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 2024-12-24.

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%	101425
	Night	10% 6% 2 5% 3 3 3 3 5% 7% 54%	98817
Transparency	Day	33% 4 10% 22% 21% 11%	105295
	Night	31% 4 10% 21% 21% 13%	103074
Seeing	Day	13% 8% 25% 31% 19% 4	14972
	Night	13% 8% 29% 35% 13% 1	24983

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	10% 8% 3 5% 2 2 2 3 4 5% 56%	2847
	11th to 20th	Day	11% 8% 1 4 2 2 3 2 5% 4 58%	2882
	21st to end	Day	13% 7% 2 5% 3 3 3 2 4 7% 50%	3487
February	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 6% 50%	1900
	11th to 20th	Day	13% 7% 2 5% 4 3 4 3 5% 6% 48%	1849
	21st to end	Day	13% 7% 2 4 3 3 3 3 5% 5% 54%	1570
March	1st to 10th	Day	14% 8% 2 6% 4 4 3 2 4 6% 47%	3275
	11th to 20th	Day	17% 10% 3 7% 4 4 4 3 4 6% 37%	3480
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 6% 49%	3814
April	1st to 10th	Day	12% 7% 2 6% 4 4 4 3 6% 7% 47%	2090
	11th to 20th	Day	15% 6% 2 6% 4 4 3 3 4 6% 47%	2202
	21st to end	Day	12% 7% 3 6% 5% 3 3 3 4 5% 50%	2340
May	1st to 10th	Day	17% 8% 2 7% 5% 4 4 3 5% 6% 38%	4082
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 5% 5% 45%	4082
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	4477
June	1st to 10th	Day	5% 5% 2 5% 4 5% 5% 4 6% 9% 51%	2470
	11th to 20th	Day	5% 5% 2 5% 4 5% 4 4 6% 9% 50%	2470
	21st to end	Day	7% 5% 1 6% 6% 5% 6% 5% 6% 10% 43%	2470
July	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 47%	4120
	11th to 20th	Day	4 4 1 4 4 4 5% 4 7% 9% 55%	4120
	21st to end	Day	8% 5% 1 5% 5% 5% 5% 5% 8% 9% 44%	4465
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 8% 68%	3859
	11th to 20th	Day	4 2 1 3 3 4 3 3 6% 9% 61%	3726
	21st to end	Day	4 4 2 5% 5% 5% 5% 4 6% 10% 50%	4130
September	1st to 10th	Day	10% 6% 2 8% 5% 6% 5% 3 4 7% 44%	2090
	11th to 20th	Day	9% 7% 2 7% 4 5% 6% 4 6% 7% 41%	2090
	21st to end	Day	10% 4 2 6% 4 4 4 4 6% 7% 49%	2090
October	1st to 10th	Day	6% 4 2 4 5% 3 4 4 5% 7% 56%	2090
	11th to 20th	Day	12% 5% 2 5% 4 4 3 2 4 5% 55%	2042
	21st to end	Day	12% 5% 2 5% 3 3 3 2 3 5% 59%	2090
November	1st to 10th	Day	14% 7% 2 5% 4 3 4 3 5% 6% 49%	1913
	11th to 20th	Day	9% 6% 1 5% 4 3 4 2 5% 7% 54%	1800
	21st to end	Day	14% 7% 3 5% 3 3 3 2 4 6% 51%	1800
December	1st to 10th	Day	14% 8% 2 5% 3 3 2 3 5% 6% 50%	1800
	11th to 20th	Day	12% 7% 2 6% 3 4 4 3 4 6% 48%	2251
	21st to end	Day	12% 6% 2 4 3 3 3 2 3 4 58%	3162
January	1st to 10th	Night	13% 6% 3 6% 2 2 2 2 3 5% 56%	4108
	11th to 20th	Night	11% 5% 2 3 2 2 1 2 2 4 67%	4116
	21st to end	Night	16% 7% 2 6% 3 3 3 2 4 6% 47%	4180
February	1st to 10th	Night	13% 7% 2 5% 3 3 3 2 4 5% 53%	2280
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2218
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 6% 50%	1884
March	1st to 10th	Night	10% 9% 2 6% 3 4 4 4 5% 6% 47%	3584
	11th to 20th	Night	16% 12% 2 6% 4 4 4 4 4 5% 38%	3480
	21st to end	Night	11% 9% 2 6% 4 3 3 3 4 6% 50%	3817
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 6% 51%	2090
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 4 5% 48%	1924
	21st to end	Night	14% 7% 2 4 3 3 3 2 4 6% 51%	1838
May	1st to 10th	Night	21% 11% 2 5% 3 3 3 3 4 6% 39%	2823
	11th to 20th	Night	9% 7% 2 5% 4 3 3 3 5% 7% 50%	2823
	21st to end	Night	12% 7% 2 5% 3 3 4 4 5% 8% 48%	3090
June	1st to 10th	Night	3 3 1 3 2 2 4 4 7% 10% 60%	1710
	11th to 20th	Night	4 4 1 4 3 3 3 3 6% 8% 61%	1710
	21st to end	Night	7% 5% 1 4 3 4 4 4 5% 10% 54%	1710
July	1st to 10th	Night	4 4 2 4 4 4 4 4 6% 10% 55%	2850
	11th to 20th	Night	3 3 1 4 3 3 4 3 6% 9% 62%	2850
	21st to end	Night	8% 7% 2 6% 4 4 4 4 6% 9% 46%	3098
August	1st to 10th	Night	1 2 1 2 2 2 2 2 4 8% 73%	3076
	11th to 20th	Night	3 2 1 3 3 3 3 3 7% 10% 62%	3113
	21st to end	Night	4 5% 1 4 3 4 4 4 7% 10% 53%	3535
September	1st to 10th	Night	8% 6% 2 6% 3 3 4 3 6% 7% 52%	2090
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 3 5% 7% 48%	2090
	21st to end	Night	7% 6% 1 5% 4 4 4 3 5% 7% 54%	2090
October	1st to 10th	Night	5% 4 2 4 2 3 3 3 5% 7% 63%	2090
	11th to 20th	Night	12% 4 1 5% 3 3 3 2 5% 5% 58%	2118
	21st to end	Night	13% 5% 2 4 3 2 2 2 3 3 62%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 2 2 4 6% 55%	2380
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 4 6% 59%	2600
	21st to end	Night	15% 7% 2 4 3 3 3 3 4 5% 52%	2600
December	1st to 10th	Night	14% 6% 2 5% 4 3 3 2 4 6% 52%	2600
	11th to 20th	Night	13% 9% 3 5% 4 3 3 3 3 5% 49%	3166
	21st to end	Night	15% 6% 2 4 2 2 2 2 3 5% 57%	4578

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	31% 3 3 16% 25% 22%	2848
	11th to 20th	Day	31% 2 5% 14% 31% 16%	2882
	21st to end	Day	36% 3 5% 14% 29% 13%	3487
February	1st to 10th	Day	37% 3 8% 13% 22% 18%	1900
	11th to 20th	Day	38% 3 6% 18% 19% 16%	1859
	21st to end	Day	33% 3 7% 16% 23% 18%	1570
March	1st to 10th	Day	39% 4 6% 14% 21% 17%	3275
	11th to 20th	Day	50% 4 7% 12% 17% 10%	3480
	21st to end	Day	34% 4 8% 17% 19% 16%	3814
April	1st to 10th	Day	37% 4 8% 18% 19% 14%	2090
	11th to 20th	Day	39% 4 7% 17% 20% 12%	2202
	21st to end	Day	38% 2 7% 21% 20% 12%	2340
May	1st to 10th	Day	47% 4 10% 14% 17% 8%	4120
	11th to 20th	Day	40% 5% 7% 19% 21% 9%	4086
	21st to end	Day	38% 5% 12% 28% 12% 4	4489
June	1st to 10th	Day	30% 5% 12% 29% 21% 3	2470
	11th to 20th	Day	31% 5% 12% 27% 20% 6%	2470
	21st to end	Day	36% 7% 12% 23% 18% 5%	2470
July	1st to 10th	Day	32% 7% 17% 37% 8%	4056
	11th to 20th	Day	24% 6% 14% 35% 17% 3	4051
	21st to end	Day	32% 8% 17% 30% 13% 1	4325
August	1st to 10th	Day	14% 5% 11% 40% 30% 1	3845
	11th to 20th	Day	20% 4 22% 38% 13% 2	3762
	21st to end	Day	29% 8% 17% 26% 19% 2	4129
September	1st to 10th	Day	40% 5% 10% 20% 17% 8%	2090
	11th to 20th	Day	39% 6% 10% 20% 19% 6%	2090
	21st to end	Day	34% 4 10% 21% 19% 13%	2090
October	1st to 10th	Day	17% 2 6% 13% 26% 35%	3480
	11th to 20th	Day	27% 3 5% 19% 24% 22%	3373
	21st to end	Day	26% 3 4 17% 37% 13%	3428
November	1st to 10th	Day	38% 3 8% 20% 20% 12%	1913
	11th to 20th	Day	32% 3 7% 18% 22% 19%	1800
	21st to end	Day	38% 3 5% 17% 22% 15%	1800
December	1st to 10th	Day	37% 2 8% 20% 24% 10%	1800
	11th to 20th	Day	37% 4 8% 12% 23% 16%	2249
	21st to end	Day	32% 2 4 16% 28% 18%	3162
January	1st to 10th	Night	33% 2 3 13% 26% 22%	4108
	11th to 20th	Night	25% 1 3 18% 26% 26%	4116
	21st to end	Night	40% 3 4 12% 27% 14%	4180
February	1st to 10th	Night	36% 3 7% 12% 22% 20%	2280
	11th to 20th	Night	36% 3 5% 17% 21% 17%	2218
	21st to end	Night	37% 3 6% 13% 22% 19%	1884
March	1st to 10th	Night	38% 4 7% 14% 23% 14%	3584
	11th to 20th	Night	49% 4 8% 12% 16% 12%	3480
	21st to end	Night	37% 3 9% 17% 17% 16%	3817
April	1st to 10th	Night	34% 4 10% 18% 16% 17%	2090
	11th to 20th	Night	39% 4 8% 15% 18% 17%	1924
	21st to end	Night	38% 2 9% 20% 20% 11%	1838
May	1st to 10th	Night	48% 4 8% 18% 13% 10%	2850
	11th to 20th	Night	33% 5% 11% 23% 18% 10%	2824
	21st to end	Night	35% 5% 14% 29% 11% 6%	3099
June	1st to 10th	Night	19% 5% 19% 34% 20% 3	1710
	11th to 20th	Night	22% 4 16% 29% 23% 5%	1710
	21st to end	Night	27% 5% 17% 28% 19% 4	1710
July	1st to 10th	Night	24% 7% 23% 39% 6% 1	2805
	11th to 20th	Night	20% 7% 18% 36% 18% 2	2796
	21st to end	Night	32% 7% 21% 29% 10% 1	2999
August	1st to 10th	Night	12% 4 14% 46% 24% 1	3066
	11th to 20th	Night	17% 5% 28% 37% 11% 2	3143
	21st to end	Night	25% 10% 20% 25% 18% 2	3535
September	1st to 10th	Night	32% 5% 14% 23% 19% 9%	2090
	11th to 20th	Night	35% 6% 15% 22% 16% 6%	2090
	21st to end	Night	30% 4 12% 21% 19% 13%	2090
October	1st to 10th	Night	15% 2 8% 15% 23% 37%	3480
	11th to 20th	Night	24% 3 8% 16% 30% 19%	3513
	21st to end	Night	25% 3 4 20% 34% 15%	4120
November	1st to 10th	Night	33% 2 8% 20% 22% 15%	2380
	11th to 20th	Night	28% 3 7% 22% 21% 18%	2600
	21st to end	Night	37% 2 7% 17% 21% 16%	2600
December	1st to 10th	Night	35% 2 8% 19% 23% 13%	2600
	11th to 20th	Night	40% 2 5% 14% 19% 20%	3167
	21st to end	Night	33% 2 4 15% 29% 17%	4578

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% 16% 40% 19% 4 1	270
	11th to 20th	Day	24% 14% 34% 21% 6%	270
	21st to end	Day	19% 14% 40% 22% 5%	297
February	1st to 10th	Day	25% 14% 35% 20% 6%	270
	11th to 20th	Day	25% 16% 38% 17% 3 1	270
	21st to end	Day	21% 19% 41% 17% 2	225
March	1st to 10th	Day	20% 19% 43% 16% 2	270
	11th to 20th	Day	20% 21% 37% 17% 6%	270
	21st to end	Day	17% 19% 44% 16% 3 1	297
April	1st to 10th	Day	20% 20% 41% 17% 3	270
	11th to 20th	Day	19% 20% 39% 16% 5% 1	400
	21st to end	Day	17% 15% 44% 18% 5%	520
May	1st to 10th	Day	16% 11% 42% 26% 5%	653
	11th to 20th	Day	20% 7% 34% 28% 9% 1	675
	21st to end	Day	16% 7% 32% 34% 10%	734
June	1st to 10th	Day	6% 4 22% 44% 21% 4	680
	11th to 20th	Day	7% 3 22% 48% 18% 3	680
	21st to end	Day	7% 3 16% 42% 28% 5%	680
July	1st to 10th	Day	10% 4 8% 40% 31% 8%	671
	11th to 20th	Day	3 2 6% 44% 38% 7%	668
	21st to end	Day	4 2 5% 42% 37% 10%	739
August	1st to 10th	Day	2 4 41% 42% 11%	535
	11th to 20th	Day	9% 1 4 38% 38% 11%	477
	21st to end	Day	6% 2 5% 35% 39% 14%	515
September	1st to 10th	Day	9% 1 8% 34% 34% 14%	300
	11th to 20th	Day	7% 1 4 34% 43% 11%	300
	21st to end	Day	11% 1 5% 38% 37% 9%	300
October	1st to 10th	Day	7% 2 15% 39% 30% 7%	300
	11th to 20th	Day	11% 3 26% 40% 16% 3	300
	21st to end	Day	18% 12% 29% 27% 12% 2	330
November	1st to 10th	Day	22% 5% 33% 30% 8% 3	300
	11th to 20th	Day	21% 9% 32% 22% 10% 5%	300
	21st to end	Day	23% 20% 30% 19% 7% 2	300
December	1st to 10th	Day	19% 10% 34% 28% 8% 2	300
	11th to 20th	Day	18% 15% 38% 20% 8% 1	300
	21st to end	Day	18% 13% 36% 25% 8%	306
January	1st to 10th	Night	16% 11% 35% 30% 8%	950
	11th to 20th	Night	17% 11% 36% 30% 5%	946
	21st to end	Night	18% 11% 34% 31% 6%	836
February	1st to 10th	Night	20% 13% 33% 28% 6%	760
	11th to 20th	Night	19% 14% 37% 26% 4	744
	21st to end	Night	18% 14% 48% 19% 1	628
March	1st to 10th	Night	18% 13% 39% 26% 3	675
	11th to 20th	Night	18% 11% 38% 29% 4	570
	21st to end	Night	14% 14% 48% 21% 3	627
April	1st to 10th	Night	10% 15% 49% 24% 3	570
	11th to 20th	Night	16% 17% 44% 21% 3	563
	21st to end	Night	17% 11% 45% 23% 4 1	570
May	1st to 10th	Night	15% 12% 45% 24% 4	570
	11th to 20th	Night	14% 6% 39% 33% 7%	561
	21st to end	Night	13% 6% 39% 32% 9% 1	617
June	1st to 10th	Night	4 2 28% 48% 16% 1	570
	11th to 20th	Night	5% 3 37% 46% 9%	570
	21st to end	Night	7% 3 19% 49% 20% 2	570
July	1st to 10th	Night	8% 3 12% 48% 27% 2	555
	11th to 20th	Night	3 1 11% 55% 27% 2	552
	21st to end	Night	5% 1 8% 56% 26% 3	624
August	1st to 10th	Night	2 1 8% 54% 32% 2	567
	11th to 20th	Night	5% 1 8% 51% 32% 3	564
	21st to end	Night	5% 3 7% 44% 37% 4	627
September	1st to 10th	Night	10% 3 10% 41% 33% 3	570
	11th to 20th	Night	12% 2 6% 42% 36% 2	570
	21st to end	Night	10% 2 11% 44% 31% 2	570
October	1st to 10th	Night	6% 2 15% 47% 28% 2	570
	11th to 20th	Night	13% 4 25% 41% 16% 2	598
	21st to end	Night	16% 8% 27% 29% 18% 2	836
November	1st to 10th	Night	15% 6% 26% 38% 14% 1	820
	11th to 20th	Night	12% 7% 31% 35% 13% 2	1000
	21st to end	Night	19% 10% 30% 30% 11% 1	1000
December	1st to 10th	Night	17% 8% 30% 36% 9% 1	1000
	11th to 20th	Night	20% 10% 32% 31% 7%	1000
	21st to end	Night	16% 11% 32% 34% 7%	1063

Page generated at 2024-12-21 03:13:52. Version ac07

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