D.A.R.C. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the D.A.R.C. Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	3 3 1 3 2 2 3 3 4 5% 71%	104929
	Night	4 3 1 3 2 2 3 3 4 5% 69%	102873
Transparency	Day	17% 3 7% 18% 27% 28%	104683
	Night	18% 3 9% 19% 25% 26%	102709
Seeing	Day	5% 7% 19% 32% 27% 10%	15418
	Night	5% 4 15% 35% 32% 8%	24869

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	8% 6% 3 5% 4 4 6% 4 5% 8% 46%	2610
	11th to 20th	Day	7% 5% 3 7% 7% 6% 7% 5% 6% 8% 39%	2860
	21st to end	Day	14% 7% 2 7% 5% 5% 6% 4 6% 8% 37%	3146
February	1st to 10th	Day	11% 6% 2 4 3 3 4 4 5% 7% 51%	1717
	11th to 20th	Day	6% 4 2 4 4 4 4 5% 5% 7% 54%	1850
	21st to end	Day	7% 6% 2 5% 4 4 3 4 6% 7% 51%	1570
March	1st to 10th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 11% 38%	3248
	11th to 20th	Day	5% 4 1 4 3 3 3 3 4 8% 61%	3490
	21st to end	Day	6% 5% 2 6% 4 5% 6% 6% 6% 8% 46%	3979
April	1st to 10th	Day	5% 6% 3 5% 4 3 4 3 6% 7% 55%	2280
	11th to 20th	Day	4 5% 1 3 3 3 3 3 5% 6% 62%	2249
	21st to end	Day	4 3 1 3 3 2 3 3 4 6% 70%	2280
May	1st to 10th	Day	3 2 1 2 2 2 2 2 3 4 78%	3800
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 85%	4142
	21st to end	Day	2 1 1 1 1 1 2 2 4 84%	4813
June	1st to 10th	Day	2 2 2 2 2 2 2 4 6% 75%	2660
	11th to 20th	Day	1 1 1 1 1 2 4 89%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	2660
July	1st to 10th	Day	1 1 1 1 94%	4420
	11th to 20th	Day	1 1 1 2 1 1 1 1 3 2 86%	4249
	21st to end	Day	1 1 1 1 1 1 2 2 91%	4455
August	1st to 10th	Day	1 1 1 1 1 1 1 1 3 4 85%	4013
	11th to 20th	Day	1 1 1 2 94%	3724
	21st to end	Day	1 1 1 1 2 2 92%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 92%	2280
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 85%	2280
	21st to end	Day	1 1 1 1 2 1 1 3 4 84%	2071
October	1st to 10th	Day	1 1 1 2 1 1 2 2 3 5% 81%	3180
	11th to 20th	Day	1 1 2 1 2 2 2 3 4 81%	3161
	21st to end	Day	2 2 1 3 2 3 3 3 3 7% 70%	3448
November	1st to 10th	Day	4 3 2 3 3 4 4 3 6% 8% 61%	1796
	11th to 20th	Day	5% 4 1 4 3 3 4 4 4 7% 62%	1710
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 7% 61%	1706
December	1st to 10th	Day	6% 5% 2 6% 5% 4 6% 4 6% 8% 49%	1520
	11th to 20th	Day	12% 9% 3 8% 6% 6% 6% 3 5% 6% 36%	1928
	21st to end	Day	8% 8% 2 7% 5% 5% 5% 5% 6% 8% 40%	2794
January	1st to 10th	Night	7% 7% 2 5% 4 3 4 3 5% 7% 52%	4334
	11th to 20th	Night	10% 7% 3 7% 5% 4 6% 4 6% 8% 40%	4110
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 6% 8% 43%	4521
February	1st to 10th	Night	9% 5% 1 4 4 3 4 2 5% 7% 56%	2461
	11th to 20th	Night	6% 3 1 3 4 3 4 4 6% 10% 57%	2218
	21st to end	Night	5% 5% 2 4 4 4 3 4 5% 6% 59%	1884
March	1st to 10th	Night	6% 5% 2 5% 5% 4 6% 5% 7% 8% 47%	3624
	11th to 20th	Night	3 3 1 4 3 3 4 3 4 5% 64%	3490
	21st to end	Night	6% 5% 2 4 3 4 4 4 6% 8% 54%	3674
April	1st to 10th	Night	4 4 1 4 3 4 4 4 5% 7% 60%	1900
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 6% 67%	1879
	21st to end	Night	2 1 1 1 2 2 3 3 4 7% 74%	1900
May	1st to 10th	Night	1 1 1 1 1 1 1 3 4 85%	3180
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 4 88%	2752
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	2762
June	1st to 10th	Night	1 1 2 2 2 3 3 4 5% 78%	1520
	11th to 20th	Night	1 1 1 1 1 2 3 89%	1520
	21st to end	Night	1 1 1 1 2 3 91%	1520
July	1st to 10th	Night	1 1 1 1 1 1 95%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	2699
	21st to end	Night	1 1 1 1 1 1 2 3 89%	3109
August	1st to 10th	Night	1 1 1 1 1 1 2 2 3 87%	2928
	11th to 20th	Night	1 1 1 1 1 1 2 92%	3126
	21st to end	Night	1 1 1 1 1 1 2 1 91%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 91%	1900
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	1900
	21st to end	Night	1 1 1 1 1 2 2 3 87%	2106
October	1st to 10th	Night	2 1 1 1 1 2 2 3 4 83%	3800
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 82%	3777
	21st to end	Night	2 2 1 2 2 1 1 2 3 5% 78%	4121
November	1st to 10th	Night	4 2 1 2 2 2 3 3 4 7% 69%	2380
	11th to 20th	Night	5% 4 1 3 3 2 3 3 5% 8% 63%	2470
	21st to end	Night	4 4 1 3 3 3 4 3 5% 7% 63%	2472
December	1st to 10th	Night	8% 5% 2 5% 3 4 4 3 4 7% 55%	2660
	11th to 20th	Night	11% 7% 2 7% 5% 6% 5% 4 7% 8% 38%	3276
	21st to end	Night	11% 7% 2 6% 4 4 4 4 7% 8% 43%	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	34% 7% 16% 30% 10% 3	2610
	11th to 20th	Day	41% 5% 14% 20% 14% 6%	2860
	21st to end	Day	43% 6% 14% 19% 15% 3	3146
February	1st to 10th	Day	33% 4 8% 24% 20% 11%	1717
	11th to 20th	Day	29% 5% 11% 22% 25% 9%	1860
	21st to end	Day	32% 6% 10% 24% 22% 6%	1570
March	1st to 10th	Day	33% 8% 19% 20% 17% 3	3248
	11th to 20th	Day	23% 4 10% 32% 19% 12%	3490
	21st to end	Day	33% 8% 13% 26% 16% 4	3979
April	1st to 10th	Day	28% 5% 11% 26% 21% 9%	2280
	11th to 20th	Day	22% 5% 9% 29% 27% 8%	2252
	21st to end	Day	18% 3 8% 32% 31% 7%	2280
May	1st to 10th	Day	12% 4 6% 24% 34% 20%	3800
	11th to 20th	Day	8% 2 3 17% 46% 24%	4145
	21st to end	Day	8% 2 4 26% 46% 14%	4814
June	1st to 10th	Day	12% 3 8% 21% 35% 21%	2660
	11th to 20th	Day	4 1 4 16% 40% 35%	2660
	21st to end	Day	6% 1 1 7% 29% 56%	2660
July	1st to 10th	Day	2 1 2 13% 82%	4351
	11th to 20th	Day	7% 1 2 5% 14% 70%	4175
	21st to end	Day	4 1 1 7% 27% 61%	4315
August	1st to 10th	Day	6% 1 1 9% 30% 52%	3998
	11th to 20th	Day	2 1 1 5% 34% 57%	3760
	21st to end	Day	3 1 2 8% 29% 57%	4179
September	1st to 10th	Day	4 1 2 7% 32% 55%	2280
	11th to 20th	Day	6% 2 2 18% 44% 28%	2280
	21st to end	Day	7% 2 2 16% 43% 30%	2071
October	1st to 10th	Day	9% 2 4 11% 25% 50%	3180
	11th to 20th	Day	9% 2 5% 25% 40% 19%	3161
	21st to end	Day	16% 2 4 17% 43% 18%	3448
November	1st to 10th	Day	21% 5% 10% 22% 27% 15%	1796
	11th to 20th	Day	21% 5% 12% 25% 28% 10%	1710
	21st to end	Day	25% 4 9% 22% 26% 14%	1706
December	1st to 10th	Day	31% 6% 12% 19% 19% 13%	1520
	11th to 20th	Day	46% 7% 9% 17% 15% 5%	1928
	21st to end	Day	38% 8% 14% 26% 11% 3	2794
January	1st to 10th	Night	31% 4 17% 30% 15% 3	4334
	11th to 20th	Night	41% 6% 15% 20% 11% 7%	4110
	21st to end	Night	39% 5% 15% 20% 16% 5%	4521
February	1st to 10th	Night	29% 4 8% 23% 24% 12%	2461
	11th to 20th	Night	22% 5% 12% 25% 28% 9%	2219
	21st to end	Night	25% 5% 11% 30% 22% 6%	1884
March	1st to 10th	Night	30% 8% 20% 21% 18% 4	3624
	11th to 20th	Night	21% 5% 13% 28% 21% 13%	3490
	21st to end	Night	26% 6% 19% 29% 16% 4	3674
April	1st to 10th	Night	23% 4 15% 27% 23% 9%	1900
	11th to 20th	Night	17% 5% 13% 29% 26% 10%	1879
	21st to end	Night	11% 3 11% 33% 32% 9%	1900
May	1st to 10th	Night	6% 3 7% 29% 35% 21%	3180
	11th to 20th	Night	6% 1 4 16% 42% 32%	2752
	21st to end	Night	5% 1 5% 25% 46% 18%	2762
June	1st to 10th	Night	11% 3 8% 19% 33% 27%	1520
	11th to 20th	Night	4 2 4 15% 37% 37%	1520
	21st to end	Night	3 1 1 9% 27% 59%	1520
July	1st to 10th	Night	2 1 3 12% 82%	2500
	11th to 20th	Night	6% 1 2 4 13% 76%	2651
	21st to end	Night	4 1 1 6% 22% 66%	3011
August	1st to 10th	Night	6% 1 1 6% 23% 63%	2919
	11th to 20th	Night	3 1 4 23% 69%	3156
	21st to end	Night	4 1 2 7% 27% 59%	3498
September	1st to 10th	Night	4 2 1 6% 24% 63%	1900
	11th to 20th	Night	6% 1 2 16% 40% 35%	1900
	21st to end	Night	6% 2 1 14% 42% 35%	2106
October	1st to 10th	Night	8% 1 4 11% 19% 57%	3800
	11th to 20th	Night	9% 3 4 24% 36% 24%	3777
	21st to end	Night	12% 2 3 18% 44% 22%	4121
November	1st to 10th	Night	16% 4 9% 24% 28% 19%	2380
	11th to 20th	Night	20% 4 12% 22% 31% 12%	2470
	21st to end	Night	21% 5% 9% 21% 29% 14%	2472
December	1st to 10th	Night	29% 5% 10% 22% 20% 13%	2660
	11th to 20th	Night	42% 6% 13% 17% 17% 5%	3276
	21st to end	Night	38% 5% 18% 22% 13% 4	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	20% 9% 26% 25% 17% 3	225
	11th to 20th	Day	9% 7% 22% 36% 21% 5%	370
	21st to end	Day	10% 8% 23% 29% 25% 5%	407
February	1st to 10th	Day	15% 11% 24% 22% 21% 7%	374
	11th to 20th	Day	9% 13% 25% 32% 18% 2	455
	21st to end	Day	10% 14% 26% 34% 14% 2	382
March	1st to 10th	Day	13% 16% 25% 32% 12% 2	460
	11th to 20th	Day	13% 13% 28% 28% 15% 3	460
	21st to end	Day	12% 12% 31% 32% 12% 1	506
April	1st to 10th	Day	10% 11% 28% 30% 18% 3	460
	11th to 20th	Day	8% 12% 29% 30% 18% 3	452
	21st to end	Day	4 14% 25% 33% 17% 7%	460
May	1st to 10th	Day	6% 10% 30% 31% 18% 4	463
	11th to 20th	Day	3 9% 26% 35% 20% 8%	488
	21st to end	Day	5% 9% 19% 33% 24% 10%	529
June	1st to 10th	Day	3 6% 22% 34% 24% 10%	490
	11th to 20th	Day	2 5% 22% 34% 28% 9%	490
	21st to end	Day	2 2 16% 38% 29% 14%	490
July	1st to 10th	Day	1 11% 36% 34% 17%	486
	11th to 20th	Day	6% 36% 39% 19%	484
	21st to end	Day	1 7% 36% 37% 18%	531
August	1st to 10th	Day	2 1 12% 38% 33% 14%	487
	11th to 20th	Day	1 13% 31% 38% 16%	477
	21st to end	Day	1 7% 32% 40% 20%	539
September	1st to 10th	Day	1 3 9% 34% 37% 17%	490
	11th to 20th	Day	2 4 18% 35% 29% 13%	490
	21st to end	Day	1 4 12% 28% 41% 13%	490
October	1st to 10th	Day	1 3 18% 34% 33% 10%	490
	11th to 20th	Day	2 6% 15% 31% 36% 10%	490
	21st to end	Day	4 5% 18% 30% 33% 10%	539
November	1st to 10th	Day	6% 7% 19% 27% 33% 9%	368
	11th to 20th	Day	4 6% 16% 31% 30% 13%	270
	21st to end	Day	5% 4 22% 34% 26% 8%	268
December	1st to 10th	Day	11% 7% 20% 30% 23% 9%	180
	11th to 20th	Day	8% 12% 18% 31% 22% 9%	180
	21st to end	Day	10% 15% 22% 28% 16% 8%	198
January	1st to 10th	Night	12% 5% 16% 33% 25% 10%	760
	11th to 20th	Night	7% 4 16% 35% 29% 9%	760
	21st to end	Night	10% 6% 15% 39% 24% 7%	836
February	1st to 10th	Night	12% 6% 15% 34% 25% 8%	760
	11th to 20th	Night	8% 8% 18% 32% 25% 8%	744
	21st to end	Night	8% 7% 19% 42% 20% 4	628
March	1st to 10th	Night	8% 9% 20% 36% 22% 5%	760
	11th to 20th	Night	7% 7% 20% 33% 27% 7%	760
	21st to end	Night	8% 8% 22% 35% 23% 4	836
April	1st to 10th	Night	5% 8% 23% 37% 23% 4	760
	11th to 20th	Night	5% 11% 23% 33% 21% 7%	751
	21st to end	Night	3 8% 26% 31% 26% 6%	760
May	1st to 10th	Night	3 5% 26% 36% 23% 7%	760
	11th to 20th	Night	2 5% 22% 38% 29% 4	519
	21st to end	Night	2 7% 22% 36% 30% 3	411
June	1st to 10th	Night	2 4 19% 44% 28% 2	380
	11th to 20th	Night	1 3 19% 45% 27% 5%	380
	21st to end	Night	1 9% 47% 36% 6%	380
July	1st to 10th	Night	1 10% 37% 46% 6%	370
	11th to 20th	Night	1 3 36% 49% 10%	462
	21st to end	Night	1 1 4 30% 51% 14%	624
August	1st to 10th	Night	8% 38% 45% 9%	608
	11th to 20th	Night	1 11% 36% 44% 8%	752
	21st to end	Night	1 1 6% 28% 56% 9%	836
September	1st to 10th	Night	1 8% 32% 47% 11%	760
	11th to 20th	Night	2 2 13% 33% 40% 9%	760
	21st to end	Night	1 2 11% 30% 43% 13%	760
October	1st to 10th	Night	4 3 10% 37% 36% 11%	760
	11th to 20th	Night	3 2 13% 32% 37% 12%	760
	21st to end	Night	4 2 11% 33% 34% 16%	836
November	1st to 10th	Night	4 4 13% 34% 32% 12%	760
	11th to 20th	Night	6% 2 11% 37% 32% 11%	760
	21st to end	Night	5% 3 15% 35% 32% 9%	760
December	1st to 10th	Night	9% 3 15% 38% 27% 8%	760
	11th to 20th	Night	12% 5% 16% 36% 26% 6%	760
	21st to end	Night	11% 9% 19% 35% 21% 5%	836

Page generated at 2024-11-02 23:11:26. Version ac07

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