Ross Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ross Creek Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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% 4 2 4 4 2 3 4 5% 7% 60%	74709
	Night	6% 5% 2 4 4 3 4 4 5% 7% 54%	75075
Transparency	Day	24% 5% 9% 20% 24% 17%	74429
	Night	27% 6% 11% 19% 21% 16%	74877
Seeing	Day	7% 11% 23% 31% 21% 7%	14303
	Night	8% 5% 15% 33% 31% 7%	23780

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	13% 7% 4 6% 4 4 5% 5% 6% 8% 37%	1627
	11th to 20th	Day	8% 6% 2 5% 4 2 4 6% 6% 7% 49%	1657
	21st to end	Day	13% 8% 3 5% 5% 3 5% 6% 5% 8% 41%	1782
February	1st to 10th	Day	11% 7% 4 5% 4 3 4 5% 5% 7% 47%	1620
	11th to 20th	Day	11% 7% 2 5% 4 4 5% 5% 6% 7% 45%	1677
	21st to end	Day	11% 8% 3 5% 5% 3 4 3 5% 6% 46%	1480
March	1st to 10th	Day	11% 8% 3 7% 7% 4 5% 4 7% 8% 36%	1931
	11th to 20th	Day	12% 6% 4 5% 6% 5% 5% 6% 5% 9% 37%	1980
	21st to end	Day	10% 5% 2 5% 6% 4 6% 5% 6% 6% 45%	2217
April	1st to 10th	Day	11% 6% 3 6% 5% 3 4 5% 5% 7% 46%	2160
	11th to 20th	Day	9% 5% 2 4 4 2 4 3 5% 7% 55%	2129
	21st to end	Day	5% 4 2 3 3 2 3 4 5% 8% 59%	2160
May	1st to 10th	Day	4 4 2 4 3 2 3 4 4 8% 62%	2192
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 8% 66%	2347
	21st to end	Day	5% 3 2 5% 3 3 3 4 4 8% 60%	2723
June	1st to 10th	Day	4 3 2 3 3 2 2 4 5% 7% 64%	2520
	11th to 20th	Day	2 2 1 1 1 1 2 3 3 7% 76%	2520
	21st to end	Day	1 2 1 1 2 1 2 2 3 5% 79%	2520
July	1st to 10th	Day	1 1 1 1 2 1 2 3 3 5% 80%	2520
	11th to 20th	Day	1 1 1 2 2 1 2 2 4 6% 78%	2347
	21st to end	Day	1 1 1 1 1 1 2 3 4 6% 79%	2546
August	1st to 10th	Day	1 2 1 3 3 2 3 3 4 6% 73%	2328
	11th to 20th	Day	1 1 1 2 1 2 2 4 6% 80%	2164
	21st to end	Day	1 1 2 2 1 2 3 3 6% 78%	2376
September	1st to 10th	Day	1 1 1 2 2 1 2 2 2 4 83%	2160
	11th to 20th	Day	2 2 2 3 3 2 3 3 5% 7% 67%	2160
	21st to end	Day	3 1 1 2 2 1 3 2 3 6% 77%	1918
October	1st to 10th	Day	3 2 1 3 4 2 2 3 4 6% 69%	1800
	11th to 20th	Day	3 3 1 3 3 2 3 4 5% 7% 65%	1800
	21st to end	Day	4 4 2 3 3 2 4 4 6% 6% 63%	1980
November	1st to 10th	Day	6% 5% 2 4 4 3 5% 4 5% 7% 54%	1822
	11th to 20th	Day	5% 5% 2 4 5% 3 5% 5% 6% 8% 52%	1710
	21st to end	Day	9% 4 2 4 4 3 3 4 5% 8% 54%	1594
December	1st to 10th	Day	8% 5% 3 6% 6% 5% 6% 5% 5% 8% 44%	1520
	11th to 20th	Day	15% 9% 5% 9% 7% 3 5% 4 5% 6% 31%	1928
	21st to end	Day	9% 11% 5% 8% 7% 4 5% 6% 5% 7% 33%	2794
January	1st to 10th	Night	11% 8% 3 6% 6% 3 5% 5% 7% 8% 38%	2534
	11th to 20th	Night	8% 6% 3 6% 6% 3 5% 5% 6% 7% 44%	2392
	21st to end	Night	13% 6% 3 6% 4 3 5% 5% 5% 7% 43%	2574
February	1st to 10th	Night	10% 6% 3 5% 5% 3 4 4 4 6% 50%	2340
	11th to 20th	Night	9% 8% 3 4 4 3 5% 5% 6% 7% 46%	2170
	21st to end	Night	10% 8% 4 4 4 3 3 4 4 7% 49%	1776
March	1st to 10th	Night	12% 8% 3 7% 5% 3 5% 6% 6% 7% 38%	2026
	11th to 20th	Night	11% 7% 4 6% 5% 3 4 4 6% 9% 42%	1980
	21st to end	Night	9% 7% 3 5% 5% 3 4 5% 5% 7% 48%	2138
April	1st to 10th	Night	10% 6% 3 4 4 3 5% 5% 6% 7% 46%	1800
	11th to 20th	Night	7% 5% 3 6% 4 2 4 3 4 6% 55%	1779
	21st to end	Night	4 3 2 3 3 2 5% 5% 5% 10% 58%	1800
May	1st to 10th	Night	2 5% 2 4 3 2 5% 5% 5% 8% 59%	1764
	11th to 20th	Night	3 4 2 4 4 2 3 6% 6% 9% 57%	1545
	21st to end	Night	3 4 2 4 4 3 5% 4 4 10% 57%	1552
June	1st to 10th	Night	3 4 1 3 3 2 3 3 4 9% 65%	1440
	11th to 20th	Night	1 2 1 3 3 1 3 4 5% 7% 72%	1440
	21st to end	Night	1 3 1 3 2 2 3 3 4 5% 75%	1440
July	1st to 10th	Night	1 1 1 2 2 3 3 4 7% 76%	1440
	11th to 20th	Night	1 1 2 1 1 3 3 4 9% 73%	1608
	21st to end	Night	2 1 3 3 1 3 6% 6% 9% 66%	1773
August	1st to 10th	Night	1 3 2 5% 4 3 4 4 6% 7% 61%	1611
	11th to 20th	Night	1 1 3 2 2 3 4 5% 10% 68%	1736
	21st to end	Night	2 1 2 3 2 4 4 5% 8% 69%	1980
September	1st to 10th	Night	1 1 1 1 3 2 3 3 4 7% 75%	1800
	11th to 20th	Night	2 3 2 4 3 3 4 4 4 5% 66%	1800
	21st to end	Night	2 2 1 2 2 1 3 3 4 6% 73%	2036
October	1st to 10th	Night	3 2 1 3 3 2 3 4 4 6% 68%	2160
	11th to 20th	Night	4 3 2 4 4 3 4 4 4 7% 61%	2160
	21st to end	Night	4 3 2 3 4 2 4 4 5% 6% 63%	2376
November	1st to 10th	Night	5% 4 2 5% 5% 2 4 4 5% 8% 57%	2253
	11th to 20th	Night	6% 5% 2 5% 4 3 4 5% 6% 8% 51%	2470
	21st to end	Night	6% 4 2 4 5% 3 5% 4 4 7% 57%	2584
December	1st to 10th	Night	8% 7% 4 5% 6% 3 4 5% 4 7% 48%	2660
	11th to 20th	Night	15% 11% 5% 8% 6% 3 4 4 5% 7% 32%	3276
	21st to end	Night	11% 9% 4 8% 8% 5% 7% 6% 6% 6% 31%	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	43% 8% 12% 19% 11% 5%	1627
	11th to 20th	Day	33% 8% 12% 17% 17% 14%	1657
	21st to end	Day	40% 7% 9% 13% 22% 9%	1782
February	1st to 10th	Day	37% 5% 9% 19% 18% 12%	1620
	11th to 20th	Day	37% 6% 9% 17% 22% 8%	1686
	21st to end	Day	39% 6% 8% 21% 20% 7%	1480
March	1st to 10th	Day	45% 7% 11% 18% 15% 4	1931
	11th to 20th	Day	44% 7% 9% 19% 17% 4	1980
	21st to end	Day	39% 8% 13% 19% 17% 4	2217
April	1st to 10th	Day	37% 6% 12% 19% 20% 5%	2160
	11th to 20th	Day	28% 6% 14% 24% 21% 6%	2132
	21st to end	Day	24% 6% 14% 28% 23% 5%	2160
May	1st to 10th	Day	21% 6% 11% 27% 29% 6%	2192
	11th to 20th	Day	19% 4 11% 30% 28% 8%	2350
	21st to end	Day	24% 6% 16% 27% 23% 4	2724
June	1st to 10th	Day	20% 6% 12% 27% 26% 10%	2520
	11th to 20th	Day	10% 4 10% 24% 34% 18%	2520
	21st to end	Day	10% 3 6% 16% 31% 34%	2520
July	1st to 10th	Day	7% 2 5% 13% 27% 46%	2451
	11th to 20th	Day	9% 3 6% 15% 25% 41%	2277
	21st to end	Day	7% 2 4 14% 32% 40%	2405
August	1st to 10th	Day	13% 3 5% 15% 31% 33%	2312
	11th to 20th	Day	7% 2 5% 16% 32% 38%	2164
	21st to end	Day	8% 2 5% 19% 31% 34%	2376
September	1st to 10th	Day	8% 2 4 11% 31% 44%	2160
	11th to 20th	Day	15% 4 8% 24% 30% 19%	2160
	21st to end	Day	11% 3 8% 21% 36% 22%	1918
October	1st to 10th	Day	17% 4 8% 17% 28% 25%	1800
	11th to 20th	Day	18% 6% 10% 19% 22% 25%	1800
	21st to end	Day	21% 6% 9% 20% 23% 22%	1980
November	1st to 10th	Day	29% 6% 9% 20% 23% 13%	1822
	11th to 20th	Day	29% 7% 11% 20% 23% 9%	1710
	21st to end	Day	30% 5% 9% 17% 30% 10%	1594
December	1st to 10th	Day	35% 8% 11% 18% 16% 11%	1520
	11th to 20th	Day	52% 7% 10% 13% 13% 6%	1928
	21st to end	Day	46% 9% 12% 19% 11% 3	2794
January	1st to 10th	Night	40% 8% 13% 20% 13% 6%	2534
	11th to 20th	Night	37% 7% 12% 15% 15% 14%	2392
	21st to end	Night	39% 7% 11% 12% 18% 13%	2574
February	1st to 10th	Night	35% 5% 9% 18% 20% 13%	2340
	11th to 20th	Night	35% 6% 11% 18% 22% 8%	2172
	21st to end	Night	33% 6% 12% 26% 17% 6%	1776
March	1st to 10th	Night	43% 7% 13% 16% 14% 5%	2026
	11th to 20th	Night	39% 6% 10% 19% 19% 7%	1980
	21st to end	Night	34% 8% 14% 19% 18% 6%	2138
April	1st to 10th	Night	34% 8% 14% 20% 18% 6%	1800
	11th to 20th	Night	30% 5% 13% 27% 18% 8%	1779
	21st to end	Night	21% 6% 18% 27% 22% 7%	1800
May	1st to 10th	Night	22% 7% 12% 24% 24% 12%	1764
	11th to 20th	Night	20% 6% 17% 25% 24% 8%	1546
	21st to end	Night	21% 9% 18% 23% 24% 4	1552
June	1st to 10th	Night	18% 5% 16% 26% 24% 11%	1440
	11th to 20th	Night	11% 5% 11% 20% 29% 24%	1440
	21st to end	Night	11% 4 8% 18% 26% 32%	1440
July	1st to 10th	Night	8% 3 4 16% 28% 41%	1400
	11th to 20th	Night	8% 3 7% 20% 24% 38%	1555
	21st to end	Night	15% 3 9% 20% 24% 30%	1674
August	1st to 10th	Night	20% 4 7% 19% 24% 26%	1602
	11th to 20th	Night	12% 2 9% 15% 28% 34%	1736
	21st to end	Night	10% 3 7% 19% 29% 30%	1980
September	1st to 10th	Night	9% 3 5% 13% 26% 45%	1800
	11th to 20th	Night	16% 5% 9% 22% 28% 20%	1800
	21st to end	Night	12% 3 8% 19% 32% 26%	2036
October	1st to 10th	Night	16% 5% 11% 16% 23% 28%	2160
	11th to 20th	Night	20% 5% 9% 20% 21% 24%	2160
	21st to end	Night	20% 4 8% 21% 24% 23%	2376
November	1st to 10th	Night	24% 7% 10% 23% 20% 16%	2253
	11th to 20th	Night	29% 6% 10% 21% 24% 10%	2470
	21st to end	Night	28% 5% 10% 19% 23% 16%	2584
December	1st to 10th	Night	35% 7% 10% 17% 18% 13%	2660
	11th to 20th	Night	52% 5% 10% 13% 15% 5%	3276
	21st to end	Night	45% 11% 14% 14% 11% 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	24% 16% 23% 20% 15% 1	288
	11th to 20th	Day	13% 14% 26% 29% 14% 4	353
	21st to end	Day	14% 13% 25% 27% 18% 4	374
February	1st to 10th	Day	14% 19% 24% 22% 16% 5%	340
	11th to 20th	Day	16% 18% 29% 25% 10% 2	381
	21st to end	Day	12% 20% 29% 24% 13% 1	346
March	1st to 10th	Day	16% 19% 32% 23% 8% 1	420
	11th to 20th	Day	17% 20% 25% 22% 12% 4	420
	21st to end	Day	13% 19% 32% 25% 9% 2	462
April	1st to 10th	Day	13% 17% 32% 23% 13% 2	420
	11th to 20th	Day	13% 19% 30% 23% 13% 2	412
	21st to end	Day	5% 20% 27% 29% 12% 7%	420
May	1st to 10th	Day	7% 16% 30% 29% 13% 4	423
	11th to 20th	Day	8% 13% 31% 27% 14% 6%	448
	21st to end	Day	6% 12% 27% 31% 17% 6%	485
June	1st to 10th	Day	4 12% 27% 30% 20% 8%	450
	11th to 20th	Day	2 10% 29% 36% 19% 4	450
	21st to end	Day	1 4 22% 39% 25% 9%	450
July	1st to 10th	Day	1 2 17% 41% 33% 6%	446
	11th to 20th	Day	1 11% 45% 31% 12%	444
	21st to end	Day	1 11% 40% 33% 15%	487
August	1st to 10th	Day	1 2 19% 41% 28% 8%	447
	11th to 20th	Day	2 16% 40% 25% 17%	437
	21st to end	Day	1 12% 42% 33% 12%	495
September	1st to 10th	Day	1 5% 12% 41% 31% 11%	450
	11th to 20th	Day	2 5% 24% 40% 20% 10%	450
	21st to end	Day	4 7% 18% 36% 29% 8%	450
October	1st to 10th	Day	2 9% 23% 31% 25% 10%	450
	11th to 20th	Day	5% 10% 19% 29% 28% 9%	450
	21st to end	Day	7% 8% 21% 33% 24% 7%	495
November	1st to 10th	Day	7% 13% 19% 25% 29% 7%	418
	11th to 20th	Day	3 10% 21% 28% 28% 10%	270
	21st to end	Day	7% 9% 21% 32% 26% 5%	214
December	1st to 10th	Day	9% 16% 18% 30% 19% 8%	180
	11th to 20th	Day	9% 22% 20% 23% 17% 9%	180
	21st to end	Day	11% 25% 23% 21% 15% 5%	198
January	1st to 10th	Night	16% 7% 18% 30% 23% 6%	760
	11th to 20th	Night	12% 7% 16% 35% 22% 8%	736
	21st to end	Night	16% 6% 15% 33% 25% 6%	792
February	1st to 10th	Night	13% 9% 14% 33% 25% 5%	720
	11th to 20th	Night	13% 9% 20% 33% 20% 5%	704
	21st to end	Night	15% 9% 21% 34% 18% 3	592
March	1st to 10th	Night	15% 11% 21% 29% 21% 3	720
	11th to 20th	Night	14% 9% 19% 28% 25% 5%	720
	21st to end	Night	11% 10% 22% 35% 19% 3	792
April	1st to 10th	Night	12% 6% 24% 33% 22% 3	720
	11th to 20th	Night	10% 12% 21% 32% 20% 6%	711
	21st to end	Night	6% 8% 26% 32% 23% 7%	720
May	1st to 10th	Night	5% 4 25% 34% 27% 5%	684
	11th to 20th	Night	6% 3 13% 36% 35% 6%	483
	21st to end	Night	4 5% 17% 40% 31% 3	389
June	1st to 10th	Night	3 4 15% 42% 31% 5%	360
	11th to 20th	Night	1 4 11% 48% 33% 2	360
	21st to end	Night	1 9% 39% 47% 3	360
July	1st to 10th	Night	1 4 37% 55% 4	350
	11th to 20th	Night	1 1 29% 56% 13%	511
	21st to end	Night	1 1 1 29% 55% 13%	591
August	1st to 10th	Night	3 4 34% 52% 7%	537
	11th to 20th	Night	1 7% 38% 46% 8%	668
	21st to end	Night	1 6% 32% 50% 10%	792
September	1st to 10th	Night	1 1 8% 30% 48% 12%	720
	11th to 20th	Night	3 1 9% 38% 41% 8%	720
	21st to end	Night	3 2 11% 32% 40% 11%	720
October	1st to 10th	Night	4 3 12% 36% 34% 11%	720
	11th to 20th	Night	6% 4 12% 30% 38% 11%	720
	21st to end	Night	5% 5% 11% 32% 31% 16%	792
November	1st to 10th	Night	6% 4 16% 30% 34% 10%	740
	11th to 20th	Night	9% 3 16% 33% 32% 7%	760
	21st to end	Night	8% 4 16% 39% 26% 7%	760
December	1st to 10th	Night	13% 6% 18% 32% 25% 6%	760
	11th to 20th	Night	17% 6% 19% 33% 21% 5%	760
	21st to end	Night	14% 12% 21% 31% 18% 4	836

Page generated at 2024-01-11 19:33:48. Version ac07

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