Morrison Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morrison Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	12% 7% 3 6% 4 4 4 3 5% 7% 43%	149322
	Night	12% 7% 3 7% 4 4 4 3 5% 7% 45%	142438
Transparency	Day	39% 5% 13% 25% 16% 2	152925
	Night	39% 4 12% 25% 17% 3	146914
Seeing	Day	16% 8% 26% 28% 17% 5%	18325
	Night	16% 8% 29% 33% 14% 1	28032

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	13% 12% 4 9% 4 4 4 4 4 5% 36%	4931
	11th to 20th	Day	7% 8% 3 9% 5% 7% 5% 3 5% 6% 42%	5310
	21st to end	Day	14% 14% 5% 9% 4 3 3 3 4 4 37%	5812
February	1st to 10th	Day	16% 8% 4 8% 6% 4 3 3 5% 6% 35%	1890
	11th to 20th	Day	14% 10% 3 8% 4 4 4 3 5% 5% 41%	1868
	21st to end	Day	13% 11% 4 6% 4 3 4 3 4 5% 42%	1730
March	1st to 10th	Day	16% 10% 3 4 3 2 2 2 3 5% 50%	5041
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 5% 7% 52%	5090
	21st to end	Day	16% 11% 3 8% 5% 4 4 3 5% 7% 33%	5623
April	1st to 10th	Day	16% 8% 3 7% 4 3 3 3 4 6% 42%	2520
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 5% 6% 41%	2494
	21st to end	Day	21% 6% 3 6% 5% 5% 3 4 6% 6% 34%	2520
May	1st to 10th	Day	15% 7% 2 6% 5% 4 4 4 6% 9% 38%	7572
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 43%	7901
	21st to end	Day	18% 8% 3 7% 5% 4 4 3 5% 6% 36%	9047
June	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 9% 46%	2940
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 47%	2940
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 7% 9% 47%	2940
July	1st to 10th	Day	10% 5% 3 6% 4 4 4 4 6% 9% 46%	8280
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 10% 49%	7888
	21st to end	Day	10% 4 2 6% 4 5% 5% 5% 7% 10% 41%	8337
August	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 48%	6014
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 7% 9% 43%	5815
	21st to end	Day	5% 4 2 4 3 4 4 4 6% 10% 55%	6080
September	1st to 10th	Day	10% 5% 2 4 4 3 4 3 4 6% 54%	2520
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 9% 51%	2515
	21st to end	Day	9% 5% 1 6% 5% 4 5% 3 5% 6% 51%	2170
October	1st to 10th	Day	12% 5% 2 6% 3 2 3 3 5% 5% 55%	2000
	11th to 20th	Day	14% 6% 2 5% 3 3 3 2 4 5% 53%	2000
	21st to end	Day	19% 7% 2 6% 3 3 3 3 4 5% 46%	2200
November	1st to 10th	Day	11% 7% 2 5% 2 4 3 2 5% 7% 51%	1984
	11th to 20th	Day	14% 7% 2 6% 3 3 2 3 4 7% 49%	1822
	21st to end	Day	18% 10% 3 7% 4 3 3 3 4 6% 41%	1680
December	1st to 10th	Day	7% 9% 3 7% 3 3 3 3 4 6% 51%	2690
	11th to 20th	Day	12% 9% 4 8% 4 3 4 3 3 6% 44%	3088
	21st to end	Day	21% 19% 7% 10% 4 3 4 3 4 4 21%	4070
January	1st to 10th	Night	13% 11% 6% 12% 5% 4 3 3 4 4 36%	7974
	11th to 20th	Night	10% 8% 4 10% 5% 5% 5% 4 4 5% 42%	7680
	21st to end	Night	14% 12% 4 8% 5% 5% 4 3 4 5% 37%	8402
February	1st to 10th	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 36%	2730
	11th to 20th	Night	15% 9% 4 6% 4 4 3 3 5% 6% 40%	2637
	21st to end	Night	14% 8% 3 7% 3 3 3 4 5% 4 45%	2076
March	1st to 10th	Night	17% 10% 3 7% 3 3 3 2 4 4 45%	5074
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 4 6% 53%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 5% 3 5% 6% 34%	5547
April	1st to 10th	Night	13% 7% 2 6% 3 4 4 3 5% 6% 46%	2100
	11th to 20th	Night	15% 6% 3 6% 4 3 4 3 5% 6% 45%	2075
	21st to end	Night	18% 7% 2 7% 4 4 4 3 5% 6% 41%	2100
May	1st to 10th	Night	15% 7% 3 8% 5% 5% 4 3 6% 7% 38%	5293
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 9% 46%	4975
	21st to end	Night	16% 7% 3 7% 4 4 4 3 5% 8% 39%	5139
June	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 8% 53%	1680
	11th to 20th	Night	5% 5% 2 4 4 4 5% 4 7% 9% 50%	1680
	21st to end	Night	4 3 1 4 5% 4 4 3 6% 9% 56%	1680
July	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 6% 10% 51%	4708
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 6% 8% 53%	5081
	21st to end	Night	7% 4 2 6% 4 4 5% 4 7% 11% 46%	5770
August	1st to 10th	Night	7% 2 1 4 3 3 4 3 5% 9% 59%	4156
	11th to 20th	Night	10% 5% 2 4 3 4 4 4 6% 8% 51%	4243
	21st to end	Night	4 3 1 4 3 4 4 3 6% 8% 60%	5046
September	1st to 10th	Night	11% 4 1 4 3 3 4 3 4 5% 59%	2100
	11th to 20th	Night	9% 4 1 6% 4 4 5% 4 6% 7% 50%	2100
	21st to end	Night	9% 5% 2 6% 3 3 3 3 4 6% 55%	2385
October	1st to 10th	Night	9% 4 2 4 2 3 3 3 5% 7% 58%	2400
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2400
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	2640
November	1st to 10th	Night	11% 5% 2 4 4 3 3 3 4 6% 56%	2534
	11th to 20th	Night	11% 7% 2 6% 4 3 3 3 5% 6% 50%	2794
	21st to end	Night	18% 9% 2 6% 4 3 3 2 4 4 45%	2940
December	1st to 10th	Night	10% 8% 3 6% 4 3 3 3 4 7% 49%	4720
	11th to 20th	Night	15% 9% 3 6% 3 3 4 3 4 6% 43%	5339
	21st to end	Night	23% 17% 5% 9% 4 4 3 3 3 4 25%	7150

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	50% 4 10% 22% 11% 2	4932
	11th to 20th	Day	44% 3 12% 23% 15% 4	5310
	21st to end	Day	51% 3 11% 17% 16% 2	5812
February	1st to 10th	Day	50% 3 12% 22% 11% 1	1890
	11th to 20th	Day	47% 3 11% 20% 15% 3	1877
	21st to end	Day	46% 3 8% 24% 17% 3	1730
March	1st to 10th	Day	40% 2 8% 25% 23% 2	5041
	11th to 20th	Day	33% 4 10% 25% 23% 6%	5090
	21st to end	Day	50% 4 16% 24% 5%	5625
April	1st to 10th	Day	45% 3 11% 26% 15% 1	2520
	11th to 20th	Day	45% 4 11% 25% 14% 3	2494
	21st to end	Day	50% 5% 12% 21% 12% 1	2520
May	1st to 10th	Day	42% 6% 15% 25% 12% 1	7610
	11th to 20th	Day	41% 5% 12% 26% 14% 2	7905
	21st to end	Day	47% 6% 9% 23% 14% 1	9061
June	1st to 10th	Day	33% 7% 15% 30% 14% 1	2940
	11th to 20th	Day	33% 6% 13% 28% 17% 3	2940
	21st to end	Day	32% 6% 17% 28% 14% 3	2940
July	1st to 10th	Day	35% 6% 14% 26% 18% 1	8211
	11th to 20th	Day	31% 6% 18% 26% 16% 3	7815
	21st to end	Day	35% 8% 21% 28% 7%	8155
August	1st to 10th	Day	30% 5% 16% 31% 16% 1	6000
	11th to 20th	Day	37% 6% 16% 30% 11% 1	5854
	21st to end	Day	24% 6% 17% 35% 15% 3	6079
September	1st to 10th	Day	31% 4 11% 27% 22% 5%	2520
	11th to 20th	Day	32% 4 10% 28% 21% 4	2515
	21st to end	Day	33% 5% 10% 26% 23% 3	2170
October	1st to 10th	Day	20% 2 6% 21% 40% 11%	3260
	11th to 20th	Day	23% 2 6% 21% 38% 9%	3240
	21st to end	Day	37% 3 8% 24% 26% 2	3538
November	1st to 10th	Day	34% 3 13% 24% 21% 5%	1984
	11th to 20th	Day	37% 2 10% 27% 20% 4	1822
	21st to end	Day	47% 3 8% 28% 14% 1	1680
December	1st to 10th	Day	36% 3 15% 30% 16% 1	2690
	11th to 20th	Day	43% 3 14% 28% 11% 1	3085
	21st to end	Day	67% 3 12% 12% 5%	4070
January	1st to 10th	Night	53% 3 12% 22% 9%	7975
	11th to 20th	Night	45% 3 9% 24% 16% 4	7680
	21st to end	Night	51% 3 12% 18% 14% 2	8402
February	1st to 10th	Night	50% 4 12% 21% 11% 2	2730
	11th to 20th	Night	45% 4 11% 22% 16% 2	2638
	21st to end	Night	42% 3 10% 24% 18% 3	2076
March	1st to 10th	Night	44% 3 7% 21% 22% 3	5074
	11th to 20th	Night	34% 4 7% 25% 26% 5%	5090
	21st to end	Night	49% 5% 12% 24% 8% 2	5547
April	1st to 10th	Night	39% 4 10% 25% 18% 3	2100
	11th to 20th	Night	40% 4 9% 27% 16% 5%	2075
	21st to end	Night	44% 4 10% 23% 17% 1	2100
May	1st to 10th	Night	44% 6% 13% 22% 14% 1	5320
	11th to 20th	Night	36% 5% 12% 26% 18% 4	4976
	21st to end	Night	44% 6% 11% 20% 18% 1	5149
June	1st to 10th	Night	28% 5% 16% 32% 16% 3	1680
	11th to 20th	Night	30% 7% 11% 27% 22% 4	1680
	21st to end	Night	26% 4 16% 30% 19% 5%	1680
July	1st to 10th	Night	29% 4 16% 29% 20% 2	4670
	11th to 20th	Night	28% 7% 15% 29% 18% 3	5034
	21st to end	Night	30% 6% 20% 35% 8% 1	5641
August	1st to 10th	Night	23% 4 18% 31% 23% 1	4146
	11th to 20th	Night	31% 5% 17% 31% 15% 2	4270
	21st to end	Night	21% 5% 16% 38% 17% 3	5046
September	1st to 10th	Night	28% 4 11% 24% 27% 7%	2100
	11th to 20th	Night	31% 6% 10% 27% 22% 5%	2100
	21st to end	Night	31% 4 8% 26% 26% 4	2385
October	1st to 10th	Night	17% 3 6% 21% 46% 7%	3920
	11th to 20th	Night	24% 2 7% 22% 31% 13%	3899
	21st to end	Night	36% 3 7% 26% 25% 2	4252
November	1st to 10th	Night	31% 3 12% 28% 21% 5%	2534
	11th to 20th	Night	36% 3 9% 26% 22% 4	2794
	21st to end	Night	44% 3 9% 25% 16% 2	2940
December	1st to 10th	Night	37% 4 11% 35% 13% 1	4720
	11th to 20th	Night	43% 3 13% 25% 15% 1	5341
	21st to end	Night	64% 3 12% 13% 6% 1	7150

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	21% 15% 35% 25% 5%	330
	11th to 20th	Day	27% 19% 38% 14% 2	330
	21st to end	Day	18% 14% 45% 20% 4	363
February	1st to 10th	Day	21% 15% 43% 17% 4	330
	11th to 20th	Day	24% 19% 36% 16% 5%	330
	21st to end	Day	21% 19% 40% 15% 4	273
March	1st to 10th	Day	21% 18% 42% 18% 2	330
	11th to 20th	Day	28% 12% 34% 21% 5%	330
	21st to end	Day	33% 16% 34% 15% 3	378
April	1st to 10th	Day	24% 14% 32% 22% 6% 1	540
	11th to 20th	Day	23% 14% 33% 21% 8% 1	532
	21st to end	Day	23% 11% 32% 25% 8% 1	540
May	1st to 10th	Day	19% 9% 32% 29% 9% 1	734
	11th to 20th	Day	20% 9% 25% 29% 14% 4	775
	21st to end	Day	17% 6% 28% 27% 17% 6%	844
June	1st to 10th	Day	9% 4 19% 35% 25% 9%	780
	11th to 20th	Day	11% 4 22% 34% 23% 6%	780
	21st to end	Day	9% 5% 26% 37% 19% 4	780
July	1st to 10th	Day	10% 2 14% 28% 33% 14%	771
	11th to 20th	Day	7% 1 17% 35% 29% 11%	768
	21st to end	Day	8% 2 15% 34% 31% 9%	849
August	1st to 10th	Day	7% 2 17% 37% 30% 7%	776
	11th to 20th	Day	10% 2 14% 37% 30% 8%	710
	21st to end	Day	9% 1 12% 33% 34% 10%	627
September	1st to 10th	Day	10% 2 14% 35% 29% 11%	570
	11th to 20th	Day	7% 2 14% 35% 31% 11%	565
	21st to end	Day	11% 5% 23% 29% 25% 8%	354
October	1st to 10th	Day	16% 6% 28% 27% 17% 7%	330
	11th to 20th	Day	13% 18% 33% 25% 11% 1	330
	21st to end	Day	28% 15% 31% 20% 6%	363
November	1st to 10th	Day	16% 12% 38% 22% 9% 3	330
	11th to 20th	Day	17% 15% 32% 27% 8% 2	330
	21st to end	Day	28% 11% 30% 24% 7%	330
December	1st to 10th	Day	13% 17% 37% 24% 7% 2	330
	11th to 20th	Day	20% 15% 38% 21% 5% 1	330
	21st to end	Day	26% 18% 35% 19% 1	363
January	1st to 10th	Night	16% 14% 38% 25% 7%	1050
	11th to 20th	Night	17% 16% 38% 23% 5%	1050
	21st to end	Night	18% 12% 40% 27% 3	1155
February	1st to 10th	Night	19% 12% 39% 26% 4	1050
	11th to 20th	Night	21% 14% 36% 26% 3	1003
	21st to end	Night	20% 13% 41% 22% 3	692
March	1st to 10th	Night	19% 14% 38% 25% 4	651
	11th to 20th	Night	22% 10% 33% 29% 6%	630
	21st to end	Night	25% 12% 36% 23% 4	693
April	1st to 10th	Night	16% 11% 38% 29% 5%	630
	11th to 20th	Night	19% 11% 33% 29% 8%	623
	21st to end	Night	20% 7% 35% 32% 6%	630
May	1st to 10th	Night	15% 6% 31% 38% 10%	630
	11th to 20th	Night	17% 5% 24% 37% 16% 1	621
	21st to end	Night	14% 5% 24% 35% 21% 1	683
June	1st to 10th	Night	9% 3 16% 45% 26% 3	630
	11th to 20th	Night	7% 3 21% 43% 25% 2	630
	21st to end	Night	5% 2 22% 51% 20% 1	630
July	1st to 10th	Night	11% 2 9% 37% 37% 4	615
	11th to 20th	Night	7% 2 11% 43% 36% 2	612
	21st to end	Night	7% 2 11% 45% 33% 2	690
August	1st to 10th	Night	7% 1 12% 47% 31% 3	627
	11th to 20th	Night	7% 1 10% 44% 35% 3	624
	21st to end	Night	6% 1 9% 40% 40% 3	693
September	1st to 10th	Night	12% 1 13% 41% 30% 3	630
	11th to 20th	Night	10% 2 14% 41% 28% 4	630
	21st to end	Night	10% 5% 21% 36% 24% 3	721
October	1st to 10th	Night	11% 3 27% 34% 23% 3	800
	11th to 20th	Night	17% 8% 31% 33% 11%	800
	21st to end	Night	22% 11% 30% 29% 9%	880
November	1st to 10th	Night	14% 8% 33% 35% 11%	820
	11th to 20th	Night	14% 10% 39% 29% 7%	904
	21st to end	Night	22% 10% 33% 28% 6%	1050
December	1st to 10th	Night	18% 11% 37% 27% 6%	1050
	11th to 20th	Night	22% 9% 33% 30% 6%	1050
	21st to end	Night	24% 12% 33% 28% 3	1155

Page generated at 2026-09-26 03:18:43. Version ac07

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