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 2024-11-05.

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	11% 7% 3 7% 5% 5% 5% 5% 6% 8% 38%	100739
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	98325
Transparency	Day	41% 7% 11% 18% 17% 6%	104391
	Night	36% 6% 13% 19% 18% 9%	102814
Seeing	Day	13% 6% 18% 24% 24% 15%	14838
	Night	14% 6% 19% 31% 24% 6%	24537

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	17% 8% 4 8% 4 5% 4 3 4 6% 37%	2847
	11th to 20th	Day	13% 7% 2 5% 3 3 3 2 3 5% 54%	2850
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 5% 35%	3197
February	1st to 10th	Day	16% 9% 3 7% 6% 4 4 3 5% 6% 37%	1900
	11th to 20th	Day	18% 8% 2 6% 4 3 4 3 5% 6% 41%	1848
	21st to end	Day	19% 7% 3 7% 4 4 4 4 5% 6% 36%	1579
March	1st to 10th	Day	23% 11% 3 8% 5% 4 4 4 5% 7% 26%	3436
	11th to 20th	Day	14% 7% 3 8% 6% 5% 6% 4 7% 9% 31%	3480
	21st to end	Day	12% 7% 3 6% 4 5% 5% 4 5% 7% 42%	3898
April	1st to 10th	Day	12% 7% 3 6% 4 5% 5% 5% 6% 7% 40%	2280
	11th to 20th	Day	13% 7% 3 6% 5% 5% 4 4 7% 8% 37%	2254
	21st to end	Day	9% 5% 2 5% 5% 5% 5% 5% 6% 9% 44%	2280
May	1st to 10th	Day	11% 6% 3 8% 6% 6% 7% 6% 6% 9% 33%	3765
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 5% 5% 7% 42%	3764
	21st to end	Day	5% 5% 2 6% 6% 6% 6% 5% 8% 10% 41%	4129
June	1st to 10th	Day	7% 5% 2 8% 6% 8% 6% 6% 9% 9% 33%	2408
	11th to 20th	Day	5% 5% 2 7% 8% 7% 8% 7% 9% 9% 32%	2470
	21st to end	Day	6% 6% 3 8% 8% 8% 9% 7% 9% 9% 26%	2470
July	1st to 10th	Day	6% 7% 3 11% 8% 9% 8% 7% 9% 10% 23%	4120
	11th to 20th	Day	7% 5% 3 10% 8% 8% 9% 7% 10% 11% 23%	4120
	21st to end	Day	11% 9% 4 11% 8% 7% 7% 5% 8% 9% 21%	4308
August	1st to 10th	Day	3 3 1 5% 4 5% 6% 4 6% 10% 54%	3785
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 6% 9% 12% 37%	3729
	21st to end	Day	11% 6% 3 9% 6% 7% 6% 5% 7% 7% 34%	4180
September	1st to 10th	Day	11% 6% 3 7% 6% 6% 7% 5% 8% 9% 32%	2280
	11th to 20th	Day	11% 6% 3 7% 5% 6% 5% 4 6% 9% 38%	2280
	21st to end	Day	9% 5% 3 6% 4 5% 4 4 6% 7% 47%	2072
October	1st to 10th	Day	7% 4 3 5% 4 4 5% 3 5% 7% 55%	1900
	11th to 20th	Day	7% 5% 2 6% 2 4 3 4 6% 7% 55%	1900
	21st to end	Day	11% 4 1 4 3 3 4 3 5% 7% 54%	2090
November	1st to 10th	Day	8% 5% 2 6% 4 3 4 4 6% 6% 51%	1898
	11th to 20th	Day	11% 5% 1 6% 4 3 4 4 4 4 52%	1900
	21st to end	Day	13% 7% 3 6% 4 4 5% 4 5% 6% 44%	1900
December	1st to 10th	Day	16% 7% 3 6% 4 4 4 3 5% 7% 41%	1900
	11th to 20th	Day	16% 10% 3 7% 4 6% 4 3 5% 6% 37%	2387
	21st to end	Day	16% 8% 2 7% 4 4 5% 3 6% 7% 37%	3135
January	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 6% 45%	4108
	11th to 20th	Night	14% 6% 2 4 4 3 3 3 4 5% 52%	4120
	21st to end	Night	25% 8% 2 5% 3 3 3 2 4 6% 39%	4464
February	1st to 10th	Night	16% 7% 3 6% 4 4 4 4 4 5% 42%	2280
	11th to 20th	Night	20% 7% 2 6% 3 4 3 3 5% 6% 42%	2218
	21st to end	Night	18% 9% 2 7% 4 4 3 3 5% 7% 39%	1884
March	1st to 10th	Night	23% 13% 3 8% 4 4 4 3 4 5% 29%	3424
	11th to 20th	Night	16% 8% 3 7% 4 5% 4 4 5% 7% 35%	3480
	21st to end	Night	14% 7% 2 6% 3 4 4 4 5% 7% 44%	3732
April	1st to 10th	Night	12% 6% 2 6% 4 3 4 4 7% 7% 45%	1900
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 5% 6% 47%	1875
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 7% 53%	1900
May	1st to 10th	Night	13% 8% 3 6% 5% 4 4 4 6% 8% 39%	3130
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 8% 41%	3129
	21st to end	Night	7% 5% 2 6% 4 4 5% 5% 8% 12% 42%	3426
June	1st to 10th	Night	5% 5% 2 6% 5% 5% 6% 6% 7% 9% 44%	1765
	11th to 20th	Night	3 4 2 6% 4 6% 5% 4 8% 13% 45%	1710
	21st to end	Night	7% 5% 2 5% 7% 5% 6% 6% 8% 11% 38%	1710
July	1st to 10th	Night	4 6% 3 8% 6% 6% 7% 6% 9% 12% 34%	2850
	11th to 20th	Night	4 4 1 5% 5% 6% 7% 6% 10% 14% 38%	2850
	21st to end	Night	6% 5% 3 8% 6% 6% 7% 6% 10% 13% 31%	3246
August	1st to 10th	Night	2 3 1 3 3 3 4 4 6% 11% 61%	3144
	11th to 20th	Night	3 3 2 6% 4 5% 6% 5% 8% 14% 44%	3100
	21st to end	Night	6% 6% 3 8% 6% 6% 5% 5% 7% 9% 40%	3476
September	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 4 8% 10% 41%	1900
	11th to 20th	Night	10% 6% 3 7% 6% 5% 5% 5% 6% 8% 39%	1900
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 5% 8% 51%	2081
October	1st to 10th	Night	6% 4 2 4 4 3 5% 3 4 6% 59%	2280
	11th to 20th	Night	6% 5% 2 4 4 3 3 4 5% 6% 59%	2280
	21st to end	Night	11% 4 2 5% 3 3 3 3 4 6% 57%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 4 3 2 5% 5% 57%	2280
	11th to 20th	Night	12% 4 2 4 4 3 3 3 5% 5% 55%	2280
	21st to end	Night	13% 8% 3 6% 3 3 4 3 4 6% 47%	2280
December	1st to 10th	Night	15% 8% 3 7% 4 3 3 3 4 6% 46%	2280
	11th to 20th	Night	13% 10% 3 5% 5% 4 4 3 5% 7% 42%	2803
	21st to end	Night	18% 10% 3 7% 4 4 3 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	47% 5% 4 15% 23% 7%	2848
	11th to 20th	Day	35% 3 4 18% 31% 9%	2850
	21st to end	Day	52% 3 4 14% 19% 7%	3197
February	1st to 10th	Day	49% 4 8% 13% 17% 10%	1900
	11th to 20th	Day	44% 5% 7% 14% 20% 10%	1858
	21st to end	Day	47% 4 8% 16% 17% 8%	1579
March	1st to 10th	Day	56% 5% 9% 15% 10% 4	3436
	11th to 20th	Day	48% 6% 9% 18% 15% 4	3480
	21st to end	Day	41% 5% 9% 17% 22% 7%	3898
April	1st to 10th	Day	41% 6% 10% 16% 18% 9%	2280
	11th to 20th	Day	43% 5% 9% 19% 16% 9%	2254
	21st to end	Day	36% 6% 11% 20% 23% 5%	2280
May	1st to 10th	Day	45% 6% 13% 20% 11% 4	3800
	11th to 20th	Day	40% 6% 9% 19% 23% 3	3768
	21st to end	Day	34% 8% 15% 31% 11% 1	4141
June	1st to 10th	Day	42% 10% 13% 22% 12% 1	2408
	11th to 20th	Day	43% 11% 13% 19% 13% 1	2470
	21st to end	Day	47% 13% 16% 18% 6% 1	2470
July	1st to 10th	Day	50% 12% 20% 15% 3	4056
	11th to 20th	Day	48% 15% 24% 9% 2	4051
	21st to end	Day	57% 10% 15% 15% 3	4172
August	1st to 10th	Day	25% 7% 15% 30% 24%	3773
	11th to 20th	Day	36% 10% 21% 26% 7% 1	3765
	21st to end	Day	47% 9% 14% 14% 15% 2	4179
September	1st to 10th	Day	44% 10% 16% 15% 12% 3	2280
	11th to 20th	Day	42% 8% 14% 16% 15% 6%	2280
	21st to end	Day	35% 6% 11% 21% 18% 10%	2072
October	1st to 10th	Day	26% 4 7% 12% 21% 30%	3160
	11th to 20th	Day	18% 4 4 20% 26% 28%	3140
	21st to end	Day	24% 3 5% 11% 40% 18%	3428
November	1st to 10th	Day	31% 6% 8% 20% 26% 10%	1898
	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	41% 4 7% 14% 26% 8%	4108
	11th to 20th	Night	34% 4 5% 19% 26% 11%	4120
	21st to end	Night	47% 3 5% 17% 19% 8%	4464
February	1st to 10th	Night	44% 5% 7% 17% 19% 10%	2280
	11th to 20th	Night	44% 4 5% 16% 19% 12%	2219
	21st to end	Night	46% 5% 9% 14% 15% 11%	1884
March	1st to 10th	Night	56% 5% 9% 13% 11% 6%	3424
	11th to 20th	Night	47% 6% 10% 16% 16% 5%	3480
	21st to end	Night	39% 5% 10% 20% 20% 6%	3732
April	1st to 10th	Night	38% 5% 12% 18% 17% 11%	1900
	11th to 20th	Night	38% 4 10% 19% 18% 11%	1875
	21st to end	Night	28% 6% 10% 22% 26% 7%	1900
May	1st to 10th	Night	40% 6% 15% 21% 13% 5%	3160
	11th to 20th	Night	39% 7% 11% 22% 20% 2	3130
	21st to end	Night	31% 7% 20% 29% 12% 1	3435
June	1st to 10th	Night	31% 11% 18% 26% 13% 1	1765
	11th to 20th	Night	29% 9% 19% 28% 13% 2	1710
	21st to end	Night	35% 11% 22% 23% 9%	1710
July	1st to 10th	Night	38% 14% 31% 14% 3	2805
	11th to 20th	Night	31% 16% 38% 11% 2	2796
	21st to end	Night	41% 17% 20% 17% 4	3143
August	1st to 10th	Night	17% 8% 21% 30% 23% 1	3132
	11th to 20th	Night	28% 10% 28% 29% 5%	3130
	21st to end	Night	39% 9% 18% 18% 14% 3	3476
September	1st to 10th	Night	35% 11% 19% 19% 10% 5%	1900
	11th to 20th	Night	40% 8% 17% 16% 13% 6%	1900
	21st to end	Night	31% 6% 12% 23% 16% 11%	2081
October	1st to 10th	Night	25% 3 8% 15% 20% 29%	3800
	11th to 20th	Night	17% 3 5% 19% 28% 28%	3779
	21st to end	Night	20% 3 5% 13% 36% 24%	4120
November	1st to 10th	Night	30% 4 9% 20% 24% 13%	2280
	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% 7% 2	270
	11th to 20th	Day	18% 14% 37% 22% 9%	270
	21st to end	Day	27% 14% 35% 20% 4	297
February	1st to 10th	Day	25% 15% 29% 23% 8% 1	270
	11th to 20th	Day	27% 14% 31% 20% 7% 1	270
	21st to end	Day	17% 23% 40% 17% 3	225
March	1st to 10th	Day	22% 16% 34% 23% 5%	270
	11th to 20th	Day	21% 17% 35% 20% 5% 2	270
	21st to end	Day	18% 15% 32% 23% 11% 1	343
April	1st to 10th	Day	18% 12% 31% 26% 12% 1	460
	11th to 20th	Day	19% 13% 30% 22% 15% 2	452
	21st to end	Day	12% 7% 26% 32% 19% 3	460
May	1st to 10th	Day	13% 6% 24% 31% 21% 5%	463
	11th to 20th	Day	13% 3 19% 27% 26% 10%	487
	21st to end	Day	7% 2 13% 31% 29% 18%	529
June	1st to 10th	Day	10% 2 11% 27% 30% 20%	618
	11th to 20th	Day	9% 3 9% 26% 36% 18%	680
	21st to end	Day	10% 3 9% 24% 30% 25%	680
July	1st to 10th	Day	9% 2 6% 20% 36% 28%	671
	11th to 20th	Day	8% 1 4 17% 37% 33%	668
	21st to end	Day	10% 2 6% 16% 36% 31%	644
August	1st to 10th	Day	5% 5% 15% 34% 41%	486
	11th to 20th	Day	9% 1 4 11% 32% 44%	479
	21st to end	Day	12% 2 6% 19% 35% 27%	539
September	1st to 10th	Day	10% 3 5% 17% 41% 23%	490
	11th to 20th	Day	9% 3 8% 21% 38% 20%	490
	21st to end	Day	12% 2 10% 24% 33% 18%	468
October	1st to 10th	Day	6% 3 19% 42% 21% 9%	300
	11th to 20th	Day	7% 5% 19% 34% 21% 13%	300
	21st to end	Day	15% 8% 19% 32% 20% 5%	330
November	1st to 10th	Day	13% 2 23% 38% 19% 5%	282
	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%	950
	21st to end	Night	22% 10% 32% 30% 6%	1003
February	1st to 10th	Night	20% 10% 31% 35% 4	760
	11th to 20th	Night	24% 10% 26% 33% 8%	743
	21st to end	Night	22% 13% 34% 28% 3	628
March	1st to 10th	Night	17% 9% 31% 33% 10%	575
	11th to 20th	Night	16% 8% 30% 35% 11%	570
	21st to end	Night	13% 9% 32% 34% 12%	627
April	1st to 10th	Night	15% 8% 31% 33% 12% 1	570
	11th to 20th	Night	18% 9% 29% 32% 12% 1	563
	21st to end	Night	8% 5% 29% 42% 15% 1	570
May	1st to 10th	Night	11% 4 22% 45% 17% 1	570
	11th to 20th	Night	12% 4 18% 38% 26% 2	561
	21st to end	Night	7% 2 12% 39% 33% 6%	617
June	1st to 10th	Night	7% 2 6% 36% 42% 7%	570
	11th to 20th	Night	3 2 6% 34% 46% 9%	570
	21st to end	Night	9% 1 5% 25% 43% 17%	570
July	1st to 10th	Night	6% 1 4 17% 52% 21%	555
	11th to 20th	Night	5% 1 3 13% 52% 25%	552
	21st to end	Night	5% 1 4 17% 54% 18%	624
August	1st to 10th	Night	5% 1 3 15% 51% 26%	567
	11th to 20th	Night	5% 1 15% 51% 27%	564
	21st to end	Night	7% 1 6% 17% 47% 22%	627
September	1st to 10th	Night	9% 2 5% 21% 46% 17%	570
	11th to 20th	Night	12% 3 5% 23% 47% 9%	570
	21st to end	Night	9% 2 9% 29% 43% 8%	736
October	1st to 10th	Night	7% 1 13% 37% 36% 5%	760
	11th to 20th	Night	9% 4 17% 40% 28% 2	760
	21st to end	Night	13% 6% 18% 38% 23% 2	836
November	1st to 10th	Night	11% 4 18% 42% 22% 2	760
	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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