Morrow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morrow Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 8% 3 7% 4 4 4 4 5% 7% 37%	131682
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	131627
Transparency	Day	44% 5% 10% 23% 15% 2	135305
	Night	45% 4 10% 22% 16% 2	136077
Seeing	Day	19% 13% 28% 23% 13% 5%	17573
	Night	19% 11% 28% 27% 14% 2	27433

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	15% 13% 6% 10% 4 4 4 3 5% 6% 30%	4691
	11th to 20th	Day	11% 12% 5% 13% 6% 4 3 3 5% 5% 33%	4710
	21st to end	Day	18% 14% 5% 10% 6% 5% 4 3 4 4 27%	5631
February	1st to 10th	Day	17% 13% 5% 8% 4 4 4 2 5% 7% 32%	2100
	11th to 20th	Day	16% 10% 4 9% 5% 4 4 3 5% 6% 34%	2048
	21st to end	Day	15% 13% 5% 9% 4 4 4 2 4 6% 33%	1730
March	1st to 10th	Day	19% 13% 4 7% 4 3 3 2 3 4 38%	4580
	11th to 20th	Day	13% 10% 3 6% 3 4 3 3 5% 7% 42%	4892
	21st to end	Day	17% 9% 4 7% 5% 5% 5% 4 6% 7% 33%	5864
April	1st to 10th	Day	15% 9% 3 6% 5% 5% 4 3 6% 8% 37%	2520
	11th to 20th	Day	16% 10% 3 6% 4 5% 4 3 5% 7% 38%	2494
	21st to end	Day	18% 7% 3 6% 4 5% 4 3 5% 7% 37%	2659
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 4 7% 8% 28%	5992
	11th to 20th	Day	20% 7% 2 7% 4 4 4 3 5% 7% 35%	5991
	21st to end	Day	15% 7% 3 7% 5% 5% 6% 5% 7% 9% 31%	6622
June	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 4 5% 8% 43%	2801
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 7% 8% 44%	2800
	21st to end	Day	9% 6% 2 5% 5% 5% 6% 5% 7% 9% 42%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 10% 42%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	6360
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 5% 7% 10% 32%	6659
August	1st to 10th	Day	9% 4 1 4 3 4 4 4 6% 11% 48%	4234
	11th to 20th	Day	9% 5% 2 5% 5% 6% 6% 5% 7% 10% 41%	3889
	21st to end	Day	5% 4 2 4 4 4 4 3 6% 9% 56%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 4 6% 49%	2375
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 6% 8% 49%	2200
	21st to end	Day	13% 5% 2 5% 4 3 3 4 5% 7% 50%	2200
October	1st to 10th	Day	11% 4 2 5% 4 4 4 3 4 7% 52%	2200
	11th to 20th	Day	12% 6% 2 5% 5% 4 3 3 5% 7% 46%	2069
	21st to end	Day	20% 8% 3 5% 4 4 4 3 5% 5% 39%	1980
November	1st to 10th	Day	10% 6% 2 5% 4 3 4 4 4 7% 52%	1852
	11th to 20th	Day	16% 10% 3 7% 4 3 4 3 5% 7% 39%	1890
	21st to end	Day	21% 10% 3 7% 4 3 3 3 4 5% 39%	1890
December	1st to 10th	Day	15% 12% 3 6% 3 3 4 3 4 4 43%	3020
	11th to 20th	Day	17% 14% 5% 8% 5% 5% 5% 3 4 6% 29%	3197
	21st to end	Day	27% 17% 5% 9% 4 4 4 3 4 5% 19%	4070
January	1st to 10th	Night	15% 14% 6% 10% 5% 4 4 4 5% 5% 29%	8232
	11th to 20th	Night	12% 14% 6% 13% 6% 6% 5% 3 3 5% 27%	8280
	21st to end	Night	18% 13% 4 7% 5% 5% 4 3 4 6% 33%	8549
February	1st to 10th	Night	20% 11% 4 9% 5% 4 4 3 5% 5% 28%	2520
	11th to 20th	Night	17% 11% 3 10% 5% 4 3 3 5% 7% 32%	2458
	21st to end	Night	20% 10% 4 8% 4 3 5% 3 5% 5% 33%	2076
March	1st to 10th	Night	21% 14% 3 6% 3 3 3 2 4 5% 36%	5493
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 4 7% 47%	5287
	21st to end	Night	17% 9% 3 9% 5% 5% 5% 4 5% 6% 32%	5336
April	1st to 10th	Night	14% 9% 2 6% 4 4 4 4 6% 6% 40%	2100
	11th to 20th	Night	16% 8% 2 6% 4 4 4 3 5% 7% 42%	2075
	21st to end	Night	16% 8% 2 6% 4 4 4 4 5% 7% 38%	1959
May	1st to 10th	Night	19% 11% 3 8% 5% 4 4 3 5% 8% 29%	4123
	11th to 20th	Night	17% 7% 2 7% 5% 4 4 4 7% 8% 34%	4122
	21st to end	Night	13% 7% 3 7% 5% 4 5% 4 6% 10% 35%	4459
June	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 7% 10% 46%	1600
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 7% 9% 48%	1600
	21st to end	Night	6% 3 1 4 4 5% 3 5% 8% 10% 52%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 4 6% 9% 46%	3618
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 4 7% 10% 45%	3617
	21st to end	Night	9% 5% 2 7% 5% 5% 6% 5% 7% 11% 38%	4199
August	1st to 10th	Night	8% 5% 2 5% 4 3 3 3 5% 9% 52%	2915
	11th to 20th	Night	7% 4 2 6% 4 5% 5% 4 7% 10% 47%	3155
	21st to end	Night	4 2 1 4 3 4 4 4 6% 9% 59%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 7% 51%	2005
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 5% 8% 55%	2200
	21st to end	Night	14% 6% 2 4 3 3 4 3 5% 7% 50%	2200
October	1st to 10th	Night	9% 4 2 5% 3 3 3 3 5% 6% 59%	2200
	11th to 20th	Night	14% 7% 3 5% 3 3 4 3 4 5% 49%	2326
	21st to end	Night	18% 8% 2 6% 4 3 4 2 4 6% 42%	2860
November	1st to 10th	Night	13% 7% 2 5% 4 3 4 3 6% 6% 49%	2666
	11th to 20th	Night	15% 8% 2 6% 4 4 3 3 4 7% 43%	2730
	21st to end	Night	20% 11% 4 7% 4 4 3 2 4 6% 36%	2730
December	1st to 10th	Night	18% 11% 3 8% 3 3 3 3 4 4 39%	4380
	11th to 20th	Night	19% 12% 5% 8% 4 4 4 4 4 6% 30%	5221
	21st to end	Night	27% 18% 5% 8% 5% 4 3 3 4 4 19%	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	55% 3 11% 22% 8% 1	4692
	11th to 20th	Day	53% 2 9% 19% 14% 2	4710
	21st to end	Day	62% 2 7% 18% 5% 5%	5631
February	1st to 10th	Day	53% 3 9% 23% 10% 2	2100
	11th to 20th	Day	51% 4 9% 21% 13% 2	2058
	21st to end	Day	53% 4 9% 21% 11% 3	1730
March	1st to 10th	Day	52% 3 4 19% 16% 6%	4580
	11th to 20th	Day	42% 4 8% 29% 15% 2	4892
	21st to end	Day	49% 5% 12% 21% 12% 1	5866
April	1st to 10th	Day	46% 5% 11% 21% 15% 1	2520
	11th to 20th	Day	46% 4 10% 24% 13% 3	2494
	21st to end	Day	47% 4 9% 25% 13% 3	2659
May	1st to 10th	Day	52% 6% 12% 19% 10%	6027
	11th to 20th	Day	48% 6% 9% 23% 13%	5995
	21st to end	Day	47% 6% 13% 18% 14% 2	6636
June	1st to 10th	Day	39% 6% 10% 25% 18% 3	2801
	11th to 20th	Day	35% 7% 13% 23% 20% 3	2800
	21st to end	Day	36% 6% 15% 21% 18% 4	2800
July	1st to 10th	Day	37% 6% 10% 23% 22% 1	6291
	11th to 20th	Day	38% 9% 12% 25% 14% 1	6285
	21st to end	Day	46% 8% 13% 21% 12% 1	6469
August	1st to 10th	Day	29% 7% 14% 34% 15% 1	4221
	11th to 20th	Day	36% 6% 12% 29% 16% 1	3926
	21st to end	Day	26% 5% 13% 35% 17% 4	4311
September	1st to 10th	Day	36% 5% 10% 24% 20% 5%	2375
	11th to 20th	Day	32% 5% 11% 28% 21% 4	2200
	21st to end	Day	34% 5% 8% 24% 25% 3	2200
October	1st to 10th	Day	22% 3 5% 29% 39% 1	3590
	11th to 20th	Day	28% 3 8% 27% 31% 3	3350
	21st to end	Day	37% 3 8% 24% 26% 2	3180
November	1st to 10th	Day	33% 4 10% 29% 19% 4	1852
	11th to 20th	Day	46% 3 8% 22% 17% 3	1890
	21st to end	Day	50% 3 9% 22% 15% 1	1890
December	1st to 10th	Day	45% 3 10% 27% 14%	3020
	11th to 20th	Day	57% 4 8% 21% 9% 1	3194
	21st to end	Day	68% 4 10% 12% 6%	4070
January	1st to 10th	Night	57% 4 10% 20% 8%	8233
	11th to 20th	Night	61% 2 8% 14% 13% 3	8280
	21st to end	Night	54% 2 11% 20% 12% 1	8549
February	1st to 10th	Night	58% 3 11% 19% 7% 1	2520
	11th to 20th	Night	53% 3 10% 21% 12% 1	2459
	21st to end	Night	54% 4 10% 19% 11% 2	2076
March	1st to 10th	Night	52% 2 5% 23% 13% 4	5493
	11th to 20th	Night	38% 4 10% 26% 22% 1	5287
	21st to end	Night	52% 5% 8% 24% 10% 1	5336
April	1st to 10th	Night	44% 4 11% 25% 14% 3	2100
	11th to 20th	Night	42% 5% 11% 19% 19% 4	2075
	21st to end	Night	44% 5% 10% 25% 14% 3	1959
May	1st to 10th	Night	52% 7% 11% 20% 8% 2	4150
	11th to 20th	Night	47% 6% 11% 25% 11%	4123
	21st to end	Night	44% 7% 15% 21% 11% 3	4467
June	1st to 10th	Night	33% 6% 10% 25% 21% 4	1600
	11th to 20th	Night	32% 6% 12% 24% 21% 6%	1600
	21st to end	Night	25% 8% 17% 24% 21% 6%	1600
July	1st to 10th	Night	35% 7% 10% 26% 20% 3	3580
	11th to 20th	Night	33% 8% 15% 28% 15% 2	3572
	21st to end	Night	38% 7% 16% 25% 12% 1	4076
August	1st to 10th	Night	29% 6% 14% 37% 13% 1	2904
	11th to 20th	Night	31% 6% 15% 29% 17% 1	3184
	21st to end	Night	21% 6% 14% 32% 24% 3	3586
September	1st to 10th	Night	32% 6% 8% 27% 20% 7%	2005
	11th to 20th	Night	27% 6% 9% 26% 28% 4	2200
	21st to end	Night	34% 5% 9% 22% 24% 5%	2200
October	1st to 10th	Night	19% 3 7% 20% 48% 4	3590
	11th to 20th	Night	26% 2 6% 25% 36% 4	3784
	21st to end	Night	34% 3 7% 23% 28% 6%	4610
November	1st to 10th	Night	36% 3 9% 25% 22% 4	2666
	11th to 20th	Night	43% 2 9% 22% 19% 4	2730
	21st to end	Night	50% 3 9% 22% 14% 1	2730
December	1st to 10th	Night	50% 3 10% 24% 13%	4380
	11th to 20th	Night	56% 4 11% 19% 10% 1	5223
	21st to end	Night	70% 4 8% 13% 5% 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	28% 20% 32% 15% 3 1	330
	11th to 20th	Day	34% 27% 28% 9% 2	330
	21st to end	Day	29% 27% 32% 10% 2	363
February	1st to 10th	Day	27% 29% 31% 11% 3	330
	11th to 20th	Day	28% 30% 31% 10% 1 1	330
	21st to end	Day	27% 22% 40% 11% 1	273
March	1st to 10th	Day	28% 27% 32% 11% 2	330
	11th to 20th	Day	29% 24% 31% 11% 3 1	446
	21st to end	Day	30% 20% 31% 13% 5%	594
April	1st to 10th	Day	24% 20% 31% 19% 5% 1	540
	11th to 20th	Day	24% 23% 25% 20% 7% 2	532
	21st to end	Day	23% 19% 35% 15% 7% 1	540
May	1st to 10th	Day	24% 17% 32% 18% 7% 2	543
	11th to 20th	Day	23% 14% 29% 21% 9% 4	567
	21st to end	Day	16% 9% 30% 25% 14% 7%	637
June	1st to 10th	Day	13% 8% 28% 27% 18% 5%	731
	11th to 20th	Day	12% 7% 26% 33% 18% 5%	730
	21st to end	Day	11% 8% 29% 28% 18% 6%	730
July	1st to 10th	Day	12% 4 21% 25% 24% 14%	721
	11th to 20th	Day	8% 5% 24% 32% 22% 8%	718
	21st to end	Day	10% 5% 20% 34% 23% 8%	681
August	1st to 10th	Day	9% 4 19% 30% 27% 11%	526
	11th to 20th	Day	13% 5% 21% 29% 24% 7%	519
	21st to end	Day	8% 4 17% 33% 27% 12%	583
September	1st to 10th	Day	10% 5% 20% 31% 27% 8%	530
	11th to 20th	Day	9% 3 16% 31% 28% 13%	530
	21st to end	Day	17% 8% 21% 25% 22% 7%	530
October	1st to 10th	Day	10% 8% 25% 33% 18% 6%	530
	11th to 20th	Day	15% 15% 33% 23% 10% 3	453
	21st to end	Day	30% 15% 36% 18% 1	363
November	1st to 10th	Day	15% 18% 35% 23% 6% 3	330
	11th to 20th	Day	27% 20% 29% 18% 5% 1	330
	21st to end	Day	31% 13% 33% 19% 4	330
December	1st to 10th	Day	23% 27% 31% 14% 4 1	330
	11th to 20th	Day	29% 18% 37% 12% 3 1	330
	21st to end	Day	33% 22% 32% 11% 2	363
January	1st to 10th	Night	23% 20% 33% 21% 4	1050
	11th to 20th	Night	26% 21% 33% 17% 2	1050
	21st to end	Night	22% 20% 36% 20% 2	1041
February	1st to 10th	Night	25% 16% 38% 19% 1	840
	11th to 20th	Night	26% 19% 34% 20% 2	823
	21st to end	Night	28% 16% 37% 18% 1	692
March	1st to 10th	Night	24% 18% 33% 22% 3	840
	11th to 20th	Night	25% 15% 34% 20% 6%	840
	21st to end	Night	24% 14% 34% 23% 5%	814
April	1st to 10th	Night	19% 14% 35% 25% 7%	630
	11th to 20th	Night	21% 14% 29% 26% 10%	623
	21st to end	Night	22% 10% 34% 27% 6% 1	630
May	1st to 10th	Night	21% 8% 33% 27% 10% 1	630
	11th to 20th	Night	16% 8% 27% 30% 16% 3	621
	21st to end	Night	11% 4 25% 34% 22% 4	683
June	1st to 10th	Night	12% 4 23% 38% 22% 2	600
	11th to 20th	Night	10% 3 21% 43% 22% 1	600
	21st to end	Night	7% 3 22% 40% 27% 2	600
July	1st to 10th	Night	12% 2 12% 34% 30% 10%	585
	11th to 20th	Night	7% 4 14% 35% 35% 5%	582
	21st to end	Night	9% 3 14% 37% 34% 4	657
August	1st to 10th	Night	9% 2 13% 36% 36% 5%	597
	11th to 20th	Night	6% 2 13% 39% 36% 4	594
	21st to end	Night	6% 1 13% 36% 38% 6%	660
September	1st to 10th	Night	12% 4 13% 33% 35% 3	605
	11th to 20th	Night	8% 3 16% 33% 35% 5%	800
	21st to end	Night	16% 5% 21% 30% 25% 3	800
October	1st to 10th	Night	9% 6% 23% 33% 24% 4	800
	11th to 20th	Night	16% 12% 28% 29% 15% 1	800
	21st to end	Night	23% 13% 32% 24% 9%	880
November	1st to 10th	Night	17% 10% 33% 28% 12% 1	820
	11th to 20th	Night	21% 15% 30% 26% 7%	840
	21st to end	Night	27% 14% 27% 25% 7%	840
December	1st to 10th	Night	25% 14% 33% 22% 5%	840
	11th to 20th	Night	28% 12% 32% 23% 5% 1	971
	21st to end	Night	32% 16% 32% 17% 3	1155

Page generated at 2026-05-29 01:53:27. Version ac07

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