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 2024-01-13.

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	18% 10% 4 7% 6% 3 4 5% 5% 7% 31%	74612
	Night	18% 10% 4 7% 5% 3 4 4 5% 6% 33%	75130
Transparency	Day	52% 6% 11% 19% 10% 1	74331
	Night	51% 6% 11% 20% 11% 2	74930
Seeing	Day	24% 13% 25% 21% 12% 5%	13617
	Night	23% 11% 24% 28% 12% 1	23403

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	20% 16% 8% 11% 7% 3 4 4 3 5% 18%	1691
	11th to 20th	Day	19% 21% 6% 13% 7% 4 3 3 3 4 17%	1644
	21st to end	Day	20% 19% 7% 10% 7% 4 3 3 3 4 19%	1782
February	1st to 10th	Day	22% 16% 7% 9% 8% 4 3 4 3 5% 19%	1620
	11th to 20th	Day	19% 17% 8% 10% 5% 3 4 5% 4 5% 21%	1573
	21st to end	Day	23% 14% 5% 10% 5% 3 3 4 3 5% 24%	1488
March	1st to 10th	Day	18% 11% 6% 8% 5% 3 4 4 4 7% 31%	1980
	11th to 20th	Day	20% 13% 4 8% 5% 3 5% 4 4 5% 28%	1980
	21st to end	Day	22% 11% 4 8% 6% 3 3 4 4 6% 28%	2178
April	1st to 10th	Day	16% 12% 5% 6% 6% 4 4 4 5% 7% 31%	2088
	11th to 20th	Day	18% 10% 3 7% 5% 3 4 4 5% 7% 34%	2134
	21st to end	Day	22% 8% 3 6% 6% 3 5% 4 5% 6% 33%	2338
May	1st to 10th	Day	21% 8% 3 6% 6% 3 5% 5% 6% 7% 29%	2340
	11th to 20th	Day	23% 7% 3 7% 6% 3 5% 4 5% 6% 30%	2301
	21st to end	Day	13% 5% 2 5% 6% 4 6% 5% 7% 10% 37%	2686
June	1st to 10th	Day	15% 7% 3 6% 5% 3 6% 6% 6% 9% 34%	2520
	11th to 20th	Day	15% 6% 2 6% 6% 4 6% 5% 7% 8% 35%	2520
	21st to end	Day	16% 5% 3 6% 6% 4 5% 5% 6% 9% 37%	2520
July	1st to 10th	Day	13% 6% 2 5% 6% 5% 5% 6% 7% 8% 38%	2461
	11th to 20th	Day	14% 6% 3 6% 4 3 4 6% 6% 11% 37%	2340
	21st to end	Day	14% 6% 3 5% 7% 4 5% 7% 7% 8% 35%	2547
August	1st to 10th	Day	13% 6% 3 5% 6% 5% 6% 7% 7% 10% 33%	2327
	11th to 20th	Day	14% 6% 3 5% 6% 4 5% 6% 7% 10% 36%	2303
	21st to end	Day	14% 6% 2 5% 6% 3 5% 6% 6% 9% 38%	2433
September	1st to 10th	Day	16% 6% 2 5% 5% 3 4 4 5% 7% 43%	2160
	11th to 20th	Day	14% 5% 2 5% 4 3 5% 4 5% 8% 45%	2010
	21st to end	Day	20% 7% 4 6% 5% 3 4 3 5% 7% 38%	1963
October	1st to 10th	Day	19% 5% 3 6% 6% 3 5% 3 5% 6% 39%	1800
	11th to 20th	Day	21% 8% 3 5% 5% 3 5% 4 4 6% 35%	1800
	21st to end	Day	26% 10% 2 6% 4 2 4 3 4 6% 33%	1980
November	1st to 10th	Day	12% 11% 4 6% 6% 3 3 4 4 6% 42%	1815
	11th to 20th	Day	17% 13% 4 7% 5% 4 3 4 5% 6% 32%	1568
	21st to end	Day	20% 15% 6% 7% 4 3 4 4 5% 5% 28%	1520
December	1st to 10th	Day	23% 18% 7% 10% 6% 4 4 4 3 5% 17%	1520
	11th to 20th	Day	21% 19% 8% 9% 5% 2 3 4 3 7% 22%	1910
	21st to end	Day	25% 21% 7% 9% 6% 3 3 3 4 5% 16%	2772
January	1st to 10th	Night	21% 18% 7% 8% 6% 4 4 5% 4 5% 20%	2469
	11th to 20th	Night	22% 18% 6% 10% 8% 3 3 3 4 4 18%	2378
	21st to end	Night	20% 16% 6% 10% 7% 3 4 3 3 4 23%	2574
February	1st to 10th	Night	22% 15% 6% 9% 6% 3 4 4 4 5% 24%	2340
	11th to 20th	Night	21% 14% 6% 10% 7% 4 4 4 4 5% 23%	2273
	21st to end	Night	22% 14% 6% 9% 6% 4 3 4 4 5% 24%	1765
March	1st to 10th	Night	18% 11% 4 8% 6% 3 3 4 5% 6% 33%	1980
	11th to 20th	Night	21% 10% 4 6% 6% 2 4 2 3 7% 34%	1980
	21st to end	Night	20% 11% 4 6% 5% 3 4 4 3 5% 33%	2178
April	1st to 10th	Night	19% 12% 3 7% 4 3 4 4 4 5% 35%	1872
	11th to 20th	Night	17% 7% 3 5% 5% 3 4 4 5% 8% 38%	1775
	21st to end	Night	16% 9% 3 7% 5% 3 4 4 6% 7% 37%	1637
May	1st to 10th	Night	19% 9% 3 6% 5% 3 5% 5% 6% 6% 33%	1620
	11th to 20th	Night	19% 8% 4 6% 5% 4 4 5% 4 8% 34%	1592
	21st to end	Night	11% 4 2 4 4 4 4 5% 6% 10% 44%	1584
June	1st to 10th	Night	13% 5% 3 5% 5% 3 5% 5% 5% 7% 44%	1440
	11th to 20th	Night	13% 5% 3 6% 5% 3 5% 5% 7% 9% 39%	1440
	21st to end	Night	13% 5% 2 4 4 2 5% 5% 7% 10% 42%	1440
July	1st to 10th	Night	10% 6% 3 6% 6% 3 3 4 7% 9% 43%	1495
	11th to 20th	Night	11% 5% 2 4 4 2 6% 5% 6% 9% 47%	1620
	21st to end	Night	12% 4 3 5% 6% 3 5% 6% 8% 10% 37%	1771
August	1st to 10th	Night	9% 5% 2 5% 6% 3 6% 6% 7% 9% 41%	1611
	11th to 20th	Night	11% 6% 3 6% 5% 3 5% 6% 5% 7% 44%	1598
	21st to end	Night	12% 5% 3 5% 4 3 4 5% 6% 8% 46%	1903
September	1st to 10th	Night	13% 5% 2 4 5% 3 4 5% 5% 7% 46%	1800
	11th to 20th	Night	11% 4 2 3 4 2 4 3 6% 8% 52%	1947
	21st to end	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 42%	1995
October	1st to 10th	Night	17% 6% 3 5% 4 3 5% 4 5% 7% 43%	2160
	11th to 20th	Night	19% 6% 2 6% 6% 2 4 5% 5% 7% 37%	2160
	21st to end	Night	27% 7% 2 5% 4 2 3 3 5% 6% 36%	2376
November	1st to 10th	Night	14% 8% 3 6% 4 3 4 4 4 6% 45%	2260
	11th to 20th	Night	19% 13% 4 6% 5% 3 4 4 4 5% 33%	2608
	21st to end	Night	21% 14% 4 5% 5% 3 4 4 4 5% 30%	2660
December	1st to 10th	Night	24% 16% 8% 8% 5% 3 4 3 4 5% 21%	2660
	11th to 20th	Night	21% 16% 6% 9% 6% 3 3 3 3 5% 24%	3285
	21st to end	Night	26% 19% 6% 8% 7% 3 4 4 4 5% 16%	4884

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	70% 5% 9% 12% 4	1692
	11th to 20th	Day	73% 4 10% 12% 2	1644
	21st to end	Day	71% 3 9% 12% 4	1782
February	1st to 10th	Day	70% 4 9% 12% 5%	1620
	11th to 20th	Day	65% 5% 9% 15% 6%	1582
	21st to end	Day	63% 5% 10% 13% 7% 2	1488
March	1st to 10th	Day	53% 6% 12% 21% 8% 1	1980
	11th to 20th	Day	58% 6% 9% 16% 9% 1	1980
	21st to end	Day	58% 5% 9% 17% 11% 1	2178
April	1st to 10th	Day	52% 5% 11% 19% 11% 2	2088
	11th to 20th	Day	50% 5% 12% 15% 16% 2	2134
	21st to end	Day	52% 5% 10% 21% 10% 2	2338
May	1st to 10th	Day	52% 7% 11% 19% 9% 2	2340
	11th to 20th	Day	54% 7% 10% 16% 11% 2	2305
	21st to end	Day	41% 10% 17% 22% 9% 2	2686
June	1st to 10th	Day	45% 8% 13% 22% 11% 1	2520
	11th to 20th	Day	44% 8% 13% 22% 12% 1	2520
	21st to end	Day	43% 7% 12% 25% 12% 1	2520
July	1st to 10th	Day	42% 9% 12% 21% 14% 3	2394
	11th to 20th	Day	41% 8% 14% 23% 14% 1	2271
	21st to end	Day	44% 8% 12% 22% 13% 1	2405
August	1st to 10th	Day	43% 9% 15% 24% 8%	2314
	11th to 20th	Day	41% 9% 15% 23% 11% 1	2303
	21st to end	Day	40% 9% 12% 24% 13% 3	2433
September	1st to 10th	Day	41% 7% 11% 21% 18% 3	2160
	11th to 20th	Day	38% 6% 12% 29% 15% 1	2010
	21st to end	Day	47% 4 12% 23% 14% 1	1963
October	1st to 10th	Day	45% 6% 12% 22% 15% 1	1800
	11th to 20th	Day	50% 5% 11% 25% 10%	1800
	21st to end	Day	53% 4 10% 19% 12% 1	1980
November	1st to 10th	Day	44% 5% 11% 21% 16% 3	1815
	11th to 20th	Day	53% 5% 9% 19% 12% 2	1568
	21st to end	Day	59% 5% 11% 18% 7% 1	1520
December	1st to 10th	Day	70% 4 9% 11% 5%	1520
	11th to 20th	Day	65% 4 11% 16% 3	1906
	21st to end	Day	72% 4 9% 11% 3	2772
January	1st to 10th	Night	67% 5% 8% 14% 6% 1	2469
	11th to 20th	Night	71% 4 11% 11% 3	2378
	21st to end	Night	67% 4 8% 15% 5% 1	2574
February	1st to 10th	Night	64% 4 9% 17% 6%	2340
	11th to 20th	Night	64% 4 10% 16% 7%	2275
	21st to end	Night	63% 5% 8% 16% 8% 1	1765
March	1st to 10th	Night	52% 4 11% 21% 11% 1	1980
	11th to 20th	Night	52% 5% 11% 21% 10% 1	1980
	21st to end	Night	53% 5% 7% 19% 13% 3	2178
April	1st to 10th	Night	52% 5% 10% 18% 13% 2	1872
	11th to 20th	Night	45% 6% 11% 19% 16% 4	1775
	21st to end	Night	46% 6% 9% 24% 11% 4	1637
May	1st to 10th	Night	49% 7% 10% 18% 12% 3	1620
	11th to 20th	Night	50% 7% 11% 17% 13% 3	1593
	21st to end	Night	34% 7% 18% 27% 12% 1	1585
June	1st to 10th	Night	38% 7% 12% 27% 15% 2	1440
	11th to 20th	Night	40% 8% 13% 25% 12% 2	1440
	21st to end	Night	36% 9% 14% 24% 16% 2	1440
July	1st to 10th	Night	39% 7% 13% 22% 15% 4	1452
	11th to 20th	Night	32% 9% 15% 25% 19% 1	1566
	21st to end	Night	38% 9% 15% 21% 13% 3	1672
August	1st to 10th	Night	35% 9% 16% 25% 15%	1600
	11th to 20th	Night	37% 8% 14% 23% 15% 1	1598
	21st to end	Night	35% 8% 13% 27% 14% 4	1903
September	1st to 10th	Night	36% 7% 11% 24% 19% 4	1800
	11th to 20th	Night	31% 6% 12% 28% 22% 1	1947
	21st to end	Night	42% 5% 11% 23% 18% 1	1995
October	1st to 10th	Night	41% 6% 11% 25% 14% 2	2160
	11th to 20th	Night	46% 6% 10% 24% 14% 1	2160
	21st to end	Night	51% 4 10% 21% 12% 2	2376
November	1st to 10th	Night	42% 5% 10% 22% 18% 4	2260
	11th to 20th	Night	53% 4 9% 19% 12% 2	2608
	21st to end	Night	56% 5% 9% 22% 8% 1	2660
December	1st to 10th	Night	67% 4 10% 14% 5%	2660
	11th to 20th	Night	65% 4 10% 17% 4	3288
	21st to end	Night	71% 6% 9% 12% 2	4884

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	32% 26% 25% 11% 5% 1	263
	11th to 20th	Day	38% 21% 31% 7% 3	248
	21st to end	Day	33% 27% 29% 7% 3 2	264
February	1st to 10th	Day	37% 21% 35% 7%	240
	11th to 20th	Day	39% 28% 27% 5%	240
	21st to end	Day	36% 24% 30% 9%	247
March	1st to 10th	Day	29% 23% 27% 18% 3	420
	11th to 20th	Day	35% 19% 28% 12% 5% 1	420
	21st to end	Day	31% 17% 27% 18% 5% 1	462
April	1st to 10th	Day	29% 20% 26% 16% 9% 1	420
	11th to 20th	Day	25% 19% 26% 16% 9% 5%	412
	21st to end	Day	30% 15% 29% 15% 8% 2	420
May	1st to 10th	Day	30% 13% 28% 16% 9% 3	423
	11th to 20th	Day	26% 11% 28% 22% 9% 4	447
	21st to end	Day	16% 10% 24% 24% 16% 8%	485
June	1st to 10th	Day	16% 10% 26% 27% 17% 4	450
	11th to 20th	Day	18% 12% 26% 23% 18% 4	450
	21st to end	Day	16% 9% 27% 29% 16% 3	450
July	1st to 10th	Day	16% 5% 15% 30% 21% 14%	445
	11th to 20th	Day	15% 4 22% 32% 18% 9%	444
	21st to end	Day	17% 4 23% 30% 19% 8%	488
August	1st to 10th	Day	15% 6% 17% 30% 22% 10%	446
	11th to 20th	Day	18% 5% 14% 29% 22% 12%	439
	21st to end	Day	16% 4 22% 27% 19% 14%	495
September	1st to 10th	Day	20% 6% 17% 27% 20% 11%	450
	11th to 20th	Day	14% 4 16% 30% 24% 11%	450
	21st to end	Day	25% 11% 21% 22% 16% 6%	450
October	1st to 10th	Day	19% 9% 24% 23% 19% 6%	450
	11th to 20th	Day	24% 15% 28% 22% 10% 2	450
	21st to end	Day	31% 15% 28% 16% 9%	495
November	1st to 10th	Day	22% 12% 30% 23% 11% 4	413
	11th to 20th	Day	32% 19% 28% 16% 5% 1	203
	21st to end	Day	31% 17% 37% 12% 3 1	180
December	1st to 10th	Day	31% 26% 28% 12% 3 1	180
	11th to 20th	Day	34% 24% 29% 11% 2	180
	21st to end	Day	38% 22% 23% 13% 4 1	198
January	1st to 10th	Night	34% 19% 27% 17% 4	760
	11th to 20th	Night	34% 15% 31% 17% 3	732
	21st to end	Night	32% 20% 26% 20% 2	792
February	1st to 10th	Night	32% 17% 27% 23% 2	720
	11th to 20th	Night	31% 20% 32% 16%	703
	21st to end	Night	30% 19% 31% 18% 2	592
March	1st to 10th	Night	25% 18% 29% 25% 4	720
	11th to 20th	Night	28% 15% 29% 21% 7% 1	720
	21st to end	Night	26% 15% 30% 23% 5% 1	792
April	1st to 10th	Night	27% 14% 30% 21% 7%	720
	11th to 20th	Night	21% 12% 31% 24% 11% 1	711
	21st to end	Night	20% 11% 32% 31% 7%	557
May	1st to 10th	Night	24% 9% 21% 35% 10% 1	540
	11th to 20th	Night	23% 9% 24% 33% 11%	531
	21st to end	Night	14% 2 23% 33% 26% 2	424
June	1st to 10th	Night	13% 4 21% 42% 20% 1	360
	11th to 20th	Night	13% 5% 23% 40% 18% 1	360
	21st to end	Night	13% 2 26% 39% 19% 1	360
July	1st to 10th	Night	10% 2 16% 42% 25% 5%	402
	11th to 20th	Night	11% 3 16% 37% 30% 4	522
	21st to end	Night	14% 4 13% 39% 29% 2	591
August	1st to 10th	Night	10% 3 13% 42% 30% 2	537
	11th to 20th	Night	13% 4 11% 37% 31% 4	534
	21st to end	Night	12% 4 16% 34% 29% 4	715
September	1st to 10th	Night	15% 4 15% 34% 28% 4	720
	11th to 20th	Night	13% 3 14% 39% 27% 5%	720
	21st to end	Night	21% 6% 20% 30% 21% 3	720
October	1st to 10th	Night	18% 7% 21% 31% 20% 2	720
	11th to 20th	Night	23% 8% 28% 30% 10% 1	720
	21st to end	Night	30% 11% 23% 29% 7% 1	792
November	1st to 10th	Night	20% 9% 26% 33% 11% 1	740
	11th to 20th	Night	28% 14% 25% 24% 8% 1	760
	21st to end	Night	30% 14% 26% 23% 6%	760
December	1st to 10th	Night	34% 15% 27% 19% 5%	760
	11th to 20th	Night	29% 19% 27% 21% 4	760
	21st to end	Night	32% 17% 27% 22% 2	836

Page generated at 2024-01-10 19:35:23. Version ac07

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