Lambert Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lambert 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	16% 8% 3 6% 5% 4 4 4 5% 7% 37%	126428
	Night	16% 8% 3 6% 4 4 4 3 5% 6% 40%	122627
Transparency	Day	44% 5% 11% 22% 15% 2	129942
	Night	43% 4 11% 23% 16% 2	127200
Seeing	Day	25% 16% 29% 19% 8% 2	16050
	Night	23% 16% 30% 23% 8% 1	25186

By Month

Percentage Cloud Cover NE (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	13% 9% 3 7% 5% 4 4 3 5% 5% 42%	3597
	11th to 20th	Day	15% 10% 4 9% 6% 5% 5% 3 4 5% 36%	3570
	21st to end	Day	18% 8% 4 7% 5% 4 4 3 4 5% 38%	4374
February	1st to 10th	Day	16% 9% 2 7% 5% 4 5% 4 5% 7% 36%	1800
	11th to 20th	Day	15% 9% 3 8% 5% 4 3 3 5% 6% 39%	1914
	21st to end	Day	18% 10% 3 6% 5% 4 5% 3 5% 6% 37%	1650
March	1st to 10th	Day	22% 8% 2 6% 4 3 3 3 4 5% 40%	4476
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 4 5% 7% 42%	4875
	21st to end	Day	18% 7% 2 6% 4 4 4 3 4 7% 40%	5869
April	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 4 5% 6% 36%	2400
	11th to 20th	Day	17% 8% 2 7% 4 4 5% 3 4 6% 39%	2374
	21st to end	Day	17% 8% 2 6% 4 5% 4 4 6% 7% 38%	2400
May	1st to 10th	Day	26% 9% 3 6% 4 4 5% 4 5% 7% 28%	5968
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 4 4 6% 32%	6318
	21st to end	Day	20% 7% 2 5% 4 4 5% 4 7% 9% 33%	6949
June	1st to 10th	Day	15% 6% 2 6% 5% 6% 5% 4 6% 8% 35%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 6% 5% 4 6% 10% 33%	2800
	21st to end	Day	13% 6% 3 8% 5% 4 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 4 7% 10% 33%	6360
	11th to 20th	Day	10% 7% 3 7% 6% 6% 7% 5% 8% 11% 31%	6360
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	6926
August	1st to 10th	Day	21% 7% 2 6% 6% 5% 5% 4 7% 7% 31%	4298
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 6% 8% 41%	3850
	21st to end	Day	12% 6% 2 5% 3 4 4 3 6% 9% 46%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 5% 4 4 5% 7% 43%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 6% 49%	2318
	21st to end	Day	18% 7% 2 7% 4 4 3 4 5% 6% 39%	2090
October	1st to 10th	Day	19% 6% 2 6% 3 4 4 3 6% 6% 40%	1941
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 5% 6% 45%	1900
	21st to end	Day	20% 6% 2 5% 4 4 5% 4 4 6% 41%	1966
November	1st to 10th	Day	14% 7% 2 5% 3 3 2 3 4 6% 52%	1761
	11th to 20th	Day	17% 7% 2 4 4 4 4 3 5% 6% 44%	1800
	21st to end	Day	17% 7% 2 6% 3 4 4 5% 5% 7% 39%	1666
December	1st to 10th	Day	14% 10% 3 7% 4 3 4 4 5% 7% 40%	2600
	11th to 20th	Day	20% 10% 3 5% 3 3 3 3 5% 5% 40%	2986
	21st to end	Day	16% 10% 3 8% 5% 5% 4 4 6% 7% 32%	3960
January	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 3 6% 6% 39%	6349
	11th to 20th	Night	13% 8% 4 9% 5% 5% 5% 3 5% 6% 37%	6315
	21st to end	Night	16% 9% 3 6% 4 4 4 4 5% 6% 39%	6566
February	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 39%	2600
	11th to 20th	Night	16% 8% 3 6% 5% 4 5% 4 5% 6% 38%	2370
	21st to end	Night	18% 8% 3 7% 4 4 3 4 5% 6% 39%	1980
March	1st to 10th	Night	18% 10% 3 6% 4 3 3 3 4 6% 40%	5364
	11th to 20th	Night	12% 8% 3 7% 4 4 4 3 4 6% 45%	5081
	21st to end	Night	16% 8% 3 7% 4 4 5% 3 5% 5% 40%	5061
April	1st to 10th	Night	18% 9% 2 7% 5% 5% 4 4 5% 6% 34%	2000
	11th to 20th	Night	18% 7% 3 6% 5% 5% 5% 3 5% 7% 37%	1975
	21st to end	Night	18% 7% 2 6% 3 3 3 4 6% 7% 41%	2000
May	1st to 10th	Night	26% 10% 3 7% 4 4 4 3 5% 7% 26%	4015
	11th to 20th	Night	22% 10% 3 8% 4 4 4 3 5% 5% 33%	3575
	21st to end	Night	21% 7% 2 6% 4 4 4 3 6% 8% 34%	3926
June	1st to 10th	Night	15% 6% 2 6% 4 5% 4 3 5% 7% 43%	1600
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 7% 9% 39%	1600
	21st to end	Night	13% 4 2 6% 5% 4 5% 4 6% 10% 43%	1600
July	1st to 10th	Night	11% 7% 2 7% 5% 4 5% 4 6% 10% 39%	3600
	11th to 20th	Night	13% 7% 3 7% 6% 4 6% 5% 7% 9% 34%	3600
	21st to end	Night	12% 6% 2 5% 3 4 5% 4 6% 8% 45%	3926
August	1st to 10th	Night	20% 7% 3 7% 5% 5% 4 4 6% 7% 32%	2830
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 5% 7% 41%	3182
	21st to end	Night	11% 4 2 5% 3 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	15% 6% 3 5% 4 4 3 3 5% 6% 46%	2000
	11th to 20th	Night	12% 6% 3 5% 3 4 3 3 4 6% 52%	2038
	21st to end	Night	17% 6% 3 6% 3 4 4 2 4 6% 44%	2090
October	1st to 10th	Night	20% 6% 3 5% 4 3 3 3 5% 6% 42%	2236
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	2280
	21st to end	Night	19% 8% 2 5% 3 3 3 3 5% 6% 44%	2631
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 4 5% 6% 50%	2537
	11th to 20th	Night	18% 8% 2 4 3 3 3 3 4 5% 46%	2600
	21st to end	Night	14% 8% 3 5% 4 4 4 4 6% 7% 42%	2730
December	1st to 10th	Night	13% 9% 4 7% 4 4 4 3 4 6% 43%	4580
	11th to 20th	Night	22% 9% 2 5% 4 3 4 3 4 5% 39%	5208
	21st to end	Night	19% 12% 3 7% 4 4 3 3 4 6% 35%	6996

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% 4 8% 29% 15% 2	3598
	11th to 20th	Day	50% 3 9% 22% 12% 5%	3600
	21st to end	Day	47% 4 10% 23% 16% 1	4374
February	1st to 10th	Day	48% 4 12% 22% 13% 1	1800
	11th to 20th	Day	46% 4 8% 22% 17% 2	1923
	21st to end	Day	49% 3 10% 22% 13% 3	1650
March	1st to 10th	Day	47% 3 10% 20% 13% 8%	4479
	11th to 20th	Day	40% 6% 12% 29% 10% 4	4875
	21st to end	Day	45% 4 10% 11% 24% 5%	5869
April	1st to 10th	Day	49% 4 8% 20% 15% 4	2400
	11th to 20th	Day	46% 5% 9% 21% 17% 3	2374
	21st to end	Day	45% 5% 8% 22% 16% 4	2400
May	1st to 10th	Day	57% 5% 12% 15% 10% 1	5979
	11th to 20th	Day	53% 5% 8% 21% 12% 1	6322
	21st to end	Day	47% 6% 11% 22% 13% 1	6949
June	1st to 10th	Day	44% 7% 12% 21% 15% 1	2800
	11th to 20th	Day	46% 6% 13% 23% 11% 2	2800
	21st to end	Day	43% 5% 13% 24% 15% 1	2800
July	1st to 10th	Day	44% 8% 12% 21% 14% 1	6290
	11th to 20th	Day	44% 8% 16% 22% 10% 1	6285
	21st to end	Day	42% 5% 10% 22% 19% 1	6728
August	1st to 10th	Day	50% 6% 13% 21% 9%	4284
	11th to 20th	Day	39% 6% 14% 28% 12% 1	3886
	21st to end	Day	35% 4 10% 32% 17% 2	4312
September	1st to 10th	Day	41% 5% 11% 23% 19% 2	2400
	11th to 20th	Day	36% 5% 8% 24% 25% 2	2318
	21st to end	Day	46% 4 10% 19% 19% 3	2090
October	1st to 10th	Day	41% 4 8% 20% 23% 5%	3227
	11th to 20th	Day	30% 3 10% 27% 26% 4	3140
	21st to end	Day	28% 4 16% 24% 25% 4	3217
November	1st to 10th	Day	36% 3 12% 24% 21% 3	1761
	11th to 20th	Day	40% 4 9% 25% 19% 3	1800
	21st to end	Day	43% 4 13% 28% 11% 2	1666
December	1st to 10th	Day	44% 3 15% 27% 10% 1	2600
	11th to 20th	Day	47% 3 13% 22% 14% 1	2986
	21st to end	Day	50% 5% 10% 19% 15% 2	3960
January	1st to 10th	Night	42% 4 11% 24% 17% 3	6349
	11th to 20th	Night	46% 3 15% 20% 11% 4	6360
	21st to end	Night	46% 3 10% 20% 17% 4	6566
February	1st to 10th	Night	46% 4 13% 24% 12% 2	2600
	11th to 20th	Night	46% 4 9% 20% 17% 4	2372
	21st to end	Night	46% 4 7% 24% 16% 3	1980
March	1st to 10th	Night	46% 3 11% 21% 13% 6%	5374
	11th to 20th	Night	39% 5% 14% 25% 13% 4	5082
	21st to end	Night	45% 5% 7% 16% 22% 5%	5061
April	1st to 10th	Night	49% 5% 9% 20% 13% 3	2000
	11th to 20th	Night	46% 6% 9% 20% 18% 1	1975
	21st to end	Night	43% 5% 8% 23% 17% 4	2000
May	1st to 10th	Night	57% 5% 12% 16% 10% 1	4021
	11th to 20th	Night	53% 6% 11% 15% 13% 1	3576
	21st to end	Night	47% 6% 13% 24% 9% 1	3927
June	1st to 10th	Night	41% 5% 11% 23% 19% 1	1600
	11th to 20th	Night	42% 6% 14% 26% 11% 1	1600
	21st to end	Night	36% 7% 15% 27% 15% 1	1600
July	1st to 10th	Night	41% 7% 15% 20% 16% 2	3560
	11th to 20th	Night	45% 7% 15% 21% 11% 1	3552
	21st to end	Night	36% 6% 12% 25% 19% 1	3809
August	1st to 10th	Night	50% 6% 15% 18% 9% 1	2820
	11th to 20th	Night	43% 5% 14% 28% 10%	3212
	21st to end	Night	32% 5% 11% 34% 18% 1	3586
September	1st to 10th	Night	40% 4 10% 25% 20% 1	2000
	11th to 20th	Night	35% 3 10% 24% 25% 3	2038
	21st to end	Night	42% 4 8% 22% 22% 1	2090
October	1st to 10th	Night	42% 4 10% 23% 20% 1	3730
	11th to 20th	Night	29% 4 10% 27% 27% 4	3779
	21st to end	Night	31% 3 14% 31% 19% 2	4330
November	1st to 10th	Night	34% 4 11% 25% 22% 3	2537
	11th to 20th	Night	41% 3 9% 23% 20% 4	2600
	21st to end	Night	40% 4 13% 29% 13% 1	2730
December	1st to 10th	Night	43% 3 13% 31% 9% 1	4580
	11th to 20th	Night	48% 3 11% 21% 16% 2	5208
	21st to end	Night	50% 5% 9% 17% 16% 3	6996

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	37% 25% 28% 10% 1 1	200
	11th to 20th	Day	40% 27% 27% 7% 1	200
	21st to end	Day	32% 27% 30% 6% 5%	310
February	1st to 10th	Day	37% 27% 28% 7% 1	300
	11th to 20th	Day	41% 25% 28% 6%	462
	21st to end	Day	35% 25% 29% 8% 3	414
March	1st to 10th	Day	32% 24% 29% 12% 3	500
	11th to 20th	Day	30% 24% 30% 11% 3 1	500
	21st to end	Day	34% 24% 28% 10% 3 1	550
April	1st to 10th	Day	35% 20% 30% 11% 4	500
	11th to 20th	Day	30% 24% 27% 12% 6% 1	492
	21st to end	Day	31% 18% 33% 13% 4 1	500
May	1st to 10th	Day	37% 17% 26% 13% 6% 1	503
	11th to 20th	Day	25% 17% 31% 19% 6% 2	527
	21st to end	Day	23% 9% 31% 26% 9% 2	573
June	1st to 10th	Day	21% 14% 31% 22% 10% 1	530
	11th to 20th	Day	23% 13% 28% 25% 11%	530
	21st to end	Day	17% 12% 32% 29% 8% 2	530
July	1st to 10th	Day	15% 8% 28% 29% 15% 5%	525
	11th to 20th	Day	11% 6% 30% 33% 16% 3	524
	21st to end	Day	14% 11% 32% 28% 12% 2	576
August	1st to 10th	Day	16% 5% 28% 31% 13% 6%	526
	11th to 20th	Day	14% 5% 29% 31% 16% 5%	519
	21st to end	Day	14% 9% 31% 27% 14% 6%	583
September	1st to 10th	Day	19% 11% 28% 22% 13% 6%	530
	11th to 20th	Day	14% 9% 28% 25% 13% 10%	522
	21st to end	Day	25% 12% 28% 21% 10% 4	490
October	1st to 10th	Day	27% 11% 26% 20% 12% 4	490
	11th to 20th	Day	20% 19% 31% 21% 7% 2	490
	21st to end	Day	26% 18% 30% 17% 8% 1	473
November	1st to 10th	Day	23% 18% 29% 20% 8% 2	395
	11th to 20th	Day	29% 21% 29% 17% 4 1	400
	21st to end	Day	32% 23% 30% 12% 3	266
December	1st to 10th	Day	34% 27% 25% 11% 5%	200
	11th to 20th	Day	43% 20% 27% 8% 4	200
	21st to end	Day	33% 20% 32% 9% 5% 1	220
January	1st to 10th	Night	25% 28% 33% 13% 2	800
	11th to 20th	Night	30% 20% 30% 18% 2	800
	21st to end	Night	23% 27% 33% 15% 2	880
February	1st to 10th	Night	27% 22% 33% 16% 2	800
	11th to 20th	Night	29% 27% 33% 10% 1	783
	21st to end	Night	31% 24% 30% 14% 2	660
March	1st to 10th	Night	28% 23% 32% 14% 3	800
	11th to 20th	Night	27% 24% 31% 15% 3 1	800
	21st to end	Night	25% 24% 32% 14% 4	880
April	1st to 10th	Night	28% 22% 31% 15% 4	800
	11th to 20th	Night	25% 20% 33% 18% 4	791
	21st to end	Night	25% 16% 32% 22% 5%	800
May	1st to 10th	Night	29% 14% 30% 20% 6%	587
	11th to 20th	Night	20% 14% 36% 22% 7%	394
	21st to end	Night	19% 7% 29% 33% 12%	433
June	1st to 10th	Night	19% 6% 31% 33% 10% 1	400
	11th to 20th	Night	14% 8% 32% 35% 11% 1	400
	21st to end	Night	14% 5% 27% 42% 12%	400
July	1st to 10th	Night	14% 4 24% 38% 18% 2	390
	11th to 20th	Night	14% 2 26% 41% 16% 1	388
	21st to end	Night	12% 5% 25% 42% 15% 1	438
August	1st to 10th	Night	13% 2 22% 39% 22% 2	538
	11th to 20th	Night	16% 4 21% 36% 22% 2	775
	21st to end	Night	15% 5% 23% 37% 19% 2	880
September	1st to 10th	Night	17% 6% 25% 35% 15% 2	800
	11th to 20th	Night	15% 7% 25% 29% 19% 5%	793
	21st to end	Night	21% 7% 26% 30% 14% 2	760
October	1st to 10th	Night	23% 11% 25% 25% 13% 2	760
	11th to 20th	Night	20% 15% 29% 27% 8%	760
	21st to end	Night	26% 16% 31% 19% 7%	836
November	1st to 10th	Night	18% 17% 33% 22% 9% 1	780
	11th to 20th	Night	25% 18% 31% 19% 6% 1	800
	21st to end	Night	22% 21% 34% 18% 5%	800
December	1st to 10th	Night	28% 19% 32% 16% 5% 1	800
	11th to 20th	Night	27% 21% 33% 17% 3	800
	21st to end	Night	28% 21% 32% 17% 2	880

Page generated at 2025-09-16 01:49:11. Version ac07

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