st to 10th	Night	10% 15% 27% 28% 17% 4	800
	11th to 20th	Night	10% 12% 25% 36% 13% 3	784
	21st to end	Night	10% 12% 34% 30% 12% 3	660
March	1st to 10th	Night	10% 13% 30% 32% 13% 2	800
	11th to 20th	Night	9% 12% 29% 29% 18% 3	800
	21st to end	Night	9% 16% 31% 29% 12% 2	880
April	1st to 10th	Night	11% 16% 27% 33% 12% 2	800
	11th to 20th	Night	8% 17% 35% 26% 13% 2	771
	21st to end	Night	11% 16% 31% 27% 13% 3	600
May	1st to 10th	Night	9% 16% 34% 29% 10% 2	600
	11th to 20th	Night	8% 13% 24% 33% 19% 3	591
	21st to end	Night	9% 14% 25% 30% 19% 2	650
June	1st to 10th	Night	5% 13% 25% 32% 21% 4	554
	11th to 20th	Night	7% 13% 24% 38% 19%	400
	21st to end	Night	4 6% 29% 36% 24% 2	400
July	1st to 10th	Night	4 5% 26% 34% 27% 4	390
	11th to 20th	Night	5% 3 23% 43% 22% 4	440
	21st to end	Night	6% 4 19% 37% 32% 3	657
August	1st to 10th	Night	6% 5% 26% 41% 20% 3	597
	11th to 20th	Night	2 3 24% 42% 25% 4	631
	21st to end	Night	4 5% 18% 40% 29% 4	880
September	1st to 10th	Night	3 6% 18% 34% 33% 6%	800
	11th to 20th	Night	4 9% 21% 29% 29% 8%	800
	21st to end	Night	4 7% 22% 32% 27% 9%	800
October	1st to 10th	Night	5% 10% 23% 35% 22% 5%	800
	11th to 20th	Night	7% 8% 21% 35% 22% 6%	792
	21st to end	Night	4 12% 24% 29% 24% 7%	836
November	1st to 10th	Night	7% 10% 24% 29% 24% 6%	780
	11th to 20th	Night	5% 9% 28% 35% 17% 6%	800
	21st to end	Night	7% 13% 24% 29% 21% 6%	800
December	1st to 10th	Night	8% 10% 24% 37% 16% 5%	800
	11th to 20th	Night	9% 9% 27% 35% 17% 3	800
	21st to end	Night	11% 15% 26% 33% 13% 2	880

Page generated at 2025-10-15 22:50:26. Version ac07

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