Landolt Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Landolt Astronomical 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	11% 6% 3 6% 6% 4 6% 6% 6% 8% 38%	75136
	Night	11% 6% 2 5% 5% 3 4 5% 5% 8% 46%	74579
Transparency	Day	42% 7% 11% 18% 16% 6%	74868
	Night	36% 6% 13% 19% 17% 8%	74369
Seeing	Day	13% 6% 17% 23% 24% 16%	13707
	Night	14% 5% 19% 32% 24% 6%	23529

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	16% 6% 3 6% 5% 4 5% 4 4 6% 41%	1707
	11th to 20th	Day	15% 8% 3 5% 4 3 4 4 4 6% 44%	1629
	21st to end	Day	20% 7% 2 6% 5% 3 4 3 3 4 43%	1818
February	1st to 10th	Day	17% 9% 4 6% 6% 4 4 4 4 6% 38%	1800
	11th to 20th	Day	18% 7% 2 5% 5% 2 5% 3 5% 7% 41%	1748
	21st to end	Day	20% 7% 3 6% 5% 3 4 5% 5% 6% 36%	1488
March	1st to 10th	Day	15% 8% 3 7% 6% 3 4 4 4 7% 39%	1980
	11th to 20th	Day	14% 7% 4 7% 7% 3 5% 4 5% 8% 37%	1980
	21st to end	Day	11% 6% 3 6% 4 3 5% 5% 5% 8% 43%	2221
April	1st to 10th	Day	12% 6% 3 6% 5% 4 6% 5% 6% 7% 41%	2160
	11th to 20th	Day	14% 7% 3 5% 6% 4 5% 5% 6% 9% 37%	2134
	21st to end	Day	9% 5% 2 5% 6% 3 6% 6% 6% 9% 44%	2160
May	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 6% 6% 9% 42%	2160
	11th to 20th	Day	10% 5% 2 6% 6% 4 6% 7% 6% 7% 42%	2124
	21st to end	Day	6% 5% 3 6% 7% 4 7% 7% 7% 10% 36%	2337
June	1st to 10th	Day	7% 5% 2 7% 7% 5% 8% 7% 8% 10% 33%	2281
	11th to 20th	Day	4 5% 3 6% 9% 6% 9% 9% 9% 9% 32%	2340
	21st to end	Day	6% 6% 3 7% 9% 6% 9% 9% 9% 9% 26%	2340
July	1st to 10th	Day	8% 7% 3 8% 8% 6% 9% 7% 8% 10% 25%	2340
	11th to 20th	Day	8% 5% 4 8% 8% 6% 9% 9% 9% 10% 24%	2340
	21st to end	Day	8% 6% 3 8% 9% 6% 8% 8% 8% 9% 28%	2456
August	1st to 10th	Day	5% 5% 3 7% 8% 5% 9% 7% 9% 12% 31%	2148
	11th to 20th	Day	10% 6% 3 7% 8% 5% 7% 8% 8% 10% 28%	2125
	21st to end	Day	10% 6% 3 7% 7% 5% 7% 6% 8% 9% 31%	2376
September	1st to 10th	Day	10% 6% 3 6% 8% 5% 7% 7% 8% 10% 33%	2160
	11th to 20th	Day	11% 6% 3 7% 6% 4 6% 5% 6% 9% 38%	2160
	21st to end	Day	10% 5% 3 6% 5% 4 5% 5% 6% 7% 46%	1964
October	1st to 10th	Day	7% 3 2 4 5% 3 4 5% 5% 7% 55%	1800
	11th to 20th	Day	8% 5% 3 5% 3 3 4 4 5% 7% 53%	1800
	21st to end	Day	12% 4 1 4 4 3 4 4 5% 7% 53%	1980
November	1st to 10th	Day	8% 4 2 6% 5% 2 4 5% 5% 7% 52%	1858
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 5% 4 5% 52%	1900
	21st to end	Day	13% 6% 3 5% 4 3 5% 5% 5% 6% 44%	1900
December	1st to 10th	Day	16% 7% 3 5% 4 3 5% 4 5% 7% 41%	1900
	11th to 20th	Day	16% 9% 4 6% 5% 4 5% 4 4 7% 37%	2387
	21st to end	Day	16% 8% 2 6% 5% 3 5% 5% 5% 7% 37%	3135
January	1st to 10th	Night	17% 6% 3 5% 5% 2 4 3 5% 7% 44%	2458
	11th to 20th	Night	15% 6% 3 4 4 2 4 4 5% 6% 47%	2354
	21st to end	Night	19% 7% 2 5% 4 2 3 4 4 5% 44%	2534
February	1st to 10th	Night	16% 7% 3 6% 4 3 4 4 4 5% 42%	2160
	11th to 20th	Night	19% 6% 2 5% 4 2 4 3 5% 7% 42%	2098
	21st to end	Night	19% 8% 2 6% 5% 3 4 4 4 7% 38%	1776
March	1st to 10th	Night	13% 8% 2 5% 4 3 4 4 5% 7% 45%	1985
	11th to 20th	Night	14% 7% 3 7% 4 4 4 5% 5% 7% 40%	1980
	21st to end	Night	11% 5% 2 5% 3 2 4 4 5% 7% 52%	2134
April	1st to 10th	Night	12% 6% 3 5% 5% 3 4 4 6% 8% 46%	1800
	11th to 20th	Night	13% 6% 2 6% 5% 2 4 4 5% 6% 48%	1775
	21st to end	Night	7% 4 2 4 4 3 4 5% 5% 8% 53%	1800
May	1st to 10th	Night	9% 5% 1 4 4 3 4 5% 7% 8% 51%	1800
	11th to 20th	Night	8% 5% 2 4 5% 3 4 5% 5% 9% 49%	1769
	21st to end	Night	6% 4 2 5% 5% 3 5% 5% 6% 11% 48%	1938
June	1st to 10th	Night	5% 4 2 5% 6% 3 6% 7% 7% 10% 44%	1673
	11th to 20th	Night	3 3 2 5% 5% 4 6% 6% 8% 13% 46%	1620
	21st to end	Night	7% 4 3 4 8% 4 6% 8% 8% 11% 39%	1620
July	1st to 10th	Night	4 5% 3 6% 5% 4 6% 6% 8% 12% 42%	1620
	11th to 20th	Night	5% 4 2 5% 5% 4 7% 6% 10% 14% 39%	1620
	21st to end	Night	5% 4 2 5% 6% 3 6% 7% 10% 14% 37%	1857
August	1st to 10th	Night	4 4 2 4 4 3 4 6% 7% 13% 51%	1790
	11th to 20th	Night	5% 5% 2 6% 6% 4 7% 7% 9% 13% 37%	1776
	21st to end	Night	7% 5% 3 6% 6% 5% 6% 7% 7% 12% 37%	1980
September	1st to 10th	Night	6% 5% 2 6% 6% 3 5% 5% 8% 10% 43%	1800
	11th to 20th	Night	9% 6% 3 6% 7% 4 5% 6% 6% 9% 41%	1800
	21st to end	Night	7% 4 2 5% 5% 3 5% 5% 5% 8% 50%	1971
October	1st to 10th	Night	6% 3 2 4 4 3 4 4 4 7% 59%	2160
	11th to 20th	Night	7% 5% 2 4 4 3 3 5% 5% 6% 58%	2160
	21st to end	Night	12% 4 2 4 3 2 3 4 4 6% 56%	2376
November	1st to 10th	Night	8% 5% 2 4 3 3 3 3 4 6% 58%	2220
	11th to 20th	Night	12% 4 2 4 4 2 3 4 5% 5% 55%	2280
	21st to end	Night	13% 7% 3 5% 4 3 4 4 4 6% 48%	2280
December	1st to 10th	Night	15% 7% 3 6% 4 2 3 3 4 6% 46%	2280
	11th to 20th	Night	14% 9% 4 5% 5% 2 4 4 5% 7% 42%	2803
	21st to end	Night	18% 10% 3 6% 5% 3 4 3 4 5% 40%	4532

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 5% 17% 22% 8%	1708
	11th to 20th	Day	42% 4 5% 14% 26% 8%	1629
	21st to end	Day	46% 2 5% 16% 19% 11%	1818
February	1st to 10th	Day	48% 4 8% 13% 17% 10%	1800
	11th to 20th	Day	44% 5% 6% 15% 20% 9%	1758
	21st to end	Day	49% 5% 7% 16% 16% 8%	1488
March	1st to 10th	Day	45% 5% 7% 19% 17% 7%	1980
	11th to 20th	Day	46% 5% 10% 12% 21% 6%	1980
	21st to end	Day	38% 5% 10% 20% 20% 6%	2221
April	1st to 10th	Day	40% 6% 11% 17% 19% 8%	2160
	11th to 20th	Day	43% 6% 8% 19% 16% 9%	2134
	21st to end	Day	36% 6% 11% 20% 23% 4	2160
May	1st to 10th	Day	37% 5% 12% 21% 18% 7%	2160
	11th to 20th	Day	38% 7% 10% 24% 18% 2	2128
	21st to end	Day	39% 9% 15% 23% 13% 1	2337
June	1st to 10th	Day	43% 10% 13% 22% 12%	2281
	11th to 20th	Day	43% 11% 13% 20% 12% 1	2340
	21st to end	Day	48% 13% 16% 18% 6% 1	2340
July	1st to 10th	Day	50% 12% 18% 15% 4	2276
	11th to 20th	Day	48% 14% 18% 16% 4 1	2271
	21st to end	Day	48% 11% 19% 17% 6% 1	2320
August	1st to 10th	Day	41% 12% 22% 18% 7%	2136
	11th to 20th	Day	48% 12% 18% 17% 4 1	2125
	21st to end	Day	47% 10% 17% 16% 8% 2	2376
September	1st to 10th	Day	43% 10% 16% 16% 12% 4	2160
	11th to 20th	Day	42% 8% 14% 16% 15% 6%	2160
	21st to end	Day	36% 6% 11% 20% 18% 8%	1964
October	1st to 10th	Day	29% 6% 9% 21% 22% 13%	1800
	11th to 20th	Day	30% 5% 6% 17% 28% 13%	1800
	21st to end	Day	31% 4 6% 16% 27% 16%	1980
November	1st to 10th	Day	31% 6% 7% 20% 26% 10%	1858
	11th to 20th	Day	34% 4 6% 17% 26% 13%	1900
	21st to end	Day	40% 4 7% 17% 22% 10%	1900
December	1st to 10th	Day	42% 4 10% 19% 17% 8%	1900
	11th to 20th	Day	48% 4 7% 12% 22% 7%	2385
	21st to end	Day	45% 5% 8% 16% 23% 3	3135
January	1st to 10th	Night	42% 3 8% 15% 24% 9%	2458
	11th to 20th	Night	38% 4 7% 18% 21% 11%	2354
	21st to end	Night	43% 3 6% 15% 22% 11%	2534
February	1st to 10th	Night	43% 5% 7% 17% 19% 10%	2160
	11th to 20th	Night	43% 4 6% 16% 19% 12%	2099
	21st to end	Night	47% 5% 9% 14% 14% 11%	1776
March	1st to 10th	Night	39% 5% 9% 18% 18% 10%	1985
	11th to 20th	Night	42% 6% 11% 16% 18% 7%	1980
	21st to end	Night	32% 5% 11% 21% 22% 8%	2134
April	1st to 10th	Night	37% 5% 12% 18% 17% 10%	1800
	11th to 20th	Night	38% 4 10% 20% 17% 11%	1775
	21st to end	Night	28% 6% 10% 22% 26% 7%	1800
May	1st to 10th	Night	29% 6% 12% 26% 19% 7%	1800
	11th to 20th	Night	32% 6% 14% 25% 21% 3	1770
	21st to end	Night	29% 8% 17% 29% 16% 2	1939
June	1st to 10th	Night	31% 11% 18% 26% 13% 1	1673
	11th to 20th	Night	28% 9% 19% 28% 13% 2	1620
	21st to end	Night	35% 11% 22% 24% 8%	1620
July	1st to 10th	Night	32% 11% 31% 19% 6%	1575
	11th to 20th	Night	31% 15% 29% 19% 4 1	1566
	21st to end	Night	31% 14% 29% 21% 6%	1754
August	1st to 10th	Night	23% 13% 32% 25% 7% 1	1778
	11th to 20th	Night	35% 13% 27% 20% 5% 1	1776
	21st to end	Night	37% 11% 23% 16% 9% 4	1980
September	1st to 10th	Night	33% 11% 20% 20% 11% 5%	1800
	11th to 20th	Night	39% 8% 17% 17% 13% 6%	1800
	21st to end	Night	32% 6% 13% 23% 16% 10%	1971
October	1st to 10th	Night	25% 5% 10% 23% 22% 14%	2160
	11th to 20th	Night	27% 5% 7% 21% 25% 15%	2160
	21st to end	Night	30% 4 4 18% 25% 19%	2376
November	1st to 10th	Night	29% 4 8% 21% 25% 13%	2220
	11th to 20th	Night	32% 4 8% 16% 26% 15%	2280
	21st to end	Night	38% 4 6% 19% 21% 11%	2280
December	1st to 10th	Night	41% 4 7% 22% 20% 7%	2280
	11th to 20th	Night	43% 4 7% 13% 19% 14%	2804
	21st to end	Night	47% 4 8% 15% 19% 7%	4532

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% 15% 36% 19% 8% 2	270
	11th to 20th	Day	17% 13% 38% 22% 9%	243
	21st to end	Day	26% 14% 35% 20% 5%	264
February	1st to 10th	Day	26% 13% 29% 24% 8% 1	240
	11th to 20th	Day	28% 13% 31% 19% 8% 1	240
	21st to end	Day	18% 20% 41% 19% 3	198
March	1st to 10th	Day	19% 16% 36% 23% 5%	240
	11th to 20th	Day	23% 16% 35% 19% 6% 2	240
	21st to end	Day	18% 15% 30% 24% 12% 1	307
April	1st to 10th	Day	18% 10% 31% 27% 13% 1	420
	11th to 20th	Day	19% 13% 28% 23% 15% 2	412
	21st to end	Day	12% 7% 25% 33% 19% 3	420
May	1st to 10th	Day	13% 5% 23% 31% 22% 5%	423
	11th to 20th	Day	13% 3 17% 28% 28% 11%	447
	21st to end	Day	7% 2 11% 29% 32% 19%	485
June	1st to 10th	Day	10% 2 11% 27% 29% 21%	571
	11th to 20th	Day	9% 2 8% 26% 37% 19%	630
	21st to end	Day	10% 3 8% 24% 30% 26%	630
July	1st to 10th	Day	9% 1 5% 19% 36% 30%	621
	11th to 20th	Day	8% 1 3 17% 38% 33%	618
	21st to end	Day	10% 1 5% 17% 35% 32%	594
August	1st to 10th	Day	5% 5% 13% 32% 44%	446
	11th to 20th	Day	10% 1 3 10% 32% 44%	439
	21st to end	Day	12% 2 6% 16% 35% 29%	495
September	1st to 10th	Day	8% 2 4 16% 43% 26%	450
	11th to 20th	Day	9% 3 8% 19% 40% 22%	450
	21st to end	Day	13% 2 8% 24% 34% 19%	430
October	1st to 10th	Day	6% 3 17% 41% 23% 10%	270
	11th to 20th	Day	8% 5% 16% 34% 23% 14%	270
	21st to end	Day	17% 9% 19% 32% 18% 5%	297
November	1st to 10th	Day	14% 2 22% 37% 20% 6%	270
	11th to 20th	Day	16% 9% 26% 29% 15% 5%	270
	21st to end	Day	17% 10% 31% 28% 11% 3	270
December	1st to 10th	Day	20% 7% 27% 26% 15% 4	270
	11th to 20th	Day	23% 12% 32% 26% 7% 1	270
	21st to end	Day	27% 10% 31% 20% 11% 1	297
January	1st to 10th	Night	21% 9% 30% 32% 7%	950
	11th to 20th	Night	18% 9% 29% 34% 9%	906
	21st to end	Night	22% 9% 32% 30% 7%	950
February	1st to 10th	Night	21% 9% 30% 36% 4	720
	11th to 20th	Night	23% 10% 26% 33% 8%	703
	21st to end	Night	22% 13% 34% 28% 3	592
March	1st to 10th	Night	16% 8% 32% 34% 10%	545
	11th to 20th	Night	16% 8% 29% 35% 11%	540
	21st to end	Night	13% 8% 32% 35% 12%	594
April	1st to 10th	Night	15% 8% 31% 34% 12% 1	540
	11th to 20th	Night	18% 8% 29% 32% 12% 1	533
	21st to end	Night	8% 4 29% 43% 14% 1	540
May	1st to 10th	Night	11% 4 21% 46% 18% 1	540
	11th to 20th	Night	12% 3 18% 38% 27% 2	531
	21st to end	Night	7% 1 11% 40% 35% 7%	584
June	1st to 10th	Night	7% 2 6% 36% 43% 7%	540
	11th to 20th	Night	3 1 5% 34% 47% 9%	540
	21st to end	Night	9% 1 5% 25% 44% 17%	540
July	1st to 10th	Night	6% 1 2 17% 52% 21%	525
	11th to 20th	Night	5% 2 3 14% 52% 25%	522
	21st to end	Night	5% 1 5% 16% 55% 18%	591
August	1st to 10th	Night	5% 1 3 14% 51% 27%	537
	11th to 20th	Night	5% 1 15% 51% 27%	534
	21st to end	Night	8% 1 5% 17% 46% 23%	594
September	1st to 10th	Night	8% 2 4 22% 48% 18%	540
	11th to 20th	Night	11% 3 5% 24% 47% 10%	540
	21st to end	Night	10% 1 8% 28% 45% 8%	697
October	1st to 10th	Night	7% 1 12% 37% 37% 5%	720
	11th to 20th	Night	9% 4 17% 39% 29% 2	720
	21st to end	Night	14% 6% 18% 36% 23% 2	792
November	1st to 10th	Night	11% 3 18% 42% 23% 2	740
	11th to 20th	Night	14% 7% 22% 39% 17% 1	760
	21st to end	Night	17% 5% 26% 34% 16% 1	760
December	1st to 10th	Night	18% 8% 25% 33% 16%	760
	11th to 20th	Night	21% 8% 26% 36% 9%	764
	21st to end	Night	26% 8% 26% 30% 9%	1045

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