Union
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union clear sky chart from 2005-11-06 to 2024-02-10.

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% 10% 4 6% 5% 3 5% 4 5% 6% 42%	78198
	Night	12% 11% 4 6% 5% 3 4 4 5% 7% 38%	80787
Transparency	Day	41% 9% 13% 18% 14% 5%	77902
	Night	42% 9% 14% 19% 12% 4	80604
Seeing	Day	14% 17% 30% 25% 12% 3	13355
	Night	15% 10% 24% 32% 16% 2	22700

By Month

Percentage Cloud Cover SW (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% 22% 9% 11% 7% 4 6% 5% 6% 6% 15%	2539
	11th to 20th	Day	14% 15% 6% 11% 7% 4 5% 4 4 7% 21%	2540
	21st to end	Day	15% 14% 5% 9% 8% 4 6% 5% 6% 6% 22%	2794
February	1st to 10th	Day	13% 16% 6% 8% 7% 3 5% 5% 6% 7% 24%	1608
	11th to 20th	Day	12% 19% 6% 9% 6% 3 5% 4 4 5% 27%	1714
	21st to end	Day	9% 15% 8% 11% 9% 4 5% 5% 5% 6% 24%	1480
March	1st to 10th	Day	11% 15% 5% 9% 8% 4 6% 6% 6% 7% 23%	1800
	11th to 20th	Day	11% 15% 5% 8% 6% 3 6% 5% 6% 7% 26%	1832
	21st to end	Day	13% 16% 6% 10% 8% 4 7% 6% 5% 5% 20%	2376
April	1st to 10th	Day	12% 17% 4 7% 8% 4 4 4 5% 7% 28%	2160
	11th to 20th	Day	12% 12% 5% 7% 6% 4 4 5% 5% 6% 34%	2129
	21st to end	Day	12% 13% 4 7% 6% 3 5% 6% 5% 6% 35%	2338
May	1st to 10th	Day	10% 10% 2 5% 5% 3 5% 5% 6% 8% 41%	2405
	11th to 20th	Day	14% 7% 3 5% 6% 4 5% 6% 6% 8% 37%	2477
	21st to end	Day	14% 9% 4 7% 6% 3 5% 5% 6% 7% 34%	2723
June	1st to 10th	Day	15% 7% 3 6% 5% 3 5% 5% 6% 8% 37%	2520
	11th to 20th	Day	14% 7% 2 4 5% 3 4 4 5% 6% 45%	2520
	21st to end	Day	6% 3 2 3 3 3 4 4 4 7% 62%	2520
July	1st to 10th	Day	2 2 1 3 3 2 4 4 5% 6% 69%	2520
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 5% 74%	2520
	21st to end	Day	2 1 1 2 2 2 3 3 4 5% 77%	2741
August	1st to 10th	Day	3 3 1 3 3 2 4 4 5% 6% 67%	2507
	11th to 20th	Day	3 2 1 2 2 1 3 3 4 6% 71%	2288
	21st to end	Day	4 2 1 3 3 2 4 4 4 5% 67%	2376
September	1st to 10th	Day	4 3 1 2 3 2 2 3 3 4 72%	2160
	11th to 20th	Day	8% 4 2 4 4 2 3 3 4 7% 58%	2066
	21st to end	Day	9% 7% 3 5% 4 2 4 3 4 6% 54%	1980
October	1st to 10th	Day	13% 7% 4 5% 5% 3 3 3 3 5% 50%	1908
	11th to 20th	Day	12% 7% 2 5% 4 3 5% 3 5% 7% 46%	1746
	21st to end	Day	14% 10% 4 7% 6% 3 6% 5% 4 6% 36%	1782
November	1st to 10th	Day	18% 14% 4 5% 6% 3 4 5% 4 7% 30%	1674
	11th to 20th	Day	13% 15% 5% 8% 8% 3 6% 5% 6% 8% 24%	1693
	21st to end	Day	12% 13% 5% 7% 6% 3 4 4 6% 8% 32%	1520
December	1st to 10th	Day	12% 17% 5% 6% 5% 4 4 5% 6% 8% 28%	1520
	11th to 20th	Day	12% 15% 5% 8% 7% 3 5% 6% 5% 9% 25%	1928
	21st to end	Day	11% 15% 5% 8% 7% 4 5% 5% 6% 8% 25%	2794
January	1st to 10th	Night	12% 18% 8% 11% 8% 4 5% 5% 7% 7% 15%	4406
	11th to 20th	Night	17% 19% 6% 9% 5% 2 3 5% 6% 9% 20%	4420
	21st to end	Night	16% 14% 4 7% 6% 4 5% 6% 7% 8% 22%	4862
February	1st to 10th	Night	15% 17% 5% 9% 7% 3 5% 5% 5% 6% 23%	2553
	11th to 20th	Night	12% 16% 5% 8% 7% 2 5% 4 5% 6% 29%	2133
	21st to end	Night	11% 17% 7% 9% 8% 3 5% 4 5% 6% 24%	1776
March	1st to 10th	Night	12% 12% 5% 9% 6% 4 5% 5% 5% 7% 30%	2160
	11th to 20th	Night	12% 13% 4 6% 7% 3 5% 5% 6% 7% 32%	2123
	21st to end	Night	12% 16% 5% 8% 7% 4 4 4 6% 7% 26%	1980
April	1st to 10th	Night	10% 13% 6% 8% 6% 4 4 4 6% 7% 32%	1800
	11th to 20th	Night	14% 10% 4 6% 5% 3 4 4 4 5% 41%	1779
	21st to end	Night	11% 13% 3 6% 5% 3 4 3 5% 6% 40%	1620
May	1st to 10th	Night	10% 8% 2 4 4 2 4 4 6% 7% 49%	1552
	11th to 20th	Night	11% 7% 3 4 5% 4 5% 6% 8% 8% 41%	1416
	21st to end	Night	13% 7% 3 6% 5% 3 5% 5% 5% 9% 38%	1552
June	1st to 10th	Night	11% 7% 2 5% 5% 3 5% 6% 5% 10% 42%	1440
	11th to 20th	Night	14% 5% 2 4 4 3 4 3 5% 7% 48%	1440
	21st to end	Night	5% 4 1 4 3 2 3 3 5% 7% 63%	1440
July	1st to 10th	Night	3 1 1 2 3 2 4 3 5% 5% 71%	1440
	11th to 20th	Night	2 1 1 2 1 1 2 3 3 6% 79%	1440
	21st to end	Night	1 1 1 1 2 2 3 3 3 6% 76%	1576
August	1st to 10th	Night	3 2 1 2 1 3 4 6% 8% 69%	1432
	11th to 20th	Night	1 1 1 2 3 2 2 3 3 6% 76%	1610
	21st to end	Night	3 2 1 3 3 2 3 3 4 6% 70%	1980
September	1st to 10th	Night	5% 2 1 2 2 1 2 3 4 5% 72%	1800
	11th to 20th	Night	6% 3 1 3 4 3 3 5% 6% 6% 61%	1893
	21st to end	Night	9% 5% 3 4 3 2 3 4 4 6% 57%	1980
October	1st to 10th	Night	12% 6% 3 4 4 2 4 4 4 6% 51%	2049
	11th to 20th	Night	12% 9% 3 6% 5% 3 3 4 4 6% 46%	2214
	21st to end	Night	14% 9% 3 6% 5% 3 4 4 5% 6% 41%	2574
November	1st to 10th	Night	18% 13% 3 5% 6% 2 4 4 6% 6% 33%	2405
	11th to 20th	Night	18% 13% 4 7% 6% 3 5% 4 5% 8% 29%	2484
	21st to end	Night	12% 13% 4 7% 6% 3 5% 4 5% 6% 35%	2660
December	1st to 10th	Night	14% 14% 4 7% 5% 3 5% 6% 7% 8% 26%	2660
	11th to 20th	Night	12% 17% 5% 9% 7% 4 5% 5% 7% 8% 22%	3276
	21st to end	Night	13% 15% 5% 8% 7% 4 5% 5% 6% 8% 24%	4862

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	62% 14% 15% 7% 1	2539
	11th to 20th	Day	58% 9% 17% 12% 4	2540
	21st to end	Day	66% 13% 11% 7% 2 1	2794
February	1st to 10th	Day	58% 13% 12% 13% 4	1608
	11th to 20th	Day	57% 10% 13% 13% 6% 1	1724
	21st to end	Day	56% 10% 14% 13% 6% 1	1480
March	1st to 10th	Day	57% 12% 14% 12% 6%	1800
	11th to 20th	Day	55% 10% 12% 18% 5%	1832
	21st to end	Day	62% 10% 13% 12% 2	2376
April	1st to 10th	Day	54% 9% 15% 16% 5% 1	2160
	11th to 20th	Day	49% 9% 13% 16% 11% 2	2132
	21st to end	Day	47% 9% 13% 19% 11% 1	2338
May	1st to 10th	Day	37% 9% 16% 27% 11%	2405
	11th to 20th	Day	42% 11% 14% 24% 8% 1	2480
	21st to end	Day	47% 10% 16% 21% 6%	2724
June	1st to 10th	Day	44% 10% 15% 24% 7%	2520
	11th to 20th	Day	36% 8% 15% 25% 13% 3	2520
	21st to end	Day	22% 7% 11% 31% 24% 5%	2520
July	1st to 10th	Day	16% 5% 12% 22% 29% 17%	2451
	11th to 20th	Day	12% 6% 9% 14% 31% 28%	2445
	21st to end	Day	11% 3 6% 12% 41% 27%	2589
August	1st to 10th	Day	17% 5% 9% 19% 34% 14%	2490
	11th to 20th	Day	14% 4 7% 24% 33% 17%	2288
	21st to end	Day	18% 5% 9% 26% 30% 12%	2376
September	1st to 10th	Day	16% 4 9% 24% 29% 17%	2160
	11th to 20th	Day	25% 6% 12% 20% 25% 11%	2066
	21st to end	Day	30% 6% 12% 26% 21% 5%	1980
October	1st to 10th	Day	37% 6% 11% 24% 19% 3	1908
	11th to 20th	Day	35% 8% 13% 26% 16% 2	1746
	21st to end	Day	45% 9% 13% 21% 11% 1	1782
November	1st to 10th	Day	52% 10% 13% 18% 6% 1	1674
	11th to 20th	Day	53% 13% 16% 15% 2	1693
	21st to end	Day	49% 11% 15% 15% 9% 1	1520
December	1st to 10th	Day	55% 13% 14% 12% 6%	1520
	11th to 20th	Day	59% 15% 14% 10% 2	1928
	21st to end	Day	54% 14% 17% 13% 3	2794
January	1st to 10th	Night	58% 16% 17% 8% 1	4406
	11th to 20th	Night	60% 8% 15% 14% 3	4420
	21st to end	Night	62% 13% 12% 10% 3	4862
February	1st to 10th	Night	57% 11% 15% 13% 3 1	2553
	11th to 20th	Night	53% 9% 14% 16% 7% 1	2134
	21st to end	Night	54% 9% 14% 16% 6% 2	1776
March	1st to 10th	Night	50% 9% 16% 18% 6% 1	2160
	11th to 20th	Night	48% 9% 17% 22% 4	2123
	21st to end	Night	52% 10% 20% 16% 2	1980
April	1st to 10th	Night	47% 9% 18% 20% 5%	1800
	11th to 20th	Night	42% 8% 16% 22% 9% 2	1779
	21st to end	Night	40% 10% 18% 19% 13% 1	1620
May	1st to 10th	Night	30% 10% 16% 31% 12% 1	1552
	11th to 20th	Night	35% 11% 17% 27% 8% 1	1416
	21st to end	Night	37% 10% 19% 24% 9%	1552
June	1st to 10th	Night	37% 9% 16% 29% 9%	1440
	11th to 20th	Night	33% 7% 15% 29% 13% 3	1440
	21st to end	Night	20% 6% 13% 30% 25% 6%	1440
July	1st to 10th	Night	14% 5% 8% 25% 28% 19%	1400
	11th to 20th	Night	9% 3 11% 22% 28% 28%	1392
	21st to end	Night	10% 3 5% 18% 38% 26%	1488
August	1st to 10th	Night	13% 4 11% 25% 33% 15%	1424
	11th to 20th	Night	11% 5% 8% 24% 34% 18%	1610
	21st to end	Night	14% 4 10% 28% 31% 13%	1980
September	1st to 10th	Night	15% 3 9% 26% 30% 17%	1800
	11th to 20th	Night	20% 6% 11% 25% 24% 13%	1893
	21st to end	Night	27% 5% 11% 27% 20% 9%	1980
October	1st to 10th	Night	33% 6% 14% 23% 21% 3	2049
	11th to 20th	Night	37% 6% 14% 22% 20% 2	2214
	21st to end	Night	41% 8% 14% 19% 17% 1	2574
November	1st to 10th	Night	49% 8% 14% 19% 9% 1	2405
	11th to 20th	Night	50% 9% 15% 20% 5%	2484
	21st to end	Night	45% 10% 15% 17% 11% 2	2660
December	1st to 10th	Night	53% 12% 15% 13% 6% 1	2660
	11th to 20th	Night	57% 12% 14% 14% 3	3276
	21st to end	Night	51% 13% 18% 13% 5%	4862

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	21% 28% 26% 15% 8% 2	180
	11th to 20th	Day	22% 28% 24% 14% 7% 4	180
	21st to end	Day	23% 18% 31% 20% 7% 2	198
February	1st to 10th	Day	23% 24% 36% 14% 3	274
	11th to 20th	Day	22% 27% 28% 18% 5%	399
	21st to end	Day	23% 22% 30% 21% 5%	346
March	1st to 10th	Day	20% 27% 33% 16% 3	420
	11th to 20th	Day	15% 25% 32% 20% 7%	420
	21st to end	Day	22% 24% 30% 17% 6%	462
April	1st to 10th	Day	22% 23% 28% 17% 8% 2	420
	11th to 20th	Day	20% 26% 28% 16% 7% 3	412
	21st to end	Day	15% 19% 34% 25% 6% 1	420
May	1st to 10th	Day	13% 18% 32% 25% 9% 3	423
	11th to 20th	Day	21% 15% 27% 23% 13% 2	448
	21st to end	Day	15% 13% 27% 28% 14% 2	485
June	1st to 10th	Day	14% 15% 27% 24% 15% 5%	450
	11th to 20th	Day	12% 18% 33% 25% 9% 2	450
	21st to end	Day	5% 10% 31% 32% 18% 4	450
July	1st to 10th	Day	2 10% 35% 36% 15% 3	446
	11th to 20th	Day	2 11% 39% 31% 16% 2	444
	21st to end	Day	1 8% 32% 34% 19% 5%	487
August	1st to 10th	Day	3 8% 32% 35% 16% 6%	447
	11th to 20th	Day	3 6% 24% 36% 21% 9%	437
	21st to end	Day	6% 12% 28% 30% 18% 6%	495
September	1st to 10th	Day	5% 11% 26% 31% 21% 7%	450
	11th to 20th	Day	9% 10% 26% 32% 19% 4	450
	21st to end	Day	11% 13% 28% 28% 18% 2	450
October	1st to 10th	Day	14% 14% 27% 26% 16% 3	450
	11th to 20th	Day	13% 19% 27% 24% 12% 4	396
	21st to end	Day	19% 20% 25% 24% 8% 4	297
November	1st to 10th	Day	23% 19% 31% 23% 4	270
	11th to 20th	Day	15% 26% 28% 24% 6% 1	261
	21st to end	Day	16% 18% 38% 21% 7%	180
December	1st to 10th	Day	19% 24% 27% 17% 9% 4	180
	11th to 20th	Day	27% 28% 31% 11% 3	180
	21st to end	Day	19% 25% 33% 16% 5% 2	198
January	1st to 10th	Night	19% 19% 32% 19% 11% 2	760
	11th to 20th	Night	23% 14% 28% 24% 10% 1	760
	21st to end	Night	21% 11% 25% 31% 11% 1	836
February	1st to 10th	Night	21% 16% 32% 24% 6% 1	758
	11th to 20th	Night	19% 13% 28% 27% 12% 1	704
	21st to end	Night	16% 20% 29% 23% 10% 2	592
March	1st to 10th	Night	18% 15% 30% 25% 11% 1	720
	11th to 20th	Night	17% 11% 30% 28% 12% 1	720
	21st to end	Night	18% 16% 27% 28% 10% 1	792
April	1st to 10th	Night	15% 11% 28% 31% 13% 2	720
	11th to 20th	Night	16% 10% 32% 25% 14% 2	711
	21st to end	Night	16% 12% 24% 34% 13% 1	540
May	1st to 10th	Night	10% 11% 27% 36% 14% 2	472
	11th to 20th	Night	15% 5% 17% 30% 30% 3	354
	21st to end	Night	12% 7% 20% 34% 25% 2	389
June	1st to 10th	Night	12% 6% 19% 37% 23% 3	360
	11th to 20th	Night	14% 8% 23% 35% 19% 1	360
	21st to end	Night	6% 3 16% 48% 26% 1	360
July	1st to 10th	Night	3 2 17% 47% 29% 3	350
	11th to 20th	Night	2 3 15% 56% 23% 1	348
	21st to end	Night	1 2 15% 52% 28% 2	394
August	1st to 10th	Night	4 2 11% 47% 34% 3	358
	11th to 20th	Night	2 1 15% 44% 32% 5%	542
	21st to end	Night	4 3 19% 43% 29% 3	792
September	1st to 10th	Night	5% 3 18% 41% 28% 5%	720
	11th to 20th	Night	4 4 15% 41% 30% 7%	720
	21st to end	Night	12% 4 21% 34% 23% 6%	720
October	1st to 10th	Night	13% 4 21% 37% 21% 3	720
	11th to 20th	Night	14% 9% 20% 33% 18% 5%	720
	21st to end	Night	19% 10% 22% 33% 13% 4	792
November	1st to 10th	Night	21% 11% 28% 31% 9% 1	740
	11th to 20th	Night	21% 15% 26% 29% 9% 1	760
	21st to end	Night	16% 15% 28% 26% 12% 3	760
December	1st to 10th	Night	19% 14% 29% 27% 9% 2	760
	11th to 20th	Night	22% 16% 29% 25% 8% 1	760
	21st to end	Night	24% 19% 30% 22% 5% 1	836

Page generated at 2024-02-07 03:11:52. Version ac07

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