Moore Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Moore Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	14% 8% 3 7% 4 4 4 4 6% 7% 38%	126644
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 39%	123794
Transparency	Day	43% 5% 10% 24% 16% 2	130299
	Night	44% 5% 10% 23% 17% 3	128210
Seeing	Day	17% 8% 24% 25% 18% 8%	16744
	Night	19% 6% 25% 31% 17% 2	26453

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	15% 15% 8% 8% 3 4 3 3 5% 6% 30%	3617
	11th to 20th	Day	14% 13% 5% 14% 4 4 4 2 3 4 33%	3733
	21st to end	Day	21% 11% 6% 7% 5% 4 4 3 4 6% 29%	4489
February	1st to 10th	Day	19% 12% 4 9% 4 3 3 2 5% 5% 34%	2000
	11th to 20th	Day	18% 9% 4 8% 5% 3 4 3 5% 5% 35%	1948
	21st to end	Day	18% 12% 4 7% 4 4 5% 3 5% 6% 32%	1650
March	1st to 10th	Day	20% 12% 4 6% 4 3 3 3 4 4 37%	4480
	11th to 20th	Day	13% 9% 2 6% 4 4 4 3 6% 7% 41%	4888
	21st to end	Day	16% 8% 3 6% 5% 5% 5% 3 6% 7% 35%	5553
April	1st to 10th	Day	16% 7% 2 6% 5% 4 4 5% 5% 7% 38%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 3 4 6% 40%	2374
	21st to end	Day	17% 7% 2 7% 5% 5% 4 3 5% 6% 38%	2575
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 6% 5% 6% 9% 26%	5862
	11th to 20th	Day	20% 8% 2 7% 4 4 5% 4 6% 7% 34%	5861
	21st to end	Day	14% 6% 2 7% 5% 6% 5% 5% 8% 10% 32%	6433
June	1st to 10th	Day	11% 5% 2 7% 6% 4 4 4 6% 7% 45%	2657
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 5% 7% 8% 44%	2800
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 6% 10% 44%	2800
July	1st to 10th	Day	12% 5% 3 6% 4 4 5% 4 7% 9% 41%	6360
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 11% 39%	6276
	21st to end	Day	10% 7% 3 9% 6% 6% 6% 6% 8% 10% 29%	6424
August	1st to 10th	Day	8% 4 2 5% 4 4 5% 3 7% 11% 48%	4234
	11th to 20th	Day	8% 5% 2 7% 4 5% 5% 6% 8% 11% 40%	3988
	21st to end	Day	5% 4 1 4 3 4 4 4 5% 8% 57%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2324
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 6% 9% 50%	2200
	21st to end	Day	14% 5% 3 5% 4 4 4 3 4 6% 48%	2200
October	1st to 10th	Day	10% 4 2 5% 3 3 4 3 4 6% 55%	2200
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	2103
	21st to end	Day	18% 8% 2 6% 3 4 3 3 5% 6% 42%	1988
November	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 4 7% 52%	1852
	11th to 20th	Day	15% 11% 3 5% 4 3 3 3 5% 6% 42%	1863
	21st to end	Day	19% 10% 3 5% 4 3 4 4 4 5% 39%	1800
December	1st to 10th	Day	19% 10% 3 6% 4 3 3 2 3 5% 42%	2930
	11th to 20th	Day	17% 13% 5% 9% 6% 5% 5% 3 4 5% 28%	3373
	21st to end	Day	23% 17% 4 8% 4 5% 4 3 4 5% 22%	4097
January	1st to 10th	Night	16% 16% 5% 12% 5% 4 4 3 5% 6% 24%	6348
	11th to 20th	Night	16% 14% 6% 12% 6% 5% 4 2 3 4 28%	6223
	21st to end	Night	21% 11% 4 6% 4 3 4 3 4 5% 35%	6463
February	1st to 10th	Night	23% 10% 4 8% 5% 4 4 4 4 5% 29%	2400
	11th to 20th	Night	19% 10% 4 9% 4 4 4 3 4 5% 34%	2338
	21st to end	Night	21% 12% 3 7% 4 4 4 3 4 5% 32%	1980
March	1st to 10th	Night	21% 13% 2 5% 4 3 4 3 4 5% 36%	5373
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 4 5% 47%	5057
	21st to end	Night	17% 8% 3 8% 5% 5% 4 4 5% 6% 34%	5405
April	1st to 10th	Night	15% 8% 3 6% 4 4 3 4 5% 7% 41%	2000
	11th to 20th	Night	17% 8% 2 6% 4 4 4 2 4 6% 42%	1975
	21st to end	Night	17% 7% 2 6% 4 5% 4 4 5% 7% 38%	1824
May	1st to 10th	Night	21% 10% 3 8% 5% 5% 5% 3 5% 8% 28%	4033
	11th to 20th	Night	17% 6% 3 7% 5% 5% 5% 4 6% 8% 34%	4032
	21st to end	Night	13% 6% 3 7% 5% 5% 5% 4 7% 9% 37%	4421
June	1st to 10th	Night	10% 4 2 5% 4 4 4 3 6% 9% 48%	1737
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 9% 49%	1600
	21st to end	Night	6% 3 2 5% 4 4 4 5% 8% 11% 49%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 4 4 3 5% 8% 48%	3618
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	3695
	21st to end	Night	10% 6% 3 8% 5% 6% 6% 5% 7% 10% 35%	4427
August	1st to 10th	Night	7% 4 2 4 3 4 4 4 6% 9% 53%	2915
	11th to 20th	Night	7% 5% 2 6% 4 5% 5% 4 7% 9% 46%	3056
	21st to end	Night	4 2 1 4 3 3 3 3 5% 10% 62%	3586
September	1st to 10th	Night	11% 5% 2 4 3 4 4 3 5% 7% 51%	2055
	11th to 20th	Night	10% 5% 2 5% 3 4 3 4 5% 8% 53%	2200
	21st to end	Night	14% 5% 2 6% 3 3 3 4 4 5% 52%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 2 3 3 5% 6% 59%	2200
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 50%	2295
	21st to end	Night	18% 7% 2 5% 4 4 3 3 4 5% 45%	2850
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 49%	2666
	11th to 20th	Night	14% 8% 2 6% 5% 3 4 3 4 7% 43%	2692
	21st to end	Night	19% 11% 3 5% 4 4 4 3 4 6% 37%	2600
December	1st to 10th	Night	19% 12% 3 7% 5% 4 3 2 3 4 40%	4250
	11th to 20th	Night	21% 11% 4 7% 4 3 3 3 4 6% 34%	4822
	21st to end	Night	24% 17% 5% 10% 5% 5% 4 3 3 4 20%	6858

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	56% 3 9% 21% 9% 1	3618
	11th to 20th	Day	58% 3 7% 18% 13% 2	3733
	21st to end	Day	57% 2 9% 21% 7% 3	4489
February	1st to 10th	Day	53% 3 9% 22% 11% 2	2000
	11th to 20th	Day	53% 4 8% 21% 12% 2	1958
	21st to end	Day	53% 4 9% 21% 11% 2	1650
March	1st to 10th	Day	52% 2 5% 18% 16% 6%	4480
	11th to 20th	Day	41% 4 8% 27% 17% 2	4888
	21st to end	Day	47% 5% 12% 23% 12% 1	5555
April	1st to 10th	Day	44% 5% 11% 23% 15% 1	2400
	11th to 20th	Day	46% 4 8% 24% 15% 3	2374
	21st to end	Day	46% 5% 10% 25% 12% 2	2575
May	1st to 10th	Day	53% 7% 12% 17% 10%	5897
	11th to 20th	Day	49% 6% 10% 23% 10%	5865
	21st to end	Day	44% 7% 16% 18% 15% 1	6447
June	1st to 10th	Day	38% 6% 10% 26% 17% 3	2657
	11th to 20th	Day	34% 7% 13% 25% 19% 2	2800
	21st to end	Day	34% 8% 14% 21% 19% 4	2800
July	1st to 10th	Day	38% 6% 12% 23% 20% 1	6291
	11th to 20th	Day	37% 8% 15% 27% 12% 1	6201
	21st to end	Day	48% 8% 14% 19% 11% 1	6240
August	1st to 10th	Day	30% 6% 13% 36% 14% 1	4221
	11th to 20th	Day	35% 7% 12% 29% 16% 1	4027
	21st to end	Day	25% 6% 11% 36% 18% 3	4311
September	1st to 10th	Day	38% 5% 10% 24% 19% 5%	2324
	11th to 20th	Day	32% 5% 11% 25% 23% 4	2200
	21st to end	Day	38% 4 8% 23% 25% 3	2200
October	1st to 10th	Day	21% 2 5% 29% 39% 3	3590
	11th to 20th	Day	26% 3 9% 28% 31% 3	3408
	21st to end	Day	33% 3 8% 28% 22% 6%	3188
November	1st to 10th	Day	34% 3 9% 29% 21% 4	1852
	11th to 20th	Day	43% 4 6% 24% 18% 4	1863
	21st to end	Day	47% 4 8% 23% 17% 1	1800
December	1st to 10th	Day	47% 2 9% 26% 15%	2930
	11th to 20th	Day	59% 4 8% 18% 10% 1	3370
	21st to end	Day	63% 5% 10% 15% 6% 1	4097
January	1st to 10th	Night	61% 5% 8% 17% 10% 1	6348
	11th to 20th	Night	62% 3 6% 15% 11% 3	6223
	21st to end	Night	52% 3 7% 23% 14% 2	6463
February	1st to 10th	Night	57% 4 10% 19% 8% 1	2400
	11th to 20th	Night	53% 4 8% 21% 13% 1	2339
	21st to end	Night	55% 4 10% 16% 12% 3	1980
March	1st to 10th	Night	52% 3 5% 23% 13% 4	5373
	11th to 20th	Night	39% 4 9% 26% 21% 2	5057
	21st to end	Night	50% 5% 8% 24% 13% 1	5405
April	1st to 10th	Night	42% 5% 10% 23% 16% 3	2000
	11th to 20th	Night	43% 3 9% 20% 19% 6%	1975
	21st to end	Night	45% 6% 10% 23% 13% 3	1824
May	1st to 10th	Night	53% 8% 10% 19% 9% 1	4060
	11th to 20th	Night	48% 7% 12% 24% 9% 1	4033
	21st to end	Night	41% 7% 15% 22% 13% 2	4429
June	1st to 10th	Night	32% 5% 11% 27% 20% 4	1737
	11th to 20th	Night	31% 6% 14% 22% 22% 5%	1600
	21st to end	Night	28% 7% 18% 22% 19% 7%	1600
July	1st to 10th	Night	34% 6% 11% 26% 20% 3	3580
	11th to 20th	Night	34% 9% 15% 28% 13% 2	3650
	21st to end	Night	43% 8% 15% 21% 13% 1	4297
August	1st to 10th	Night	26% 7% 14% 38% 14% 1	2904
	11th to 20th	Night	33% 7% 12% 30% 18% 1	3083
	21st to end	Night	18% 5% 14% 35% 24% 4	3586
September	1st to 10th	Night	32% 5% 11% 24% 22% 6%	2055
	11th to 20th	Night	30% 4 10% 27% 24% 5%	2200
	21st to end	Night	35% 5% 7% 24% 26% 4	2200
October	1st to 10th	Night	21% 3 7% 19% 48% 3	3590
	11th to 20th	Night	25% 3 7% 25% 35% 5%	3729
	21st to end	Night	32% 3 7% 22% 30% 7%	4600
November	1st to 10th	Night	37% 4 8% 25% 23% 4	2666
	11th to 20th	Night	41% 4 7% 22% 21% 5%	2692
	21st to end	Night	49% 3 8% 23% 16% 1	2600
December	1st to 10th	Night	50% 3 10% 23% 14%	4250
	11th to 20th	Night	51% 3 11% 21% 12% 1	4824
	21st to end	Night	68% 4 8% 14% 6%	6858

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	24% 15% 37% 18% 6% 1	300
	11th to 20th	Day	25% 21% 36% 14% 5%	300
	21st to end	Day	27% 15% 38% 18% 2	330
February	1st to 10th	Day	25% 16% 40% 14% 4	300
	11th to 20th	Day	28% 20% 36% 10% 5%	300
	21st to end	Day	32% 13% 36% 14% 4	249
March	1st to 10th	Day	23% 18% 38% 16% 5%	300
	11th to 20th	Day	23% 15% 37% 17% 6% 2	460
	21st to end	Day	27% 14% 35% 18% 6% 1	550
April	1st to 10th	Day	23% 14% 28% 22% 12% 1	500
	11th to 20th	Day	24% 12% 25% 25% 11% 3	492
	21st to end	Day	20% 11% 35% 22% 10% 2	500
May	1st to 10th	Day	22% 9% 29% 24% 12% 4	503
	11th to 20th	Day	22% 6% 22% 27% 15% 8%	527
	21st to end	Day	14% 4 23% 26% 20% 12%	573
June	1st to 10th	Day	14% 4 20% 27% 24% 11%	587
	11th to 20th	Day	9% 5% 14% 36% 25% 11%	730
	21st to end	Day	10% 4 19% 32% 24% 12%	730
July	1st to 10th	Day	15% 3 13% 24% 24% 20%	721
	11th to 20th	Day	10% 3 13% 31% 30% 13%	686
	21st to end	Day	12% 2 11% 29% 28% 17%	576
August	1st to 10th	Day	10% 2 13% 28% 28% 20%	526
	11th to 20th	Day	12% 3 15% 28% 30% 13%	519
	21st to end	Day	8% 1 10% 30% 33% 17%	583
September	1st to 10th	Day	14% 3 13% 28% 30% 12%	530
	11th to 20th	Day	10% 2 12% 27% 34% 16%	530
	21st to end	Day	17% 5% 17% 26% 25% 11%	530
October	1st to 10th	Day	11% 3 18% 36% 25% 7%	530
	11th to 20th	Day	12% 10% 30% 29% 15% 5%	530
	21st to end	Day	24% 10% 34% 22% 9%	371
November	1st to 10th	Day	15% 11% 32% 29% 9% 3	330
	11th to 20th	Day	21% 17% 30% 23% 7% 2	321
	21st to end	Day	25% 10% 33% 24% 8% 1	300
December	1st to 10th	Day	22% 17% 31% 22% 7% 1	300
	11th to 20th	Day	22% 12% 41% 16% 9%	300
	21st to end	Day	31% 14% 32% 20% 3	330
January	1st to 10th	Night	23% 13% 32% 26% 5%	1000
	11th to 20th	Night	29% 11% 33% 23% 4	941
	21st to end	Night	24% 10% 39% 25% 2	880
February	1st to 10th	Night	28% 9% 33% 26% 3	800
	11th to 20th	Night	24% 12% 37% 25% 1	783
	21st to end	Night	27% 11% 37% 23% 2	660
March	1st to 10th	Night	25% 11% 32% 27% 5%	800
	11th to 20th	Night	23% 8% 35% 26% 8% 1	800
	21st to end	Night	24% 9% 29% 30% 6% 1	856
April	1st to 10th	Night	19% 8% 35% 30% 9%	600
	11th to 20th	Night	19% 7% 30% 31% 11% 1	593
	21st to end	Night	18% 6% 35% 32% 8% 2	600
May	1st to 10th	Night	21% 5% 32% 28% 14% 1	600
	11th to 20th	Night	16% 5% 17% 40% 18% 4	591
	21st to end	Night	10% 3 17% 39% 27% 5%	650
June	1st to 10th	Night	11% 1 17% 40% 27% 4	600
	11th to 20th	Night	9% 3 11% 43% 31% 4	600
	21st to end	Night	6% 1 14% 43% 32% 5%	600
July	1st to 10th	Night	13% 2 8% 28% 39% 10%	585
	11th to 20th	Night	10% 1 6% 36% 41% 6%	582
	21st to end	Night	11% 2 7% 36% 36% 8%	657
August	1st to 10th	Night	9% 1 8% 34% 43% 6%	597
	11th to 20th	Night	8% 2 7% 40% 36% 7%	594
	21st to end	Night	6% 1 7% 36% 44% 6%	660
September	1st to 10th	Night	13% 1 8% 37% 35% 5%	655
	11th to 20th	Night	12% 2 13% 30% 39% 4	800
	21st to end	Night	15% 2 17% 36% 27% 3	800
October	1st to 10th	Night	12% 3 17% 35% 27% 5%	800
	11th to 20th	Night	15% 6% 26% 36% 16% 2	800
	21st to end	Night	21% 6% 32% 29% 12%	880
November	1st to 10th	Night	16% 6% 28% 33% 15% 2	820
	11th to 20th	Night	19% 10% 32% 28% 11%	829
	21st to end	Night	23% 10% 25% 31% 11% 1	800
December	1st to 10th	Night	26% 7% 33% 29% 5% 1	800
	11th to 20th	Night	27% 8% 33% 25% 6% 1	800
	21st to end	Night	31% 10% 30% 24% 4	1040

Page generated at 2025-11-15 00:55:55. Version ac07

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