Morton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morton Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	6% 8% 2 6% 4 4 4 3 5% 6% 52%	135278
	Night	7% 8% 3 6% 3 3 4 3 5% 6% 51%	132552
Transparency	Day	30% 5% 9% 21% 21% 15%	130024
	Night	32% 4 10% 23% 19% 12%	128809
Seeing	Day	10% 5% 18% 32% 27% 9%	16214
	Night	12% 4 14% 40% 27% 4	26753

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	7% 17% 5% 11% 5% 4 3 4 6% 7% 32%	4098
	11th to 20th	Day	8% 14% 4 10% 8% 6% 5% 3 4 6% 32%	4100
	21st to end	Day	6% 15% 4 8% 3 3 3 3 4 5% 45%	4510
February	1st to 10th	Day	12% 14% 4 7% 6% 5% 5% 4 5% 6% 33%	1800
	11th to 20th	Day	10% 15% 3 9% 4 4 4 4 6% 6% 33%	1825
	21st to end	Day	8% 16% 4 10% 5% 4 3 3 6% 7% 33%	1650
March	1st to 10th	Day	6% 16% 5% 9% 4 4 4 3 5% 7% 36%	4855
	11th to 20th	Day	6% 11% 3 10% 7% 5% 5% 4 6% 7% 37%	5000
	21st to end	Day	11% 18% 4 10% 5% 4 5% 3 4 5% 30%	5486
April	1st to 10th	Day	9% 11% 3 7% 5% 5% 5% 3 6% 7% 41%	2220
	11th to 20th	Day	9% 11% 3 7% 5% 5% 4 4 6% 6% 42%	2513
	21st to end	Day	10% 9% 2 6% 5% 4 4 4 6% 7% 45%	2600
May	1st to 10th	Day	8% 10% 3 6% 4 4 4 3 5% 6% 47%	5880
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 6% 7% 47%	5841
	21st to end	Day	6% 5% 2 4 3 3 4 4 6% 7% 54%	6423
June	1st to 10th	Day	4 4 2 5% 3 4 4 4 6% 8% 56%	2600
	11th to 20th	Day	6% 4 1 4 3 3 3 3 4 6% 64%	2718
	21st to end	Day	3 3 1 3 3 3 3 3 4 6% 70%	2600
July	1st to 10th	Day	1 2 1 3 3 2 3 2 4 4 74%	5879
	11th to 20th	Day	2 2 1 4 4 4 5% 4 7% 7% 60%	5880
	21st to end	Day	2 2 1 3 2 3 3 3 5% 6% 69%	6401
August	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 6% 69%	5779
	11th to 20th	Day	2 2 1 4 3 3 3 2 4 5% 70%	5798
	21st to end	Day	4 3 1 5% 4 4 5% 4 5% 7% 58%	6390
September	1st to 10th	Day	3 2 1 3 3 3 3 3 6% 6% 67%	2260
	11th to 20th	Day	5% 3 1 3 3 3 4 3 5% 7% 63%	2200
	21st to end	Day	6% 3 1 3 2 2 3 3 4 6% 68%	2200
October	1st to 10th	Day	6% 4 1 3 2 3 3 2 4 4 68%	3435
	11th to 20th	Day	6% 7% 2 5% 3 3 3 3 5% 5% 57%	3250
	21st to end	Day	7% 8% 3 6% 3 3 2 3 4 5% 58%	3548
November	1st to 10th	Day	11% 7% 3 6% 4 3 3 3 5% 6% 50%	1863
	11th to 20th	Day	13% 7% 3 6% 4 4 3 3 4 7% 45%	1863
	21st to end	Day	9% 11% 4 7% 5% 5% 4 3 4 6% 42%	1675
December	1st to 10th	Day	7% 7% 2 5% 3 3 3 3 5% 10% 52%	2620
	11th to 20th	Day	9% 12% 4 8% 5% 4 4 4 5% 9% 35%	3026
	21st to end	Day	12% 13% 4 9% 6% 4 4 4 6% 6% 32%	4492
January	1st to 10th	Night	7% 13% 4 8% 5% 5% 4 3 6% 8% 38%	5867
	11th to 20th	Night	10% 14% 4 9% 4 4 4 4 6% 9% 33%	5880
	21st to end	Night	7% 13% 4 7% 3 3 3 2 4 6% 49%	6468
February	1st to 10th	Night	11% 14% 3 7% 4 4 4 4 5% 7% 38%	2600
	11th to 20th	Night	9% 12% 4 7% 5% 4 4 3 5% 5% 42%	2458
	21st to end	Night	8% 12% 4 9% 4 4 4 4 6% 6% 39%	1980
March	1st to 10th	Night	10% 16% 4 7% 4 4 4 3 4 6% 38%	5038
	11th to 20th	Night	6% 11% 4 9% 4 4 4 4 6% 7% 40%	5000
	21st to end	Night	11% 14% 3 7% 4 4 4 3 5% 5% 40%	5489
April	1st to 10th	Night	12% 11% 3 4 3 3 3 2 4 6% 48%	2179
	11th to 20th	Night	8% 8% 2 5% 5% 5% 4 4 5% 7% 48%	1832
	21st to end	Night	10% 8% 3 6% 4 4 4 3 6% 6% 47%	1800
May	1st to 10th	Night	9% 7% 2 5% 4 4 4 4 5% 7% 49%	4100
	11th to 20th	Night	6% 4 2 5% 3 3 3 4 6% 8% 57%	4074
	21st to end	Night	7% 5% 2 4 3 3 3 3 5% 7% 58%	4474
June	1st to 10th	Night	6% 4 2 5% 3 3 4 3 6% 7% 57%	1782
	11th to 20th	Night	5% 4 1 3 2 3 3 2 4 5% 67%	1648
	21st to end	Night	4 2 2 3 2 3 3 3 5% 6% 68%	1800
July	1st to 10th	Night	3 3 2 4 3 3 3 3 3 5% 69%	4100
	11th to 20th	Night	3 4 1 5% 4 6% 6% 5% 6% 8% 53%	4100
	21st to end	Night	2 3 1 4 4 3 4 4 5% 6% 64%	4472
August	1st to 10th	Night	2 2 1 4 2 3 4 3 5% 7% 66%	4033
	11th to 20th	Night	2 3 2 5% 3 4 4 3 5% 6% 62%	4051
	21st to end	Night	6% 5% 2 6% 4 4 4 4 5% 5% 56%	4586
September	1st to 10th	Night	3 2 1 4 3 3 4 3 4 6% 66%	2135
	11th to 20th	Night	6% 4 2 3 3 3 3 3 5% 7% 62%	2200
	21st to end	Night	6% 4 1 3 2 2 2 2 3 5% 69%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 3 3 4 71%	3760
	11th to 20th	Night	10% 6% 2 3 3 3 3 3 4 5% 59%	3898
	21st to end	Night	6% 5% 2 5% 3 3 3 3 5% 6% 58%	4252
November	1st to 10th	Night	9% 8% 2 4 3 3 3 3 5% 7% 53%	2655
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 4 7% 52%	2691
	21st to end	Night	9% 10% 3 6% 4 3 3 3 5% 6% 48%	2700
December	1st to 10th	Night	6% 6% 2 4 2 2 3 3 5% 9% 57%	4570
	11th to 20th	Night	12% 14% 3 5% 3 3 3 3 5% 6% 43%	5181
	21st to end	Night	10% 12% 4 9% 5% 4 5% 4 6% 6% 33%	6499

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	47% 9% 16% 20% 7% 1	4099
	11th to 20th	Day	48% 11% 14% 19% 7% 2	4100
	21st to end	Day	42% 5% 15% 20% 17% 1	4510
February	1st to 10th	Day	48% 8% 14% 18% 10% 1	1800
	11th to 20th	Day	47% 5% 13% 24% 10% 1	1834
	21st to end	Day	45% 3 13% 25% 12% 2	1650
March	1st to 10th	Day	41% 4 11% 26% 14% 4	4855
	11th to 20th	Day	36% 5% 11% 35% 12% 1	5000
	21st to end	Day	52% 3 14% 22% 7% 1	5486
April	1st to 10th	Day	39% 2 13% 24% 17% 6%	2220
	11th to 20th	Day	37% 2 11% 26% 20% 4	2516
	21st to end	Day	34% 2 10% 29% 21% 3	2600
May	1st to 10th	Day	32% 4 7% 28% 22% 7%	5880
	11th to 20th	Day	26% 6% 8% 29% 27% 5%	5844
	21st to end	Day	24% 4 6% 28% 33% 6%	6424
June	1st to 10th	Day	23% 2 7% 22% 33% 13%	2600
	11th to 20th	Day	21% 2 5% 17% 29% 27%	2718
	21st to end	Day	16% 2 1 11% 29% 41%	2600
July	1st to 10th	Day	13% 3 3 7% 15% 61%	5816
	11th to 20th	Day	18% 4 5% 11% 25% 37%	5811
	21st to end	Day	13% 3 5% 9% 22% 48%	6217
August	1st to 10th	Day	18% 3 6% 19% 27% 28%	4208
	11th to 20th	Day	19% 4 4 14% 32% 28%	4197
	21st to end	Day	16% 3 4 15% 29% 33%	4609
September	1st to 10th	Day	15% 3 5% 18% 29% 30%	2260
	11th to 20th	Day	19% 3 6% 21% 28% 23%	2200
	21st to end	Day	18% 2 5% 22% 32% 21%	2200
October	1st to 10th	Day	19% 2 5% 15% 38% 21%	3435
	11th to 20th	Day	24% 4 8% 27% 28% 9%	3250
	21st to end	Day	27% 4 11% 29% 24% 4	3548
November	1st to 10th	Day	34% 4 11% 27% 20% 5%	1863
	11th to 20th	Day	37% 5% 11% 31% 15% 1	1863
	21st to end	Day	40% 8% 13% 24% 14% 1	1675
December	1st to 10th	Day	27% 7% 14% 42% 9% 1	2620
	11th to 20th	Day	43% 9% 16% 23% 9%	3024
	21st to end	Day	49% 11% 11% 19% 10%	4492
January	1st to 10th	Night	44% 7% 15% 25% 8% 1	5867
	11th to 20th	Night	47% 9% 16% 20% 8% 1	5880
	21st to end	Night	40% 4 15% 26% 14%	6468
February	1st to 10th	Night	46% 5% 17% 22% 8% 2	2600
	11th to 20th	Night	41% 5% 13% 25% 14% 1	2459
	21st to end	Night	42% 5% 11% 27% 12% 3	1980
March	1st to 10th	Night	45% 4 9% 27% 13% 2	5038
	11th to 20th	Night	38% 4 9% 31% 16% 2	5000
	21st to end	Night	42% 4 16% 22% 15% 1	5489
April	1st to 10th	Night	36% 3 10% 25% 19% 7%	2179
	11th to 20th	Night	33% 3 10% 26% 24% 5%	1832
	21st to end	Night	34% 3 11% 28% 22% 3	1800
May	1st to 10th	Night	30% 5% 8% 25% 28% 5%	4097
	11th to 20th	Night	22% 4 7% 31% 31% 6%	4075
	21st to end	Night	23% 3 6% 25% 37% 5%	4474
June	1st to 10th	Night	24% 3 6% 21% 30% 15%	1782
	11th to 20th	Night	19% 2 6% 17% 30% 26%	1648
	21st to end	Night	16% 3 2 11% 29% 40%	1800
July	1st to 10th	Night	19% 3 3 6% 12% 58%	4055
	11th to 20th	Night	22% 5% 6% 10% 21% 37%	4046
	21st to end	Night	19% 3 5% 10% 20% 44%	4344
August	1st to 10th	Night	19% 5% 7% 17% 24% 27%	2928
	11th to 20th	Night	22% 5% 7% 14% 25% 28%	2928
	21st to end	Night	18% 3 5% 11% 27% 35%	3298
September	1st to 10th	Night	18% 4 5% 15% 25% 33%	2135
	11th to 20th	Night	21% 3 6% 19% 28% 25%	2200
	21st to end	Night	20% 2 4 18% 32% 25%	2200
October	1st to 10th	Night	17% 2 6% 14% 35% 26%	3760
	11th to 20th	Night	26% 3 5% 28% 28% 10%	3898
	21st to end	Night	24% 4 9% 32% 27% 4	4252
November	1st to 10th	Night	29% 3 11% 30% 22% 5%	2655
	11th to 20th	Night	31% 5% 13% 32% 16% 3	2691
	21st to end	Night	36% 6% 11% 31% 14% 2	2700
December	1st to 10th	Night	24% 5% 18% 39% 13% 1	4570
	11th to 20th	Night	41% 6% 21% 25% 8%	5182
	21st to end	Night	47% 9% 15% 20% 8%	6499

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	24% 3 17% 32% 16% 8%	300
	11th to 20th	Day	15% 4 19% 36% 20% 7%	300
	21st to end	Day	12% 5% 18% 37% 21% 7%	330
February	1st to 10th	Day	22% 8% 19% 35% 12% 4	300
	11th to 20th	Day	16% 7% 20% 36% 16% 4	370
	21st to end	Day	12% 6% 23% 38% 18% 4	414
March	1st to 10th	Day	15% 6% 20% 36% 17% 6%	500
	11th to 20th	Day	13% 6% 21% 34% 23% 4	500
	21st to end	Day	18% 7% 24% 31% 17% 2	550
April	1st to 10th	Day	12% 8% 23% 31% 20% 7%	500
	11th to 20th	Day	13% 9% 27% 28% 18% 4	492
	21st to end	Day	13% 8% 25% 28% 20% 6%	500
May	1st to 10th	Day	10% 8% 22% 35% 20% 6%	503
	11th to 20th	Day	9% 6% 23% 27% 26% 9%	528
	21st to end	Day	9% 5% 18% 28% 29% 12%	573
June	1st to 10th	Day	4 6% 15% 31% 35% 9%	530
	11th to 20th	Day	7% 5% 21% 33% 27% 7%	648
	21st to end	Day	3 5% 21% 33% 30% 8%	530
July	1st to 10th	Day	2 4 11% 34% 38% 12%	526
	11th to 20th	Day	5% 3 9% 37% 39% 8%	524
	21st to end	Day	3 2 11% 30% 41% 14%	575
August	1st to 10th	Day	5% 2 14% 33% 35% 11%	527
	11th to 20th	Day	3 3 16% 26% 38% 14%	517
	21st to end	Day	5% 3 12% 29% 38% 12%	583
September	1st to 10th	Day	4 2 11% 34% 35% 15%	530
	11th to 20th	Day	5% 6% 17% 28% 30% 13%	530
	21st to end	Day	6% 4 15% 29% 34% 13%	530
October	1st to 10th	Day	7% 5% 19% 31% 27% 12%	430
	11th to 20th	Day	8% 5% 19% 28% 24% 15%	330
	21st to end	Day	9% 6% 20% 35% 25% 6%	363
November	1st to 10th	Day	17% 5% 16% 31% 22% 10%	330
	11th to 20th	Day	12% 6% 18% 36% 17% 10%	321
	21st to end	Day	15% 5% 13% 33% 25% 9%	300
December	1st to 10th	Day	13% 4 14% 35% 24% 11%	300
	11th to 20th	Day	15% 7% 11% 40% 23% 4	300
	21st to end	Day	22% 8% 16% 32% 18% 4	330
January	1st to 10th	Night	18% 4 14% 39% 22% 4	800
	11th to 20th	Night	17% 4 14% 36% 26% 4	800
	21st to end	Night	15% 4 13% 42% 22% 4	880
February	1st to 10th	Night	17% 5% 16% 45% 15% 3	800
	11th to 20th	Night	14% 3 18% 45% 18% 2	784
	21st to end	Night	13% 5% 19% 46% 17% 1	660
March	1st to 10th	Night	17% 4 18% 43% 16% 3	800
	11th to 20th	Night	11% 5% 16% 43% 23% 3	800
	21st to end	Night	15% 6% 18% 44% 16% 2	880
April	1st to 10th	Night	18% 4 17% 40% 18% 2	779
	11th to 20th	Night	10% 4 22% 48% 15% 1	593
	21st to end	Night	12% 5% 21% 38% 23% 2	600
May	1st to 10th	Night	10% 5% 17% 45% 22% 2	600
	11th to 20th	Night	9% 4 13% 39% 32% 3	591
	21st to end	Night	9% 5% 14% 37% 31% 4	650
June	1st to 10th	Night	8% 5% 12% 39% 34% 3	600
	11th to 20th	Night	6% 5% 19% 42% 26% 2	600
	21st to end	Night	5% 2 15% 40% 36% 3	600
July	1st to 10th	Night	4 2 10% 35% 42% 6%	585
	11th to 20th	Night	4 1 8% 42% 39% 5%	582
	21st to end	Night	5% 2 6% 31% 52% 4	657
August	1st to 10th	Night	4 1 11% 42% 39% 2	597
	11th to 20th	Night	4 2 10% 39% 40% 5%	594
	21st to end	Night	4 3 7% 39% 43% 3	660
September	1st to 10th	Night	4 2 9% 38% 39% 7%	735
	11th to 20th	Night	7% 3 13% 35% 36% 6%	800
	21st to end	Night	8% 2 13% 33% 39% 7%	800
October	1st to 10th	Night	10% 3 16% 40% 25% 6%	800
	11th to 20th	Night	10% 4 14% 38% 29% 6%	800
	21st to end	Night	11% 2 15% 41% 28% 3	880
November	1st to 10th	Night	13% 3 17% 38% 26% 4	820
	11th to 20th	Night	13% 3 18% 43% 20% 4	828
	21st to end	Night	14% 4 14% 40% 22% 7%	900
December	1st to 10th	Night	15% 3 14% 44% 21% 3	1000
	11th to 20th	Night	21% 5% 14% 41% 18% 2	1000
	21st to end	Night	20% 4 16% 42% 15% 2	898

Page generated at 2025-11-14 22:56:01. Version ac07

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