Morris Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morris Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	137438
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 34%	135451
Transparency	Day	47% 5% 12% 22% 13% 1	141006
	Night	48% 4 11% 22% 13% 2	139959
Seeing	Day	17% 11% 27% 24% 15% 6%	17981
	Night	19% 9% 29% 29% 13% 1	27784

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% 15% 9% 16% 6% 4 4 3 5% 5% 18%	4691
	11th to 20th	Day	11% 14% 8% 16% 7% 8% 7% 5% 5% 4 17%	4710
	21st to end	Day	18% 17% 7% 11% 6% 5% 5% 4 4 4 17%	5308
February	1st to 10th	Day	16% 15% 5% 13% 7% 5% 5% 3 4 5% 21%	2029
	11th to 20th	Day	15% 12% 5% 11% 7% 5% 4 4 4 6% 25%	2048
	21st to end	Day	15% 16% 6% 9% 4 4 3 4 5% 6% 27%	1730
March	1st to 10th	Day	19% 14% 7% 10% 5% 4 3 2 3 4 30%	4580
	11th to 20th	Day	11% 10% 4 7% 5% 4 4 4 6% 7% 40%	4948
	21st to end	Day	18% 14% 5% 10% 6% 5% 5% 3 5% 6% 23%	5770
April	1st to 10th	Day	18% 12% 4 7% 5% 5% 4 4 5% 6% 29%	2520
	11th to 20th	Day	17% 9% 3 8% 4 5% 5% 3 5% 7% 34%	2514
	21st to end	Day	18% 9% 3 7% 5% 5% 4 3 5% 7% 34%	2730
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 5% 7% 31%	7616
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	7626
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 7% 37%	8708
June	1st to 10th	Day	11% 5% 3 6% 5% 5% 6% 4 6% 8% 42%	2940
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 5% 7% 9% 44%	2940
	21st to end	Day	10% 6% 2 6% 6% 5% 5% 5% 7% 9% 40%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 42%	6500
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	6431
	21st to end	Day	9% 7% 2 8% 5% 5% 5% 5% 6% 8% 38%	6793
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	4234
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 6% 9% 46%	4117
	21st to end	Day	7% 4 2 5% 4 4 4 4 7% 10% 49%	4312
September	1st to 10th	Day	10% 5% 2 6% 6% 4 4 3 5% 7% 48%	2356
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 6% 9% 49%	2200
	21st to end	Day	13% 7% 3 5% 4 4 5% 3 5% 6% 45%	2200
October	1st to 10th	Day	13% 6% 2 7% 5% 3 4 4 5% 6% 45%	2200
	11th to 20th	Day	16% 6% 2 7% 4 4 5% 4 6% 7% 39%	2007
	21st to end	Day	19% 9% 3 9% 4 5% 5% 4 5% 7% 30%	1980
November	1st to 10th	Day	12% 7% 2 5% 4 4 4 4 5% 7% 46%	1852
	11th to 20th	Day	18% 14% 4 8% 5% 4 4 2 5% 6% 31%	1890
	21st to end	Day	20% 13% 5% 8% 5% 4 4 3 4 5% 28%	1890
December	1st to 10th	Day	11% 14% 4 11% 5% 5% 5% 3 5% 6% 32%	2970
	11th to 20th	Day	17% 15% 7% 11% 6% 5% 4 3 4 5% 23%	3088
	21st to end	Day	28% 20% 7% 11% 5% 5% 4 3 4 4 10%	4070
January	1st to 10th	Night	15% 15% 6% 14% 7% 5% 5% 3 4 5% 20%	8232
	11th to 20th	Night	10% 17% 7% 17% 8% 5% 5% 3 4 4 18%	8280
	21st to end	Night	17% 12% 6% 9% 5% 5% 5% 4 5% 5% 25%	8896
February	1st to 10th	Night	18% 14% 5% 10% 6% 4 5% 3 4 6% 26%	2588
	11th to 20th	Night	13% 13% 4 10% 6% 5% 4 3 4 6% 31%	2458
	21st to end	Night	19% 13% 4 10% 4 4 4 3 5% 5% 30%	2076
March	1st to 10th	Night	17% 12% 4 7% 4 4 4 3 4 6% 35%	5493
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 4 6% 7% 42%	5231
	21st to end	Night	18% 10% 3 8% 4 4 4 3 5% 6% 34%	5430
April	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 5% 7% 36%	2100
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 5% 6% 39%	2054
	21st to end	Night	18% 9% 2 7% 4 4 4 4 6% 7% 36%	1890
May	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 32%	5237
	11th to 20th	Night	16% 6% 2 6% 4 3 4 3 6% 9% 42%	5234
	21st to end	Night	13% 7% 2 6% 5% 4 5% 4 6% 9% 40%	5444
June	1st to 10th	Night	9% 6% 3 5% 4 4 4 4 5% 7% 49%	1680
	11th to 20th	Night	9% 5% 3 6% 3 4 5% 4 6% 9% 46%	1680
	21st to end	Night	9% 5% 2 6% 5% 4 5% 5% 6% 8% 44%	1680
July	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 4 6% 10% 46%	3698
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 10% 48%	3657
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4072
August	1st to 10th	Night	10% 5% 3 6% 4 4 4 4 6% 10% 45%	2915
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 6% 7% 49%	2929
	21st to end	Night	8% 4 3 5% 4 4 4 4 6% 7% 51%	3586
September	1st to 10th	Night	10% 4 2 5% 5% 3 4 3 5% 8% 51%	2022
	11th to 20th	Night	8% 4 1 4 3 3 3 3 5% 7% 58%	2200
	21st to end	Night	13% 8% 2 5% 3 3 4 3 4 5% 48%	2200
October	1st to 10th	Night	11% 5% 2 5% 4 3 4 2 6% 6% 52%	2200
	11th to 20th	Night	16% 7% 2 7% 4 4 3 3 4 6% 43%	2386
	21st to end	Night	19% 8% 2 5% 5% 4 5% 3 5% 6% 38%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 4 3 3 5% 6% 45%	2666
	11th to 20th	Night	14% 12% 3 7% 5% 4 5% 4 5% 6% 35%	2730
	21st to end	Night	19% 13% 4 9% 5% 4 3 4 4 6% 28%	2730
December	1st to 10th	Night	13% 11% 4 8% 6% 4 5% 4 6% 7% 33%	4428
	11th to 20th	Night	18% 17% 8% 11% 6% 4 4 3 4 4 20%	5339
	21st to end	Night	27% 18% 7% 11% 5% 5% 4 3 4 4 12%	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	67% 4 12% 14% 3	4692
	11th to 20th	Day	70% 3 8% 9% 9% 1	4710
	21st to end	Day	69% 4 9% 13% 5%	5308
February	1st to 10th	Day	65% 3 11% 13% 7% 1	2029
	11th to 20th	Day	59% 5% 9% 19% 7% 1	2058
	21st to end	Day	58% 4 12% 18% 7% 1	1730
March	1st to 10th	Day	60% 3 6% 22% 9%	4580
	11th to 20th	Day	44% 3 11% 33% 8% 1	4948
	21st to end	Day	61% 3 14% 16% 5% 1	5772
April	1st to 10th	Day	55% 4 11% 18% 11%	2520
	11th to 20th	Day	49% 5% 10% 23% 11% 2	2514
	21st to end	Day	49% 5% 12% 21% 11% 3	2730
May	1st to 10th	Day	51% 5% 11% 20% 12%	7651
	11th to 20th	Day	44% 5% 14% 21% 15% 1	7630
	21st to end	Day	44% 5% 9% 23% 15% 5%	8722
June	1st to 10th	Day	39% 5% 11% 25% 17% 2	2940
	11th to 20th	Day	36% 5% 13% 24% 19% 3	2940
	21st to end	Day	39% 7% 16% 20% 15% 3	2940
July	1st to 10th	Day	36% 6% 12% 28% 16% 2	6431
	11th to 20th	Day	33% 7% 16% 28% 16% 1	6356
	21st to end	Day	39% 8% 13% 21% 17% 2	6598
August	1st to 10th	Day	31% 8% 16% 29% 15% 1	4221
	11th to 20th	Day	33% 6% 14% 29% 17% 1	4156
	21st to end	Day	29% 5% 14% 29% 19% 3	4311
September	1st to 10th	Day	36% 4 14% 24% 18% 3	2356
	11th to 20th	Day	32% 5% 10% 29% 22% 2	2200
	21st to end	Day	39% 4 10% 25% 21% 2	2200
October	1st to 10th	Day	28% 4 10% 33% 24% 1	3590
	11th to 20th	Day	35% 6% 11% 22% 26% 1	3236
	21st to end	Day	44% 3 10% 28% 13% 1	3180
November	1st to 10th	Day	37% 6% 13% 27% 17% 1	1852
	11th to 20th	Day	55% 3 11% 20% 11% 2	1890
	21st to end	Day	58% 4 11% 20% 7% 1	1890
December	1st to 10th	Day	53% 4 16% 21% 5%	2970
	11th to 20th	Day	64% 4 12% 17% 4	3085
	21st to end	Day	79% 4 7% 7% 3	4070
January	1st to 10th	Night	65% 4 10% 16% 5%	8233
	11th to 20th	Night	70% 3 8% 10% 8% 1	8280
	21st to end	Night	61% 4 12% 16% 7%	8896
February	1st to 10th	Night	61% 4 11% 16% 8% 1	2588
	11th to 20th	Night	55% 3 10% 21% 9% 1	2459
	21st to end	Night	54% 4 12% 17% 10% 2	2076
March	1st to 10th	Night	51% 3 9% 27% 9% 1	5493
	11th to 20th	Night	40% 5% 11% 29% 13% 2	5231
	21st to end	Night	51% 4 11% 24% 9% 1	5430
April	1st to 10th	Night	47% 5% 11% 22% 14% 1	2100
	11th to 20th	Night	45% 4 10% 22% 16% 3	2054
	21st to end	Night	47% 5% 10% 24% 12% 3	1890
May	1st to 10th	Night	53% 4 8% 19% 13% 2	5264
	11th to 20th	Night	40% 5% 12% 26% 17% 1	5235
	21st to end	Night	41% 5% 11% 21% 17% 5%	5454
June	1st to 10th	Night	34% 4 12% 26% 21% 3	1680
	11th to 20th	Night	35% 5% 12% 25% 20% 3	1680
	21st to end	Night	35% 7% 16% 23% 16% 4	1680
July	1st to 10th	Night	34% 6% 11% 27% 18% 4	3660
	11th to 20th	Night	30% 8% 17% 24% 20% 1	3612
	21st to end	Night	37% 8% 15% 20% 17% 3	3955
August	1st to 10th	Night	34% 6% 14% 31% 13% 1	2904
	11th to 20th	Night	31% 6% 14% 31% 17% 1	2956
	21st to end	Night	30% 8% 11% 27% 21% 3	3586
September	1st to 10th	Night	32% 6% 12% 26% 19% 6%	2022
	11th to 20th	Night	27% 4 9% 31% 28% 3	2200
	21st to end	Night	38% 4 9% 25% 22% 2	2200
October	1st to 10th	Night	23% 3 7% 34% 32% 1	3590
	11th to 20th	Night	34% 4 8% 24% 29% 1	3896
	21st to end	Night	36% 4 8% 30% 20% 1	4610
November	1st to 10th	Night	40% 4 10% 26% 19% 1	2666
	11th to 20th	Night	49% 4 12% 21% 13% 2	2730
	21st to end	Night	57% 3 11% 21% 8%	2730
December	1st to 10th	Night	50% 5% 14% 25% 6% 1	4428
	11th to 20th	Night	67% 4 11% 11% 7% 1	5341
	21st to end	Night	76% 3 9% 8% 3	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	22% 18% 38% 16% 5% 1	330
	11th to 20th	Day	25% 18% 37% 17% 3 1	330
	21st to end	Day	27% 16% 35% 17% 6%	363
February	1st to 10th	Day	23% 18% 34% 20% 5%	330
	11th to 20th	Day	25% 19% 36% 16% 4 1	330
	21st to end	Day	23% 22% 39% 14% 2	273
March	1st to 10th	Day	22% 20% 41% 14% 3 1	330
	11th to 20th	Day	28% 15% 30% 20% 7% 1	476
	21st to end	Day	29% 17% 30% 17% 5% 1	594
April	1st to 10th	Day	23% 14% 34% 19% 8% 1	540
	11th to 20th	Day	20% 20% 28% 19% 10% 3	532
	21st to end	Day	21% 16% 31% 22% 9% 2	540
May	1st to 10th	Day	23% 12% 31% 21% 9% 3	543
	11th to 20th	Day	20% 10% 27% 25% 14% 5%	567
	21st to end	Day	13% 6% 28% 26% 19% 8%	726
June	1st to 10th	Day	12% 7% 28% 28% 19% 6%	780
	11th to 20th	Day	13% 7% 28% 29% 18% 6%	780
	21st to end	Day	11% 7% 27% 29% 18% 7%	780
July	1st to 10th	Day	10% 4 18% 29% 25% 15%	771
	11th to 20th	Day	8% 3 21% 31% 24% 13%	744
	21st to end	Day	9% 3 17% 32% 27% 12%	738
August	1st to 10th	Day	7% 3 11% 30% 32% 17%	526
	11th to 20th	Day	12% 4 15% 29% 26% 13%	519
	21st to end	Day	8% 2 14% 30% 30% 16%	583
September	1st to 10th	Day	12% 5% 17% 25% 28% 12%	530
	11th to 20th	Day	8% 3 14% 22% 34% 19%	530
	21st to end	Day	17% 10% 19% 22% 24% 8%	530
October	1st to 10th	Day	12% 6% 25% 28% 19% 9%	530
	11th to 20th	Day	17% 14% 30% 27% 10% 2	460
	21st to end	Day	28% 12% 33% 23% 3 1	363
November	1st to 10th	Day	13% 15% 33% 25% 11% 3	330
	11th to 20th	Day	25% 19% 30% 17% 8% 1	330
	21st to end	Day	29% 16% 34% 17% 4	330
December	1st to 10th	Day	22% 16% 37% 18% 7%	330
	11th to 20th	Day	21% 19% 37% 17% 5% 1	330
	21st to end	Day	32% 16% 34% 15% 3	363
January	1st to 10th	Night	24% 15% 35% 22% 5%	1050
	11th to 20th	Night	23% 16% 38% 19% 4	1050
	21st to end	Night	23% 15% 36% 23% 4	1097
February	1st to 10th	Night	26% 11% 33% 26% 3	840
	11th to 20th	Night	21% 12% 41% 22% 3	823
	21st to end	Night	24% 14% 39% 20% 2	692
March	1st to 10th	Night	21% 13% 35% 25% 5%	840
	11th to 20th	Night	19% 13% 36% 26% 7%	840
	21st to end	Night	25% 11% 35% 25% 5%	856
April	1st to 10th	Night	21% 12% 35% 26% 6% 1	630
	11th to 20th	Night	19% 12% 34% 27% 7% 1	623
	21st to end	Night	22% 8% 35% 26% 8% 1	630
May	1st to 10th	Night	23% 7% 31% 32% 7%	630
	11th to 20th	Night	20% 5% 26% 32% 15% 1	621
	21st to end	Night	13% 4 22% 43% 16% 1	683
June	1st to 10th	Night	10% 4 25% 42% 19%	630
	11th to 20th	Night	11% 4 25% 40% 19% 1	630
	21st to end	Night	11% 3 23% 38% 23% 2	630
July	1st to 10th	Night	10% 3 12% 42% 29% 4	615
	11th to 20th	Night	9% 3 15% 39% 30% 4	597
	21st to end	Night	8% 2 14% 42% 32% 2	657
August	1st to 10th	Night	7% 2 11% 43% 33% 4	597
	11th to 20th	Night	8% 3 15% 40% 31% 3	594
	21st to end	Night	8% 3 14% 36% 36% 4	660
September	1st to 10th	Night	10% 4 16% 36% 31% 3	622
	11th to 20th	Night	10% 3 14% 38% 32% 4	800
	21st to end	Night	16% 5% 22% 32% 23% 2	800
October	1st to 10th	Night	12% 5% 24% 35% 21% 3	800
	11th to 20th	Night	20% 12% 28% 29% 10% 1	800
	21st to end	Night	23% 12% 33% 25% 6%	880
November	1st to 10th	Night	17% 11% 35% 26% 10% 1	820
	11th to 20th	Night	20% 15% 32% 26% 6% 1	840
	21st to end	Night	26% 13% 30% 25% 6%	840
December	1st to 10th	Night	25% 11% 35% 23% 5%	862
	11th to 20th	Night	24% 14% 34% 23% 5%	1050
	21st to end	Night	32% 13% 32% 20% 3	1155

Page generated at 2026-07-13 03:14:50. Version ac07

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