UBarU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UBarU Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	10% 6% 2 5% 4 4 4 3 5% 7% 50%	131960
	Night	11% 6% 2 5% 3 3 3 3 5% 6% 53%	130552
Transparency	Day	34% 4 9% 21% 20% 12%	135791
	Night	32% 4 10% 20% 21% 14%	134785
Seeing	Day	13% 8% 25% 31% 18% 4	16648
	Night	13% 8% 29% 35% 13% 1	26727

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	9% 6% 3 5% 3 2 3 3 4 5% 58%	5289
	11th to 20th	Day	11% 7% 2 5% 2 2 3 2 4 4 58%	5370
	21st to end	Day	19% 6% 2 6% 3 3 3 2 4 6% 46%	6468
February	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 6% 51%	2100
	11th to 20th	Day	13% 7% 2 5% 4 3 4 4 5% 6% 49%	2049
	21st to end	Day	12% 6% 2 4 3 3 3 2 5% 5% 55%	1730
March	1st to 10th	Day	11% 6% 2 4 3 3 3 2 4 6% 56%	4814
	11th to 20th	Day	12% 7% 2 5% 3 3 3 2 3 5% 55%	5090
	21st to end	Day	10% 7% 2 5% 4 4 4 3 5% 6% 51%	5583
April	1st to 10th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 46%	2310
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 4 6% 47%	2389
	21st to end	Day	11% 7% 3 6% 5% 3 3 3 4 5% 49%	2463
May	1st to 10th	Day	16% 7% 3 7% 4 4 4 4 5% 7% 40%	5862
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	5862
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	6434
June	1st to 10th	Day	5% 4 2 5% 4 5% 5% 4 6% 9% 51%	2600
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 4 5% 9% 51%	2600
	21st to end	Day	6% 5% 1 6% 6% 5% 6% 5% 6% 10% 44%	2600
July	1st to 10th	Day	14% 6% 3 6% 5% 5% 5% 4 6% 8% 37%	5900
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 4 7% 8% 49%	5900
	21st to end	Day	6% 5% 2 5% 4 4 5% 4 7% 9% 49%	6422
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 8% 69%	3981
	11th to 20th	Day	4 2 1 3 3 4 3 3 6% 9% 61%	3846
	21st to end	Day	4 4 2 5% 5% 5% 5% 4 6% 10% 50%	4260
September	1st to 10th	Day	10% 6% 2 8% 5% 5% 5% 3 5% 7% 45%	2200
	11th to 20th	Day	8% 7% 2 7% 4 5% 6% 4 6% 8% 42%	2200
	21st to end	Day	9% 4 2 6% 4 4 4 4 6% 7% 50%	2200
October	1st to 10th	Day	6% 4 2 4 4 3 4 4 5% 7% 57%	2200
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 5% 56%	2149
	21st to end	Day	11% 5% 2 5% 3 3 3 2 3 5% 59%	2200
November	1st to 10th	Day	13% 6% 2 4 4 3 3 3 4 6% 51%	2011
	11th to 20th	Day	9% 6% 1 5% 3 3 4 2 5% 6% 54%	1890
	21st to end	Day	14% 8% 3 5% 3 3 3 2 4 6% 50%	1890
December	1st to 10th	Day	21% 9% 3 6% 2 3 2 2 4 5% 43%	3030
	11th to 20th	Day	12% 8% 2 7% 4 3 4 4 5% 6% 46%	3481
	21st to end	Day	10% 6% 2 4 2 2 2 2 3 4 61%	4587
January	1st to 10th	Night	11% 5% 2 6% 2 3 2 2 3 5% 59%	7634
	11th to 20th	Night	9% 4 2 3 2 2 2 2 4 6% 66%	7662
	21st to end	Night	24% 7% 2 5% 3 3 3 2 3 5% 44%	7746
February	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 4 5% 54%	2520
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2458
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 5% 51%	2076
March	1st to 10th	Night	10% 7% 2 5% 3 3 3 3 6% 7% 50%	5269
	11th to 20th	Night	11% 8% 2 5% 3 3 3 3 4 5% 54%	5090
	21st to end	Night	12% 10% 2 5% 3 3 3 3 4 6% 51%	5588
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 5% 6% 50%	2310
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 48%	2088
	21st to end	Night	14% 8% 2 5% 4 3 3 2 4 5% 50%	1935
May	1st to 10th	Night	20% 10% 2 6% 3 3 3 3 4 5% 40%	4053
	11th to 20th	Night	7% 6% 2 5% 4 4 4 3 6% 7% 52%	4053
	21st to end	Night	13% 8% 2 6% 3 3 4 4 6% 10% 41%	4443
June	1st to 10th	Night	4 3 1 3 3 2 4 4 7% 11% 60%	1800
	11th to 20th	Night	5% 4 1 4 3 4 3 3 6% 8% 60%	1800
	21st to end	Night	7% 5% 1 4 4 4 4 4 5% 10% 53%	1800
July	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 10% 43%	4078
	11th to 20th	Night	4 3 1 5% 3 3 4 3 6% 9% 57%	4077
	21st to end	Night	6% 6% 2 5% 3 4 4 4 6% 9% 51%	4449
August	1st to 10th	Night	1 2 1 2 2 2 2 2 4 8% 74%	3173
	11th to 20th	Night	3 2 1 3 3 3 3 3 7% 10% 62%	3213
	21st to end	Night	4 5% 1 4 3 4 4 4 7% 10% 53%	3646
September	1st to 10th	Night	8% 6% 2 6% 3 3 4 3 6% 8% 51%	2200
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 3 6% 8% 48%	2200
	21st to end	Night	7% 6% 2 5% 4 3 4 4 6% 7% 54%	2200
October	1st to 10th	Night	5% 4 2 4 2 3 3 3 5% 7% 64%	2200
	11th to 20th	Night	11% 4 1 4 3 3 3 2 5% 5% 59%	2230
	21st to end	Night	13% 4 1 4 3 2 2 2 3 3 63%	2640
November	1st to 10th	Night	12% 6% 2 4 3 3 2 2 4 6% 57%	2502
	11th to 20th	Night	9% 6% 2 3 3 3 3 3 4 6% 59%	2730
	21st to end	Night	15% 7% 2 4 3 3 3 3 4 5% 51%	2730
December	1st to 10th	Night	21% 7% 2 5% 3 3 3 3 4 5% 43%	4380
	11th to 20th	Night	17% 9% 2 4 3 3 3 2 3 4 49%	4946
	21st to end	Night	14% 6% 2 4 2 2 2 2 3 4 60%	6633

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	30% 3 4 22% 26% 14%	5290
	11th to 20th	Day	32% 2 3 16% 27% 21%	5370
	21st to end	Day	41% 2 5% 14% 25% 13%	6468
February	1st to 10th	Day	36% 3 7% 12% 23% 19%	2100
	11th to 20th	Day	37% 3 6% 19% 19% 15%	2059
	21st to end	Day	33% 2 6% 16% 25% 18%	1730
March	1st to 10th	Day	31% 3 5% 16% 17% 27%	4814
	11th to 20th	Day	34% 3 5% 10% 15% 33%	5090
	21st to end	Day	34% 4 7% 19% 20% 16%	5585
April	1st to 10th	Day	38% 4 8% 18% 18% 14%	2310
	11th to 20th	Day	40% 4 7% 17% 20% 13%	2389
	21st to end	Day	38% 3 8% 21% 19% 12%	2463
May	1st to 10th	Day	44% 4 9% 17% 19% 7%	5900
	11th to 20th	Day	35% 5% 10% 17% 22% 12%	5866
	21st to end	Day	42% 5% 10% 25% 14% 3	6447
June	1st to 10th	Day	29% 5% 12% 30% 21% 3	2600
	11th to 20th	Day	31% 5% 12% 26% 21% 6%	2600
	21st to end	Day	35% 7% 13% 24% 17% 5%	2600
July	1st to 10th	Day	43% 7% 16% 28% 6%	5836
	11th to 20th	Day	30% 7% 15% 33% 14% 2	5831
	21st to end	Day	29% 8% 17% 27% 18% 1	6240
August	1st to 10th	Day	14% 5% 10% 40% 30% 1	3967
	11th to 20th	Day	20% 4 22% 38% 12% 2	3882
	21st to end	Day	29% 8% 17% 25% 19% 2	4259
September	1st to 10th	Day	40% 5% 10% 19% 18% 8%	2200
	11th to 20th	Day	38% 6% 11% 21% 19% 5%	2200
	21st to end	Day	33% 4 10% 21% 19% 13%	2200
October	1st to 10th	Day	17% 3 7% 13% 26% 34%	3590
	11th to 20th	Day	26% 3 5% 20% 24% 22%	3480
	21st to end	Day	25% 3 4 17% 37% 14%	3538
November	1st to 10th	Day	36% 2 7% 19% 22% 13%	2011
	11th to 20th	Day	32% 3 7% 18% 22% 19%	1890
	21st to end	Day	39% 2 5% 17% 23% 14%	1890
December	1st to 10th	Day	47% 2 5% 17% 19% 10%	3030
	11th to 20th	Day	39% 4 7% 12% 23% 14%	3479
	21st to end	Day	29% 2 3 16% 28% 21%	4587
January	1st to 10th	Night	31% 2 5% 17% 29% 15%	7635
	11th to 20th	Night	23% 1 3 17% 30% 26%	7662
	21st to end	Night	46% 2 3 10% 23% 16%	7746
February	1st to 10th	Night	34% 3 7% 13% 23% 20%	2520
	11th to 20th	Night	36% 3 6% 18% 21% 16%	2458
	21st to end	Night	36% 3 6% 13% 24% 19%	2076
March	1st to 10th	Night	34% 3 6% 18% 20% 19%	5269
	11th to 20th	Night	35% 3 5% 11% 17% 29%	5090
	21st to end	Night	36% 4 7% 18% 19% 16%	5588
April	1st to 10th	Night	36% 4 10% 18% 15% 17%	2310
	11th to 20th	Night	39% 4 8% 15% 18% 16%	2088
	21st to end	Night	39% 2 9% 19% 19% 11%	1935
May	1st to 10th	Night	47% 4 7% 18% 14% 11%	4080
	11th to 20th	Night	31% 5% 12% 19% 19% 14%	4054
	21st to end	Night	39% 5% 14% 26% 11% 4	4452
June	1st to 10th	Night	20% 5% 19% 35% 19% 3	1800
	11th to 20th	Night	23% 4 16% 29% 23% 5%	1800
	21st to end	Night	28% 5% 18% 28% 18% 4	1800
July	1st to 10th	Night	37% 8% 22% 29% 4	4035
	11th to 20th	Night	23% 7% 20% 34% 14% 1	4026
	21st to end	Night	29% 8% 20% 27% 14% 3	4320
August	1st to 10th	Night	12% 4 14% 46% 24% 1	3163
	11th to 20th	Night	17% 5% 28% 37% 10% 2	3243
	21st to end	Night	25% 10% 20% 25% 18% 2	3646
September	1st to 10th	Night	33% 5% 13% 22% 19% 8%	2200
	11th to 20th	Night	34% 6% 15% 23% 16% 6%	2200
	21st to end	Night	29% 4 12% 22% 19% 13%	2200
October	1st to 10th	Night	15% 2 8% 15% 23% 36%	3590
	11th to 20th	Night	23% 3 8% 17% 30% 19%	3625
	21st to end	Night	25% 3 4 19% 34% 15%	4252
November	1st to 10th	Night	31% 2 8% 20% 24% 15%	2502
	11th to 20th	Night	28% 3 7% 22% 21% 18%	2730
	21st to end	Night	37% 2 7% 17% 21% 16%	2730
December	1st to 10th	Night	45% 3 7% 16% 16% 13%	4380
	11th to 20th	Night	41% 3 5% 12% 21% 19%	4947
	21st to end	Night	31% 2 4 16% 29% 18%	6633

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	19% 15% 42% 18% 5% 1	330
	11th to 20th	Day	22% 15% 35% 21% 6%	330
	21st to end	Day	20% 15% 40% 19% 5%	363
February	1st to 10th	Day	24% 12% 35% 22% 7%	330
	11th to 20th	Day	22% 17% 39% 18% 2 1	330
	21st to end	Day	19% 18% 42% 18% 3	273
March	1st to 10th	Day	22% 21% 40% 16% 2	330
	11th to 20th	Day	19% 20% 38% 18% 5%	330
	21st to end	Day	15% 17% 43% 20% 4 1	363
April	1st to 10th	Day	21% 19% 38% 19% 2	330
	11th to 20th	Day	19% 20% 39% 16% 5%	459
	21st to end	Day	17% 15% 45% 18% 5%	563
May	1st to 10th	Day	17% 11% 42% 26% 5%	703
	11th to 20th	Day	19% 8% 36% 27% 9% 1	725
	21st to end	Day	16% 8% 33% 33% 10%	789
June	1st to 10th	Day	6% 4 24% 44% 20% 3	730
	11th to 20th	Day	8% 3 23% 47% 17% 2	730
	21st to end	Day	6% 3 17% 40% 28% 5%	730
July	1st to 10th	Day	11% 4 9% 39% 29% 7%	721
	11th to 20th	Day	5% 2 7% 43% 37% 6%	718
	21st to end	Day	4 2 6% 41% 36% 10%	794
August	1st to 10th	Day	2 4 41% 42% 11%	577
	11th to 20th	Day	8% 1 4 37% 39% 12%	517
	21st to end	Day	5% 2 5% 34% 39% 14%	557
September	1st to 10th	Day	10% 1 8% 35% 33% 14%	330
	11th to 20th	Day	7% 1 4 35% 43% 10%	330
	21st to end	Day	10% 2 5% 37% 37% 9%	330
October	1st to 10th	Day	6% 2 14% 39% 31% 8%	330
	11th to 20th	Day	10% 2 26% 39% 19% 3	330
	21st to end	Day	16% 13% 29% 28% 12% 2	363
November	1st to 10th	Day	20% 5% 32% 30% 11% 3	330
	11th to 20th	Day	20% 9% 31% 26% 9% 5%	330
	21st to end	Day	23% 20% 30% 19% 7% 2	330
December	1st to 10th	Day	18% 10% 35% 28% 7% 2	330
	11th to 20th	Day	18% 15% 37% 21% 8% 1	330
	21st to end	Day	17% 13% 35% 26% 9%	363
January	1st to 10th	Night	16% 11% 35% 30% 8%	1050
	11th to 20th	Night	16% 12% 36% 30% 5%	1045
	21st to end	Night	20% 12% 33% 29% 6%	924
February	1st to 10th	Night	19% 13% 34% 28% 6%	840
	11th to 20th	Night	19% 14% 36% 27% 4	824
	21st to end	Night	18% 14% 46% 21% 1	692
March	1st to 10th	Night	19% 14% 39% 25% 3	745
	11th to 20th	Night	18% 12% 38% 29% 4	630
	21st to end	Night	14% 13% 47% 23% 3	693
April	1st to 10th	Night	11% 15% 48% 23% 3	630
	11th to 20th	Night	18% 16% 43% 21% 3	611
	21st to end	Night	18% 12% 43% 23% 4 1	600
May	1st to 10th	Night	16% 12% 45% 24% 3	600
	11th to 20th	Night	14% 6% 40% 33% 7%	591
	21st to end	Night	13% 6% 40% 32% 9% 1	650
June	1st to 10th	Night	5% 2 30% 47% 16% 1	600
	11th to 20th	Night	5% 4 37% 44% 9%	600
	21st to end	Night	7% 3 21% 49% 19% 2	600
July	1st to 10th	Night	9% 3 12% 48% 26% 2	585
	11th to 20th	Night	4 1 12% 54% 27% 2	582
	21st to end	Night	5% 1 9% 55% 26% 3	657
August	1st to 10th	Night	2 1 8% 54% 32% 3	597
	11th to 20th	Night	5% 1 8% 51% 32% 3	594
	21st to end	Night	5% 3 7% 45% 37% 4	660
September	1st to 10th	Night	10% 4 9% 40% 35% 3	600
	11th to 20th	Night	11% 2 7% 43% 35% 2	600
	21st to end	Night	9% 2 11% 45% 31% 2	600
October	1st to 10th	Night	6% 2 14% 47% 29% 2	600
	11th to 20th	Night	13% 4 24% 41% 17% 1	630
	21st to end	Night	16% 8% 27% 30% 17% 2	880
November	1st to 10th	Night	14% 7% 25% 39% 14% 1	862
	11th to 20th	Night	12% 7% 30% 36% 13% 2	1050
	21st to end	Night	20% 10% 30% 30% 11% 1	1050
December	1st to 10th	Night	16% 8% 31% 35% 8% 1	1050
	11th to 20th	Night	19% 10% 32% 31% 7%	1050
	21st to end	Night	16% 11% 32% 34% 7%	1155

Page generated at 2026-04-14 01:44:25. Version ac07

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