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 2024-11-05.

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%	101361
	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	97696
Transparency	Day	44% 5% 11% 23% 15% 2	104893
	Night	43% 5% 11% 24% 15% 2	102253
Seeing	Day	25% 16% 29% 19% 8% 2	14938
	Night	22% 16% 30% 23% 8% 1	24106

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	14% 9% 3 5% 4 4 4 3 4 3 45%	2517
	11th to 20th	Day	15% 10% 4 9% 6% 5% 5% 3 4 6% 33%	2520
	21st to end	Day	21% 11% 5% 7% 5% 4 3 3 4 5% 32%	3060
February	1st to 10th	Day	16% 9% 2 7% 5% 4 5% 4 5% 7% 36%	1710
	11th to 20th	Day	14% 9% 3 7% 5% 4 3 3 5% 7% 39%	1815
	21st to end	Day	19% 10% 3 6% 5% 4 5% 3 5% 5% 36%	1570
March	1st to 10th	Day	26% 10% 2 6% 4 3 3 2 3 5% 34%	3116
	11th to 20th	Day	10% 7% 2 6% 6% 5% 5% 4 6% 8% 41%	3403
	21st to end	Day	21% 8% 2 5% 4 3 3 3 4 5% 42%	4093
April	1st to 10th	Day	17% 10% 3 7% 5% 4 4 3 5% 6% 37%	2280
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 3 5% 6% 40%	2254
	21st to end	Day	17% 8% 2 6% 5% 5% 4 4 6% 7% 37%	2280
May	1st to 10th	Day	27% 9% 2 6% 4 3 5% 3 4 7% 30%	4160
	11th to 20th	Day	23% 9% 3 8% 5% 5% 4 4 4 6% 28%	4398
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 6% 9% 42%	4837
June	1st to 10th	Day	15% 6% 2 6% 5% 6% 5% 4 6% 8% 35%	2660
	11th to 20th	Day	15% 7% 2 6% 5% 7% 5% 4 6% 10% 33%	2660
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 5% 9% 39%	2660
July	1st to 10th	Day	9% 6% 3 8% 7% 7% 6% 5% 8% 10% 32%	4440
	11th to 20th	Day	10% 6% 3 6% 5% 5% 6% 4 8% 10% 37%	4440
	21st to end	Day	14% 9% 3 8% 6% 6% 5% 4 6% 7% 32%	4814
August	1st to 10th	Day	21% 7% 2 6% 6% 5% 5% 4 7% 7% 30%	4165
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 6% 8% 41%	3730
	21st to end	Day	12% 6% 2 5% 3 4 4 3 6% 9% 46%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 5% 4 4 5% 7% 43%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 6% 50%	2223
	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% 51%	1708
	11th to 20th	Day	18% 7% 2 4 4 4 4 3 5% 6% 44%	1710
	21st to end	Day	16% 7% 3 6% 4 4 4 5% 5% 7% 40%	1583
December	1st to 10th	Day	19% 10% 3 7% 4 3 4 4 5% 6% 36%	1520
	11th to 20th	Day	20% 8% 3 5% 4 4 4 3 6% 5% 39%	1906
	21st to end	Day	17% 12% 4 9% 5% 6% 5% 4 6% 6% 26%	2772
January	1st to 10th	Night	16% 8% 3 5% 4 4 4 3 6% 6% 42%	4429
	11th to 20th	Night	15% 9% 4 8% 4 4 4 3 5% 6% 37%	4440
	21st to end	Night	22% 11% 3 6% 4 4 4 4 5% 5% 33%	4594
February	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 40%	2470
	11th to 20th	Night	16% 8% 3 6% 5% 4 5% 4 4 6% 39%	2249
	21st to end	Night	18% 8% 3 7% 4 4 3 4 5% 6% 39%	1884
March	1st to 10th	Night	22% 12% 3 6% 4 3 2 2 4 6% 35%	3724
	11th to 20th	Night	11% 8% 2 7% 4 4 5% 3 5% 7% 44%	3553
	21st to end	Night	16% 9% 3 6% 4 4 4 3 4 5% 42%	3537
April	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 4 5% 7% 35%	1900
	11th to 20th	Night	17% 7% 3 6% 5% 5% 5% 3 5% 7% 38%	1875
	21st to end	Night	19% 7% 2 6% 3 3 3 4 6% 7% 41%	1900
May	1st to 10th	Night	26% 9% 3 7% 4 3 4 3 5% 7% 30%	2809
	11th to 20th	Night	24% 9% 3 7% 3 4 4 3 4 6% 32%	2495
	21st to end	Night	13% 7% 2 5% 4 4 4 4 6% 8% 41%	2738
June	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 44%	1520
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 7% 9% 40%	1520
	21st to end	Night	12% 3 2 6% 5% 4 5% 4 6% 10% 44%	1520
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	2520
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 5% 7% 10% 39%	2520
	21st to end	Night	15% 8% 2 6% 4 4 5% 4 7% 8% 37%	2739
August	1st to 10th	Night	20% 7% 3 7% 5% 5% 4 4 6% 7% 32%	2743
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 5% 7% 41%	3083
	21st to end	Night	11% 4 2 5% 3 4 4 3 5% 8% 50%	3476
September	1st to 10th	Night	15% 6% 3 5% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	1956
	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%	2470
	11th to 20th	Night	19% 8% 2 4 3 3 3 3 4 5% 46%	2470
	21st to end	Night	14% 7% 3 5% 4 4 4 4 6% 7% 43%	2593
December	1st to 10th	Night	18% 8% 2 7% 4 4 4 3 5% 7% 36%	2660
	11th to 20th	Night	17% 8% 2 5% 5% 4 4 4 5% 7% 38%	3288
	21st to end	Night	19% 11% 4 8% 4 4 4 3 4 6% 33%	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% 3 9% 33% 10% 2	2518
	11th to 20th	Day	52% 4 10% 21% 10% 2	2520
	21st to end	Day	55% 4 9% 20% 11% 1	3060
February	1st to 10th	Day	48% 4 12% 22% 12% 1	1710
	11th to 20th	Day	46% 5% 8% 22% 17% 2	1824
	21st to end	Day	49% 4 9% 22% 12% 3	1570
March	1st to 10th	Day	54% 3 9% 23% 10% 2	3119
	11th to 20th	Day	38% 7% 12% 30% 12% 2	3403
	21st to end	Day	46% 3 7% 15% 24% 5%	4093
April	1st to 10th	Day	48% 4 9% 21% 15% 4	2280
	11th to 20th	Day	45% 5% 9% 21% 17% 3	2254
	21st to end	Day	45% 5% 8% 21% 16% 4	2280
May	1st to 10th	Day	55% 4 11% 16% 12% 2	4174
	11th to 20th	Day	56% 5% 10% 19% 8% 1	4402
	21st to end	Day	36% 6% 12% 26% 18% 2	4837
June	1st to 10th	Day	44% 7% 12% 21% 15% 1	2660
	11th to 20th	Day	45% 6% 13% 24% 11% 2	2660
	21st to end	Day	43% 5% 13% 24% 15% 1	2660
July	1st to 10th	Day	44% 8% 12% 21% 13% 1	4370
	11th to 20th	Day	40% 6% 16% 24% 13% 1	4365
	21st to end	Day	50% 6% 10% 18% 15% 2	4661
August	1st to 10th	Day	50% 7% 13% 20% 9%	4151
	11th to 20th	Day	40% 6% 14% 28% 12% 1	3766
	21st to end	Day	36% 4 10% 32% 16% 2	4180
September	1st to 10th	Day	41% 5% 11% 23% 19% 2	2280
	11th to 20th	Day	36% 4 8% 24% 26% 2	2223
	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	37% 3 12% 25% 20% 3	1708
	11th to 20th	Day	41% 4 9% 24% 19% 3	1710
	21st to end	Day	43% 4 12% 28% 11% 2	1583
December	1st to 10th	Day	49% 3 15% 19% 13% 1	1520
	11th to 20th	Day	46% 4 14% 23% 11% 1	1906
	21st to end	Day	56% 6% 10% 19% 8%	2772
January	1st to 10th	Night	41% 4 11% 30% 11% 3	4429
	11th to 20th	Night	48% 4 12% 24% 11% 2	4440
	21st to end	Night	52% 4 9% 22% 12% 1	4594
February	1st to 10th	Night	45% 4 13% 24% 12% 2	2470
	11th to 20th	Night	46% 5% 9% 20% 17% 4	2251
	21st to end	Night	45% 4 7% 24% 15% 4	1884
March	1st to 10th	Night	52% 3 9% 22% 12% 2	3734
	11th to 20th	Night	37% 5% 13% 27% 16% 2	3554
	21st to end	Night	44% 5% 7% 17% 21% 7%	3537
April	1st to 10th	Night	48% 5% 10% 21% 14% 3	1900
	11th to 20th	Night	45% 5% 9% 20% 19% 1	1875
	21st to end	Night	43% 6% 8% 23% 17% 4	1900
May	1st to 10th	Night	54% 5% 10% 20% 10% 1	2815
	11th to 20th	Night	54% 6% 14% 15% 10% 1	2496
	21st to end	Night	39% 7% 14% 26% 12% 2	2739
June	1st to 10th	Night	40% 5% 12% 24% 18% 1	1520
	11th to 20th	Night	40% 6% 15% 27% 11% 1	1520
	21st to end	Night	35% 7% 14% 28% 15% 1	1520
July	1st to 10th	Night	39% 8% 16% 20% 15% 2	2480
	11th to 20th	Night	39% 7% 16% 25% 12% 1	2472
	21st to end	Night	44% 7% 13% 16% 18% 1	2651
August	1st to 10th	Night	51% 6% 16% 17% 9% 1	2733
	11th to 20th	Night	43% 5% 14% 28% 10%	3113
	21st to end	Night	32% 5% 11% 34% 18% 1	3476
September	1st to 10th	Night	40% 4 10% 25% 20% 1	1900
	11th to 20th	Night	34% 3 10% 25% 25% 3	1956
	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	35% 4 11% 25% 22% 3	2470
	11th to 20th	Night	41% 3 10% 24% 18% 4	2470
	21st to end	Night	39% 4 13% 30% 13% 1	2593
December	1st to 10th	Night	48% 4 12% 23% 12% 1	2660
	11th to 20th	Night	45% 5% 12% 20% 16% 3	3288
	21st to end	Night	50% 6% 11% 21% 11%	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	36% 23% 29% 11% 1 1	180
	11th to 20th	Day	39% 26% 27% 7% 1	180
	21st to end	Day	30% 26% 31% 7% 5%	279
February	1st to 10th	Day	35% 27% 30% 7% 1	270
	11th to 20th	Day	38% 26% 30% 6%	423
	21st to end	Day	35% 24% 30% 9% 3	382
March	1st to 10th	Day	31% 24% 29% 13% 3	460
	11th to 20th	Day	31% 25% 29% 11% 3 1	460
	21st to end	Day	35% 23% 28% 10% 4 1	506
April	1st to 10th	Day	33% 20% 32% 11% 4	460
	11th to 20th	Day	28% 23% 29% 13% 6% 1	452
	21st to end	Day	31% 17% 33% 13% 4 1	460
May	1st to 10th	Day	37% 16% 26% 13% 6% 1	463
	11th to 20th	Day	24% 17% 31% 20% 6% 2	487
	21st to end	Day	21% 9% 30% 28% 10% 2	529
June	1st to 10th	Day	20% 14% 31% 22% 11% 1	490
	11th to 20th	Day	21% 12% 29% 26% 12%	490
	21st to end	Day	17% 12% 32% 28% 9% 2	490
July	1st to 10th	Day	16% 7% 27% 29% 16% 5%	485
	11th to 20th	Day	12% 5% 30% 33% 16% 3	484
	21st to end	Day	14% 12% 33% 27% 12% 2	532
August	1st to 10th	Day	16% 6% 29% 30% 13% 6%	486
	11th to 20th	Day	14% 5% 29% 31% 16% 5%	479
	21st to end	Day	14% 9% 30% 27% 14% 6%	539
September	1st to 10th	Day	21% 11% 27% 21% 14% 7%	490
	11th to 20th	Day	14% 10% 28% 24% 14% 10%	490
	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	24% 18% 27% 21% 9% 2	378
	11th to 20th	Day	30% 21% 28% 16% 4 1	370
	21st to end	Day	29% 23% 31% 14% 3	243
December	1st to 10th	Day	33% 27% 25% 10% 5%	180
	11th to 20th	Day	41% 21% 28% 8% 3	180
	21st to end	Day	34% 21% 31% 8% 5% 1	198
January	1st to 10th	Night	24% 27% 33% 13% 2	760
	11th to 20th	Night	29% 21% 29% 18% 3	760
	21st to end	Night	23% 27% 33% 15% 2	836
February	1st to 10th	Night	26% 22% 33% 16% 2	760
	11th to 20th	Night	27% 27% 34% 10% 1	743
	21st to end	Night	32% 22% 30% 14% 2	628
March	1st to 10th	Night	28% 22% 32% 15% 3	760
	11th to 20th	Night	27% 25% 30% 15% 3 1	760
	21st to end	Night	25% 24% 32% 14% 5%	836
April	1st to 10th	Night	26% 23% 32% 16% 4	760
	11th to 20th	Night	24% 19% 34% 19% 5%	751
	21st to end	Night	25% 16% 32% 21% 5%	760
May	1st to 10th	Night	29% 14% 30% 20% 6%	558
	11th to 20th	Night	20% 14% 38% 22% 7%	374
	21st to end	Night	18% 7% 28% 34% 13%	411
June	1st to 10th	Night	19% 6% 31% 34% 10% 1	380
	11th to 20th	Night	13% 7% 32% 36% 12% 1	380
	21st to end	Night	14% 5% 27% 42% 11%	380
July	1st to 10th	Night	13% 4 25% 37% 18% 2	370
	11th to 20th	Night	14% 2 26% 41% 16% 1	368
	21st to end	Night	12% 5% 25% 42% 15% 1	416
August	1st to 10th	Night	13% 2 23% 39% 20% 3	511
	11th to 20th	Night	16% 4 20% 35% 22% 2	736
	21st to end	Night	15% 5% 22% 37% 19% 2	836
September	1st to 10th	Night	17% 6% 24% 35% 15% 2	760
	11th to 20th	Night	15% 7% 26% 28% 19% 5%	760
	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% 32% 22% 9% 1	760
	11th to 20th	Night	25% 18% 31% 19% 6% 1	760
	21st to end	Night	20% 21% 34% 19% 5%	760
December	1st to 10th	Night	28% 19% 32% 16% 5% 1	760
	11th to 20th	Night	26% 21% 34% 17% 3	760
	21st to end	Night	27% 22% 33% 16% 2	836

Page generated at 2024-11-03 02:11:14. Version ac07

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