Centennial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Centennial 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	6% 5% 2 5% 4 3 4 4 5% 7% 53%	74892
	Night	7% 6% 3 5% 5% 3 5% 5% 5% 7% 50%	74856
Transparency	Day	30% 5% 11% 22% 20% 11%	74600
	Night	33% 5% 12% 22% 18% 9%	74670
Seeing	Day	9% 9% 23% 28% 22% 8%	13383
	Night	10% 6% 24% 37% 20% 3	22968

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% 12% 4 10% 7% 3 4 5% 6% 7% 32%	1519
	11th to 20th	Day	8% 8% 5% 9% 7% 4 5% 5% 5% 7% 38%	1456
	21st to end	Day	9% 12% 4 7% 7% 4 5% 6% 6% 8% 33%	1659
February	1st to 10th	Day	8% 8% 4 9% 5% 3 4 4 7% 9% 39%	1620
	11th to 20th	Day	9% 10% 4 6% 4 3 6% 4 5% 7% 42%	1736
	21st to end	Day	6% 11% 5% 7% 6% 4 5% 5% 5% 8% 38%	1480
March	1st to 10th	Day	7% 9% 5% 6% 6% 4 6% 5% 7% 9% 36%	1800
	11th to 20th	Day	7% 7% 3 7% 6% 3 6% 5% 6% 9% 42%	1975
	21st to end	Day	10% 8% 4 6% 6% 3 5% 5% 5% 9% 39%	2178
April	1st to 10th	Day	9% 7% 3 5% 6% 3 6% 4 6% 9% 43%	2145
	11th to 20th	Day	7% 7% 3 5% 5% 3 5% 4 5% 8% 48%	2230
	21st to end	Day	8% 5% 2 5% 5% 3 4 4 5% 7% 52%	2340
May	1st to 10th	Day	6% 4 2 4 5% 3 5% 5% 5% 8% 52%	2340
	11th to 20th	Day	7% 4 2 4 4 3 4 6% 7% 10% 49%	2301
	21st to end	Day	7% 4 3 5% 6% 4 6% 5% 7% 9% 44%	2615
June	1st to 10th	Day	5% 4 2 3 4 3 4 5% 7% 9% 53%	2520
	11th to 20th	Day	5% 3 1 3 4 3 4 4 5% 7% 61%	2520
	21st to end	Day	1 2 1 2 3 2 3 3 5% 6% 74%	2520
July	1st to 10th	Day	2 2 1 2 2 2 4 4 5% 7% 69%	2520
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 5% 74%	2520
	21st to end	Day	3 2 1 3 2 3 4 4 5% 6% 67%	2670
August	1st to 10th	Day	5% 3 1 3 3 3 4 4 5% 6% 62%	2328
	11th to 20th	Day	2 1 2 2 1 3 3 4 6% 75%	2299
	21st to end	Day	2 2 1 2 3 2 2 4 4 6% 72%	2479
September	1st to 10th	Day	3 2 1 2 3 2 2 3 3 6% 74%	2098
	11th to 20th	Day	4 3 1 3 2 2 3 3 4 6% 69%	1980
	21st to end	Day	7% 3 1 3 3 2 3 3 3 5% 66%	1980
October	1st to 10th	Day	9% 4 1 3 4 2 4 4 5% 6% 57%	1980
	11th to 20th	Day	6% 3 2 4 4 2 3 3 4 7% 61%	1800
	21st to end	Day	5% 3 1 4 4 2 5% 5% 5% 8% 57%	1782
November	1st to 10th	Day	9% 7% 3 5% 5% 3 4 4 5% 7% 49%	1674
	11th to 20th	Day	9% 6% 3 5% 5% 3 5% 6% 7% 9% 44%	1710
	21st to end	Day	7% 9% 4 6% 6% 4 5% 5% 4 6% 45%	1710
December	1st to 10th	Day	11% 9% 3 8% 5% 3 5% 4 4 6% 42%	1686
	11th to 20th	Day	10% 9% 5% 7% 6% 3 4 7% 7% 10% 33%	1928
	21st to end	Day	9% 9% 5% 8% 6% 3 5% 4 6% 8% 38%	2794
January	1st to 10th	Night	10% 10% 3 8% 6% 3 5% 5% 5% 8% 35%	2646
	11th to 20th	Night	10% 10% 3 7% 6% 4 5% 6% 5% 8% 36%	2548
	21st to end	Night	11% 10% 5% 7% 5% 3 5% 5% 5% 6% 37%	2696
February	1st to 10th	Night	9% 9% 4 7% 6% 4 5% 5% 6% 9% 37%	2340
	11th to 20th	Night	9% 8% 3 6% 4 2 5% 5% 5% 8% 45%	2107
	21st to end	Night	7% 8% 3 5% 5% 3 6% 7% 7% 9% 39%	1776
March	1st to 10th	Night	7% 8% 3 5% 6% 3 7% 5% 7% 8% 40%	2160
	11th to 20th	Night	6% 7% 3 6% 5% 3 6% 6% 6% 8% 43%	1985
	21st to end	Night	9% 8% 4 7% 6% 4 6% 6% 6% 6% 39%	2178
April	1st to 10th	Night	8% 7% 3 5% 6% 3 5% 6% 6% 7% 45%	1813
	11th to 20th	Night	7% 7% 3 5% 5% 4 5% 4 5% 6% 48%	1678
	21st to end	Night	6% 5% 2 5% 4 3 4 3 5% 8% 54%	1620
May	1st to 10th	Night	7% 5% 2 5% 5% 3 4 5% 7% 6% 51%	1620
	11th to 20th	Night	6% 5% 2 5% 6% 3 5% 6% 6% 9% 49%	1594
	21st to end	Night	7% 6% 2 5% 5% 3 6% 6% 7% 8% 43%	1656
June	1st to 10th	Night	5% 2 2 4 4 3 6% 6% 6% 9% 53%	1440
	11th to 20th	Night	4 3 2 2 4 3 4 4 5% 6% 64%	1440
	21st to end	Night	2 2 1 2 2 1 3 3 4 6% 73%	1440
July	1st to 10th	Night	1 2 1 2 3 2 5% 5% 5% 8% 67%	1440
	11th to 20th	Night	1 1 1 2 2 2 4 3 5% 6% 75%	1440
	21st to end	Night	2 2 1 2 4 2 4 4 5% 5% 68%	1643
August	1st to 10th	Night	3 2 2 3 4 2 4 4 6% 6% 63%	1611
	11th to 20th	Night	2 2 1 2 3 2 2 4 6% 7% 69%	1602
	21st to end	Night	3 1 1 2 4 3 3 3 5% 5% 71%	1875
September	1st to 10th	Night	3 2 1 2 2 2 3 3 3 5% 73%	1862
	11th to 20th	Night	4 2 1 3 3 2 3 3 5% 6% 67%	1980
	21st to end	Night	6% 3 1 3 3 2 3 3 4 4 68%	1980
October	1st to 10th	Night	7% 3 2 4 4 2 4 3 4 7% 61%	1980
	11th to 20th	Night	8% 3 1 3 4 2 3 3 5% 7% 60%	2157
	21st to end	Night	5% 4 2 4 4 3 5% 4 4 8% 57%	2574
November	1st to 10th	Night	10% 6% 3 4 5% 3 4 4 6% 7% 48%	2405
	11th to 20th	Night	9% 6% 2 6% 5% 3 4 5% 6% 7% 47%	2470
	21st to end	Night	7% 6% 3 5% 5% 4 4 5% 5% 8% 47%	2470
December	1st to 10th	Night	10% 9% 4 7% 5% 3 4 5% 5% 8% 40%	2492
	11th to 20th	Night	12% 10% 4 7% 5% 4 6% 5% 6% 8% 32%	3276
	21st to end	Night	10% 9% 4 7% 6% 4 4 5% 6% 6% 39%	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	52% 8% 19% 17% 2 1	1519
	11th to 20th	Day	44% 9% 20% 20% 6% 1	1456
	21st to end	Day	49% 10% 19% 17% 5%	1659
February	1st to 10th	Day	44% 8% 20% 22% 5% 1	1620
	11th to 20th	Day	44% 7% 15% 23% 11% 1	1746
	21st to end	Day	46% 6% 13% 23% 10% 1	1480
March	1st to 10th	Day	43% 8% 15% 23% 9% 2	1800
	11th to 20th	Day	38% 6% 16% 27% 12% 2	1975
	21st to end	Day	45% 6% 16% 22% 10% 1	2178
April	1st to 10th	Day	39% 6% 13% 22% 17% 4	2145
	11th to 20th	Day	35% 6% 10% 28% 18% 3	2233
	21st to end	Day	32% 5% 12% 27% 23% 2	2340
May	1st to 10th	Day	30% 7% 7% 27% 26% 4	2340
	11th to 20th	Day	29% 6% 12% 31% 17% 4	2304
	21st to end	Day	35% 7% 13% 27% 16% 2	2615
June	1st to 10th	Day	27% 6% 13% 30% 22% 3	2520
	11th to 20th	Day	24% 4 9% 21% 32% 11%	2520
	21st to end	Day	12% 3 5% 20% 37% 23%	2520
July	1st to 10th	Day	13% 4 4 18% 33% 29%	2451
	11th to 20th	Day	13% 3 3 10% 30% 42%	2445
	21st to end	Day	16% 4 3 12% 24% 40%	2521
August	1st to 10th	Day	22% 4 5% 16% 25% 28%	2313
	11th to 20th	Day	12% 3 3 17% 37% 28%	2299
	21st to end	Day	14% 4 3 17% 37% 25%	2479
September	1st to 10th	Day	15% 3 5% 14% 30% 33%	2098
	11th to 20th	Day	19% 3 6% 23% 28% 21%	1980
	21st to end	Day	22% 3 5% 22% 35% 14%	1980
October	1st to 10th	Day	28% 4 10% 26% 23% 9%	1980
	11th to 20th	Day	25% 3 9% 34% 23% 5%	1800
	21st to end	Day	26% 4 15% 32% 20% 3	1782
November	1st to 10th	Day	36% 7% 12% 27% 16% 2	1674
	11th to 20th	Day	36% 6% 18% 30% 8% 1	1710
	21st to end	Day	40% 5% 17% 25% 12%	1710
December	1st to 10th	Day	44% 8% 17% 22% 8% 2	1686
	11th to 20th	Day	45% 7% 26% 19% 3	1928
	21st to end	Day	44% 6% 19% 21% 10%	2794
January	1st to 10th	Night	47% 7% 20% 19% 5% 1	2646
	11th to 20th	Night	46% 8% 18% 21% 6% 1	2548
	21st to end	Night	48% 7% 18% 22% 5%	2696
February	1st to 10th	Night	45% 8% 19% 23% 5% 1	2340
	11th to 20th	Night	38% 6% 17% 26% 13% 1	2108
	21st to end	Night	39% 7% 16% 27% 10% 1	1776
March	1st to 10th	Night	39% 8% 15% 26% 9% 2	2160
	11th to 20th	Night	37% 5% 14% 29% 15% 2	1985
	21st to end	Night	44% 6% 16% 22% 11% 1	2178
April	1st to 10th	Night	37% 6% 10% 25% 16% 6%	1813
	11th to 20th	Night	36% 5% 9% 27% 19% 3	1678
	21st to end	Night	29% 4 11% 28% 26% 2	1620
May	1st to 10th	Night	30% 6% 7% 23% 30% 5%	1620
	11th to 20th	Night	31% 6% 11% 26% 22% 4	1595
	21st to end	Night	36% 6% 12% 26% 17% 3	1656
June	1st to 10th	Night	25% 5% 15% 27% 24% 4	1440
	11th to 20th	Night	21% 5% 7% 21% 32% 13%	1440
	21st to end	Night	14% 2 4 17% 31% 31%	1440
July	1st to 10th	Night	14% 5% 3 13% 31% 34%	1400
	11th to 20th	Night	11% 3 3 9% 24% 50%	1392
	21st to end	Night	16% 3 3 10% 23% 44%	1552
August	1st to 10th	Night	20% 4 6% 16% 21% 34%	1602
	11th to 20th	Night	13% 5% 3 14% 31% 34%	1602
	21st to end	Night	16% 3 3 12% 33% 33%	1875
September	1st to 10th	Night	15% 3 3 11% 31% 36%	1862
	11th to 20th	Night	18% 4 5% 21% 28% 24%	1980
	21st to end	Night	19% 5% 5% 17% 36% 19%	1980
October	1st to 10th	Night	25% 3 9% 23% 27% 12%	1980
	11th to 20th	Night	25% 4 8% 28% 30% 6%	2157
	21st to end	Night	27% 4 13% 31% 21% 4	2574
November	1st to 10th	Night	36% 5% 12% 24% 19% 4	2405
	11th to 20th	Night	35% 5% 15% 30% 14% 1	2470
	21st to end	Night	34% 6% 16% 30% 12% 1	2470
December	1st to 10th	Night	44% 7% 20% 21% 7% 1	2492
	11th to 20th	Night	49% 6% 18% 24% 4	3276
	21st to end	Night	44% 6% 18% 22% 9%	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	18% 14% 26% 22% 19% 2	180
	11th to 20th	Day	9% 9% 29% 26% 19% 8%	164
	21st to end	Day	16% 10% 24% 26% 17% 6%	210
February	1st to 10th	Day	18% 14% 32% 22% 13% 2	240
	11th to 20th	Day	15% 16% 30% 23% 13% 3	401
	21st to end	Day	10% 20% 32% 28% 10% 1	346
March	1st to 10th	Day	11% 12% 37% 27% 12% 1	420
	11th to 20th	Day	9% 13% 31% 30% 15% 2	420
	21st to end	Day	17% 17% 30% 25% 9% 2	462
April	1st to 10th	Day	13% 19% 28% 20% 16% 5%	420
	11th to 20th	Day	11% 20% 29% 24% 12% 4	412
	21st to end	Day	10% 17% 32% 21% 16% 4	420
May	1st to 10th	Day	4 16% 25% 31% 20% 5%	423
	11th to 20th	Day	12% 8% 23% 29% 19% 10%	448
	21st to end	Day	6% 10% 21% 25% 24% 14%	485
June	1st to 10th	Day	5% 10% 22% 28% 20% 15%	450
	11th to 20th	Day	6% 10% 28% 30% 18% 7%	450
	21st to end	Day	2 4 23% 34% 27% 10%	450
July	1st to 10th	Day	2 2 17% 39% 30% 10%	446
	11th to 20th	Day	2 2 19% 40% 32% 6%	444
	21st to end	Day	3 2 13% 36% 34% 13%	487
August	1st to 10th	Day	6% 2 18% 34% 31% 10%	447
	11th to 20th	Day	3 3 15% 29% 34% 16%	437
	21st to end	Day	1 3 17% 31% 32% 15%	495
September	1st to 10th	Day	3 4 18% 29% 27% 20%	450
	11th to 20th	Day	5% 4 18% 29% 28% 16%	450
	21st to end	Day	8% 6% 20% 27% 28% 11%	450
October	1st to 10th	Day	10% 6% 21% 27% 23% 14%	450
	11th to 20th	Day	8% 11% 19% 21% 31% 10%	283
	21st to end	Day	9% 9% 23% 30% 23% 5%	297
November	1st to 10th	Day	14% 9% 29% 25% 21% 3	270
	11th to 20th	Day	11% 10% 19% 29% 24% 7%	270
	21st to end	Day	13% 9% 18% 26% 27% 6%	270
December	1st to 10th	Day	17% 10% 25% 26% 16% 5%	258
	11th to 20th	Day	17% 11% 23% 30% 15% 4	180
	21st to end	Day	19% 12% 21% 28% 15% 6%	198
January	1st to 10th	Night	16% 9% 23% 32% 19% 1	760
	11th to 20th	Night	14% 9% 21% 34% 19% 2	728
	21st to end	Night	15% 8% 22% 32% 20% 3	792
February	1st to 10th	Night	14% 10% 30% 32% 14%	720
	11th to 20th	Night	14% 8% 30% 33% 14% 1	704
	21st to end	Night	11% 11% 31% 32% 14% 1	592
March	1st to 10th	Night	10% 9% 32% 32% 15% 2	720
	11th to 20th	Night	8% 8% 32% 35% 14% 2	720
	21st to end	Night	14% 10% 33% 32% 10% 2	792
April	1st to 10th	Night	11% 10% 29% 34% 15% 2	720
	11th to 20th	Night	9% 11% 34% 32% 12% 1	610
	21st to end	Night	7% 9% 31% 35% 16% 2	540
May	1st to 10th	Night	8% 8% 28% 36% 18% 2	540
	11th to 20th	Night	8% 6% 18% 39% 25% 4	531
	21st to end	Night	10% 5% 24% 31% 25% 4	494
June	1st to 10th	Night	7% 5% 22% 38% 26% 4	360
	11th to 20th	Night	4 4 27% 46% 18% 1	360
	21st to end	Night	2 2 22% 49% 23% 3	360
July	1st to 10th	Night	2 1 11% 59% 25% 3	350
	11th to 20th	Night	1 21% 52% 25% 1	348
	21st to end	Night	2 1 10% 49% 35% 4	461
August	1st to 10th	Night	3 1 14% 50% 30% 2	537
	11th to 20th	Night	2 1 13% 49% 31% 3	534
	21st to end	Night	3 1 16% 47% 30% 3	687
September	1st to 10th	Night	4 2 15% 43% 30% 6%	720
	11th to 20th	Night	5% 2 14% 40% 32% 7%	720
	21st to end	Night	7% 4 16% 38% 27% 9%	720
October	1st to 10th	Night	8% 5% 21% 40% 23% 4	720
	11th to 20th	Night	9% 5% 20% 35% 26% 5%	720
	21st to end	Night	7% 7% 24% 36% 22% 4	792
November	1st to 10th	Night	12% 6% 24% 34% 22% 3	740
	11th to 20th	Night	11% 8% 26% 38% 15% 3	760
	21st to end	Night	10% 7% 26% 32% 21% 4	760
December	1st to 10th	Night	14% 7% 28% 33% 16% 3	760
	11th to 20th	Night	19% 6% 22% 34% 15% 3	760
	21st to end	Night	19% 11% 27% 31% 10% 1	836

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