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 2025-03-12.

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%	108640
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 35%	110512
Transparency	Day	47% 5% 12% 22% 13% 1	112253
	Night	49% 4 11% 21% 13% 2	115039
Seeing	Day	17% 10% 27% 24% 16% 6%	16053
	Night	19% 9% 29% 29% 13% 1	26047

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% 14% 10% 17% 6% 4 4 3 5% 4 20%	3617
	11th to 20th	Day	13% 15% 8% 14% 6% 7% 6% 5% 5% 4 16%	3620
	21st to end	Day	22% 18% 8% 11% 5% 4 3 3 4 3 19%	4095
February	1st to 10th	Day	16% 15% 5% 12% 7% 5% 5% 3 4 5% 21%	1932
	11th to 20th	Day	16% 12% 5% 11% 7% 5% 4 4 4 6% 25%	1948
	21st to end	Day	16% 16% 6% 8% 4 4 3 4 5% 6% 27%	1650
March	1st to 10th	Day	20% 16% 5% 9% 5% 4 3 2 4 5% 25%	3220
	11th to 20th	Day	13% 8% 4 8% 6% 4 4 4 6% 8% 36%	3393
	21st to end	Day	19% 11% 6% 10% 6% 4 5% 3 5% 6% 26%	3941
April	1st to 10th	Day	17% 12% 4 7% 5% 5% 4 4 5% 6% 31%	2280
	11th to 20th	Day	18% 9% 3 7% 4 4 4 3 5% 7% 35%	2272
	21st to end	Day	19% 9% 3 7% 5% 4 4 3 5% 7% 33%	2470
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 3 6% 8% 31%	4082
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 6% 7% 38%	4081
	21st to end	Day	10% 6% 2 6% 5% 4 6% 5% 7% 8% 42%	4653
June	1st to 10th	Day	11% 5% 3 6% 5% 5% 6% 4 6% 8% 42%	2660
	11th to 20th	Day	10% 6% 2 5% 4 4 4 5% 7% 9% 45%	2660
	21st to end	Day	10% 5% 2 6% 6% 5% 5% 5% 7% 8% 40%	2660
July	1st to 10th	Day	14% 6% 3 7% 4 5% 5% 4 6% 9% 38%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 47%	4440
	21st to end	Day	7% 6% 2 8% 5% 6% 5% 5% 6% 8% 40%	4724
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	4104
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 4 6% 9% 46%	3989
	21st to end	Day	7% 4 2 5% 4 4 4 4 7% 10% 49%	4180
September	1st to 10th	Day	11% 5% 2 6% 6% 4 4 3 5% 7% 47%	2238
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 9% 48%	2090
	21st to end	Day	13% 6% 2 5% 4 4 5% 3 5% 7% 45%	2090
October	1st to 10th	Day	13% 6% 2 7% 5% 3 4 4 5% 6% 45%	2090
	11th to 20th	Day	16% 6% 2 7% 4 4 5% 4 6% 7% 38%	1907
	21st to end	Day	19% 9% 3 9% 5% 5% 5% 4 5% 7% 29%	1881
November	1st to 10th	Day	12% 7% 2 5% 4 4 4 4 5% 6% 47%	1762
	11th to 20th	Day	18% 14% 4 8% 5% 4 4 3 5% 6% 31%	1800
	21st to end	Day	20% 13% 4 8% 5% 4 4 4 5% 5% 29%	1800
December	1st to 10th	Day	12% 13% 4 10% 5% 5% 5% 3 4 6% 33%	2881
	11th to 20th	Day	17% 15% 7% 11% 6% 5% 4 3 4 5% 23%	3008
	21st to end	Day	28% 20% 7% 11% 5% 5% 4 3 4 4 10%	3982
January	1st to 10th	Night	16% 15% 7% 14% 6% 5% 4 3 4 5% 20%	6348
	11th to 20th	Night	13% 20% 8% 16% 8% 5% 5% 3 3 4 17%	6360
	21st to end	Night	21% 13% 7% 10% 5% 5% 4 4 4 4 26%	6876
February	1st to 10th	Night	19% 14% 5% 10% 6% 4 4 3 4 6% 26%	2465
	11th to 20th	Night	14% 14% 5% 9% 7% 5% 4 3 4 6% 30%	2338
	21st to end	Night	19% 13% 3 9% 4 4 3 3 6% 5% 30%	1980
March	1st to 10th	Night	18% 12% 4 7% 5% 4 3 2 4 6% 35%	3853
	11th to 20th	Night	11% 6% 2 6% 4 5% 5% 4 6% 6% 45%	3595
	21st to end	Night	18% 9% 3 9% 4 4 4 3 5% 6% 34%	3704
April	1st to 10th	Night	15% 9% 2 7% 5% 4 4 4 5% 8% 36%	1900
	11th to 20th	Night	19% 8% 2 5% 4 4 4 4 5% 6% 38%	1856
	21st to end	Night	19% 9% 2 7% 4 4 4 4 6% 6% 36%	1710
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	2813
	11th to 20th	Night	20% 6% 2 6% 4 3 3 3 6% 8% 39%	2812
	21st to end	Night	11% 5% 2 6% 5% 4 5% 4 6% 9% 44%	2905
June	1st to 10th	Night	9% 6% 3 5% 4 4 4 4 5% 7% 50%	1520
	11th to 20th	Night	9% 5% 3 6% 3 4 5% 4 6% 9% 47%	1520
	21st to end	Night	8% 5% 3 6% 5% 4 5% 5% 7% 8% 44%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 3 6% 9% 45%	2530
	11th to 20th	Night	7% 4 2 4 4 3 4 3 5% 9% 55%	2530
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	2836
August	1st to 10th	Night	11% 5% 3 6% 4 4 4 4 6% 10% 44%	2825
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 6% 7% 49%	2837
	21st to end	Night	8% 4 3 5% 4 4 4 4 6% 7% 51%	3476
September	1st to 10th	Night	10% 4 2 5% 5% 3 4 3 5% 8% 51%	1921
	11th to 20th	Night	8% 4 1 4 3 3 3 3 5% 7% 57%	2090
	21st to end	Night	13% 8% 2 5% 3 3 4 3 4 5% 48%	2090
October	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 6% 6% 51%	2090
	11th to 20th	Night	16% 8% 2 7% 4 4 3 3 4 6% 43%	2267
	21st to end	Night	20% 8% 2 5% 5% 4 5% 3 5% 6% 37%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 5% 6% 45%	2536
	11th to 20th	Night	15% 12% 3 7% 5% 4 5% 4 5% 6% 34%	2600
	21st to end	Night	19% 13% 4 9% 5% 4 4 4 4 6% 28%	2600
December	1st to 10th	Night	13% 11% 4 8% 6% 5% 5% 4 6% 7% 33%	4297
	11th to 20th	Night	18% 17% 8% 11% 6% 4 4 3 4 4 20%	5199
	21st to end	Night	27% 18% 7% 11% 5% 5% 4 3 4 4 12%	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	67% 4 9% 17% 4	3618
	11th to 20th	Day	71% 3 9% 7% 8% 2	3620
	21st to end	Day	70% 3 8% 12% 6%	4095
February	1st to 10th	Day	66% 3 10% 13% 7% 1	1932
	11th to 20th	Day	60% 5% 9% 19% 6% 1	1958
	21st to end	Day	58% 4 12% 18% 8% 1	1650
March	1st to 10th	Day	63% 3 8% 19% 7%	3220
	11th to 20th	Day	47% 4 11% 30% 8% 1	3393
	21st to end	Day	60% 2 17% 18% 3	3941
April	1st to 10th	Day	53% 4 11% 19% 11%	2280
	11th to 20th	Day	49% 4 10% 23% 11% 3	2272
	21st to end	Day	51% 4 12% 19% 11% 3	2470
May	1st to 10th	Day	51% 4 13% 24% 8%	4120
	11th to 20th	Day	45% 4 16% 26% 9% 1	4085
	21st to end	Day	38% 5% 11% 25% 20% 2	4666
June	1st to 10th	Day	39% 5% 12% 26% 16% 3	2660
	11th to 20th	Day	35% 5% 13% 25% 20% 3	2660
	21st to end	Day	39% 7% 16% 20% 15% 3	2660
July	1st to 10th	Day	42% 6% 13% 22% 15% 3	4371
	11th to 20th	Day	28% 7% 15% 30% 19% 1	4365
	21st to end	Day	38% 8% 15% 17% 20% 3	4574
August	1st to 10th	Day	32% 8% 16% 28% 15% 1	4091
	11th to 20th	Day	33% 6% 15% 29% 17% 1	4028
	21st to end	Day	29% 5% 14% 29% 19% 3	4179
September	1st to 10th	Day	37% 5% 14% 23% 18% 4	2238
	11th to 20th	Day	33% 5% 11% 28% 22% 2	2090
	21st to end	Day	39% 4 10% 25% 20% 2	2090
October	1st to 10th	Day	28% 4 11% 33% 24% 1	3480
	11th to 20th	Day	35% 6% 11% 21% 26% 1	3136
	21st to end	Day	45% 3 10% 28% 12% 1	3081
November	1st to 10th	Day	37% 5% 13% 27% 17% 1	1762
	11th to 20th	Day	55% 3 11% 19% 11% 2	1800
	21st to end	Day	58% 4 11% 20% 7% 1	1800
December	1st to 10th	Day	52% 4 17% 22% 5%	2881
	11th to 20th	Day	64% 3 12% 17% 4	3005
	21st to end	Day	79% 4 7% 7% 3	3982
January	1st to 10th	Night	66% 3 9% 16% 5%	6348
	11th to 20th	Night	74% 2 7% 9% 8% 1	6360
	21st to end	Night	63% 3 11% 15% 8% 1	6876
February	1st to 10th	Night	61% 4 11% 15% 8% 1	2465
	11th to 20th	Night	56% 3 9% 21% 10% 1	2339
	21st to end	Night	54% 4 12% 18% 10% 2	1980
March	1st to 10th	Night	52% 2 10% 25% 9% 1	3853
	11th to 20th	Night	38% 5% 10% 29% 15% 2	3595
	21st to end	Night	52% 4 13% 24% 6% 1	3704
April	1st to 10th	Night	46% 5% 12% 22% 14% 1	1900
	11th to 20th	Night	46% 4 9% 22% 17% 3	1856
	21st to end	Night	48% 4 10% 22% 13% 3	1710
May	1st to 10th	Night	52% 3 11% 22% 10% 1	2840
	11th to 20th	Night	44% 3 14% 27% 10% 1	2813
	21st to end	Night	36% 5% 13% 23% 19% 3	2913
June	1st to 10th	Night	33% 4 13% 28% 20% 3	1520
	11th to 20th	Night	34% 6% 12% 25% 20% 3	1520
	21st to end	Night	35% 7% 16% 23% 15% 4	1520
July	1st to 10th	Night	36% 5% 12% 25% 16% 6%	2490
	11th to 20th	Night	24% 6% 16% 27% 24% 2	2482
	21st to end	Night	36% 7% 15% 17% 20% 4	2746
August	1st to 10th	Night	35% 6% 15% 30% 13% 1	2814
	11th to 20th	Night	31% 6% 14% 31% 17% 1	2864
	21st to end	Night	31% 7% 11% 27% 21% 3	3476
September	1st to 10th	Night	32% 6% 12% 25% 19% 6%	1921
	11th to 20th	Night	27% 4 9% 30% 27% 3	2090
	21st to end	Night	38% 4 9% 25% 21% 3	2090
October	1st to 10th	Night	23% 3 7% 34% 32% 1	3480
	11th to 20th	Night	35% 4 8% 24% 29% 1	3777
	21st to end	Night	36% 4 8% 30% 20% 1	4467
November	1st to 10th	Night	40% 4 10% 26% 19% 1	2536
	11th to 20th	Night	49% 3 12% 20% 13% 2	2600
	21st to end	Night	57% 3 11% 21% 7%	2600
December	1st to 10th	Night	49% 5% 14% 26% 6%	4297
	11th to 20th	Night	67% 4 11% 10% 7% 1	5201
	21st to end	Night	76% 3 9% 8% 3	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	20% 19% 39% 16% 5% 1	300
	11th to 20th	Day	25% 15% 37% 18% 4 1	300
	21st to end	Day	28% 14% 35% 17% 6%	330
February	1st to 10th	Day	24% 19% 34% 19% 4	300
	11th to 20th	Day	26% 18% 36% 15% 4 1	300
	21st to end	Day	24% 21% 39% 14% 2	249
March	1st to 10th	Day	22% 20% 39% 15% 3 1	300
	11th to 20th	Day	26% 13% 31% 21% 7% 1	405
	21st to end	Day	32% 14% 28% 18% 6% 1	506
April	1st to 10th	Day	20% 14% 34% 21% 9% 2	460
	11th to 20th	Day	21% 18% 29% 18% 11% 3	452
	21st to end	Day	24% 16% 30% 20% 8% 2	460
May	1st to 10th	Day	22% 11% 32% 22% 10% 3	463
	11th to 20th	Day	21% 8% 26% 25% 15% 5%	487
	21st to end	Day	12% 5% 28% 27% 20% 9%	628
June	1st to 10th	Day	11% 7% 29% 28% 19% 6%	680
	11th to 20th	Day	12% 5% 28% 29% 19% 6%	680
	21st to end	Day	11% 7% 28% 28% 19% 8%	680
July	1st to 10th	Day	11% 4 18% 28% 24% 15%	671
	11th to 20th	Day	7% 3 22% 31% 23% 14%	668
	21st to end	Day	9% 3 18% 32% 27% 12%	686
August	1st to 10th	Day	7% 3 12% 30% 31% 17%	486
	11th to 20th	Day	13% 3 16% 31% 24% 13%	479
	21st to end	Day	8% 2 14% 30% 30% 17%	539
September	1st to 10th	Day	13% 5% 17% 26% 29% 10%	490
	11th to 20th	Day	9% 3 15% 22% 34% 18%	490
	21st to end	Day	17% 11% 20% 22% 24% 7%	490
October	1st to 10th	Day	13% 7% 26% 28% 19% 9%	490
	11th to 20th	Day	18% 14% 30% 25% 10% 2	424
	21st to end	Day	28% 12% 34% 22% 3 1	330
November	1st to 10th	Day	13% 13% 34% 25% 12% 3	300
	11th to 20th	Day	25% 17% 31% 17% 9% 1	300
	21st to end	Day	28% 16% 34% 18% 4	300
December	1st to 10th	Day	22% 17% 37% 17% 7%	300
	11th to 20th	Day	21% 17% 39% 17% 5% 1	300
	21st to end	Day	33% 15% 33% 16% 3	330
January	1st to 10th	Night	24% 15% 34% 21% 5%	1000
	11th to 20th	Night	24% 15% 37% 20% 4	1000
	21st to end	Night	23% 14% 36% 23% 4	1045
February	1st to 10th	Night	27% 11% 34% 25% 3	800
	11th to 20th	Night	21% 12% 42% 22% 3	783
	21st to end	Night	25% 13% 39% 20% 3	660
March	1st to 10th	Night	21% 13% 34% 27% 6% 1	800
	11th to 20th	Night	18% 12% 37% 26% 7% 1	767
	21st to end	Night	25% 9% 34% 25% 5%	774
April	1st to 10th	Night	20% 12% 35% 27% 6% 1	570
	11th to 20th	Night	20% 11% 34% 27% 7% 1	563
	21st to end	Night	23% 8% 35% 25% 8% 1	570
May	1st to 10th	Night	22% 6% 31% 33% 8%	570
	11th to 20th	Night	22% 4 24% 33% 16% 2	561
	21st to end	Night	13% 4 22% 43% 17% 1	617
June	1st to 10th	Night	10% 3 25% 43% 18%	570
	11th to 20th	Night	10% 3 25% 40% 20% 1	570
	21st to end	Night	11% 3 23% 38% 24% 2	570
July	1st to 10th	Night	9% 3 13% 42% 29% 4	555
	11th to 20th	Night	9% 2 15% 39% 30% 4	552
	21st to end	Night	8% 2 15% 42% 32% 2	624
August	1st to 10th	Night	8% 2 11% 43% 32% 4	567
	11th to 20th	Night	8% 2 15% 40% 30% 4	564
	21st to end	Night	8% 3 14% 34% 37% 4	627
September	1st to 10th	Night	10% 4 16% 36% 31% 3	591
	11th to 20th	Night	11% 3 14% 38% 31% 4	760
	21st to end	Night	16% 5% 23% 32% 21% 2	760
October	1st to 10th	Night	13% 5% 25% 34% 20% 3	760
	11th to 20th	Night	20% 12% 29% 28% 11% 1	760
	21st to end	Night	24% 12% 32% 25% 7%	836
November	1st to 10th	Night	18% 11% 34% 26% 11% 1	780
	11th to 20th	Night	21% 15% 32% 26% 6% 1	800
	21st to end	Night	25% 13% 31% 26% 6%	800
December	1st to 10th	Night	25% 11% 35% 24% 5%	821
	11th to 20th	Night	25% 14% 33% 23% 5%	1000
	21st to end	Night	32% 13% 32% 20% 3	1100

Page generated at 2025-03-09 08:08:47. Version ac07

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