Limber Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Limber Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	13% 6% 2 5% 4 4 4 3 5% 7% 46%	126227
	Night	14% 6% 2 5% 3 3 3 3 5% 7% 50%	123122
Transparency	Day	37% 4 9% 21% 18% 11%	130045
	Night	36% 4 10% 20% 19% 13%	127367
Seeing	Day	15% 9% 25% 30% 17% 3	15954
	Night	17% 8% 26% 35% 13% 1	25825

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	15% 8% 2 5% 2 2 2 2 3 5% 51%	4077
	11th to 20th	Day	16% 7% 2 3 2 2 2 2 4 5% 53%	4100
	21st to end	Day	25% 8% 2 4 2 2 2 2 4 7% 43%	4994
February	1st to 10th	Day	17% 9% 2 4 3 3 4 3 4 4 46%	2000
	11th to 20th	Day	19% 7% 2 5% 3 3 4 3 4 4 45%	1949
	21st to end	Day	18% 6% 2 4 3 2 3 3 4 5% 50%	1650
March	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 54%	4777
	11th to 20th	Day	16% 7% 2 5% 3 3 3 2 3 4 51%	4980
	21st to end	Day	18% 8% 2 5% 3 3 3 2 4 6% 45%	5462
April	1st to 10th	Day	15% 8% 2 6% 4 3 3 3 4 6% 46%	2200
	11th to 20th	Day	18% 7% 2 6% 4 3 3 4 5% 5% 43%	2254
	21st to end	Day	16% 7% 2 6% 5% 3 4 4 5% 6% 43%	2480
May	1st to 10th	Day	20% 7% 3 6% 4 4 5% 3 5% 7% 36%	5862
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 8% 47%	5862
	21st to end	Day	13% 8% 3 7% 5% 5% 6% 4 7% 8% 36%	6434
June	1st to 10th	Day	8% 4 1 5% 5% 4 5% 5% 7% 9% 46%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 6% 8% 45%	2600
	21st to end	Day	9% 6% 2 7% 5% 4 7% 6% 7% 9% 39%	2600
July	1st to 10th	Day	14% 8% 2 7% 4 5% 5% 4 6% 8% 36%	5900
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	5900
	21st to end	Day	7% 5% 2 7% 5% 5% 5% 5% 8% 9% 42%	6422
August	1st to 10th	Day	1 2 1 2 3 3 4 3 5% 9% 66%	4017
	11th to 20th	Day	4 2 1 3 3 4 4 3 7% 12% 57%	3846
	21st to end	Day	4 4 1 6% 5% 5% 5% 4 7% 11% 46%	4199
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 7% 40%	2200
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 42%	2178
	21st to end	Day	8% 4 2 6% 5% 4 6% 4 7% 7% 48%	2090
October	1st to 10th	Day	7% 5% 2 5% 4 3 4 4 5% 7% 55%	2090
	11th to 20th	Day	14% 5% 1 5% 3 3 4 3 5% 6% 51%	2022
	21st to end	Day	13% 4 2 5% 3 3 3 3 4 4 58%	2090
November	1st to 10th	Day	16% 7% 3 6% 3 3 3 3 5% 5% 45%	1958
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 4 5% 50%	1815
	21st to end	Day	17% 7% 3 5% 4 3 3 3 3 5% 47%	1800
December	1st to 10th	Day	26% 9% 3 6% 3 2 2 2 3 6% 37%	2940
	11th to 20th	Day	17% 8% 2 5% 4 4 4 3 4 5% 44%	3391
	21st to end	Day	15% 5% 1 4 2 2 3 2 2 4 59%	4488
January	1st to 10th	Night	17% 5% 2 4 3 2 2 2 4 6% 52%	5888
	11th to 20th	Night	12% 4 1 3 2 2 3 3 4 5% 63%	5900
	21st to end	Night	29% 5% 2 4 3 2 3 2 4 5% 41%	6006
February	1st to 10th	Night	17% 7% 2 4 3 3 3 2 4 6% 51%	2400
	11th to 20th	Night	19% 7% 2 4 3 3 2 3 3 4 50%	2338
	21st to end	Night	18% 7% 2 4 3 3 3 3 4 6% 47%	1980
March	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 5% 5% 46%	5081
	11th to 20th	Night	15% 8% 2 5% 3 3 3 2 3 5% 52%	4980
	21st to end	Night	20% 9% 2 4 3 3 3 3 4 5% 45%	5467
April	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 5% 6% 48%	2200
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 5% 47%	2091
	21st to end	Night	20% 6% 2 5% 3 3 2 3 4 4 46%	1916
May	1st to 10th	Night	21% 10% 2 6% 4 3 4 2 4 6% 38%	4053
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 50%	4053
	21st to end	Night	13% 7% 2 6% 4 4 4 4 6% 9% 39%	4443
June	1st to 10th	Night	7% 3 1 3 3 3 4 3 6% 9% 58%	1800
	11th to 20th	Night	7% 4 1 4 3 4 3 4 5% 9% 57%	1800
	21st to end	Night	7% 5% 2 5% 4 4 4 5% 6% 10% 49%	1800
July	1st to 10th	Night	11% 7% 2 6% 5% 4 4 4 6% 11% 39%	4078
	11th to 20th	Night	5% 5% 2 5% 4 5% 5% 4 7% 10% 48%	4077
	21st to end	Night	8% 6% 2 6% 4 5% 4 4 6% 9% 46%	4449
August	1st to 10th	Night	2 1 1 2 2 2 3 3 5% 8% 71%	3137
	11th to 20th	Night	3 2 1 3 2 3 4 4 6% 11% 60%	3213
	21st to end	Night	4 4 2 4 4 3 5% 4 7% 10% 54%	3693
September	1st to 10th	Night	8% 5% 3 6% 4 5% 5% 3 6% 8% 46%	2200
	11th to 20th	Night	10% 6% 2 6% 4 3 4 4 6% 8% 46%	2178
	21st to end	Night	8% 5% 2 6% 4 3 4 3 6% 8% 50%	2090
October	1st to 10th	Night	7% 4 1 4 3 3 3 3 5% 6% 61%	2090
	11th to 20th	Night	13% 5% 2 4 3 2 3 4 4 6% 54%	2137
	21st to end	Night	15% 4 1 3 3 2 2 2 2 5% 61%	2508
November	1st to 10th	Night	13% 7% 2 5% 3 3 3 2 4 6% 52%	2340
	11th to 20th	Night	12% 5% 1 4 3 3 3 3 4 6% 55%	2580
	21st to end	Night	19% 4 2 4 3 3 3 3 5% 6% 47%	2600
December	1st to 10th	Night	25% 9% 2 5% 3 2 3 3 3 5% 38%	4250
	11th to 20th	Night	22% 8% 2 5% 2 2 2 2 3 4 47%	4816
	21st to end	Night	17% 6% 1 3 2 2 2 2 3 4 57%	6490

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	37% 2 2 19% 25% 14%	4078
	11th to 20th	Day	36% 1 4 15% 23% 21%	4100
	21st to end	Day	45% 2 3 11% 26% 13%	4994
February	1st to 10th	Day	43% 3 4 13% 20% 17%	2000
	11th to 20th	Day	43% 3 5% 18% 18% 13%	1959
	21st to end	Day	40% 1 6% 18% 19% 16%	1650
March	1st to 10th	Day	35% 2 4 18% 17% 24%	4777
	11th to 20th	Day	40% 2 4 9% 15% 31%	4980
	21st to end	Day	41% 2 7% 18% 20% 11%	5464
April	1st to 10th	Day	42% 3 7% 17% 18% 13%	2200
	11th to 20th	Day	45% 3 5% 17% 17% 13%	2254
	21st to end	Day	44% 3 7% 19% 16% 11%	2480
May	1st to 10th	Day	49% 3 9% 19% 15% 5%	5900
	11th to 20th	Day	36% 3 11% 21% 20% 9%	5866
	21st to end	Day	45% 4 9% 28% 12% 1	6447
June	1st to 10th	Day	32% 5% 13% 27% 19% 3	2600
	11th to 20th	Day	36% 4 11% 25% 19% 4	2600
	21st to end	Day	40% 7% 11% 25% 14% 3	2600
July	1st to 10th	Day	45% 6% 16% 28% 5% 1	5836
	11th to 20th	Day	34% 6% 18% 29% 12% 1	5831
	21st to end	Day	34% 7% 17% 25% 15% 2	6240
August	1st to 10th	Day	15% 5% 11% 45% 24%	4003
	11th to 20th	Day	20% 5% 25% 37% 11% 2	3882
	21st to end	Day	30% 8% 18% 24% 18% 2	4198
September	1st to 10th	Day	42% 4 10% 19% 17% 7%	2200
	11th to 20th	Day	39% 6% 10% 21% 18% 5%	2178
	21st to end	Day	34% 4 10% 22% 18% 12%	2090
October	1st to 10th	Day	19% 3 6% 12% 25% 36%	3480
	11th to 20th	Day	27% 4 7% 18% 22% 21%	3340
	21st to end	Day	28% 3 4 17% 33% 13%	3428
November	1st to 10th	Day	43% 3 6% 20% 16% 12%	1958
	11th to 20th	Day	37% 3 7% 16% 20% 17%	1815
	21st to end	Day	42% 3 5% 17% 19% 14%	1800
December	1st to 10th	Day	52% 2 5% 18% 14% 10%	2940
	11th to 20th	Day	43% 3 7% 13% 19% 14%	3389
	21st to end	Day	32% 2 3 16% 28% 18%	4488
January	1st to 10th	Night	35% 2 4 18% 24% 16%	5888
	11th to 20th	Night	26% 1 3 18% 27% 23%	5900
	21st to end	Night	48% 1 2 11% 21% 16%	6006
February	1st to 10th	Night	38% 2 6% 14% 21% 19%	2400
	11th to 20th	Night	41% 3 5% 17% 19% 16%	2338
	21st to end	Night	41% 3 5% 15% 19% 17%	1980
March	1st to 10th	Night	41% 3 5% 18% 17% 17%	5081
	11th to 20th	Night	39% 2 5% 12% 14% 27%	4980
	21st to end	Night	43% 3 8% 19% 17% 12%	5467
April	1st to 10th	Night	39% 3 9% 17% 16% 16%	2200
	11th to 20th	Night	42% 3 8% 14% 17% 16%	2091
	21st to end	Night	44% 4 8% 18% 16% 11%	1916
May	1st to 10th	Night	50% 3 7% 17% 14% 8%	4080
	11th to 20th	Night	34% 4 11% 21% 20% 11%	4054
	21st to end	Night	43% 5% 12% 28% 10% 2	4452
June	1st to 10th	Night	25% 4 17% 32% 19% 3	1800
	11th to 20th	Night	26% 5% 15% 29% 21% 5%	1800
	21st to end	Night	32% 6% 19% 27% 14% 3	1800
July	1st to 10th	Night	40% 9% 22% 25% 4 1	4035
	11th to 20th	Night	30% 8% 22% 28% 10% 1	4026
	21st to end	Night	34% 8% 20% 23% 12% 2	4320
August	1st to 10th	Night	13% 4 17% 46% 20%	3127
	11th to 20th	Night	20% 6% 31% 34% 8% 1	3243
	21st to end	Night	26% 9% 22% 25% 16% 2	3693
September	1st to 10th	Night	36% 6% 14% 20% 17% 7%	2200
	11th to 20th	Night	36% 8% 14% 22% 15% 5%	2178
	21st to end	Night	31% 5% 12% 22% 18% 12%	2090
October	1st to 10th	Night	17% 2 7% 16% 21% 37%	3480
	11th to 20th	Night	24% 3 9% 15% 30% 18%	3545
	21st to end	Night	27% 2 6% 17% 33% 15%	4120
November	1st to 10th	Night	37% 3 6% 19% 21% 14%	2340
	11th to 20th	Night	32% 3 7% 19% 20% 18%	2580
	21st to end	Night	39% 3 7% 17% 19% 15%	2600
December	1st to 10th	Night	51% 2 6% 15% 14% 12%	4250
	11th to 20th	Night	44% 2 4 11% 20% 19%	4817
	21st to end	Night	34% 2 3 14% 28% 19%	6490

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	24% 18% 35% 20% 4	300
	11th to 20th	Day	26% 18% 29% 21% 6%	300
	21st to end	Day	23% 20% 33% 19% 5%	330
February	1st to 10th	Day	28% 18% 27% 23% 5%	300
	11th to 20th	Day	28% 22% 33% 13% 4	300
	21st to end	Day	27% 19% 39% 13% 2	249
March	1st to 10th	Day	27% 19% 40% 13% 1	300
	11th to 20th	Day	20% 23% 42% 13% 3	300
	21st to end	Day	22% 21% 38% 17% 2	330
April	1st to 10th	Day	23% 20% 37% 18% 2	300
	11th to 20th	Day	20% 23% 34% 20% 3	372
	21st to end	Day	20% 17% 43% 17% 3	580
May	1st to 10th	Day	20% 15% 38% 23% 4	703
	11th to 20th	Day	16% 14% 34% 30% 5% 1	725
	21st to end	Day	18% 7% 33% 31% 11%	789
June	1st to 10th	Day	8% 4 24% 43% 19% 2	730
	11th to 20th	Day	9% 3 26% 44% 16% 2	730
	21st to end	Day	10% 4 18% 38% 26% 5%	730
July	1st to 10th	Day	13% 3 13% 36% 28% 6%	721
	11th to 20th	Day	7% 2 9% 41% 37% 3	718
	21st to end	Day	5% 2 10% 39% 37% 8%	794
August	1st to 10th	Day	3 1 5% 44% 38% 10%	600
	11th to 20th	Day	6% 1 7% 43% 35% 8%	517
	21st to end	Day	5% 2 7% 37% 40% 10%	522
September	1st to 10th	Day	12% 2 11% 31% 35% 10%	330
	11th to 20th	Day	6% 2 5% 39% 44% 5%	324
	21st to end	Day	8% 2 15% 33% 34% 8%	300
October	1st to 10th	Day	5% 3 18% 41% 27% 5%	300
	11th to 20th	Day	13% 4 23% 42% 16% 2	300
	21st to end	Day	19% 14% 26% 25% 13% 3	330
November	1st to 10th	Day	18% 11% 35% 23% 11% 2	300
	11th to 20th	Day	21% 11% 32% 25% 8% 2	300
	21st to end	Day	25% 17% 35% 15% 8% 1	300
December	1st to 10th	Day	20% 13% 32% 26% 8% 1	300
	11th to 20th	Day	22% 17% 32% 23% 5% 1	300
	21st to end	Day	22% 13% 34% 24% 6% 1	330
January	1st to 10th	Night	20% 12% 26% 35% 8%	1000
	11th to 20th	Night	21% 13% 31% 29% 7%	1000
	21st to end	Night	24% 11% 27% 31% 7%	889
February	1st to 10th	Night	24% 14% 28% 27% 7% 1	800
	11th to 20th	Night	27% 14% 31% 24% 4	784
	21st to end	Night	25% 13% 38% 23% 2	660
March	1st to 10th	Night	23% 15% 34% 25% 3	676
	11th to 20th	Night	22% 13% 36% 26% 3	600
	21st to end	Night	21% 14% 38% 23% 3	660
April	1st to 10th	Night	16% 16% 40% 26% 2	600
	11th to 20th	Night	21% 15% 41% 22% 2	593
	21st to end	Night	24% 12% 38% 21% 5% 1	600
May	1st to 10th	Night	18% 13% 41% 26% 3	600
	11th to 20th	Night	16% 10% 36% 32% 6% 1	591
	21st to end	Night	17% 8% 37% 30% 8%	650
June	1st to 10th	Night	7% 3 31% 43% 16% 1	600
	11th to 20th	Night	8% 2 38% 42% 10%	600
	21st to end	Night	8% 3 25% 41% 22% 1	600
July	1st to 10th	Night	11% 2 16% 48% 22% 1	585
	11th to 20th	Night	4 2 14% 54% 24% 2	582
	21st to end	Night	6% 2 13% 51% 25% 3	657
August	1st to 10th	Night	2 1 12% 51% 33% 1	597
	11th to 20th	Night	5% 2 11% 52% 29% 2	594
	21st to end	Night	6% 2 8% 45% 36% 3	660
September	1st to 10th	Night	9% 1 11% 45% 30% 4	600
	11th to 20th	Night	11% 2 11% 40% 33% 2	594
	21st to end	Night	10% 2 14% 41% 30% 4	570
October	1st to 10th	Night	7% 2 13% 52% 24% 3	570
	11th to 20th	Night	15% 5% 23% 39% 18% 1	617
	21st to end	Night	17% 7% 24% 29% 19% 2	836
November	1st to 10th	Night	15% 8% 22% 36% 18% 2	780
	11th to 20th	Night	15% 8% 24% 38% 14% 1	980
	21st to end	Night	23% 11% 25% 30% 11% 1	1000
December	1st to 10th	Night	22% 8% 25% 35% 10% 1	1000
	11th to 20th	Night	26% 11% 25% 30% 8%	1000
	21st to end	Night	22% 9% 26% 34% 8%	1100

Page generated at 2025-09-16 00:49:10. Version ac07

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