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 2025-05-26.

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% 8% 3 6% 4 4 4 3 5% 7% 44%	114238
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 45%	115251
Transparency	Day	39% 4 12% 25% 17% 3	117882
	Night	40% 4 11% 24% 18% 3	119749
Seeing	Day	16% 8% 26% 28% 17% 5%	16369
	Night	16% 8% 29% 33% 13% 1	26306

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	14% 12% 4 10% 4 5% 4 4 4 6% 33%	3801
	11th to 20th	Day	9% 9% 4 10% 5% 7% 5% 3 4 4 41%	4080
	21st to end	Day	15% 14% 5% 9% 4 4 4 3 4 4 36%	4488
February	1st to 10th	Day	17% 9% 4 9% 6% 4 3 4 5% 6% 34%	1800
	11th to 20th	Day	14% 10% 3 8% 5% 4 4 3 4 5% 40%	1777
	21st to end	Day	13% 12% 4 6% 4 3 4 3 4 5% 41%	1650
March	1st to 10th	Day	16% 10% 3 4 3 2 2 2 3 5% 51%	4931
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 5% 6% 52%	4980
	21st to end	Day	16% 11% 3 8% 5% 4 4 3 5% 7% 33%	5501
April	1st to 10th	Day	16% 8% 3 7% 4 3 3 3 4 6% 43%	2400
	11th to 20th	Day	17% 8% 3 7% 4 3 4 3 5% 6% 41%	2374
	21st to end	Day	22% 6% 3 6% 5% 5% 4 4 6% 5% 34%	2400
May	1st to 10th	Day	13% 8% 2 7% 5% 4 5% 4 7% 9% 35%	4194
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 5% 7% 39%	4365
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 5% 7% 44%	4895
June	1st to 10th	Day	7% 4 2 6% 5% 4 5% 5% 6% 10% 47%	2660
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 48%	2660
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 9% 48%	2660
July	1st to 10th	Day	13% 6% 3 6% 4 4 4 4 6% 9% 41%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	4230
	21st to end	Day	8% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	4465
August	1st to 10th	Day	4 4 2 4 4 4 5% 5% 7% 11% 51%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 4 6% 8% 45%	3953
	21st to end	Day	6% 4 2 4 4 4 5% 4 6% 9% 53%	4180
September	1st to 10th	Day	10% 5% 2 4 4 4 4 3 4 6% 53%	2280
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 4 9% 52%	2275
	21st to end	Day	8% 5% 1 6% 5% 4 5% 3 5% 6% 53%	1982
October	1st to 10th	Day	12% 5% 2 6% 3 2 3 3 5% 4 55%	1900
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 4 5% 53%	1900
	21st to end	Day	18% 7% 2 6% 3 3 3 3 4 5% 47%	2090
November	1st to 10th	Day	12% 7% 2 5% 2 4 3 2 5% 7% 50%	1888
	11th to 20th	Day	15% 7% 2 6% 3 2 2 3 4 7% 49%	1735
	21st to end	Day	17% 10% 3 6% 4 3 3 3 4 6% 42%	1600
December	1st to 10th	Day	7% 9% 3 7% 3 3 3 3 4 6% 52%	2610
	11th to 20th	Day	12% 9% 4 8% 4 3 4 2 4 6% 44%	3008
	21st to end	Day	21% 20% 7% 10% 4 3 4 3 4 4 21%	3982
January	1st to 10th	Night	14% 11% 7% 14% 5% 4 4 2 4 4 32%	6146
	11th to 20th	Night	12% 9% 4 12% 5% 5% 5% 4 4 4 36%	5900
	21st to end	Night	15% 12% 5% 7% 4 4 4 3 4 5% 37%	6490
February	1st to 10th	Night	15% 11% 4 8% 6% 4 4 3 4 6% 36%	2600
	11th to 20th	Night	15% 9% 4 6% 4 4 4 3 5% 6% 40%	2508
	21st to end	Night	15% 9% 4 7% 3 3 3 3 5% 4 45%	1980
March	1st to 10th	Night	16% 10% 3 7% 3 3 3 2 4 4 45%	4963
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 4 6% 53%	4980
	21st to end	Night	16% 10% 4 8% 5% 4 5% 3 5% 6% 34%	5427
April	1st to 10th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 47%	2000
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 3 5% 6% 46%	1975
	21st to end	Night	18% 7% 2 7% 4 5% 4 3 5% 6% 40%	2000
May	1st to 10th	Night	15% 8% 2 7% 5% 5% 4 4 6% 8% 38%	2927
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 4 10% 41%	2758
	21st to end	Night	10% 5% 2 5% 3 4 4 3 6% 9% 50%	2788
June	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 8% 54%	1520
	11th to 20th	Night	5% 5% 2 4 5% 4 5% 4 7% 9% 50%	1520
	21st to end	Night	4 3 1 4 5% 4 4 3 6% 9% 56%	1520
July	1st to 10th	Night	13% 6% 2 6% 3 3 4 3 5% 9% 45%	2530
	11th to 20th	Night	4 2 1 4 4 4 4 3 5% 9% 60%	2736
	21st to end	Night	7% 3 2 5% 5% 4 5% 5% 6% 10% 48%	3098
August	1st to 10th	Night	4 2 1 3 2 3 4 4 6% 10% 62%	2836
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 5% 7% 52%	2881
	21st to end	Night	4 3 1 4 3 4 4 4 6% 9% 57%	3476
September	1st to 10th	Night	11% 4 1 4 3 3 4 3 4 5% 59%	1900
	11th to 20th	Night	9% 5% 1 5% 3 4 4 4 6% 7% 51%	1900
	21st to end	Night	8% 5% 2 6% 3 3 3 3 4 6% 56%	2178
October	1st to 10th	Night	9% 4 1 4 2 3 3 3 5% 7% 58%	2280
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 54%	2280
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 4 6% 56%	2410
	11th to 20th	Night	12% 7% 2 6% 4 3 3 3 5% 6% 50%	2661
	21st to end	Night	17% 8% 2 5% 4 3 3 2 4 4 46%	2800
December	1st to 10th	Night	10% 8% 2 6% 4 3 3 3 4 7% 50%	4580
	11th to 20th	Night	16% 9% 3 6% 3 3 4 3 4 6% 43%	5199
	21st to end	Night	23% 17% 6% 9% 4 4 4 3 3 4 25%	6996

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	53% 4 10% 20% 10% 2	3802
	11th to 20th	Day	48% 3 11% 21% 14% 4	4080
	21st to end	Day	52% 3 10% 14% 18% 3	4488
February	1st to 10th	Day	52% 3 12% 21% 11% 1	1800
	11th to 20th	Day	47% 3 11% 20% 16% 3	1786
	21st to end	Day	47% 2 8% 23% 17% 3	1650
March	1st to 10th	Day	39% 2 8% 25% 23% 2	4931
	11th to 20th	Day	33% 4 9% 25% 23% 6%	4980
	21st to end	Day	51% 4 16% 23% 5%	5503
April	1st to 10th	Day	44% 3 11% 27% 15% 1	2400
	11th to 20th	Day	45% 3 11% 25% 13% 3	2374
	21st to end	Day	50% 5% 12% 21% 12% 1	2400
May	1st to 10th	Day	43% 6% 15% 23% 12% 1	4232
	11th to 20th	Day	45% 5% 13% 27% 10% 1	4369
	21st to end	Day	38% 6% 10% 28% 17% 1	4908
June	1st to 10th	Day	31% 7% 16% 30% 15% 1	2660
	11th to 20th	Day	32% 6% 13% 29% 17% 3	2660
	21st to end	Day	31% 6% 17% 28% 15% 4	2660
July	1st to 10th	Day	40% 6% 12% 25% 15% 2	4371
	11th to 20th	Day	28% 6% 15% 32% 19% 1	4157
	21st to end	Day	33% 9% 18% 29% 12% 1	4325
August	1st to 10th	Day	26% 5% 18% 34% 16% 1	4090
	11th to 20th	Day	36% 5% 13% 28% 16% 1	3992
	21st to end	Day	26% 6% 18% 32% 14% 4	4179
September	1st to 10th	Day	32% 4 11% 25% 22% 6%	2280
	11th to 20th	Day	31% 4 10% 27% 23% 4	2275
	21st to end	Day	31% 5% 10% 28% 23% 4	1982
October	1st to 10th	Day	20% 2 6% 21% 40% 11%	3160
	11th to 20th	Day	23% 2 5% 21% 39% 9%	3140
	21st to end	Day	37% 3 8% 24% 26% 2	3428
November	1st to 10th	Day	34% 3 13% 23% 21% 5%	1888
	11th to 20th	Day	37% 2 10% 26% 21% 4	1735
	21st to end	Day	46% 3 8% 28% 14% 1	1600
December	1st to 10th	Day	35% 3 15% 30% 16% 1	2610
	11th to 20th	Day	43% 3 14% 28% 11% 1	3005
	21st to end	Day	68% 3 12% 12% 5%	3982
January	1st to 10th	Night	57% 3 9% 21% 9% 1	6146
	11th to 20th	Night	51% 3 10% 20% 13% 3	5900
	21st to end	Night	51% 3 10% 18% 16% 2	6490
February	1st to 10th	Night	51% 4 12% 20% 11% 3	2600
	11th to 20th	Night	45% 4 11% 21% 17% 2	2509
	21st to end	Night	43% 3 10% 23% 18% 4	1980
March	1st to 10th	Night	44% 3 8% 21% 22% 3	4963
	11th to 20th	Night	33% 4 7% 25% 26% 5%	4980
	21st to end	Night	50% 5% 12% 23% 8% 2	5427
April	1st to 10th	Night	38% 4 10% 26% 19% 3	2000
	11th to 20th	Night	40% 3 9% 27% 16% 5%	1975
	21st to end	Night	45% 4 10% 23% 17% 1	2000
May	1st to 10th	Night	44% 6% 15% 19% 15% 1	2954
	11th to 20th	Night	41% 5% 14% 29% 11% 1	2759
	21st to end	Night	32% 5% 13% 24% 24% 2	2796
June	1st to 10th	Night	27% 5% 16% 31% 17% 4	1520
	11th to 20th	Night	29% 7% 11% 27% 22% 4	1520
	21st to end	Night	25% 4 16% 30% 20% 5%	1520
July	1st to 10th	Night	37% 4 12% 27% 16% 3	2490
	11th to 20th	Night	21% 5% 14% 37% 21% 2	2686
	21st to end	Night	29% 7% 21% 30% 13% 1	2999
August	1st to 10th	Night	19% 4 20% 34% 21% 1	2826
	11th to 20th	Night	32% 5% 13% 28% 19% 2	2908
	21st to end	Night	23% 5% 17% 34% 16% 4	3476
September	1st to 10th	Night	27% 4 11% 24% 27% 7%	1900
	11th to 20th	Night	30% 5% 9% 26% 24% 6%	1900
	21st to end	Night	30% 4 7% 29% 27% 4	2178
October	1st to 10th	Night	17% 3 6% 21% 46% 8%	3800
	11th to 20th	Night	24% 2 6% 22% 32% 13%	3779
	21st to end	Night	35% 3 8% 26% 26% 2	4120
November	1st to 10th	Night	31% 3 12% 28% 21% 5%	2410
	11th to 20th	Night	36% 3 8% 26% 23% 4	2661
	21st to end	Night	43% 3 9% 25% 17% 2	2800
December	1st to 10th	Night	36% 3 11% 36% 13% 1	4580
	11th to 20th	Night	43% 3 13% 25% 15% 1	5201
	21st to end	Night	65% 3 12% 13% 5% 1	6996

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% 14% 35% 24% 5%	300
	11th to 20th	Day	29% 16% 38% 14% 2	300
	21st to end	Day	18% 13% 44% 21% 4	330
February	1st to 10th	Day	23% 13% 44% 16% 4	300
	11th to 20th	Day	24% 19% 35% 16% 6%	300
	21st to end	Day	22% 19% 39% 15% 4	249
March	1st to 10th	Day	18% 17% 43% 20% 2	300
	11th to 20th	Day	27% 12% 33% 22% 5%	300
	21st to end	Day	35% 15% 31% 15% 3	344
April	1st to 10th	Day	23% 14% 32% 23% 6% 1	500
	11th to 20th	Day	23% 14% 32% 22% 8% 1	492
	21st to end	Day	23% 10% 31% 26% 9% 1	500
May	1st to 10th	Day	19% 9% 32% 30% 10% 1	685
	11th to 20th	Day	20% 8% 24% 29% 15% 4	725
	21st to end	Day	16% 6% 29% 25% 18% 6%	760
June	1st to 10th	Day	7% 3 18% 36% 26% 10%	680
	11th to 20th	Day	10% 2 22% 35% 24% 7%	680
	21st to end	Day	7% 5% 26% 38% 19% 5%	680
July	1st to 10th	Day	10% 2 14% 27% 33% 14%	671
	11th to 20th	Day	7% 1 17% 35% 30% 10%	668
	21st to end	Day	8% 2 14% 35% 31% 9%	739
August	1st to 10th	Day	7% 2 15% 37% 32% 7%	676
	11th to 20th	Day	10% 2 14% 37% 29% 8%	616
	21st to end	Day	9% 2 12% 31% 35% 11%	539
September	1st to 10th	Day	10% 2 13% 34% 29% 11%	490
	11th to 20th	Day	8% 3 14% 35% 29% 11%	485
	21st to end	Day	9% 5% 26% 28% 25% 7%	300
October	1st to 10th	Day	15% 6% 28% 26% 17% 7%	300
	11th to 20th	Day	13% 17% 34% 25% 10% 1	300
	21st to end	Day	26% 15% 31% 22% 6%	330
November	1st to 10th	Day	17% 11% 35% 23% 10% 4	300
	11th to 20th	Day	18% 16% 31% 25% 8% 2	300
	21st to end	Day	27% 10% 31% 25% 7%	300
December	1st to 10th	Day	12% 17% 37% 25% 7% 2	300
	11th to 20th	Day	20% 16% 35% 23% 5% 1	300
	21st to end	Day	27% 19% 34% 18% 2	330
January	1st to 10th	Night	16% 14% 37% 25% 7%	1000
	11th to 20th	Night	18% 16% 37% 24% 5% 1	1000
	21st to end	Night	18% 12% 39% 28% 3	1100
February	1st to 10th	Night	20% 12% 38% 26% 4	1000
	11th to 20th	Night	21% 14% 35% 26% 3	954
	21st to end	Night	20% 13% 41% 22% 3	660
March	1st to 10th	Night	18% 14% 38% 26% 4	620
	11th to 20th	Night	22% 11% 32% 29% 6% 1	600
	21st to end	Night	26% 12% 35% 23% 4	660
April	1st to 10th	Night	16% 11% 39% 30% 5%	600
	11th to 20th	Night	19% 11% 31% 30% 8%	593
	21st to end	Night	20% 6% 36% 33% 6%	600
May	1st to 10th	Night	16% 6% 31% 38% 10%	600
	11th to 20th	Night	17% 5% 23% 38% 16% 1	591
	21st to end	Night	14% 5% 24% 34% 21% 1	634
June	1st to 10th	Night	8% 3 15% 45% 26% 3	570
	11th to 20th	Night	5% 3 21% 43% 25% 2	570
	21st to end	Night	4 2 21% 52% 20% 1	570
July	1st to 10th	Night	12% 2 10% 37% 35% 4	555
	11th to 20th	Night	6% 1 12% 43% 35% 3	552
	21st to end	Night	7% 2 12% 44% 34% 2	624
August	1st to 10th	Night	7% 1 10% 47% 32% 3	567
	11th to 20th	Night	7% 1 11% 44% 34% 3	564
	21st to end	Night	6% 1 10% 39% 40% 4	627
September	1st to 10th	Night	12% 1 13% 41% 30% 4	570
	11th to 20th	Night	11% 2 14% 42% 27% 5%	570
	21st to end	Night	9% 5% 23% 36% 23% 3	658
October	1st to 10th	Night	12% 3 26% 33% 22% 3	760
	11th to 20th	Night	17% 8% 32% 32% 10%	760
	21st to end	Night	21% 11% 29% 30% 9%	836
November	1st to 10th	Night	14% 7% 32% 35% 11%	780
	11th to 20th	Night	15% 10% 38% 28% 8%	861
	21st to end	Night	21% 11% 33% 28% 7%	1000
December	1st to 10th	Night	19% 11% 37% 28% 6% 1	1000
	11th to 20th	Night	22% 9% 32% 31% 6%	1000
	21st to end	Night	24% 13% 33% 28% 3	1100

Page generated at 2025-05-23 08:21:03. Version ac07

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