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 2024-11-04.

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%	100973
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	97955
Transparency	Day	37% 8% 12% 22% 16% 4	104623
	Night	38% 7% 11% 22% 18% 5%	102444
Seeing	Day	18% 10% 25% 25% 16% 6%	15034
	Night	17% 7% 25% 34% 16% 2	24776

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% 6% 2 6% 4 3 4 3 4 5% 47%	2836
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 5% 5% 44%	2840
	21st to end	Day	16% 9% 2 7% 4 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 5% 6% 42%	1710
	11th to 20th	Day	18% 6% 2 6% 4 4 3 3 4 7% 43%	1663
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 5% 6% 38%	1634
March	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 5% 6% 37%	3435
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 8% 51%	3480
	21st to end	Day	17% 8% 3 7% 5% 4 4 4 5% 6% 38%	3815
April	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 7% 49%	2243
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 48%	2254
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 5% 42%	2308
May	1st to 10th	Day	16% 7% 3 7% 5% 6% 6% 5% 6% 9% 31%	4082
	11th to 20th	Day	21% 7% 2 6% 5% 4 5% 4 6% 8% 34%	4081
	21st to end	Day	9% 4 2 5% 4 4 5% 5% 7% 11% 44%	4697
June	1st to 10th	Day	10% 4 2 6% 4 4 5% 5% 7% 10% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 6% 6% 5% 5% 7% 9% 40%	2660
	21st to end	Day	7% 4 2 5% 5% 5% 6% 5% 8% 10% 44%	2660
July	1st to 10th	Day	6% 3 2 6% 5% 5% 7% 5% 9% 11% 41%	4224
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 6% 9% 12% 35%	4120
	21st to end	Day	10% 7% 3 7% 7% 6% 6% 5% 7% 10% 32%	4466
August	1st to 10th	Day	14% 5% 3 7% 5% 5% 5% 5% 9% 12% 32%	4104
	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 3 2 6% 5% 56%	1887
	11th to 20th	Day	15% 4 2 4 3 3 4 3 5% 5% 51%	1691
	21st to end	Day	14% 4 2 4 4 3 3 3 5% 6% 52%	1520
December	1st to 10th	Day	15% 7% 3 5% 4 5% 5% 3 5% 7% 42%	1520
	11th to 20th	Day	15% 5% 2 5% 3 3 3 2 4 6% 52%	1910
	21st to end	Day	19% 6% 2 7% 4 4 4 3 6% 6% 38%	2816
January	1st to 10th	Night	12% 7% 2 5% 3 3 4 3 4 6% 50%	4109
	11th to 20th	Night	17% 8% 2 4 3 3 3 2 5% 5% 49%	4120
	21st to end	Night	19% 9% 3 6% 3 3 3 2 4 5% 43%	4532
February	1st to 10th	Night	13% 6% 3 5% 4 5% 5% 4 6% 6% 43%	2470
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 4 5% 43%	2403
	21st to end	Night	15% 7% 3 7% 4 4 3 3 5% 7% 44%	1813
March	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 6% 37%	3418
	11th to 20th	Night	11% 4 1 5% 3 3 3 4 5% 8% 52%	3480
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 6% 49%	3817
April	1st to 10th	Night	12% 8% 2 6% 4 4 3 3 5% 7% 46%	1935
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 6% 9% 49%	1875
	21st to end	Night	15% 5% 1 5% 4 5% 4 3 5% 7% 45%	1889
May	1st to 10th	Night	19% 8% 2 6% 4 4 5% 4 5% 9% 33%	2813
	11th to 20th	Night	25% 6% 2 6% 4 4 4 4 6% 7% 33%	2812
	21st to end	Night	11% 5% 2 6% 4 4 4 4 7% 9% 44%	2842
June	1st to 10th	Night	10% 4 2 5% 5% 4 5% 4 8% 9% 45%	1520
	11th to 20th	Night	10% 6% 2 5% 4 5% 5% 6% 7% 9% 42%	1520
	21st to end	Night	8% 4 2 5% 5% 4 5% 6% 6% 9% 45%	1520
July	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 5% 8% 11% 40%	2730
	11th to 20th	Night	7% 6% 3 8% 6% 5% 6% 5% 8% 11% 35%	2840
	21st to end	Night	9% 8% 4 10% 7% 6% 7% 5% 7% 9% 30%	3087
August	1st to 10th	Night	15% 6% 2 7% 6% 5% 5% 5% 7% 10% 32%	2825
	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	12% 6% 2 5% 4 4 3 3 3 5% 54%	2264
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 50%	2484
	21st to end	Night	13% 4 2 5% 4 4 4 3 5% 6% 50%	2660
December	1st to 10th	Night	14% 9% 3 5% 4 5% 4 4 4 6% 43%	2660
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 6% 48%	3285
	21st to end	Night	16% 5% 2 5% 4 4 5% 4 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	45% 7% 9% 19% 13% 7%	3435
	11th to 20th	Day	29% 7% 9% 25% 22% 8%	3480
	21st to end	Day	45% 8% 9% 18% 15% 4	3815
April	1st to 10th	Day	33% 7% 11% 22% 18% 9%	2243
	11th to 20th	Day	34% 6% 12% 20% 21% 6%	2254
	21st to end	Day	42% 6% 11% 22% 14% 4	2308
May	1st to 10th	Day	45% 11% 13% 17% 12% 2	4120
	11th to 20th	Day	45% 10% 13% 20% 11% 1	4085
	21st to end	Day	30% 11% 16% 22% 19% 2	4711
June	1st to 10th	Day	33% 9% 16% 25% 15% 1	2660
	11th to 20th	Day	36% 12% 15% 21% 15% 2	2660
	21st to end	Day	32% 11% 15% 26% 15% 2	2660
July	1st to 10th	Day	32% 11% 21% 19% 11% 6%	4159
	11th to 20th	Day	36% 10% 19% 25% 9% 1	4051
	21st to end	Day	45% 10% 13% 20% 12% 1	4324
August	1st to 10th	Day	43% 9% 18% 24% 6%	4091
	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% 4 9% 23% 28% 5%	1887
	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	44% 8% 9% 20% 15% 3	3418
	11th to 20th	Night	28% 6% 9% 22% 28% 7%	3480
	21st to end	Night	37% 6% 10% 22% 20% 5%	3817
April	1st to 10th	Night	37% 6% 11% 17% 22% 7%	1935
	11th to 20th	Night	31% 7% 14% 20% 22% 6%	1875
	21st to end	Night	36% 7% 13% 24% 15% 5%	1889
May	1st to 10th	Night	46% 8% 14% 18% 11% 3	2840
	11th to 20th	Night	46% 11% 12% 22% 8% 1	2813
	21st to end	Night	33% 11% 15% 24% 14% 2	2849
June	1st to 10th	Night	32% 11% 14% 26% 16% 1	1520
	11th to 20th	Night	35% 10% 18% 21% 13% 2	1520
	21st to end	Night	32% 11% 15% 21% 17% 3	1520
July	1st to 10th	Night	36% 12% 23% 14% 13% 3	2685
	11th to 20th	Night	40% 10% 20% 22% 7% 1	2786
	21st to end	Night	48% 10% 15% 18% 10%	2988
August	1st to 10th	Night	45% 9% 17% 22% 6% 1	2814
	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% 22% 25% 7%	2264
	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% 19% 32% 20% 7% 1	460
	21st to end	Day	24% 15% 29% 25% 6% 1	506
April	1st to 10th	Day	22% 18% 29% 19% 9% 2	460
	11th to 20th	Day	19% 14% 29% 22% 12% 4	452
	21st to end	Day	22% 12% 29% 21% 13% 2	460
May	1st to 10th	Day	21% 14% 27% 23% 11% 4	463
	11th to 20th	Day	21% 8% 25% 26% 14% 5%	487
	21st to end	Day	19% 3 25% 26% 21% 6%	529
June	1st to 10th	Day	11% 4 27% 28% 21% 9%	490
	11th to 20th	Day	11% 7% 23% 31% 17% 11%	490
	21st to end	Day	10% 5% 24% 34% 21% 7%	490
July	1st to 10th	Day	10% 2 18% 29% 30% 11%	485
	11th to 20th	Day	8% 2 12% 37% 25% 14%	484
	21st to end	Day	11% 4 16% 35% 24% 11%	532
August	1st to 10th	Day	12% 4 20% 26% 26% 12%	486
	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	18% 9% 26% 31% 13% 4	467
	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% 33% 27% 6% 1	760
	21st to end	Night	17% 11% 35% 31% 6%	836
April	1st to 10th	Night	16% 14% 35% 28% 7%	760
	11th to 20th	Night	14% 11% 32% 32% 10% 1	751
	21st to end	Night	18% 6% 29% 38% 9%	749
May	1st to 10th	Night	19% 6% 26% 33% 16% 1	570
	11th to 20th	Night	22% 2 19% 38% 18% 1	561
	21st to end	Night	16% 1 15% 37% 27% 4	483
June	1st to 10th	Night	10% 2 15% 44% 28% 2	380
	11th to 20th	Night	11% 2 13% 42% 31% 2	380
	21st to end	Night	10% 1 17% 46% 23% 2	380
July	1st to 10th	Night	12% 2 10% 34% 36% 7%	484
	11th to 20th	Night	5% 1 10% 35% 44% 5%	552
	21st to end	Night	9% 3 11% 39% 33% 5%	624
August	1st to 10th	Night	13% 2 8% 39% 35% 4	567
	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	15% 6% 26% 37% 14% 2	755
	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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