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 2024-03-23.

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% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	78796
	Night	14% 6% 2 5% 4 3 4 4 5% 7% 45%	81072
Transparency	Day	38% 8% 12% 22% 16% 4	78517
	Night	38% 7% 11% 22% 18% 4	80870
Seeing	Day	18% 10% 25% 25% 16% 6%	14130
	Night	17% 7% 25% 34% 16% 2	24022

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% 5% 2 5% 4 3 4 3 4 5% 47%	2836
	11th to 20th	Day	13% 8% 3 6% 5% 3 5% 4 5% 5% 44%	2840
	21st to end	Day	16% 8% 3 6% 5% 3 5% 4 4 6% 40%	3124
February	1st to 10th	Day	11% 7% 3 6% 6% 4 4 5% 5% 6% 42%	1710
	11th to 20th	Day	18% 6% 2 5% 4 3 5% 4 4 7% 43%	1663
	21st to end	Day	17% 7% 3 6% 4 3 5% 4 5% 6% 38%	1634
March	1st to 10th	Day	14% 5% 2 4 4 3 4 3 4 7% 49%	2090
	11th to 20th	Day	15% 6% 3 5% 4 3 5% 5% 5% 7% 42%	2090
	21st to end	Day	14% 8% 4 7% 6% 3 4 4 5% 6% 39%	2200
April	1st to 10th	Day	10% 4 2 5% 4 3 4 5% 6% 7% 50%	2124
	11th to 20th	Day	13% 5% 3 5% 4 3 4 4 5% 7% 48%	2134
	21st to end	Day	16% 6% 2 5% 6% 3 5% 5% 5% 6% 42%	2186
May	1st to 10th	Day	14% 6% 3 5% 5% 4 5% 6% 6% 9% 38%	2340
	11th to 20th	Day	17% 6% 3 5% 5% 3 5% 6% 6% 8% 37%	2301
	21st to end	Day	14% 5% 2 5% 5% 3 5% 5% 6% 10% 39%	2655
June	1st to 10th	Day	10% 3 2 5% 5% 3 5% 5% 7% 10% 44%	2520
	11th to 20th	Day	10% 5% 2 5% 6% 4 5% 6% 7% 9% 39%	2520
	21st to end	Day	8% 3 2 4 5% 4 6% 7% 7% 10% 44%	2520
July	1st to 10th	Day	10% 5% 3 5% 6% 4 6% 6% 7% 10% 38%	2402
	11th to 20th	Day	5% 4 2 5% 6% 4 8% 8% 9% 11% 39%	2340
	21st to end	Day	9% 5% 3 5% 6% 4 8% 7% 7% 11% 35%	2547
August	1st to 10th	Day	10% 5% 3 6% 6% 5% 5% 7% 9% 10% 34%	2327
	11th to 20th	Day	12% 4 2 6% 6% 4 7% 7% 7% 10% 35%	2260
	21st to end	Day	10% 4 2 5% 6% 3 6% 6% 6% 9% 41%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 7% 47%	2160
	11th to 20th	Day	11% 6% 2 5% 5% 2 5% 5% 6% 7% 45%	2085
	21st to end	Day	21% 6% 2 4 5% 4 4 5% 6% 6% 38%	1918
October	1st to 10th	Day	15% 4 2 5% 5% 3 4 4 4 7% 48%	1800
	11th to 20th	Day	15% 5% 2 3 4 3 4 3 4 6% 52%	1800
	21st to end	Day	14% 5% 2 4 3 2 3 4 4 5% 52%	1980
November	1st to 10th	Day	13% 4 2 3 3 2 3 3 6% 5% 56%	1857
	11th to 20th	Day	15% 4 2 4 4 2 4 4 5% 5% 51%	1691
	21st to end	Day	14% 4 2 3 4 3 3 4 5% 6% 52%	1520
December	1st to 10th	Day	15% 7% 3 4 5% 3 5% 5% 5% 7% 42%	1520
	11th to 20th	Day	16% 4 2 4 3 2 3 3 4 6% 52%	1910
	21st to end	Day	20% 6% 2 5% 5% 3 5% 4 6% 6% 38%	2816
January	1st to 10th	Night	12% 7% 2 4 3 2 4 4 4 6% 50%	4109
	11th to 20th	Night	17% 7% 2 4 4 2 3 3 4 5% 49%	4120
	21st to end	Night	19% 8% 3 5% 4 2 3 3 4 5% 43%	4532
February	1st to 10th	Night	13% 6% 3 5% 4 4 4 6% 6% 6% 43%	2470
	11th to 20th	Night	20% 6% 2 4 4 3 4 4 4 5% 43%	2403
	21st to end	Night	15% 6% 3 6% 5% 3 4 4 5% 7% 44%	1813
March	1st to 10th	Night	14% 4 2 4 5% 2 4 4 5% 5% 50%	2090
	11th to 20th	Night	17% 6% 2 5% 4 2 4 4 5% 7% 45%	2090
	21st to end	Night	15% 5% 3 6% 4 4 4 4 4 6% 45%	2201
April	1st to 10th	Night	12% 6% 3 5% 5% 3 4 4 4 7% 46%	1834
	11th to 20th	Night	12% 5% 2 5% 3 2 3 4 5% 9% 50%	1775
	21st to end	Night	16% 5% 1 4 4 3 4 4 5% 7% 45%	1790
May	1st to 10th	Night	17% 5% 2 5% 5% 3 5% 5% 5% 8% 42%	1620
	11th to 20th	Night	19% 5% 2 5% 5% 3 4 5% 7% 8% 36%	1592
	21st to end	Night	14% 5% 2 6% 5% 3 5% 5% 6% 8% 41%	1618
June	1st to 10th	Night	10% 3 2 4 5% 3 4 6% 8% 10% 45%	1440
	11th to 20th	Night	10% 5% 3 5% 4 3 6% 7% 7% 8% 43%	1440
	21st to end	Night	9% 3 2 5% 5% 3 5% 7% 6% 10% 45%	1440
July	1st to 10th	Night	11% 6% 3 5% 5% 3 5% 6% 7% 10% 41%	1552
	11th to 20th	Night	5% 4 2 5% 6% 4 5% 6% 8% 10% 44%	1620
	21st to end	Night	7% 5% 3 5% 5% 3 8% 7% 8% 11% 38%	1771
August	1st to 10th	Night	10% 5% 3 6% 7% 5% 5% 7% 7% 9% 38%	1611
	11th to 20th	Night	13% 6% 3 5% 6% 3 5% 7% 6% 8% 38%	1621
	21st to end	Night	10% 6% 3 7% 6% 4 6% 6% 6% 9% 37%	1980
September	1st to 10th	Night	13% 6% 2 5% 5% 3 5% 5% 6% 6% 45%	1800
	11th to 20th	Night	13% 6% 2 6% 4 3 3 4 4 7% 47%	1871
	21st to end	Night	21% 6% 2 5% 3 3 4 3 5% 6% 40%	2041
October	1st to 10th	Night	17% 6% 2 5% 4 3 3 4 5% 7% 46%	2160
	11th to 20th	Night	15% 5% 2 5% 4 2 4 4 6% 5% 48%	2160
	21st to end	Night	14% 4 1 4 3 2 4 4 5% 6% 52%	2376
November	1st to 10th	Night	12% 6% 3 5% 4 3 3 3 3 5% 54%	2221
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 4 6% 51%	2484
	21st to end	Night	13% 3 2 4 4 3 4 4 5% 7% 50%	2660
December	1st to 10th	Night	14% 9% 3 4 5% 3 4 5% 4 6% 43%	2660
	11th to 20th	Night	16% 6% 2 5% 4 2 4 4 3 5% 48%	3285
	21st to end	Night	16% 5% 2 5% 5% 3 5% 5% 6% 7% 41%	4822

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	37% 7% 8% 29% 14% 6%	2837
	11th to 20th	Day	41% 6% 8% 19% 20% 7%	2840
	21st to end	Day	45% 6% 9% 22% 15% 3	3124
February	1st to 10th	Day	41% 6% 10% 24% 15% 4	1710
	11th to 20th	Day	41% 6% 8% 21% 19% 5%	1672
	21st to end	Day	45% 7% 10% 16% 17% 5%	1634
March	1st to 10th	Day	35% 6% 9% 23% 21% 6%	2090
	11th to 20th	Day	41% 7% 10% 18% 17% 7%	2090
	21st to end	Day	45% 7% 9% 18% 15% 5%	2200
April	1st to 10th	Day	32% 7% 11% 23% 18% 9%	2124
	11th to 20th	Day	34% 6% 12% 19% 22% 6%	2134
	21st to end	Day	42% 6% 11% 22% 14% 5%	2186
May	1st to 10th	Day	39% 10% 15% 18% 14% 3	2340
	11th to 20th	Day	43% 10% 14% 21% 10% 2	2305
	21st to end	Day	39% 9% 15% 25% 12%	2655
June	1st to 10th	Day	34% 9% 16% 25% 15% 1	2520
	11th to 20th	Day	37% 12% 15% 19% 15% 2	2520
	21st to end	Day	32% 11% 15% 26% 14% 1	2520
July	1st to 10th	Day	38% 12% 13% 19% 15% 3	2337
	11th to 20th	Day	32% 12% 17% 24% 14% 2	2271
	21st to end	Day	38% 12% 14% 22% 13% 1	2405
August	1st to 10th	Day	41% 10% 17% 23% 9% 1	2314
	11th to 20th	Day	40% 12% 14% 20% 12% 1	2260
	21st to end	Day	36% 9% 19% 21% 12% 2	2376
September	1st to 10th	Day	37% 8% 14% 17% 20% 4	2160
	11th to 20th	Day	35% 8% 9% 26% 19% 4	2085
	21st to end	Day	45% 7% 10% 19% 17% 3	1918
October	1st to 10th	Day	36% 5% 11% 26% 19% 4	1800
	11th to 20th	Day	34% 5% 10% 25% 22% 4	1800
	21st to end	Day	34% 4 8% 26% 22% 6%	1980
November	1st to 10th	Day	30% 4 9% 23% 27% 5%	1857
	11th to 20th	Day	34% 5% 10% 20% 22% 8%	1691
	21st to end	Day	33% 5% 11% 24% 23% 4	1520
December	1st to 10th	Day	41% 6% 12% 23% 14% 4	1520
	11th to 20th	Day	34% 5% 9% 29% 16% 7%	1906
	21st to end	Day	43% 7% 11% 26% 12% 2	2816
January	1st to 10th	Night	35% 5% 10% 23% 22% 5%	4109
	11th to 20th	Night	39% 4 8% 21% 22% 6%	4120
	21st to end	Night	44% 5% 7% 23% 16% 5%	4532
February	1st to 10th	Night	39% 6% 10% 25% 17% 3	2470
	11th to 20th	Night	43% 4 7% 20% 19% 7%	2405
	21st to end	Night	40% 6% 10% 21% 17% 6%	1813
March	1st to 10th	Night	34% 6% 8% 25% 21% 5%	2090
	11th to 20th	Night	38% 6% 10% 20% 21% 5%	2090
	21st to end	Night	40% 6% 11% 20% 16% 6%	2201
April	1st to 10th	Night	37% 6% 11% 17% 21% 7%	1834
	11th to 20th	Night	32% 6% 13% 20% 23% 6%	1775
	21st to end	Night	36% 6% 13% 24% 15% 6%	1790
May	1st to 10th	Night	40% 8% 13% 22% 13% 4	1620
	11th to 20th	Night	42% 11% 13% 20% 12% 2	1593
	21st to end	Night	40% 10% 13% 23% 12% 1	1619
June	1st to 10th	Night	31% 11% 15% 27% 15% 1	1440
	11th to 20th	Night	37% 10% 18% 20% 13% 2	1440
	21st to end	Night	33% 11% 15% 21% 17% 3	1440
July	1st to 10th	Night	37% 12% 16% 16% 15% 4	1507
	11th to 20th	Night	30% 11% 22% 25% 10% 2	1566
	21st to end	Night	34% 11% 19% 24% 12%	1672
August	1st to 10th	Night	38% 12% 17% 21% 11% 1	1600
	11th to 20th	Night	41% 11% 15% 21% 12% 1	1621
	21st to end	Night	40% 10% 17% 21% 10% 3	1980
September	1st to 10th	Night	38% 9% 11% 22% 15% 6%	1800
	11th to 20th	Night	36% 8% 11% 23% 19% 3	1871
	21st to end	Night	44% 6% 11% 20% 16% 3	2041
October	1st to 10th	Night	38% 5% 11% 24% 18% 3	2160
	11th to 20th	Night	37% 5% 8% 23% 22% 5%	2160
	21st to end	Night	32% 5% 10% 23% 24% 5%	2376
November	1st to 10th	Night	35% 5% 8% 21% 24% 7%	2221
	11th to 20th	Night	36% 4 8% 21% 24% 7%	2484
	21st to end	Night	35% 4 9% 25% 21% 6%	2660
December	1st to 10th	Night	42% 5% 10% 23% 16% 4	2660
	11th to 20th	Night	38% 4 9% 22% 20% 6%	3288
	21st to end	Night	38% 8% 12% 25% 14% 2	4822

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	25% 18% 34% 17% 6%	270
	11th to 20th	Day	27% 24% 30% 16% 4	270
	21st to end	Day	24% 20% 35% 16% 4 1	297
February	1st to 10th	Day	24% 20% 40% 12% 4	270
	11th to 20th	Day	36% 22% 31% 9% 1	270
	21st to end	Day	29% 17% 30% 20% 4	374
March	1st to 10th	Day	23% 16% 31% 22% 6% 1	460
	11th to 20th	Day	22% 18% 32% 20% 7% 1	460
	21st to end	Day	23% 15% 29% 25% 6% 1	470
April	1st to 10th	Day	22% 17% 28% 20% 10% 2	420
	11th to 20th	Day	19% 12% 29% 23% 13% 4	412
	21st to end	Day	23% 11% 29% 21% 13% 2	420
May	1st to 10th	Day	22% 14% 28% 23% 10% 3	423
	11th to 20th	Day	21% 8% 26% 28% 13% 5%	447
	21st to end	Day	20% 3 23% 26% 21% 7%	485
June	1st to 10th	Day	12% 4 26% 27% 22% 9%	450
	11th to 20th	Day	12% 8% 24% 30% 16% 11%	450
	21st to end	Day	10% 5% 24% 33% 21% 7%	450
July	1st to 10th	Day	11% 2 17% 28% 30% 11%	445
	11th to 20th	Day	8% 2 12% 37% 26% 15%	444
	21st to end	Day	10% 3 15% 35% 25% 11%	488
August	1st to 10th	Day	12% 4 19% 26% 27% 13%	446
	11th to 20th	Day	12% 2 18% 26% 27% 16%	439
	21st to end	Day	9% 3 14% 29% 26% 18%	495
September	1st to 10th	Day	17% 3 12% 26% 24% 19%	450
	11th to 20th	Day	15% 4 14% 28% 28% 11%	450
	21st to end	Day	26% 6% 21% 24% 17% 5%	450
October	1st to 10th	Day	20% 6% 18% 31% 18% 8%	450
	11th to 20th	Day	16% 12% 25% 28% 17% 3	450
	21st to end	Day	17% 11% 30% 29% 11% 3	495
November	1st to 10th	Day	18% 9% 26% 31% 13% 4	455
	11th to 20th	Day	20% 11% 28% 23% 15% 2	322
	21st to end	Day	16% 13% 36% 23% 11% 2	180
December	1st to 10th	Day	18% 14% 35% 24% 7% 1	180
	11th to 20th	Day	23% 10% 33% 21% 12% 1	180
	21st to end	Day	24% 12% 34% 20% 8% 2	213
January	1st to 10th	Night	18% 10% 38% 30% 4	760
	11th to 20th	Night	21% 10% 32% 33% 4	760
	21st to end	Night	17% 11% 39% 30% 3	836
February	1st to 10th	Night	17% 10% 35% 34% 4	760
	11th to 20th	Night	25% 12% 36% 26% 1	743
	21st to end	Night	20% 12% 37% 28% 4	628
March	1st to 10th	Night	16% 12% 34% 33% 5%	760
	11th to 20th	Night	21% 12% 32% 27% 6% 1	760
	21st to end	Night	18% 11% 34% 31% 6%	801
April	1st to 10th	Night	16% 14% 34% 29% 7%	720
	11th to 20th	Night	14% 10% 32% 31% 11% 1	711
	21st to end	Night	19% 6% 28% 37% 9%	710
May	1st to 10th	Night	18% 6% 27% 33% 15% 1	540
	11th to 20th	Night	21% 2 19% 38% 18% 1	531
	21st to end	Night	17% 1 14% 38% 27% 4	458
June	1st to 10th	Night	10% 1 14% 43% 29% 2	360
	11th to 20th	Night	11% 2 13% 41% 31% 2	360
	21st to end	Night	10% 1 16% 46% 24% 2	360
July	1st to 10th	Night	12% 2 11% 33% 36% 7%	457
	11th to 20th	Night	6% 1 9% 35% 45% 5%	522
	21st to end	Night	8% 3 11% 39% 35% 5%	591
August	1st to 10th	Night	12% 1 7% 40% 36% 4	537
	11th to 20th	Night	14% 1 10% 32% 37% 6%	557
	21st to end	Night	12% 2 11% 34% 35% 5%	792
September	1st to 10th	Night	15% 2 13% 32% 30% 7%	720
	11th to 20th	Night	15% 3 13% 32% 33% 5%	720
	21st to end	Night	25% 3 17% 30% 23% 3	720
October	1st to 10th	Night	19% 4 17% 31% 25% 4	720
	11th to 20th	Night	17% 5% 24% 33% 18% 1	720
	21st to end	Night	17% 7% 24% 40% 12% 1	792
November	1st to 10th	Night	15% 6% 26% 37% 14% 2	740
	11th to 20th	Night	17% 7% 28% 36% 11% 1	760
	21st to end	Night	15% 7% 30% 37% 10% 1	760
December	1st to 10th	Night	19% 8% 32% 33% 7% 1	760
	11th to 20th	Night	17% 7% 30% 40% 6%	760
	21st to end	Night	18% 8% 29% 40% 5%	836

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