Union Street Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union Street Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	17% 11% 4 8% 5% 4 4 4 5% 7% 31%	126976
	Night	18% 12% 4 9% 5% 4 4 3 5% 6% 31%	124367
Transparency	Day	51% 6% 10% 20% 11% 1	130580
	Night	53% 5% 10% 20% 11% 2	128832
Seeing	Day	24% 14% 25% 20% 12% 5%	16187
	Night	23% 11% 24% 28% 12% 1	25938

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	18% 19% 7% 21% 5% 3 3 2 3 4 15%	4036
	11th to 20th	Day	14% 24% 10% 16% 9% 6% 2 2 2 2 14%	4060
	21st to end	Day	21% 21% 5% 11% 6% 4 3 3 4 4 18%	4466
February	1st to 10th	Day	21% 17% 6% 10% 7% 4 3 3 3 5% 21%	1800
	11th to 20th	Day	19% 18% 6% 11% 5% 3 3 4 4 5% 22%	1753
	21st to end	Day	22% 15% 5% 11% 4 3 4 3 4 5% 24%	1659
March	1st to 10th	Day	16% 15% 5% 9% 4 3 4 3 4 6% 32%	4935
	11th to 20th	Day	14% 10% 4 8% 5% 5% 4 3 5% 6% 36%	4980
	21st to end	Day	19% 11% 3 9% 6% 5% 4 3 4 5% 30%	5465
April	1st to 10th	Day	18% 13% 4 7% 5% 5% 4 3 5% 6% 29%	2320
	11th to 20th	Day	17% 12% 3 8% 4 4 4 3 5% 7% 34%	2374
	21st to end	Day	21% 9% 3 7% 5% 4 4 3 5% 7% 33%	2598
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 7% 27%	5862
	11th to 20th	Day	20% 7% 3 8% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	21% 7% 2 7% 5% 5% 4 4 6% 8% 31%	6839
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 6% 8% 34%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2800
	21st to end	Day	15% 5% 2 6% 6% 5% 5% 4 6% 9% 38%	2800
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 6% 8% 41%	6189
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 7% 11% 33%	5900
	21st to end	Day	13% 6% 3 7% 5% 5% 4 5% 6% 8% 38%	6424
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 4 7% 9% 30%	4234
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 8% 37%	4174
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	4414
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 4 7% 46%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	2232
	21st to end	Day	22% 8% 4 7% 4 4 3 3 5% 6% 35%	2181
October	1st to 10th	Day	18% 5% 2 6% 4 4 4 3 6% 6% 42%	2000
	11th to 20th	Day	20% 8% 3 6% 5% 4 5% 3 4 6% 37%	2000
	21st to end	Day	24% 10% 2 6% 3 3 3 2 4 6% 35%	2200
November	1st to 10th	Day	12% 11% 3 7% 5% 4 4 3 4 7% 42%	2010
	11th to 20th	Day	17% 15% 3 8% 4 5% 4 3 5% 5% 31%	1732
	21st to end	Day	20% 16% 6% 8% 4 3 4 3 4 5% 26%	1680
December	1st to 10th	Day	18% 15% 8% 16% 5% 5% 4 3 3 4 18%	2680
	11th to 20th	Day	24% 19% 6% 9% 4 3 3 3 3 5% 22%	3070
	21st to end	Day	22% 19% 5% 11% 5% 5% 4 3 4 5% 17%	4048
January	1st to 10th	Night	17% 19% 7% 22% 5% 3 3 2 3 3 15%	5926
	11th to 20th	Night	16% 21% 8% 14% 8% 5% 3 2 3 4 16%	5900
	21st to end	Night	23% 16% 6% 12% 6% 4 4 2 3 3 22%	6490
February	1st to 10th	Night	21% 16% 5% 10% 5% 4 3 3 4 5% 25%	2600
	11th to 20th	Night	20% 15% 5% 11% 6% 5% 4 3 4 5% 24%	2533
	21st to end	Night	20% 14% 6% 11% 5% 4 4 3 4 5% 25%	1967
March	1st to 10th	Night	22% 12% 4 8% 4 3 3 2 4 5% 33%	4918
	11th to 20th	Night	15% 11% 4 6% 4 3 3 2 4 6% 42%	4980
	21st to end	Night	17% 11% 3 8% 4 5% 4 4 4 5% 34%	5467
April	1st to 10th	Night	19% 15% 3 7% 4 4 3 3 4 5% 33%	2079
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 8% 36%	1975
	21st to end	Night	16% 10% 3 7% 4 3 4 4 6% 6% 38%	1818
May	1st to 10th	Night	27% 10% 4 7% 4 4 4 4 5% 6% 24%	4033
	11th to 20th	Night	19% 8% 3 6% 5% 4 4 3 5% 8% 34%	4032
	21st to end	Night	23% 8% 2 7% 4 4 4 4 6% 8% 32%	4010
June	1st to 10th	Night	13% 6% 2 7% 4 4 4 4 5% 7% 43%	1600
	11th to 20th	Night	12% 6% 3 6% 5% 5% 4 4 7% 8% 39%	1600
	21st to end	Night	12% 6% 2 5% 4 4 5% 5% 7% 11% 41%	1600
July	1st to 10th	Night	13% 7% 3 7% 4 4 3 3 6% 8% 40%	3751
	11th to 20th	Night	15% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	4057
	21st to end	Night	12% 6% 2 7% 5% 4 5% 4 6% 8% 40%	4427
August	1st to 10th	Night	15% 8% 3 7% 4 5% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	16% 6% 2 7% 4 4 4 4 5% 6% 42%	2875
	21st to end	Night	10% 5% 2 6% 4 5% 5% 4 7% 9% 44%	3449
September	1st to 10th	Night	13% 5% 2 5% 4 3 5% 4 5% 6% 47%	2000
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 6% 7% 52%	2164
	21st to end	Night	19% 7% 2 7% 4 4 4 3 5% 6% 39%	2217
October	1st to 10th	Night	16% 6% 2 6% 3 4 4 3 5% 6% 45%	2400
	11th to 20th	Night	18% 6% 2 6% 5% 4 4 3 5% 7% 39%	2400
	21st to end	Night	25% 8% 2 6% 3 3 3 2 4 6% 38%	2640
November	1st to 10th	Night	14% 8% 2 6% 4 3 4 3 4 6% 45%	2505
	11th to 20th	Night	19% 14% 3 6% 4 4 4 3 4 5% 32%	2884
	21st to end	Night	22% 15% 4 6% 4 4 4 3 4 5% 29%	2940
December	1st to 10th	Night	20% 16% 11% 12% 4 3 3 3 4 5% 19%	4720
	11th to 20th	Night	23% 17% 7% 9% 4 3 3 3 3 5% 23%	5345
	21st to end	Night	23% 18% 5% 11% 5% 4 4 3 4 5% 18%	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	75% 4 8% 9% 3	4037
	11th to 20th	Day	79% 3 5% 8% 3 2	4060
	21st to end	Day	71% 4 7% 12% 6%	4466
February	1st to 10th	Day	67% 5% 8% 14% 6%	1800
	11th to 20th	Day	64% 5% 10% 15% 6%	1762
	21st to end	Day	62% 5% 10% 14% 7% 2	1659
March	1st to 10th	Day	54% 5% 9% 24% 8% 1	4935
	11th to 20th	Day	48% 5% 11% 25% 9% 2	4980
	21st to end	Day	55% 5% 7% 19% 12% 1	5465
April	1st to 10th	Day	55% 5% 10% 18% 11% 2	2320
	11th to 20th	Day	49% 6% 12% 17% 15% 2	2374
	21st to end	Day	50% 5% 10% 23% 10% 2	2598
May	1st to 10th	Day	55% 7% 10% 18% 8% 2	5897
	11th to 20th	Day	51% 8% 9% 22% 9% 1	5865
	21st to end	Day	48% 8% 13% 20% 10% 2	6854
June	1st to 10th	Day	44% 8% 13% 22% 11% 1	2800
	11th to 20th	Day	44% 8% 14% 21% 12% 1	2800
	21st to end	Day	42% 8% 12% 24% 13% 1	2800
July	1st to 10th	Day	41% 6% 9% 26% 16% 2	6122
	11th to 20th	Day	44% 7% 14% 23% 11%	5831
	21st to end	Day	42% 7% 9% 23% 19% 1	6240
August	1st to 10th	Day	48% 8% 14% 21% 8%	4221
	11th to 20th	Day	42% 7% 11% 27% 13%	4213
	21st to end	Day	35% 8% 12% 29% 14% 2	4414
September	1st to 10th	Day	38% 6% 10% 23% 20% 3	2400
	11th to 20th	Day	37% 6% 12% 28% 16% 1	2232
	21st to end	Day	50% 4 11% 21% 14% 1	2181
October	1st to 10th	Day	35% 4 8% 32% 20% 1	3260
	11th to 20th	Day	44% 5% 10% 22% 13% 5%	3240
	21st to end	Day	37% 4 10% 28% 20% 1	3538
November	1st to 10th	Day	43% 5% 11% 21% 17% 3	2010
	11th to 20th	Day	55% 5% 8% 19% 12% 2	1732
	21st to end	Day	61% 4 11% 18% 6% 1	1680
December	1st to 10th	Day	71% 4 13% 9% 4	2680
	11th to 20th	Day	66% 4 12% 15% 4	3066
	21st to end	Day	70% 5% 8% 13% 4	4048
January	1st to 10th	Night	75% 4 7% 11% 4	5926
	11th to 20th	Night	74% 4 7% 10% 4 1	5900
	21st to end	Night	69% 4 7% 13% 6% 1	6490
February	1st to 10th	Night	63% 4 9% 18% 6%	2600
	11th to 20th	Night	63% 4 10% 16% 7%	2535
	21st to end	Night	62% 5% 8% 16% 8% 1	1967
March	1st to 10th	Night	55% 4 8% 23% 9% 1	4918
	11th to 20th	Night	44% 4 12% 29% 8% 3	4980
	21st to end	Night	50% 5% 9% 20% 15% 2	5467
April	1st to 10th	Night	53% 5% 10% 18% 13% 2	2079
	11th to 20th	Night	46% 6% 11% 19% 15% 3	1975
	21st to end	Night	45% 6% 8% 25% 11% 3	1818
May	1st to 10th	Night	59% 7% 9% 13% 9% 3	4060
	11th to 20th	Night	48% 7% 9% 25% 10% 1	4033
	21st to end	Night	49% 7% 12% 22% 8% 3	4017
June	1st to 10th	Night	39% 8% 11% 26% 15% 2	1600
	11th to 20th	Night	40% 8% 13% 25% 12% 1	1600
	21st to end	Night	37% 8% 14% 24% 16% 2	1600
July	1st to 10th	Night	42% 6% 9% 23% 17% 3	3710
	11th to 20th	Night	44% 8% 16% 20% 11% 1	4006
	21st to end	Night	41% 7% 10% 19% 22% 2	4297
August	1st to 10th	Night	46% 8% 15% 22% 9%	2904
	11th to 20th	Night	41% 6% 10% 26% 16% 1	2902
	21st to end	Night	35% 7% 13% 31% 13% 2	3449
September	1st to 10th	Night	35% 6% 10% 24% 21% 4	2000
	11th to 20th	Night	31% 6% 11% 28% 23% 1	2164
	21st to end	Night	45% 6% 10% 21% 17% 1	2217
October	1st to 10th	Night	36% 4 8% 27% 23% 2	3920
	11th to 20th	Night	38% 4 10% 25% 15% 8%	3899
	21st to end	Night	37% 3 8% 28% 23% 2	4252
November	1st to 10th	Night	41% 4 10% 22% 19% 4	2505
	11th to 20th	Night	54% 4 8% 19% 12% 2	2884
	21st to end	Night	57% 4 9% 21% 7% 1	2940
December	1st to 10th	Night	69% 3 11% 14% 3	4720
	11th to 20th	Night	66% 3 14% 14% 3	5348
	21st to end	Night	69% 5% 9% 14% 3	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	34% 28% 23% 10% 5% 1	295
	11th to 20th	Day	41% 22% 29% 6% 3	300
	21st to end	Day	33% 28% 28% 8% 2 1	330
February	1st to 10th	Day	34% 24% 32% 9% 1	300
	11th to 20th	Day	40% 28% 26% 6% 1	300
	21st to end	Day	35% 26% 30% 9%	304
March	1st to 10th	Day	29% 22% 27% 17% 4	500
	11th to 20th	Day	34% 21% 27% 12% 5% 1	500
	21st to end	Day	31% 19% 29% 16% 4 1	550
April	1st to 10th	Day	32% 19% 26% 15% 7% 1	500
	11th to 20th	Day	26% 21% 27% 15% 8% 4	492
	21st to end	Day	29% 15% 30% 16% 8% 2	500
May	1st to 10th	Day	28% 15% 29% 17% 9% 2	503
	11th to 20th	Day	24% 13% 27% 21% 9% 5%	527
	21st to end	Day	19% 11% 25% 23% 15% 7%	573
June	1st to 10th	Day	16% 12% 27% 26% 15% 4	530
	11th to 20th	Day	17% 11% 25% 26% 17% 4	530
	21st to end	Day	14% 10% 27% 27% 18% 4	530
July	1st to 10th	Day	15% 6% 16% 29% 20% 13%	525
	11th to 20th	Day	15% 6% 23% 30% 18% 9%	524
	21st to end	Day	16% 5% 22% 31% 20% 7%	576
August	1st to 10th	Day	15% 6% 20% 30% 21% 9%	526
	11th to 20th	Day	17% 5% 15% 30% 22% 11%	519
	21st to end	Day	14% 4 22% 27% 20% 13%	583
September	1st to 10th	Day	18% 7% 17% 27% 19% 11%	530
	11th to 20th	Day	13% 3 15% 30% 27% 12%	530
	21st to end	Day	28% 9% 22% 22% 15% 5%	530
October	1st to 10th	Day	18% 9% 24% 23% 19% 7%	530
	11th to 20th	Day	23% 14% 29% 21% 10% 2	530
	21st to end	Day	30% 16% 28% 16% 9% 1	583
November	1st to 10th	Day	21% 14% 31% 22% 9% 3	488
	11th to 20th	Day	34% 17% 30% 14% 4 1	247
	21st to end	Day	37% 16% 33% 11% 3	220
December	1st to 10th	Day	31% 31% 25% 10% 2 1	220
	11th to 20th	Day	33% 25% 30% 10% 2	220
	21st to end	Day	40% 19% 25% 12% 4	242
January	1st to 10th	Night	34% 19% 27% 16% 3	805
	11th to 20th	Night	33% 17% 31% 17% 3	800
	21st to end	Night	32% 20% 27% 20% 2	880
February	1st to 10th	Night	32% 17% 26% 23% 2	800
	11th to 20th	Night	32% 20% 31% 16%	783
	21st to end	Night	29% 18% 32% 19% 2	660
March	1st to 10th	Night	26% 18% 29% 23% 4	800
	11th to 20th	Night	28% 16% 28% 21% 6% 1	800
	21st to end	Night	26% 16% 31% 22% 5% 1	880
April	1st to 10th	Night	29% 15% 30% 20% 6%	800
	11th to 20th	Night	23% 13% 30% 24% 10% 1	791
	21st to end	Night	19% 11% 31% 31% 7%	618
May	1st to 10th	Night	25% 8% 22% 34% 10% 1	600
	11th to 20th	Night	23% 9% 24% 32% 11%	591
	21st to end	Night	15% 2 23% 32% 25% 2	472
June	1st to 10th	Night	13% 5% 21% 41% 19% 1	400
	11th to 20th	Night	13% 5% 22% 43% 18% 1	400
	21st to end	Night	13% 2 26% 38% 21% 1	400
July	1st to 10th	Night	10% 2 17% 43% 23% 4	449
	11th to 20th	Night	12% 4 16% 35% 30% 4	582
	21st to end	Night	14% 4 12% 39% 29% 2	657
August	1st to 10th	Night	11% 3 13% 43% 28% 2	597
	11th to 20th	Night	13% 4 11% 38% 30% 4	594
	21st to end	Night	12% 4 15% 36% 29% 4	795
September	1st to 10th	Night	15% 4 16% 35% 26% 4	800
	11th to 20th	Night	13% 3 13% 39% 28% 5%	800
	21st to end	Night	22% 6% 20% 29% 20% 3	800
October	1st to 10th	Night	18% 7% 20% 31% 22% 3	800
	11th to 20th	Night	22% 8% 28% 31% 11% 1	800
	21st to end	Night	28% 11% 23% 30% 7% 1	880
November	1st to 10th	Night	20% 9% 27% 33% 10% 1	820
	11th to 20th	Night	29% 15% 25% 24% 7% 1	840
	21st to end	Night	31% 14% 27% 23% 6%	840
December	1st to 10th	Night	32% 16% 27% 20% 5%	840
	11th to 20th	Night	30% 19% 28% 20% 3	840
	21st to end	Night	32% 17% 28% 22% 2	924

Page generated at 2025-12-30 01:14:04. Version ac07

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