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 2024-11-12.

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% 32%	101190
	Night	19% 12% 4 8% 5% 4 4 3 5% 6% 32%	97935
Transparency	Day	50% 6% 11% 21% 11% 1	104838
	Night	51% 5% 10% 21% 11% 2	102426
Seeing	Day	24% 13% 25% 21% 12% 5%	14782
	Night	23% 11% 24% 28% 12% 1	24445

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	21% 21% 8% 11% 4 3 3 2 3 5% 19%	2808
	11th to 20th	Day	13% 23% 10% 15% 7% 4 3 2 3 3 17%	2840
	21st to end	Day	26% 21% 6% 12% 5% 4 3 2 3 3 14%	3124
February	1st to 10th	Day	21% 17% 5% 10% 7% 4 3 3 3 5% 21%	1710
	11th to 20th	Day	18% 18% 6% 11% 5% 4 3 4 4 6% 22%	1663
	21st to end	Day	23% 15% 5% 10% 4 3 4 3 4 5% 24%	1578
March	1st to 10th	Day	20% 14% 5% 8% 4 3 4 3 4 6% 30%	3435
	11th to 20th	Day	16% 12% 4 9% 5% 5% 4 3 5% 5% 32%	3480
	21st to end	Day	20% 11% 3 8% 5% 5% 4 3 4 6% 31%	3815
April	1st to 10th	Day	17% 13% 3 7% 5% 5% 5% 3 6% 6% 30%	2204
	11th to 20th	Day	17% 12% 3 8% 4 4 4 3 5% 7% 34%	2254
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 5% 7% 32%	2468
May	1st to 10th	Day	25% 9% 3 7% 4 4 4 4 5% 6% 28%	4082
	11th to 20th	Day	21% 8% 3 8% 5% 5% 5% 4 5% 7% 31%	4081
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 7% 9% 37%	4756
June	1st to 10th	Day	15% 8% 2 7% 5% 4 6% 5% 6% 8% 34%	2660
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	15% 5% 2 6% 5% 5% 5% 4 6% 9% 37%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	4325
	11th to 20th	Day	12% 6% 3 7% 4 5% 4 4 7% 9% 39%	4120
	21st to end	Day	14% 7% 3 7% 6% 5% 5% 5% 6% 8% 36%	4466
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 4 6% 9% 30%	4104
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 9% 36%	4044
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	4279
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2121
	21st to end	Day	21% 8% 4 6% 4 4 3 3 5% 7% 36%	2072
October	1st to 10th	Day	19% 6% 2 6% 5% 4 4 3 6% 6% 40%	1900
	11th to 20th	Day	20% 8% 3 6% 4 4 5% 3 4 6% 36%	1900
	21st to end	Day	25% 10% 2 6% 3 3 3 2 4 6% 35%	2090
November	1st to 10th	Day	12% 11% 3 7% 5% 3 3 3 4 6% 43%	1912
	11th to 20th	Day	16% 15% 3 7% 4 5% 3 3 5% 6% 32%	1577
	21st to end	Day	20% 16% 5% 7% 4 3 4 3 4 5% 27%	1520
December	1st to 10th	Day	22% 19% 6% 10% 5% 5% 3 3 3 5% 17%	1520
	11th to 20th	Day	21% 20% 7% 9% 5% 3 2 3 3 6% 21%	1910
	21st to end	Day	24% 22% 6% 10% 5% 4 3 2 4 5% 16%	2772
January	1st to 10th	Night	19% 23% 7% 10% 5% 4 4 3 4 4 18%	4132
	11th to 20th	Night	17% 23% 7% 12% 6% 5% 4 3 3 4 16%	4120
	21st to end	Night	28% 18% 6% 12% 5% 3 3 2 2 3 17%	4532
February	1st to 10th	Night	21% 16% 5% 10% 5% 4 3 3 4 4 25%	2470
	11th to 20th	Night	20% 15% 5% 10% 5% 5% 4 3 3 5% 24%	2403
	21st to end	Night	21% 14% 6% 10% 4 4 3 3 4 5% 25%	1872
March	1st to 10th	Night	23% 12% 3 7% 4 3 3 2 4 6% 33%	3418
	11th to 20th	Night	20% 14% 5% 6% 4 3 3 2 4 6% 34%	3480
	21st to end	Night	19% 10% 3 7% 4 4 3 3 4 5% 38%	3817
April	1st to 10th	Night	18% 14% 2 7% 4 4 3 3 4 5% 34%	1975
	11th to 20th	Night	18% 8% 3 6% 5% 4 4 3 5% 8% 37%	1875
	21st to end	Night	16% 10% 3 7% 4 3 4 4 6% 6% 38%	1727
May	1st to 10th	Night	28% 11% 3 7% 4 4 4 3 5% 6% 26%	2813
	11th to 20th	Night	19% 9% 4 7% 5% 4 4 4 4 7% 34%	2812
	21st to end	Night	14% 7% 2 7% 4 4 4 4 7% 9% 37%	2794
June	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 43%	1520
	11th to 20th	Night	12% 6% 2 6% 5% 5% 4 4 7% 9% 40%	1520
	21st to end	Night	13% 6% 2 5% 4 3 5% 4 7% 11% 40%	1520
July	1st to 10th	Night	12% 7% 3 7% 5% 4 3 4 6% 8% 41%	2631
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	2840
	21st to end	Night	14% 7% 3 7% 5% 4 5% 4 6% 8% 37%	3087
August	1st to 10th	Night	15% 8% 3 7% 4 5% 5% 4 7% 9% 33%	2825
	11th to 20th	Night	16% 6% 2 7% 4 4 4 4 5% 6% 41%	2785
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	3343
September	1st to 10th	Night	13% 5% 2 5% 4 4 5% 4 5% 7% 47%	1900
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 6% 7% 52%	2056
	21st to end	Night	18% 7% 2 6% 4 4 3 3 5% 7% 40%	2106
October	1st to 10th	Night	17% 6% 3 6% 3 4 4 3 5% 6% 44%	2280
	11th to 20th	Night	18% 7% 2 7% 5% 3 5% 3 5% 7% 38%	2280
	21st to end	Night	25% 7% 2 6% 4 3 3 3 5% 6% 37%	2508
November	1st to 10th	Night	14% 8% 2 6% 4 3 4 2 4 6% 45%	2383
	11th to 20th	Night	19% 14% 3 6% 5% 5% 4 3 5% 5% 32%	2622
	21st to end	Night	21% 15% 4 6% 5% 4 4 3 5% 5% 30%	2660
December	1st to 10th	Night	23% 18% 7% 9% 4 4 3 3 4 5% 20%	2660
	11th to 20th	Night	21% 17% 6% 10% 5% 4 3 3 4 5% 23%	3285
	21st to end	Night	25% 19% 5% 10% 6% 4 4 3 4 5% 15%	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	2809
	11th to 20th	Day	74% 3 7% 11% 4 1	2840
	21st to end	Day	77% 4 6% 10% 2	3124
February	1st to 10th	Day	67% 5% 8% 14% 6%	1710
	11th to 20th	Day	63% 5% 10% 15% 7%	1672
	21st to end	Day	62% 5% 10% 14% 7% 2	1578
March	1st to 10th	Day	55% 5% 10% 23% 7%	3435
	11th to 20th	Day	53% 7% 10% 23% 7% 1	3480
	21st to end	Day	54% 5% 9% 20% 10% 1	3815
April	1st to 10th	Day	53% 5% 11% 18% 11% 2	2204
	11th to 20th	Day	49% 5% 12% 16% 16% 2	2254
	21st to end	Day	51% 5% 10% 22% 10% 2	2468
May	1st to 10th	Day	55% 7% 9% 20% 8% 1	4120
	11th to 20th	Day	51% 8% 10% 22% 8% 1	4085
	21st to end	Day	40% 9% 15% 23% 11% 3	4770
June	1st to 10th	Day	45% 9% 13% 22% 11% 1	2660
	11th to 20th	Day	43% 8% 14% 22% 12% 1	2660
	21st to end	Day	43% 7% 12% 25% 12% 1	2660
July	1st to 10th	Day	40% 7% 12% 23% 15% 2	4258
	11th to 20th	Day	40% 7% 13% 26% 14%	4051
	21st to end	Day	45% 7% 10% 21% 16% 1	4324
August	1st to 10th	Day	49% 8% 14% 21% 8%	4091
	11th to 20th	Day	42% 7% 11% 26% 12%	4083
	21st to end	Day	35% 9% 12% 28% 14% 2	4279
September	1st to 10th	Day	39% 6% 10% 22% 19% 3	2280
	11th to 20th	Day	37% 6% 12% 28% 16% 1	2121
	21st to end	Day	49% 4 11% 22% 14% 1	2072
October	1st to 10th	Day	35% 4 8% 31% 19% 1	3160
	11th to 20th	Day	44% 5% 10% 23% 13% 5%	3140
	21st to end	Day	37% 4 10% 29% 20% 1	3428
November	1st to 10th	Day	44% 5% 11% 21% 17% 3	1912
	11th to 20th	Day	53% 5% 9% 19% 12% 2	1577
	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	69% 5% 9% 13% 4	4132
	11th to 20th	Night	72% 5% 10% 10% 3	4120
	21st to end	Night	74% 4 5% 13% 3 1	4532
February	1st to 10th	Night	63% 4 9% 18% 7%	2470
	11th to 20th	Night	63% 4 10% 17% 7%	2405
	21st to end	Night	62% 5% 8% 15% 9% 1	1872
March	1st to 10th	Night	53% 4 9% 25% 8%	3418
	11th to 20th	Night	52% 5% 10% 26% 6% 1	3480
	21st to end	Night	47% 5% 9% 24% 13% 2	3817
April	1st to 10th	Night	52% 5% 10% 18% 13% 2	1975
	11th to 20th	Night	45% 6% 11% 19% 16% 3	1875
	21st to end	Night	46% 7% 9% 25% 11% 3	1727
May	1st to 10th	Night	58% 6% 8% 15% 10% 2	2840
	11th to 20th	Night	49% 7% 10% 23% 9% 1	2813
	21st to end	Night	40% 8% 14% 23% 11% 4	2801
June	1st to 10th	Night	38% 8% 12% 26% 15% 2	1520
	11th to 20th	Night	39% 8% 14% 26% 12% 2	1520
	21st to end	Night	36% 8% 14% 24% 15% 2	1520
July	1st to 10th	Night	41% 6% 10% 23% 16% 4	2588
	11th to 20th	Night	39% 8% 14% 23% 15% 1	2786
	21st to end	Night	44% 7% 10% 16% 19% 3	2988
August	1st to 10th	Night	46% 8% 15% 22% 9%	2814
	11th to 20th	Night	42% 6% 10% 26% 16% 1	2812
	21st to end	Night	35% 7% 13% 31% 13% 2	3343
September	1st to 10th	Night	36% 6% 10% 23% 20% 4	1900
	11th to 20th	Night	31% 6% 11% 28% 23% 1	2056
	21st to end	Night	44% 5% 10% 22% 17% 1	2106
October	1st to 10th	Night	37% 4 8% 28% 22% 2	3800
	11th to 20th	Night	38% 4 10% 25% 15% 8%	3779
	21st to end	Night	37% 3 8% 28% 23% 2	4120
November	1st to 10th	Night	41% 4 10% 22% 19% 4	2383
	11th to 20th	Night	54% 4 9% 19% 12% 2	2622
	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	31% 27% 25% 11% 5% 1	263
	11th to 20th	Day	40% 21% 29% 6% 3	270
	21st to end	Day	33% 26% 29% 8% 2 1	297
February	1st to 10th	Day	34% 21% 34% 10% 1	270
	11th to 20th	Day	38% 29% 27% 6% 1	270
	21st to end	Day	36% 25% 29% 10%	277
March	1st to 10th	Day	29% 22% 27% 17% 4	460
	11th to 20th	Day	35% 20% 28% 12% 5% 1	460
	21st to end	Day	31% 18% 28% 17% 5% 1	506
April	1st to 10th	Day	29% 20% 26% 16% 8% 1	460
	11th to 20th	Day	25% 20% 27% 15% 9% 4	452
	21st to end	Day	30% 15% 30% 16% 8% 2	460
May	1st to 10th	Day	30% 14% 28% 17% 9% 3	463
	11th to 20th	Day	24% 13% 27% 22% 10% 6%	487
	21st to end	Day	16% 10% 26% 25% 16% 8%	529
June	1st to 10th	Day	16% 11% 27% 26% 16% 4	490
	11th to 20th	Day	16% 11% 26% 25% 18% 4	490
	21st to end	Day	15% 10% 28% 28% 16% 3	490
July	1st to 10th	Day	15% 6% 16% 29% 20% 13%	485
	11th to 20th	Day	15% 5% 23% 31% 18% 9%	484
	21st to end	Day	17% 4 23% 30% 19% 7%	532
August	1st to 10th	Day	16% 6% 19% 30% 21% 9%	486
	11th to 20th	Day	18% 6% 15% 29% 21% 11%	479
	21st to end	Day	15% 3 20% 28% 20% 14%	539
September	1st to 10th	Day	19% 7% 17% 27% 20% 10%	490
	11th to 20th	Day	14% 4 16% 30% 26% 11%	490
	21st to end	Day	27% 10% 22% 21% 15% 5%	490
October	1st to 10th	Day	19% 9% 25% 22% 18% 6%	490
	11th to 20th	Day	23% 14% 28% 22% 10% 3	490
	21st to end	Day	29% 15% 28% 17% 9% 1	539
November	1st to 10th	Day	20% 13% 31% 22% 10% 3	450
	11th to 20th	Day	31% 18% 28% 17% 5% 1	206
	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	760
	21st to end	Night	32% 19% 26% 20% 2	836
February	1st to 10th	Night	31% 17% 26% 24% 2	760
	11th to 20th	Night	31% 20% 32% 17%	743
	21st to end	Night	30% 18% 31% 19% 2	628
March	1st to 10th	Night	25% 18% 28% 24% 4	760
	11th to 20th	Night	28% 16% 28% 21% 7% 1	760
	21st to end	Night	26% 15% 31% 23% 5% 1	836
April	1st to 10th	Night	28% 14% 31% 21% 6%	760
	11th to 20th	Night	22% 12% 31% 24% 10% 1	751
	21st to end	Night	20% 11% 31% 32% 7%	587
May	1st to 10th	Night	24% 9% 21% 35% 10% 1	570
	11th to 20th	Night	23% 9% 24% 33% 12%	561
	21st to end	Night	13% 2 23% 33% 26% 2	448
June	1st to 10th	Night	12% 5% 21% 41% 19% 1	380
	11th to 20th	Night	13% 5% 22% 42% 18% 1	380
	21st to end	Night	13% 2 27% 38% 19% 1	380
July	1st to 10th	Night	10% 3 17% 42% 24% 4	426
	11th to 20th	Night	11% 3 17% 37% 29% 4	552
	21st to end	Night	14% 4 13% 39% 28% 2	624
August	1st to 10th	Night	11% 3 13% 42% 29% 2	567
	11th to 20th	Night	13% 4 12% 37% 30% 4	564
	21st to end	Night	12% 3 15% 36% 29% 5%	755
September	1st to 10th	Night	15% 4 15% 35% 27% 4	760
	11th to 20th	Night	13% 3 14% 39% 27% 4	760
	21st to end	Night	21% 6% 20% 30% 20% 3	760
October	1st to 10th	Night	18% 7% 21% 31% 21% 2	760
	11th to 20th	Night	22% 8% 28% 30% 11% 1	760
	21st to end	Night	28% 11% 23% 30% 7% 1	836
November	1st to 10th	Night	20% 9% 27% 33% 11% 1	780
	11th to 20th	Night	28% 14% 25% 24% 7% 1	765
	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 2024-11-08 22:45:35. Version ac07

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