Robert and Elisabeth Moore Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert and Elisabeth Moore Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	7% 7% 3 6% 4 4 4 3 5% 6% 50%	140696
	Night	8% 8% 3 7% 5% 5% 5% 4 5% 7% 43%	139099
Transparency	Day	35% 3 10% 23% 20% 9%	135403
	Night	41% 4 11% 22% 16% 7%	135396
Seeing	Day	10% 8% 20% 26% 23% 13%	16173
	Night	12% 7% 19% 36% 23% 3	25376

By Month

Percentage Cloud Cover SW (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% 18% 6% 14% 6% 6% 5% 4 5% 6% 15%	4722
	11th to 20th	Day	11% 11% 6% 9% 7% 7% 7% 5% 6% 6% 27%	4740
	21st to end	Day	14% 13% 6% 10% 6% 5% 7% 5% 5% 5% 24%	5187
February	1st to 10th	Day	14% 14% 6% 12% 8% 5% 5% 4 4 5% 23%	1822
	11th to 20th	Day	12% 13% 4 9% 5% 6% 4 4 4 6% 31%	1920
	21st to end	Day	9% 9% 3 9% 7% 5% 6% 5% 7% 8% 32%	1730
March	1st to 10th	Day	6% 7% 3 7% 5% 6% 6% 5% 8% 10% 37%	4618
	11th to 20th	Day	7% 9% 3 7% 5% 4 4 3 5% 6% 47%	4835
	21st to end	Day	9% 9% 3 9% 6% 6% 6% 5% 7% 9% 30%	6073
April	1st to 10th	Day	7% 8% 3 7% 5% 5% 5% 5% 7% 9% 39%	2520
	11th to 20th	Day	6% 7% 2 6% 4 5% 5% 4 6% 9% 46%	2489
	21st to end	Day	6% 6% 2 6% 4 4 4 3 6% 8% 51%	2688
May	1st to 10th	Day	7% 4 1 4 3 3 4 3 5% 7% 58%	6330
	11th to 20th	Day	5% 5% 2 5% 5% 5% 5% 4 6% 6% 53%	6277
	21st to end	Day	4 5% 2 5% 4 4 5% 4 6% 8% 55%	6902
June	1st to 10th	Day	9% 5% 2 6% 4 4 5% 3 5% 8% 50%	2800
	11th to 20th	Day	6% 4 2 4 4 4 5% 4 5% 8% 55%	2800
	21st to end	Day	3 3 1 3 3 3 3 3 4 6% 68%	2800
July	1st to 10th	Day	2 2 1 2 2 1 2 2 3 4 80%	6319
	11th to 20th	Day	1 1 1 2 2 1 2 1 2 3 84%	6320
	21st to end	Day	1 1 1 2 1 2 2 2 4 6% 78%	6880
August	1st to 10th	Day	3 3 1 4 2 3 2 2 4 6% 69%	6210
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 6% 75%	5846
	21st to end	Day	4 4 1 4 3 3 3 3 5% 6% 65%	5984
September	1st to 10th	Day	2 1 2 2 2 3 2 5% 6% 73%	2400
	11th to 20th	Day	4 4 1 4 4 4 4 4 6% 6% 60%	2351
	21st to end	Day	6% 4 2 5% 4 4 3 3 5% 6% 59%	2200
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 4 7% 64%	3322
	11th to 20th	Day	5% 4 2 5% 4 4 6% 5% 7% 10% 47%	3229
	21st to end	Day	10% 7% 2 8% 5% 6% 6% 5% 6% 7% 40%	3200
November	1st to 10th	Day	12% 9% 3 10% 6% 5% 5% 4 5% 7% 33%	1854
	11th to 20th	Day	12% 15% 4 10% 7% 6% 6% 4 6% 6% 24%	1748
	21st to end	Day	11% 14% 6% 14% 8% 7% 5% 4 5% 6% 20%	1680
December	1st to 10th	Day	9% 11% 7% 14% 10% 9% 6% 4 5% 5% 20%	2700
	11th to 20th	Day	16% 20% 8% 14% 7% 5% 4 3 4 4 15%	3108
	21st to end	Day	15% 16% 7% 17% 8% 6% 5% 3 3 4 17%	4092
January	1st to 10th	Night	18% 21% 7% 11% 6% 5% 5% 3 5% 5% 13%	8172
	11th to 20th	Night	10% 10% 4 7% 6% 6% 7% 5% 7% 8% 32%	8220
	21st to end	Night	11% 13% 6% 11% 6% 5% 7% 5% 6% 7% 23%	8994
February	1st to 10th	Night	13% 13% 4 8% 5% 6% 6% 5% 5% 7% 29%	2794
	11th to 20th	Night	10% 12% 3 8% 6% 5% 5% 4 6% 6% 34%	2586
	21st to end	Night	9% 10% 4 7% 7% 5% 5% 4 6% 6% 37%	2076
March	1st to 10th	Night	6% 7% 3 6% 4 5% 6% 5% 8% 9% 40%	5496
	11th to 20th	Night	8% 8% 3 7% 5% 4 4 4 5% 6% 44%	5381
	21st to end	Night	7% 8% 3 9% 6% 7% 7% 5% 9% 9% 31%	5144
April	1st to 10th	Night	10% 9% 3 7% 5% 4 5% 4 6% 8% 40%	2100
	11th to 20th	Night	8% 6% 3 6% 5% 5% 4 4 5% 6% 49%	2079
	21st to end	Night	6% 5% 2 7% 5% 4 5% 4 7% 7% 47%	1930
May	1st to 10th	Night	6% 4 1 4 3 4 4 4 6% 6% 57%	3651
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 6% 7% 53%	3616
	21st to end	Night	5% 4 2 5% 3 4 4 4 6% 8% 55%	3972
June	1st to 10th	Night	8% 4 1 4 4 4 6% 4 7% 9% 50%	1600
	11th to 20th	Night	6% 6% 3 6% 5% 4 4 3 5% 6% 54%	1600
	21st to end	Night	3 3 1 5% 5% 3 4 4 4 6% 63%	1600
July	1st to 10th	Night	1 2 1 2 1 1 2 2 4 5% 78%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 84%	3640
	21st to end	Night	2 2 1 2 1 1 2 2 4 6% 78%	3970
August	1st to 10th	Night	3 3 1 3 3 3 3 3 5% 7% 66%	3582
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 5% 72%	3996
	21st to end	Night	3 3 1 4 3 4 4 3 5% 6% 66%	5015
September	1st to 10th	Night	3 2 1 2 2 2 3 3 3 5% 73%	2000
	11th to 20th	Night	4 3 1 4 4 3 4 3 6% 6% 62%	2048
	21st to end	Night	6% 3 1 4 3 3 5% 2 5% 8% 60%	2200
October	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 5% 67%	3873
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 6% 8% 53%	3928
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	4600
November	1st to 10th	Night	14% 8% 3 8% 5% 6% 5% 4 5% 6% 36%	2665
	11th to 20th	Night	14% 11% 3 9% 6% 5% 5% 5% 5% 6% 30%	2869
	21st to end	Night	13% 11% 4 8% 6% 7% 6% 4 5% 8% 28%	2940
December	1st to 10th	Night	7% 11% 7% 10% 7% 7% 6% 5% 6% 8% 27%	4700
	11th to 20th	Night	17% 18% 7% 13% 7% 6% 6% 4 5% 5% 12%	5316
	21st to end	Night	15% 16% 5% 11% 7% 7% 5% 4 4 6% 20%	7106

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	69% 5% 11% 10% 4	4723
	11th to 20th	Day	56% 6% 16% 18% 4	4740
	21st to end	Day	60% 7% 13% 14% 5%	5187
February	1st to 10th	Day	62% 5% 17% 12% 5%	1822
	11th to 20th	Day	54% 5% 15% 19% 7%	1929
	21st to end	Day	48% 6% 14% 21% 10% 1	1730
March	1st to 10th	Day	40% 4 11% 27% 18% 1	4618
	11th to 20th	Day	39% 3 12% 30% 15%	4835
	21st to end	Day	50% 4 15% 22% 9% 1	6073
April	1st to 10th	Day	40% 4 13% 27% 15% 2	2520
	11th to 20th	Day	34% 3 13% 28% 17% 5%	2492
	21st to end	Day	32% 3 12% 30% 20% 4	2688
May	1st to 10th	Day	26% 2 4 28% 32% 8%	6330
	11th to 20th	Day	29% 5% 8% 27% 23% 8%	6280
	21st to end	Day	28% 2 14% 30% 22% 4	6904
June	1st to 10th	Day	33% 3 7% 28% 22% 6%	2800
	11th to 20th	Day	28% 2 8% 30% 25% 6%	2800
	21st to end	Day	19% 2 7% 31% 29% 12%	2800
July	1st to 10th	Day	12% 1 3 15% 31% 37%	6251
	11th to 20th	Day	9% 1 2 10% 33% 45%	6245
	21st to end	Day	10% 2 4 23% 36% 24%	6681
August	1st to 10th	Day	15% 2 3 14% 37% 29%	4524
	11th to 20th	Day	11% 2 4 34% 35% 13%	4236
	21st to end	Day	19% 3 6% 28% 33% 12%	4311
September	1st to 10th	Day	13% 2 8% 29% 32% 16%	2400
	11th to 20th	Day	25% 3 10% 27% 27% 8%	2351
	21st to end	Day	27% 3 13% 35% 18% 4	2200
October	1st to 10th	Day	22% 2 11% 38% 23% 3	3322
	11th to 20th	Day	29% 5% 23% 29% 14% 1	3229
	21st to end	Day	41% 5% 15% 27% 11% 1	3200
November	1st to 10th	Day	50% 5% 16% 21% 7% 1	1854
	11th to 20th	Day	59% 5% 16% 16% 4 1	1748
	21st to end	Day	64% 4 15% 11% 4	1680
December	1st to 10th	Day	65% 6% 13% 13% 2	2700
	11th to 20th	Day	75% 4 10% 9% 2	3108
	21st to end	Day	73% 3 10% 12% 2	4092
January	1st to 10th	Night	73% 4 10% 9% 3	8172
	11th to 20th	Night	47% 7% 18% 24% 4	8220
	21st to end	Night	58% 6% 17% 14% 5%	8994
February	1st to 10th	Night	54% 5% 20% 17% 4 1	2794
	11th to 20th	Night	48% 6% 14% 24% 8% 1	2588
	21st to end	Night	48% 4 14% 23% 9% 1	2076
March	1st to 10th	Night	38% 4 8% 31% 19% 1	5496
	11th to 20th	Night	40% 3 10% 31% 15%	5381
	21st to end	Night	48% 4 13% 28% 7% 1	5144
April	1st to 10th	Night	42% 4 13% 26% 14% 1	2100
	11th to 20th	Night	36% 3 9% 27% 19% 6%	2079
	21st to end	Night	35% 4 10% 27% 20% 5%	1930
May	1st to 10th	Night	27% 2 5% 23% 37% 7%	3648
	11th to 20th	Night	31% 3 6% 29% 26% 4	3616
	21st to end	Night	27% 2 13% 35% 19% 2	3972
June	1st to 10th	Night	31% 3 9% 27% 24% 7%	1600
	11th to 20th	Night	32% 3 5% 29% 25% 6%	1600
	21st to end	Night	22% 3 5% 26% 29% 15%	1600
July	1st to 10th	Night	11% 1 2 18% 27% 41%	3600
	11th to 20th	Night	7% 2 2 10% 32% 48%	3592
	21st to end	Night	10% 2 3 19% 36% 31%	3857
August	1st to 10th	Night	17% 2 3 10% 33% 36%	2601
	11th to 20th	Night	13% 2 4 31% 32% 17%	2883
	21st to end	Night	16% 3 5% 27% 31% 19%	3608
September	1st to 10th	Night	15% 3 7% 23% 32% 21%	2000
	11th to 20th	Night	22% 3 9% 26% 26% 14%	2048
	21st to end	Night	24% 3 8% 31% 26% 9%	2200
October	1st to 10th	Night	21% 2 8% 35% 31% 2	3873
	11th to 20th	Night	29% 3 16% 27% 22% 2	3928
	21st to end	Night	36% 4 12% 32% 15% 2	4600
November	1st to 10th	Night	48% 5% 14% 23% 9%	2665
	11th to 20th	Night	54% 4 15% 20% 6% 1	2869
	21st to end	Night	54% 4 18% 17% 6% 1	2940
December	1st to 10th	Night	54% 9% 17% 16% 4	4700
	11th to 20th	Night	74% 6% 12% 6% 2	5316
	21st to end	Night	65% 4 13% 12% 5%	7106

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	27% 9% 21% 27% 14% 3	220
	11th to 20th	Day	20% 7% 17% 31% 20% 5%	220
	21st to end	Day	17% 6% 21% 30% 22% 4	242
February	1st to 10th	Day	19% 13% 25% 26% 14% 4	362
	11th to 20th	Day	19% 9% 20% 27% 19% 5%	465
	21st to end	Day	15% 11% 24% 29% 17% 4	446
March	1st to 10th	Day	14% 13% 29% 24% 16% 5%	540
	11th to 20th	Day	13% 12% 21% 27% 19% 8%	540
	21st to end	Day	15% 12% 26% 28% 16% 3	594
April	1st to 10th	Day	12% 14% 23% 24% 16% 10%	540
	11th to 20th	Day	9% 14% 29% 26% 15% 8%	532
	21st to end	Day	6% 10% 26% 27% 21% 9%	540
May	1st to 10th	Day	6% 9% 24% 27% 22% 12%	508
	11th to 20th	Day	10% 9% 23% 23% 22% 13%	528
	21st to end	Day	9% 5% 19% 25% 26% 16%	573
June	1st to 10th	Day	9% 8% 18% 23% 25% 17%	530
	11th to 20th	Day	7% 11% 27% 28% 18% 9%	530
	21st to end	Day	2 5% 19% 26% 30% 18%	530
July	1st to 10th	Day	3 2 11% 28% 40% 16%	526
	11th to 20th	Day	1 3 14% 29% 33% 21%	524
	21st to end	Day	1 2 11% 24% 33% 28%	575
August	1st to 10th	Day	3 3 10% 23% 34% 26%	527
	11th to 20th	Day	2 2 11% 24% 31% 31%	517
	21st to end	Day	4 6% 14% 26% 28% 21%	583
September	1st to 10th	Day	4 3 11% 26% 33% 25%	530
	11th to 20th	Day	5% 5% 16% 26% 28% 22%	530
	21st to end	Day	7% 7% 16% 27% 28% 15%	530
October	1st to 10th	Day	10% 5% 19% 26% 24% 15%	530
	11th to 20th	Day	10% 10% 16% 25% 25% 14%	510
	21st to end	Day	17% 8% 22% 31% 15% 7%	363
November	1st to 10th	Day	18% 9% 24% 28% 18% 3	330
	11th to 20th	Day	22% 11% 20% 25% 19% 2	256
	21st to end	Day	9% 12% 20% 29% 25% 5%	220
December	1st to 10th	Day	15% 9% 23% 25% 16% 11%	220
	11th to 20th	Day	19% 12% 22% 30% 15% 2	220
	21st to end	Day	17% 6% 32% 29% 13% 3	242
January	1st to 10th	Night	26% 9% 19% 28% 16% 2	840
	11th to 20th	Night	18% 7% 17% 35% 20% 3	840
	21st to end	Night	17% 5% 13% 38% 24% 3	924
February	1st to 10th	Night	18% 9% 20% 32% 20% 2	840
	11th to 20th	Night	13% 9% 20% 35% 20% 3	824
	21st to end	Night	13% 14% 20% 35% 17% 1	692
March	1st to 10th	Night	13% 8% 24% 35% 18% 2	840
	11th to 20th	Night	11% 10% 21% 34% 21% 2	840
	21st to end	Night	12% 9% 30% 34% 14% 1	924
April	1st to 10th	Night	15% 7% 20% 38% 17% 3	840
	11th to 20th	Night	10% 10% 29% 30% 19% 3	831
	21st to end	Night	8% 7% 27% 36% 21% 2	670
May	1st to 10th	Night	7% 6% 24% 38% 23% 3	405
	11th to 20th	Night	10% 6% 16% 36% 30% 3	394
	21st to end	Night	8% 3 20% 38% 27% 4	433
June	1st to 10th	Night	9% 5% 24% 35% 23% 4	400
	11th to 20th	Night	9% 6% 33% 34% 17% 2	400
	21st to end	Night	3 4 23% 39% 29% 3	400
July	1st to 10th	Night	4 2 18% 45% 29% 2	390
	11th to 20th	Night	4 2 20% 46% 27% 2	388
	21st to end	Night	1 2 17% 41% 36% 3	438
August	1st to 10th	Night	4 2 12% 42% 36% 4	398
	11th to 20th	Night	3 1 13% 36% 37% 10%	561
	21st to end	Night	3 2 14% 41% 34% 6%	880
September	1st to 10th	Night	3 3 14% 39% 37% 5%	800
	11th to 20th	Night	5% 2 12% 36% 38% 8%	800
	21st to end	Night	6% 4 17% 32% 35% 7%	800
October	1st to 10th	Night	10% 3 15% 39% 29% 4	800
	11th to 20th	Night	11% 8% 17% 33% 26% 6%	800
	21st to end	Night	10% 7% 23% 34% 21% 5%	880
November	1st to 10th	Night	16% 7% 21% 36% 19% 1	820
	11th to 20th	Night	18% 10% 21% 33% 17% 2	840
	21st to end	Night	16% 6% 17% 38% 20% 4	840
December	1st to 10th	Night	15% 8% 17% 35% 21% 3	840
	11th to 20th	Night	22% 9% 18% 35% 15% 1	840
	21st to end	Night	24% 7% 19% 35% 13% 1	924

Page generated at 2026-04-28 23:21:44. Version ac07

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