UI Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UI Observatory clear sky chart from 2005-11-06 to 2024-03-08.

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	13% 9% 3 6% 5% 3 5% 4 5% 7% 39%	78195
	Night	15% 10% 3 6% 5% 3 5% 4 5% 6% 37%	81299
Transparency	Day	31% 35% 9% 14% 9% 3	77922
	Night	34% 34% 9% 13% 8% 3	81152
Seeing	Day	16% 5% 19% 27% 23% 10%	13688
	Night	19% 5% 16% 36% 22% 4	23158

By Month

Percentage Cloud Cover NW (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	16% 21% 8% 11% 6% 3 5% 6% 5% 5% 15%	2539
	11th to 20th	Day	19% 14% 6% 8% 7% 3 5% 5% 5% 6% 21%	2516
	21st to end	Day	22% 18% 5% 9% 7% 4 6% 4 4 5% 17%	2794
February	1st to 10th	Day	19% 17% 5% 9% 6% 3 5% 4 5% 7% 20%	1529
	11th to 20th	Day	17% 17% 5% 9% 6% 4 5% 4 4 6% 23%	1816
	21st to end	Day	15% 19% 5% 8% 6% 4 5% 5% 5% 6% 22%	1570
March	1st to 10th	Day	17% 14% 4 7% 6% 3 5% 5% 6% 6% 26%	1871
	11th to 20th	Day	16% 12% 4 8% 6% 4 6% 5% 6% 8% 24%	1862
	21st to end	Day	18% 15% 5% 8% 7% 4 6% 5% 5% 5% 21%	2333
April	1st to 10th	Day	14% 12% 4 8% 6% 4 6% 6% 6% 8% 26%	2160
	11th to 20th	Day	12% 12% 4 8% 6% 3 5% 5% 6% 6% 32%	2188
	21st to end	Day	14% 9% 4 7% 7% 4 6% 5% 5% 7% 33%	2340
May	1st to 10th	Day	9% 8% 3 5% 5% 3 5% 5% 6% 9% 43%	2415
	11th to 20th	Day	13% 7% 3 5% 6% 4 6% 5% 6% 9% 35%	2477
	21st to end	Day	12% 8% 4 6% 6% 4 6% 5% 7% 8% 35%	2723
June	1st to 10th	Day	14% 8% 3 6% 6% 4 5% 5% 5% 7% 37%	2618
	11th to 20th	Day	14% 6% 3 5% 6% 4 5% 5% 5% 8% 39%	2700
	21st to end	Day	6% 4 2 4 4 3 4 4 4 6% 59%	2618
July	1st to 10th	Day	3 2 1 2 3 2 4 3 5% 6% 69%	2520
	11th to 20th	Day	3 2 1 2 2 2 3 4 4 6% 71%	2520
	21st to end	Day	1 1 1 1 2 1 3 3 3 5% 79%	2741
August	1st to 10th	Day	4 3 1 3 3 1 2 3 4 7% 69%	2507
	11th to 20th	Day	5% 2 1 3 3 2 2 3 4 7% 68%	2333
	21st to end	Day	5% 4 2 3 3 2 3 4 5% 8% 61%	2422
September	1st to 10th	Day	4 2 1 3 3 2 2 3 5% 6% 68%	2160
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 5% 7% 52%	2044
	21st to end	Day	9% 5% 2 6% 4 3 4 4 6% 8% 49%	1980
October	1st to 10th	Day	15% 8% 3 6% 5% 2 4 3 4 5% 44%	1937
	11th to 20th	Day	13% 6% 3 6% 4 3 4 4 6% 5% 44%	1692
	21st to end	Day	17% 10% 3 7% 5% 3 4 5% 5% 6% 34%	1782
November	1st to 10th	Day	25% 9% 3 5% 7% 4 5% 4 5% 8% 25%	1674
	11th to 20th	Day	20% 13% 4 9% 6% 4 6% 4 5% 6% 23%	1674
	21st to end	Day	18% 13% 5% 7% 6% 3 6% 6% 6% 7% 23%	1448
December	1st to 10th	Day	20% 14% 6% 7% 7% 4 6% 4 6% 7% 19%	1330
	11th to 20th	Day	19% 17% 6% 9% 8% 4 5% 5% 4 5% 19%	1682
	21st to end	Day	18% 14% 5% 7% 6% 3 5% 4 6% 8% 23%	2680
January	1st to 10th	Night	21% 17% 5% 9% 8% 4 5% 4 5% 6% 17%	4406
	11th to 20th	Night	19% 16% 5% 8% 4 3 4 4 5% 7% 25%	4382
	21st to end	Night	21% 14% 5% 8% 7% 4 5% 5% 5% 6% 21%	4862
February	1st to 10th	Night	19% 17% 5% 7% 5% 3 5% 4 5% 4 24%	2647
	11th to 20th	Night	16% 14% 5% 7% 5% 3 4 4 6% 6% 30%	2252
	21st to end	Night	15% 17% 4 8% 6% 4 5% 5% 5% 6% 25%	1884
March	1st to 10th	Night	16% 11% 4 7% 6% 4 6% 5% 5% 6% 29%	2249
	11th to 20th	Night	15% 9% 4 7% 6% 4 5% 5% 6% 7% 32%	2097
	21st to end	Night	17% 11% 4 7% 7% 4 6% 6% 6% 7% 26%	2021
April	1st to 10th	Night	13% 11% 4 6% 6% 5% 6% 5% 6% 7% 32%	1800
	11th to 20th	Night	15% 9% 4 7% 5% 2 4 3 6% 6% 38%	1721
	21st to end	Night	11% 9% 4 7% 7% 3 5% 5% 6% 8% 37%	1620
May	1st to 10th	Night	9% 5% 3 5% 6% 3 5% 4 5% 7% 48%	1544
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 6% 7% 9% 39%	1416
	21st to end	Night	12% 5% 3 6% 5% 4 6% 6% 7% 8% 38%	1552
June	1st to 10th	Night	12% 6% 2 5% 6% 4 5% 4 7% 10% 40%	1337
	11th to 20th	Night	12% 7% 2 6% 6% 4 4 5% 5% 7% 43%	1260
	21st to end	Night	6% 4 2 4 4 2 4 4 6% 6% 56%	1335
July	1st to 10th	Night	3 2 1 3 3 1 3 4 5% 6% 69%	1440
	11th to 20th	Night	2 1 1 2 3 2 3 3 6% 7% 70%	1440
	21st to end	Night	1 2 1 2 2 2 2 3 3 5% 75%	1576
August	1st to 10th	Night	4 3 1 3 2 1 3 3 5% 8% 67%	1432
	11th to 20th	Night	3 2 1 3 4 3 3 4 5% 5% 68%	1568
	21st to end	Night	6% 3 1 3 3 2 3 4 5% 6% 66%	1933
September	1st to 10th	Night	5% 3 1 2 3 1 3 4 4 8% 66%	1800
	11th to 20th	Night	7% 4 2 4 4 3 4 5% 4 6% 56%	1916
	21st to end	Night	10% 5% 2 4 3 3 4 4 5% 8% 51%	1980
October	1st to 10th	Night	11% 6% 3 4 5% 2 4 4 6% 6% 49%	2020
	11th to 20th	Night	14% 7% 3 5% 4 3 4 5% 4 5% 45%	2268
	21st to end	Night	16% 7% 2 6% 6% 4 5% 4 4 5% 40%	2574
November	1st to 10th	Night	20% 11% 4 6% 6% 4 5% 4 5% 7% 29%	2405
	11th to 20th	Night	22% 11% 4 7% 6% 4 4 4 5% 6% 29%	2503
	21st to end	Night	19% 10% 4 7% 5% 3 4 5% 6% 6% 31%	2709
December	1st to 10th	Night	18% 12% 4 8% 6% 3 6% 4 5% 7% 27%	2850
	11th to 20th	Night	21% 15% 5% 9% 8% 4 6% 5% 5% 5% 17%	3510
	21st to end	Night	18% 13% 4 6% 6% 4 5% 5% 5% 6% 26%	4990

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	55% 25% 14% 6%	2539
	11th to 20th	Day	50% 25% 10% 12% 3 1	2540
	21st to end	Day	57% 22% 13% 7% 1	2794
February	1st to 10th	Day	41% 36% 11% 9% 3	1529
	11th to 20th	Day	41% 36% 9% 10% 4	1826
	21st to end	Day	42% 35% 8% 11% 4	1570
March	1st to 10th	Day	37% 37% 11% 12% 3	1871
	11th to 20th	Day	41% 36% 10% 11% 2	1862
	21st to end	Day	43% 36% 11% 8% 2	2333
April	1st to 10th	Day	38% 37% 8% 13% 4	2160
	11th to 20th	Day	33% 35% 9% 13% 8% 2	2188
	21st to end	Day	34% 37% 9% 15% 5% 1	2340
May	1st to 10th	Day	27% 36% 8% 21% 8%	2415
	11th to 20th	Day	27% 37% 9% 17% 9% 1	2480
	21st to end	Day	34% 38% 9% 14% 5% 1	2724
June	1st to 10th	Day	37% 37% 7% 13% 6%	2618
	11th to 20th	Day	30% 36% 10% 16% 6% 1	2700
	21st to end	Day	17% 35% 9% 22% 14% 2	2618
July	1st to 10th	Day	11% 36% 6% 16% 21% 10%	2451
	11th to 20th	Day	10% 36% 4 13% 23% 14%	2445
	21st to end	Day	7% 37% 2 13% 29% 13%	2589
August	1st to 10th	Day	13% 35% 6% 17% 23% 7%	2490
	11th to 20th	Day	12% 34% 5% 14% 24% 11%	2333
	21st to end	Day	14% 36% 5% 19% 19% 6%	2422
September	1st to 10th	Day	12% 34% 7% 18% 20% 10%	2160
	11th to 20th	Day	18% 35% 8% 19% 15% 5%	2044
	21st to end	Day	22% 35% 10% 19% 11% 3	1980
October	1st to 10th	Day	26% 36% 9% 17% 11% 1	1937
	11th to 20th	Day	27% 36% 11% 18% 8% 1	1692
	21st to end	Day	31% 37% 11% 15% 5%	1782
November	1st to 10th	Day	37% 40% 10% 9% 3 1	1674
	11th to 20th	Day	42% 39% 9% 10% 1	1674
	21st to end	Day	38% 41% 11% 8% 2	1448
December	1st to 10th	Day	39% 40% 9% 9% 2 1	1330
	11th to 20th	Day	48% 33% 12% 6% 1	1684
	21st to end	Day	45% 24% 18% 10% 3	2680
January	1st to 10th	Night	55% 25% 11% 8%	4406
	11th to 20th	Night	48% 24% 12% 11% 4 1	4420
	21st to end	Night	54% 23% 13% 9% 2	4862
February	1st to 10th	Night	40% 35% 11% 11% 3 1	2647
	11th to 20th	Night	36% 35% 14% 10% 5%	2253
	21st to end	Night	40% 35% 10% 11% 4	1884
March	1st to 10th	Night	36% 37% 10% 13% 4	2249
	11th to 20th	Night	37% 36% 9% 15% 3	2098
	21st to end	Night	40% 37% 9% 11% 3	2021
April	1st to 10th	Night	35% 36% 8% 16% 5%	1800
	11th to 20th	Night	32% 36% 8% 15% 7% 1	1721
	21st to end	Night	31% 37% 10% 12% 8% 1	1620
May	1st to 10th	Night	26% 36% 6% 19% 12% 1	1544
	11th to 20th	Night	24% 37% 7% 19% 11% 2	1416
	21st to end	Night	29% 38% 10% 15% 7% 1	1552
June	1st to 10th	Night	32% 38% 7% 15% 7% 1	1337
	11th to 20th	Night	31% 35% 7% 17% 9% 2	1260
	21st to end	Night	20% 35% 6% 18% 16% 4	1335
July	1st to 10th	Night	9% 36% 4 13% 24% 13%	1400
	11th to 20th	Night	9% 36% 4 12% 20% 19%	1392
	21st to end	Night	9% 36% 2 8% 23% 22%	1488
August	1st to 10th	Night	13% 35% 4 12% 25% 11%	1424
	11th to 20th	Night	13% 34% 4 10% 23% 15%	1568
	21st to end	Night	14% 35% 5% 16% 21% 11%	1933
September	1st to 10th	Night	12% 35% 4 17% 21% 12%	1800
	11th to 20th	Night	17% 36% 5% 17% 18% 7%	1916
	21st to end	Night	21% 35% 8% 18% 11% 7%	1980
October	1st to 10th	Night	22% 35% 9% 19% 13% 1	2020
	11th to 20th	Night	27% 36% 8% 17% 11% 1	2268
	21st to end	Night	27% 37% 9% 15% 10% 1	2574
November	1st to 10th	Night	35% 40% 9% 12% 3 1	2405
	11th to 20th	Night	39% 39% 10% 10% 3	2503
	21st to end	Night	35% 39% 12% 12% 3	2709
December	1st to 10th	Night	36% 41% 10% 10% 2 1	2850
	11th to 20th	Night	47% 33% 10% 10% 1	3507
	21st to end	Night	43% 25% 18% 11% 4	4990

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	28% 7% 22% 20% 18% 4	180
	11th to 20th	Day	28% 3 23% 27% 12% 7%	180
	21st to end	Day	29% 4 26% 28% 13% 1	198
February	1st to 10th	Day	28% 10% 28% 25% 8% 2	189
	11th to 20th	Day	28% 11% 18% 27% 14% 3	439
	21st to end	Day	23% 10% 23% 29% 12% 3	382
March	1st to 10th	Day	21% 9% 26% 27% 14% 4	449
	11th to 20th	Day	20% 7% 18% 28% 20% 7%	420
	21st to end	Day	20% 10% 27% 25% 16% 3	462
April	1st to 10th	Day	21% 9% 20% 26% 18% 6%	420
	11th to 20th	Day	19% 9% 28% 23% 14% 7%	412
	21st to end	Day	15% 8% 22% 29% 20% 6%	420
May	1st to 10th	Day	15% 4 20% 27% 27% 6%	423
	11th to 20th	Day	19% 4 16% 25% 19% 17%	448
	21st to end	Day	14% 3 15% 23% 28% 17%	485
June	1st to 10th	Day	16% 4 15% 25% 27% 14%	548
	11th to 20th	Day	16% 6% 23% 28% 21% 7%	630
	21st to end	Day	7% 3 16% 28% 32% 14%	548
July	1st to 10th	Day	4 2 14% 30% 37% 14%	446
	11th to 20th	Day	4 3 19% 30% 30% 14%	444
	21st to end	Day	2 2 17% 28% 35% 17%	487
August	1st to 10th	Day	6% 2 15% 27% 29% 20%	447
	11th to 20th	Day	4 3 9% 27% 35% 23%	437
	21st to end	Day	9% 3 17% 24% 29% 17%	495
September	1st to 10th	Day	6% 2 19% 26% 32% 15%	450
	11th to 20th	Day	10% 4 15% 27% 27% 17%	450
	21st to end	Day	12% 6% 18% 30% 27% 8%	450
October	1st to 10th	Day	16% 7% 15% 29% 25% 8%	450
	11th to 20th	Day	14% 8% 17% 30% 22% 9%	342
	21st to end	Day	25% 5% 20% 32% 15% 4	297
November	1st to 10th	Day	25% 5% 27% 31% 11%	270
	11th to 20th	Day	23% 8% 25% 25% 16% 3	252
	21st to end	Day	23% 3 23% 33% 15% 3	180
December	1st to 10th	Day	31% 5% 21% 24% 14% 6%	180
	11th to 20th	Day	32% 7% 24% 24% 8% 3	180
	21st to end	Day	32% 7% 24% 26% 9% 4	198
January	1st to 10th	Night	31% 8% 17% 29% 13% 2	760
	11th to 20th	Night	26% 7% 16% 33% 15% 4	760
	21st to end	Night	22% 5% 17% 36% 18% 2	836
February	1st to 10th	Night	27% 8% 19% 34% 11% 2	760
	11th to 20th	Night	23% 6% 18% 34% 16% 2	744
	21st to end	Night	23% 10% 17% 34% 14% 2	628
March	1st to 10th	Night	21% 4 19% 35% 18% 3	750
	11th to 20th	Night	18% 7% 19% 32% 22% 2	720
	21st to end	Night	23% 5% 19% 37% 15% 2	792
April	1st to 10th	Night	20% 5% 14% 38% 20% 3	720
	11th to 20th	Night	19% 6% 22% 34% 16% 3	653
	21st to end	Night	16% 4 18% 37% 21% 3	540
May	1st to 10th	Night	12% 3 21% 35% 25% 3	464
	11th to 20th	Night	13% 3 11% 33% 30% 10%	354
	21st to end	Night	14% 3 13% 32% 32% 6%	389
June	1st to 10th	Night	12% 4 13% 36% 28% 7%	360
	11th to 20th	Night	12% 6% 23% 32% 23% 4	360
	21st to end	Night	8% 2 13% 40% 33% 5%	360
July	1st to 10th	Night	4 1 11% 38% 39% 7%	350
	11th to 20th	Night	3 1 16% 44% 33% 3	348
	21st to end	Night	2 2 11% 37% 42% 6%	394
August	1st to 10th	Night	5% 2 9% 36% 41% 7%	358
	11th to 20th	Night	3 1 8% 35% 43% 10%	500
	21st to end	Night	6% 2 14% 38% 36% 4	745
September	1st to 10th	Night	6% 1 13% 42% 30% 8%	720
	11th to 20th	Night	9% 2 10% 34% 38% 8%	720
	21st to end	Night	13% 2 14% 38% 29% 5%	720
October	1st to 10th	Night	14% 2 13% 43% 25% 4	720
	11th to 20th	Night	18% 5% 12% 34% 26% 5%	720
	21st to end	Night	20% 4 18% 37% 17% 5%	792
November	1st to 10th	Night	25% 5% 15% 39% 16% 1	740
	11th to 20th	Night	26% 6% 17% 34% 14% 2	760
	21st to end	Night	22% 6% 16% 33% 20% 3	809
December	1st to 10th	Night	24% 4 15% 38% 16% 2	950
	11th to 20th	Night	29% 5% 16% 36% 13% 1	950
	21st to end	Night	33% 8% 19% 29% 10% 1	912

Page generated at 2024-03-05 04:24:01. Version ac07

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