Leander McCormick Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leander McCormick Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	125290
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	122741
Transparency	Day	37% 7% 12% 22% 17% 5%	128896
	Night	37% 6% 11% 22% 18% 5%	127204
Seeing	Day	18% 10% 25% 25% 15% 6%	15987
	Night	17% 7% 25% 34% 15% 2	25732

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	14% 6% 2 6% 4 4 4 3 4 5% 50%	4056
	11th to 20th	Day	12% 7% 2 5% 4 3 3 3 4 4 52%	4060
	21st to end	Day	14% 7% 2 6% 3 4 4 3 4 6% 47%	4466
February	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 5% 6% 42%	1800
	11th to 20th	Day	18% 6% 2 6% 4 4 3 3 4 6% 42%	1753
	21st to end	Day	17% 7% 3 6% 4 4 5% 3 5% 6% 40%	1718
March	1st to 10th	Day	16% 7% 2 5% 4 5% 5% 4 5% 7% 40%	4935
	11th to 20th	Day	10% 6% 2 5% 3 4 4 3 5% 7% 51%	4980
	21st to end	Day	14% 6% 3 6% 5% 4 4 4 5% 6% 41%	5465
April	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 48%	2361
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 6% 7% 48%	2374
	21st to end	Day	15% 6% 2 6% 5% 4 4 4 6% 6% 42%	2430
May	1st to 10th	Day	13% 6% 3 6% 5% 6% 6% 5% 7% 10% 33%	5862
	11th to 20th	Day	21% 7% 2 6% 5% 4 5% 4 6% 9% 32%	5861
	21st to end	Day	15% 5% 2 6% 4 4 4 4 7% 10% 40%	6752
June	1st to 10th	Day	10% 4 2 6% 4 4 5% 5% 7% 10% 43%	2800
	11th to 20th	Day	10% 5% 2 6% 6% 6% 5% 5% 7% 9% 39%	2800
	21st to end	Day	7% 4 2 5% 5% 5% 6% 5% 8% 10% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	6046
	11th to 20th	Day	8% 6% 3 7% 6% 6% 7% 6% 10% 13% 28%	5900
	21st to end	Day	8% 5% 2 6% 6% 6% 6% 5% 8% 11% 37%	6424
August	1st to 10th	Day	14% 5% 3 7% 5% 5% 5% 5% 9% 11% 31%	4156
	11th to 20th	Day	8% 4 2 7% 5% 5% 6% 5% 8% 10% 40%	3959
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 9% 48%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 6% 48%	2280
	11th to 20th	Day	12% 6% 2 6% 4 3 5% 4 6% 7% 45%	2200
	21st to end	Day	22% 6% 2 5% 4 5% 4 3 5% 6% 36%	2024
October	1st to 10th	Day	15% 4 2 5% 4 4 3 3 4 7% 49%	1900
	11th to 20th	Day	15% 5% 2 4 3 3 3 2 4 5% 54%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	2090
November	1st to 10th	Day	12% 4 2 4 3 3 4 2 6% 5% 55%	1957
	11th to 20th	Day	15% 4 2 5% 3 3 4 3 5% 5% 51%	1779
	21st to end	Day	14% 4 2 4 4 3 3 3 5% 6% 51%	1600
December	1st to 10th	Day	10% 5% 2 4 4 4 4 3 4 6% 52%	2600
	11th to 20th	Day	19% 6% 2 5% 2 2 3 3 5% 6% 47%	2990
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 6% 39%	4032
January	1st to 10th	Night	11% 6% 2 4 3 3 3 3 4 6% 55%	5889
	11th to 20th	Night	13% 6% 2 4 3 2 2 2 4 4 57%	5900
	21st to end	Night	14% 7% 2 6% 3 3 3 2 4 4 51%	6490
February	1st to 10th	Night	13% 6% 3 6% 4 5% 5% 4 6% 6% 43%	2600
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 4 5% 43%	2533
	21st to end	Night	14% 6% 2 6% 4 4 3 3 5% 7% 44%	1905
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 4 5% 47%	4918
	11th to 20th	Night	9% 4 1 5% 3 3 3 3 5% 8% 54%	4980
	21st to end	Night	11% 6% 2 6% 4 5% 5% 3 5% 6% 46%	5467
April	1st to 10th	Night	13% 8% 2 6% 4 4 3 3 5% 7% 45%	2037
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 6% 9% 48%	1975
	21st to end	Night	15% 5% 1 5% 4 5% 4 3 6% 7% 45%	1988
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 9% 34%	4033
	11th to 20th	Night	24% 6% 2 6% 4 4 4 4 7% 7% 31%	4032
	21st to end	Night	15% 7% 2 6% 4 4 5% 4 6% 8% 39%	4086
June	1st to 10th	Night	10% 4 2 5% 5% 4 5% 4 8% 9% 45%	1600
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 6% 7% 9% 42%	1600
	21st to end	Night	8% 4 2 5% 5% 4 5% 5% 6% 9% 46%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 6% 5% 8% 10% 38%	3906
	11th to 20th	Night	10% 9% 4 10% 6% 6% 7% 6% 8% 10% 26%	4057
	21st to end	Night	6% 7% 3 9% 6% 6% 7% 5% 9% 10% 31%	4427
August	1st to 10th	Night	15% 6% 2 7% 6% 5% 5% 5% 7% 10% 32%	2861
	11th to 20th	Night	9% 6% 3 8% 7% 5% 6% 6% 7% 8% 35%	2836
	21st to end	Night	11% 6% 3 6% 4 5% 4 4 5% 8% 43%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 3 5% 7% 45%	1900
	11th to 20th	Night	14% 6% 2 7% 3 4 4 3 5% 7% 46%	1975
	21st to end	Night	23% 6% 2 6% 3 3 4 3 5% 6% 39%	2155
October	1st to 10th	Night	18% 5% 2 5% 3 3 3 3 5% 7% 46%	2280
	11th to 20th	Night	15% 4 2 5% 3 3 4 3 6% 5% 50%	2280
	21st to end	Night	13% 4 1 5% 3 3 3 3 5% 6% 54%	2508
November	1st to 10th	Night	11% 7% 2 5% 4 4 3 3 3 5% 53%	2341
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 50%	2615
	21st to end	Night	13% 4 2 4 4 4 4 3 5% 6% 51%	2800
December	1st to 10th	Night	9% 6% 2 4 3 4 4 4 5% 7% 53%	4580
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 6% 44%	5205
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 7% 42%	6906

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	35% 5% 6% 23% 20% 10%	4057
	11th to 20th	Day	35% 4 6% 19% 22% 14%	4060
	21st to end	Day	39% 5% 7% 25% 19% 4	4466
February	1st to 10th	Day	42% 6% 10% 24% 15% 3	1800
	11th to 20th	Day	43% 6% 7% 20% 19% 5%	1762
	21st to end	Day	44% 6% 10% 17% 18% 6%	1718
March	1st to 10th	Day	41% 6% 8% 18% 16% 10%	4935
	11th to 20th	Day	31% 6% 8% 25% 21% 9%	4980
	21st to end	Day	42% 7% 9% 17% 16% 7%	5465
April	1st to 10th	Day	34% 7% 10% 22% 18% 9%	2361
	11th to 20th	Day	34% 6% 12% 21% 21% 6%	2374
	21st to end	Day	41% 7% 11% 23% 14% 4	2430
May	1st to 10th	Day	42% 10% 15% 19% 12% 1	5897
	11th to 20th	Day	47% 9% 12% 19% 12% 1	5865
	21st to end	Day	37% 9% 14% 19% 18% 2	6767
June	1st to 10th	Day	33% 9% 17% 25% 16% 1	2800
	11th to 20th	Day	37% 11% 15% 21% 15% 2	2800
	21st to end	Day	31% 11% 16% 26% 15% 1	2800
July	1st to 10th	Day	33% 11% 20% 19% 13% 4	5981
	11th to 20th	Day	41% 12% 20% 21% 6% 1	5831
	21st to end	Day	38% 9% 14% 24% 15%	6240
August	1st to 10th	Day	43% 9% 18% 24% 6%	4143
	11th to 20th	Day	36% 9% 15% 25% 15% 1	3998
	21st to end	Day	30% 7% 19% 26% 16% 1	4180
September	1st to 10th	Day	36% 8% 13% 16% 22% 4	2280
	11th to 20th	Day	35% 8% 9% 26% 19% 4	2200
	21st to end	Day	47% 7% 10% 18% 16% 3	2024
October	1st to 10th	Day	34% 4 8% 19% 31% 5%	3160
	11th to 20th	Day	24% 4 7% 23% 31% 11%	3140
	21st to end	Day	20% 3 9% 30% 24% 12%	3428
November	1st to 10th	Day	30% 5% 10% 24% 27% 5%	1957
	11th to 20th	Day	34% 5% 10% 21% 22% 8%	1779
	21st to end	Day	33% 5% 11% 25% 23% 4	1600
December	1st to 10th	Day	34% 5% 10% 25% 18% 9%	2600
	11th to 20th	Day	38% 4 14% 24% 16% 5%	2986
	21st to end	Day	44% 6% 11% 23% 15% 1	4032
January	1st to 10th	Night	31% 4 7% 21% 26% 10%	5889
	11th to 20th	Night	32% 3 6% 21% 26% 12%	5900
	21st to end	Night	37% 4 6% 26% 21% 6%	6490
February	1st to 10th	Night	40% 6% 11% 24% 17% 3	2600
	11th to 20th	Night	44% 4 7% 19% 19% 7%	2535
	21st to end	Night	39% 5% 10% 21% 17% 7%	1905
March	1st to 10th	Night	37% 6% 7% 21% 20% 9%	4918
	11th to 20th	Night	28% 5% 9% 22% 29% 8%	4980
	21st to end	Night	38% 5% 9% 21% 21% 6%	5467
April	1st to 10th	Night	38% 6% 11% 17% 21% 7%	2037
	11th to 20th	Night	32% 7% 13% 20% 22% 5%	1975
	21st to end	Night	36% 7% 13% 24% 16% 5%	1988
May	1st to 10th	Night	46% 8% 16% 18% 10% 3	4060
	11th to 20th	Night	47% 9% 12% 21% 9% 1	4033
	21st to end	Night	41% 9% 14% 23% 11% 2	4093
June	1st to 10th	Night	32% 10% 14% 26% 16% 1	1600
	11th to 20th	Night	36% 10% 18% 21% 13% 2	1600
	21st to end	Night	32% 11% 15% 22% 17% 3	1600
July	1st to 10th	Night	39% 11% 18% 15% 14% 2	3863
	11th to 20th	Night	51% 11% 17% 16% 5% 1	4006
	21st to end	Night	43% 10% 14% 21% 11%	4297
August	1st to 10th	Night	45% 9% 17% 22% 6% 1	2850
	11th to 20th	Night	42% 10% 13% 23% 12% 1	2863
	21st to end	Night	37% 8% 15% 29% 10% 1	3476
September	1st to 10th	Night	38% 8% 11% 21% 15% 7%	1900
	11th to 20th	Night	37% 7% 11% 23% 19% 3	1975
	21st to end	Night	46% 6% 11% 19% 15% 3	2155
October	1st to 10th	Night	41% 4 9% 18% 21% 7%	3800
	11th to 20th	Night	25% 4 8% 22% 30% 11%	3779
	21st to end	Night	20% 3 8% 32% 27% 10%	4120
November	1st to 10th	Night	34% 5% 8% 21% 25% 7%	2341
	11th to 20th	Night	36% 4 8% 21% 24% 7%	2615
	21st to end	Night	34% 4 9% 25% 22% 6%	2800
December	1st to 10th	Night	31% 4 10% 27% 24% 4	4580
	11th to 20th	Night	43% 4 11% 18% 17% 7%	5208
	21st to end	Night	39% 7% 12% 26% 16% 1	6906

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	26% 19% 33% 16% 6%	300
	11th to 20th	Day	26% 25% 30% 15% 4	300
	21st to end	Day	23% 21% 36% 16% 4 1	330
February	1st to 10th	Day	24% 20% 40% 12% 4	300
	11th to 20th	Day	38% 22% 31% 8% 1	300
	21st to end	Day	28% 17% 31% 20% 4	406
March	1st to 10th	Day	23% 17% 31% 22% 6% 1	500
	11th to 20th	Day	22% 18% 33% 19% 7% 1	500
	21st to end	Day	24% 15% 30% 24% 6% 1	550
April	1st to 10th	Day	24% 19% 29% 18% 9% 2	500
	11th to 20th	Day	19% 14% 30% 22% 11% 4	492
	21st to end	Day	21% 12% 29% 22% 13% 2	500
May	1st to 10th	Day	20% 15% 29% 23% 10% 3	503
	11th to 20th	Day	21% 8% 26% 26% 14% 5%	527
	21st to end	Day	20% 5% 25% 25% 19% 6%	573
June	1st to 10th	Day	11% 4 27% 28% 21% 8%	530
	11th to 20th	Day	12% 7% 23% 30% 18% 10%	530
	21st to end	Day	10% 4 22% 33% 22% 8%	530
July	1st to 10th	Day	10% 2 18% 30% 29% 11%	525
	11th to 20th	Day	8% 2 13% 37% 25% 15%	524
	21st to end	Day	10% 3 15% 34% 26% 12%	576
August	1st to 10th	Day	13% 4 20% 26% 25% 12%	502
	11th to 20th	Day	11% 2 19% 28% 25% 15%	479
	21st to end	Day	9% 2 14% 30% 26% 18%	539
September	1st to 10th	Day	16% 4 13% 27% 24% 17%	490
	11th to 20th	Day	16% 3 16% 27% 28% 11%	490
	21st to end	Day	29% 6% 22% 23% 16% 5%	490
October	1st to 10th	Day	19% 6% 19% 32% 18% 7%	490
	11th to 20th	Day	16% 12% 24% 28% 17% 4	490
	21st to end	Day	16% 10% 30% 29% 12% 3	539
November	1st to 10th	Day	17% 8% 27% 31% 13% 4	495
	11th to 20th	Day	19% 12% 30% 23% 14% 2	350
	21st to end	Day	16% 16% 35% 22% 10% 2	200
December	1st to 10th	Day	19% 16% 35% 24% 6% 1	200
	11th to 20th	Day	23% 11% 32% 22% 12% 1	200
	21st to end	Day	24% 12% 32% 22% 8% 2	237
January	1st to 10th	Night	18% 10% 39% 29% 4	800
	11th to 20th	Night	20% 10% 34% 32% 4	800
	21st to end	Night	17% 10% 40% 30% 3	880
February	1st to 10th	Night	17% 11% 36% 33% 3	800
	11th to 20th	Night	26% 12% 36% 26% 1	783
	21st to end	Night	19% 12% 37% 28% 4	660
March	1st to 10th	Night	16% 14% 34% 32% 4	800
	11th to 20th	Night	21% 12% 33% 28% 6% 1	800
	21st to end	Night	17% 11% 36% 30% 6%	880
April	1st to 10th	Night	17% 14% 35% 27% 7%	800
	11th to 20th	Night	14% 11% 32% 32% 10% 1	791
	21st to end	Night	18% 6% 29% 38% 9%	788
May	1st to 10th	Night	18% 6% 27% 34% 15% 1	600
	11th to 20th	Night	21% 2 19% 38% 17% 1	591
	21st to end	Night	17% 1 16% 37% 25% 4	509
June	1st to 10th	Night	10% 3 15% 44% 27% 2	400
	11th to 20th	Night	12% 2 13% 42% 30% 2	400
	21st to end	Night	10% 1 17% 45% 24% 3	400
July	1st to 10th	Night	12% 2 10% 34% 35% 7%	511
	11th to 20th	Night	6% 2 10% 34% 43% 5%	582
	21st to end	Night	8% 2 11% 39% 34% 5%	657
August	1st to 10th	Night	13% 2 8% 39% 34% 4	579
	11th to 20th	Night	13% 1 10% 34% 36% 5%	589
	21st to end	Night	12% 2 11% 34% 36% 5%	836
September	1st to 10th	Night	15% 2 13% 33% 30% 7%	760
	11th to 20th	Night	16% 2 12% 32% 33% 5%	760
	21st to end	Night	26% 3 17% 30% 22% 2	760
October	1st to 10th	Night	20% 4 17% 31% 24% 4	760
	11th to 20th	Night	17% 6% 24% 34% 18% 1	760
	21st to end	Night	16% 6% 24% 40% 13% 1	836
November	1st to 10th	Night	14% 6% 27% 38% 13% 2	780
	11th to 20th	Night	17% 7% 28% 37% 10% 1	800
	21st to end	Night	15% 7% 31% 37% 9% 1	800
December	1st to 10th	Night	19% 7% 34% 33% 7% 1	800
	11th to 20th	Night	18% 7% 30% 40% 6%	800
	21st to end	Night	18% 8% 29% 41% 5%	880

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