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 2024-01-12.

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% 7% 3 5% 5% 3 5% 5% 5% 7% 38%	74745
	Night	16% 7% 3 5% 5% 3 4 4 5% 7% 41%	74969
Transparency	Day	45% 5% 11% 22% 15% 2	74450
	Night	43% 5% 11% 23% 16% 2	74798
Seeing	Day	25% 16% 29% 19% 8% 2	13801
	Night	22% 16% 30% 23% 8% 1	23101

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% 8% 4 5% 6% 3 5% 5% 5% 5% 41%	1517
	11th to 20th	Day	16% 8% 4 6% 6% 4 6% 4 5% 7% 35%	1454
	21st to end	Day	13% 8% 3 5% 6% 3 4 5% 5% 7% 40%	1746
February	1st to 10th	Day	17% 9% 2 6% 6% 3 5% 5% 5% 7% 34%	1620
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 4 5% 6% 39%	1717
	21st to end	Day	19% 9% 3 5% 6% 3 4 4 5% 6% 36%	1480
March	1st to 10th	Day	14% 7% 4 6% 6% 3 4 4 5% 7% 41%	1797
	11th to 20th	Day	15% 8% 2 5% 6% 3 6% 5% 5% 8% 36%	1931
	21st to end	Day	20% 8% 3 5% 5% 3 4 4 4 6% 38%	2331
April	1st to 10th	Day	17% 9% 3 6% 5% 3 4 4 5% 6% 38%	2160
	11th to 20th	Day	16% 7% 3 6% 5% 3 5% 4 4 7% 41%	2134
	21st to end	Day	18% 7% 2 5% 6% 3 5% 5% 5% 7% 37%	2160
May	1st to 10th	Day	22% 8% 3 5% 6% 3 5% 4 4 6% 33%	2366
	11th to 20th	Day	19% 6% 3 6% 6% 4 5% 5% 4 8% 35%	2478
	21st to end	Day	14% 7% 3 5% 5% 4 5% 4 5% 9% 39%	2725
June	1st to 10th	Day	16% 5% 3 5% 6% 4 6% 5% 7% 8% 36%	2520
	11th to 20th	Day	16% 7% 2 5% 6% 5% 5% 5% 6% 10% 32%	2520
	21st to end	Day	13% 6% 3 6% 6% 3 5% 5% 5% 9% 39%	2520
July	1st to 10th	Day	12% 6% 3 6% 7% 4 5% 5% 6% 10% 36%	2520
	11th to 20th	Day	11% 4 2 5% 6% 4 6% 6% 8% 10% 39%	2520
	21st to end	Day	11% 6% 3 6% 6% 4 6% 5% 7% 8% 37%	2744
August	1st to 10th	Day	12% 6% 2 6% 7% 4 6% 6% 8% 8% 36%	2373
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 7% 6% 8% 37%	2126
	21st to end	Day	14% 6% 3 5% 4 4 5% 5% 6% 8% 41%	2376
September	1st to 10th	Day	15% 6% 2 5% 5% 3 5% 4 5% 7% 42%	2160
	11th to 20th	Day	13% 4 2 5% 5% 3 4 4 4 6% 50%	2106
	21st to end	Day	17% 7% 2 6% 5% 3 4 5% 5% 7% 40%	1980
October	1st to 10th	Day	20% 6% 3 5% 4 3 4 4 6% 6% 39%	1839
	11th to 20th	Day	15% 7% 2 4 5% 3 4 5% 5% 7% 44%	1800
	21st to end	Day	21% 6% 2 5% 5% 3 5% 5% 4 6% 38%	1863
November	1st to 10th	Day	15% 7% 2 4 4 3 2 3 4 6% 51%	1671
	11th to 20th	Day	18% 7% 2 3 4 3 4 5% 5% 6% 44%	1710
	21st to end	Day	17% 6% 3 5% 4 3 4 5% 5% 7% 40%	1583
December	1st to 10th	Day	19% 9% 3 6% 6% 2 4 4 4 7% 36%	1520
	11th to 20th	Day	20% 7% 2 5% 5% 3 5% 4 5% 5% 39%	1906
	21st to end	Day	17% 12% 4 9% 6% 4 5% 5% 5% 6% 26%	2772
January	1st to 10th	Night	15% 8% 2 4 4 3 4 5% 6% 6% 42%	2649
	11th to 20th	Night	17% 8% 4 6% 6% 3 4 4 5% 8% 36%	2546
	21st to end	Night	12% 9% 4 5% 5% 2 4 5% 6% 6% 42%	2608
February	1st to 10th	Night	17% 9% 3 5% 5% 2 4 4 5% 6% 39%	2340
	11th to 20th	Night	16% 8% 3 6% 5% 3 5% 5% 4 6% 38%	2127
	21st to end	Night	19% 7% 3 6% 5% 3 3 4 5% 5% 39%	1776
March	1st to 10th	Night	16% 7% 3 5% 5% 3 3 4 5% 8% 43%	2150
	11th to 20th	Night	17% 8% 3 5% 4 2 5% 4 5% 6% 41%	2025
	21st to end	Night	17% 7% 3 4 4 3 5% 5% 5% 7% 39%	2023
April	1st to 10th	Night	17% 8% 3 6% 6% 3 6% 5% 6% 7% 35%	1800
	11th to 20th	Night	17% 7% 3 5% 5% 4 5% 3 5% 8% 38%	1775
	21st to end	Night	19% 6% 3 5% 4 3 3 5% 5% 7% 40%	1800
May	1st to 10th	Night	21% 8% 4 6% 5% 3 4 5% 5% 6% 33%	1609
	11th to 20th	Night	18% 6% 3 7% 3 3 4 5% 5% 7% 39%	1415
	21st to end	Night	14% 7% 2 5% 5% 3 4 5% 6% 7% 42%	1550
June	1st to 10th	Night	17% 5% 2 5% 4 4 4 4 5% 7% 43%	1440
	11th to 20th	Night	13% 7% 4 6% 4 3 4 5% 6% 10% 39%	1440
	21st to end	Night	12% 3 2 4 6% 3 4 5% 6% 10% 45%	1440
July	1st to 10th	Night	12% 5% 2 6% 6% 3 4 4 7% 9% 42%	1440
	11th to 20th	Night	12% 4 3 5% 5% 3 4 6% 7% 10% 42%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 5% 7% 11% 42%	1574
August	1st to 10th	Night	10% 5% 3 6% 5% 4 5% 6% 7% 8% 42%	1557
	11th to 20th	Night	11% 8% 3 7% 5% 3 5% 6% 6% 8% 37%	1760
	21st to end	Night	14% 5% 2 5% 6% 3 4 5% 5% 8% 43%	1980
September	1st to 10th	Night	16% 6% 3 5% 5% 3 3 4 5% 6% 44%	1800
	11th to 20th	Night	13% 5% 3 4 4 2 3 3 4 7% 52%	1853
	21st to end	Night	16% 5% 3 5% 4 3 4 3 4 6% 46%	1980
October	1st to 10th	Night	21% 6% 3 4 4 2 3 4 4 6% 42%	2118
	11th to 20th	Night	17% 6% 2 5% 4 2 3 4 4 5% 47%	2160
	21st to end	Night	20% 8% 2 5% 4 2 3 3 4 6% 42%	2492
November	1st to 10th	Night	13% 5% 2 5% 4 2 3 5% 5% 6% 50%	2407
	11th to 20th	Night	19% 7% 2 4 4 2 3 4 4 6% 46%	2470
	21st to end	Night	14% 6% 3 5% 4 3 5% 4 6% 7% 43%	2593
December	1st to 10th	Night	19% 8% 3 6% 5% 3 4 4 5% 7% 36%	2660
	11th to 20th	Night	18% 8% 2 5% 5% 3 5% 5% 5% 7% 38%	3288
	21st to end	Night	20% 10% 4 6% 5% 3 4 4 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	44% 4 10% 27% 12% 3	1518
	11th to 20th	Day	50% 4 13% 20% 11% 3	1454
	21st to end	Day	44% 4 10% 24% 16% 2	1746
February	1st to 10th	Day	50% 4 11% 21% 12% 2	1620
	11th to 20th	Day	47% 5% 8% 22% 16% 3	1726
	21st to end	Day	50% 3 9% 22% 12% 3	1480
March	1st to 10th	Day	43% 3 12% 24% 14% 3	1800
	11th to 20th	Day	45% 5% 11% 23% 15% 2	1931
	21st to end	Day	49% 3 8% 18% 17% 5%	2331
April	1st to 10th	Day	47% 4 9% 21% 15% 4	2160
	11th to 20th	Day	44% 5% 9% 22% 17% 3	2134
	21st to end	Day	46% 5% 8% 21% 15% 4	2160
May	1st to 10th	Day	52% 4 8% 19% 16% 2	2366
	11th to 20th	Day	48% 5% 10% 21% 14% 3	2482
	21st to end	Day	43% 5% 15% 22% 14% 2	2725
June	1st to 10th	Day	45% 7% 13% 21% 14% 1	2520
	11th to 20th	Day	47% 6% 12% 23% 11% 2	2520
	21st to end	Day	43% 5% 12% 25% 14% 1	2520
July	1st to 10th	Day	43% 6% 12% 22% 16% 1	2450
	11th to 20th	Day	38% 7% 15% 25% 14% 1	2445
	21st to end	Day	43% 6% 10% 21% 17% 2	2591
August	1st to 10th	Day	42% 8% 13% 23% 14%	2359
	11th to 20th	Day	43% 7% 13% 24% 12% 1	2126
	21st to end	Day	41% 5% 10% 24% 18% 3	2376
September	1st to 10th	Day	42% 5% 11% 23% 17% 2	2160
	11th to 20th	Day	36% 4 8% 23% 26% 3	2106
	21st to end	Day	43% 5% 10% 19% 20% 3	1980
October	1st to 10th	Day	45% 5% 10% 22% 15% 2	1839
	11th to 20th	Day	40% 4 11% 25% 18% 1	1800
	21st to end	Day	46% 5% 9% 22% 15% 2	1863
November	1st to 10th	Day	37% 3 12% 25% 20% 4	1671
	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% 25% 15% 4	2649
	11th to 20th	Night	48% 4 14% 23% 10% 2	2546
	21st to end	Night	41% 4 9% 25% 19% 2	2608
February	1st to 10th	Night	46% 4 13% 23% 12% 2	2340
	11th to 20th	Night	46% 5% 9% 19% 16% 4	2129
	21st to end	Night	46% 4 6% 25% 16% 3	1776
March	1st to 10th	Night	41% 4 12% 25% 15% 4	2160
	11th to 20th	Night	45% 3 10% 22% 18% 3	2026
	21st to end	Night	43% 5% 10% 18% 19% 5%	2023
April	1st to 10th	Night	48% 5% 10% 21% 14% 3	1800
	11th to 20th	Night	45% 5% 9% 21% 19% 1	1775
	21st to end	Night	44% 5% 8% 23% 16% 4	1800
May	1st to 10th	Night	51% 5% 9% 18% 15% 2	1609
	11th to 20th	Night	45% 6% 12% 18% 18% 2	1416
	21st to end	Night	41% 6% 17% 23% 11% 2	1551
June	1st to 10th	Night	41% 5% 12% 24% 17% 2	1440
	11th to 20th	Night	41% 6% 14% 26% 11% 1	1440
	21st to end	Night	34% 7% 15% 28% 15% 1	1440
July	1st to 10th	Night	39% 8% 14% 22% 15% 2	1400
	11th to 20th	Night	37% 8% 15% 27% 12% 1	1392
	21st to end	Night	33% 9% 16% 22% 17% 2	1486
August	1st to 10th	Night	37% 8% 15% 23% 15% 1	1547
	11th to 20th	Night	43% 7% 14% 23% 12% 1	1760
	21st to end	Night	40% 6% 11% 21% 21% 2	1980
September	1st to 10th	Night	41% 5% 11% 25% 18% 1	1800
	11th to 20th	Night	35% 3 10% 24% 25% 3	1853
	21st to end	Night	40% 4 8% 23% 23% 1	1980
October	1st to 10th	Night	44% 4 11% 24% 15% 2	2118
	11th to 20th	Night	39% 4 12% 28% 15% 2	2160
	21st to end	Night	44% 4 11% 24% 16% 1	2492
November	1st to 10th	Night	35% 5% 11% 24% 21% 3	2407
	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	35% 23% 29% 11% 1 1	180
	11th to 20th	Day	38% 25% 28% 9% 1	164
	21st to end	Day	30% 29% 30% 6% 5%	248
February	1st to 10th	Day	39% 28% 26% 6% 1	240
	11th to 20th	Day	38% 26% 31% 6%	385
	21st to end	Day	36% 25% 28% 8% 2	346
March	1st to 10th	Day	30% 25% 30% 12% 3	420
	11th to 20th	Day	30% 24% 30% 11% 3 1	420
	21st to end	Day	34% 23% 28% 10% 4 1	462
April	1st to 10th	Day	32% 20% 31% 12% 5%	420
	11th to 20th	Day	26% 23% 30% 14% 7% 1	412
	21st to end	Day	33% 17% 32% 13% 4 1	420
May	1st to 10th	Day	37% 16% 26% 13% 6% 1	423
	11th to 20th	Day	24% 18% 32% 19% 5% 2	447
	21st to end	Day	22% 8% 29% 29% 10% 2	485
June	1st to 10th	Day	20% 15% 31% 22% 11% 1	450
	11th to 20th	Day	23% 13% 28% 24% 11%	450
	21st to end	Day	16% 12% 33% 28% 9% 2	450
July	1st to 10th	Day	16% 7% 26% 29% 16% 5%	445
	11th to 20th	Day	12% 5% 29% 34% 17% 3	444
	21st to end	Day	15% 11% 32% 28% 13% 2	488
August	1st to 10th	Day	14% 5% 28% 32% 14% 7%	446
	11th to 20th	Day	15% 3 29% 31% 16% 5%	439
	21st to end	Day	15% 8% 30% 26% 15% 6%	495
September	1st to 10th	Day	21% 10% 28% 21% 14% 7%	450
	11th to 20th	Day	15% 10% 30% 22% 13% 10%	450
	21st to end	Day	23% 13% 29% 20% 11% 4	450
October	1st to 10th	Day	29% 10% 26% 19% 12% 4	450
	11th to 20th	Day	20% 19% 31% 22% 7% 1	450
	21st to end	Day	28% 18% 30% 17% 7% 1	436
November	1st to 10th	Day	24% 18% 27% 22% 9% 2	365
	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	27% 21% 30% 19% 3	728
	21st to end	Night	22% 27% 34% 16% 2	792
February	1st to 10th	Night	27% 23% 32% 16% 2	720
	11th to 20th	Night	27% 27% 34% 10% 1	703
	21st to end	Night	32% 23% 30% 14% 2	592
March	1st to 10th	Night	28% 22% 32% 15% 3	720
	11th to 20th	Night	28% 23% 31% 15% 3 1	720
	21st to end	Night	24% 24% 32% 15% 5%	792
April	1st to 10th	Night	26% 23% 31% 16% 4	720
	11th to 20th	Night	23% 19% 34% 19% 5%	711
	21st to end	Night	26% 16% 32% 22% 5%	720
May	1st to 10th	Night	28% 14% 31% 21% 6%	529
	11th to 20th	Night	19% 15% 38% 22% 6%	354
	21st to end	Night	18% 6% 27% 35% 13% 1	389
June	1st to 10th	Night	20% 6% 30% 33% 10% 1	360
	11th to 20th	Night	13% 8% 31% 35% 12% 1	360
	21st to end	Night	13% 5% 27% 44% 12%	360
July	1st to 10th	Night	13% 4 25% 37% 19% 2	350
	11th to 20th	Night	14% 2 26% 41% 17% 1	348
	21st to end	Night	11% 5% 25% 42% 15% 2	394
August	1st to 10th	Night	12% 2 23% 41% 20% 3	483
	11th to 20th	Night	16% 3 20% 36% 22% 2	696
	21st to end	Night	16% 5% 21% 37% 19% 2	792
September	1st to 10th	Night	18% 6% 23% 37% 14% 2	720
	11th to 20th	Night	16% 8% 27% 27% 17% 5%	720
	21st to end	Night	20% 7% 27% 30% 14% 2	720
October	1st to 10th	Night	23% 10% 25% 25% 14% 2	720
	11th to 20th	Night	21% 16% 29% 27% 8%	720
	21st to end	Night	27% 17% 31% 18% 6%	792
November	1st to 10th	Night	19% 17% 32% 22% 9% 1	740
	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

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