Jack C. Davis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jack C. Davis Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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	1 3 1 4 3 3 3 3 5% 7% 65%	105168
	Night	2 4 2 4 3 3 4 4 6% 7% 62%	102678
Transparency	Day	17% 7% 10% 19% 25% 22%	104912
	Night	19% 8% 12% 21% 23% 16%	102523
Seeing	Day	8% 16% 27% 26% 18% 5%	15067
	Night	5% 9% 23% 35% 23% 5%	24413

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	1 5% 2 6% 5% 5% 5% 5% 8% 11% 46%	2539
	11th to 20th	Day	1 5% 2 7% 5% 5% 7% 5% 8% 8% 45%	2586
	21st to end	Day	3 8% 3 9% 6% 6% 7% 5% 9% 8% 36%	3146
February	1st to 10th	Day	2 5% 3 5% 5% 4 5% 4 8% 9% 51%	1720
	11th to 20th	Day	1 5% 2 6% 4 5% 5% 5% 8% 9% 49%	1850
	21st to end	Day	1 6% 1 7% 4 4 4 5% 9% 10% 51%	1570
March	1st to 10th	Day	1 7% 4 11% 6% 5% 7% 7% 9% 10% 34%	3138
	11th to 20th	Day	3 1 4 4 3 4 3 6% 7% 64%	3422
	21st to end	Day	1 8% 3 7% 4 5% 4 5% 7% 9% 48%	4086
April	1st to 10th	Day	2 5% 2 6% 4 5% 5% 4 7% 8% 52%	2280
	11th to 20th	Day	2 5% 1 4 4 3 4 4 6% 9% 57%	2249
	21st to end	Day	2 5% 1 4 4 4 4 3 6% 7% 61%	2280
May	1st to 10th	Day	2 3 1 4 2 3 3 2 4 5% 70%	4003
	11th to 20th	Day	2 2 1 3 3 3 3 2 4 6% 72%	4284
	21st to end	Day	3 3 2 4 3 3 4 3 5% 6% 65%	4813
June	1st to 10th	Day	2 3 1 4 3 3 3 4 6% 8% 62%	2660
	11th to 20th	Day	1 2 1 2 1 2 3 2 4 5% 76%	2660
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 78%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 3 89%	4420
	11th to 20th	Day	2 3 1 3 2 2 3 2 4 4 74%	4420
	21st to end	Day	1 1 1 1 2 2 2 2 3 4 80%	4703
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 77%	4035
	11th to 20th	Day	1 1 1 1 2 2 2 3 87%	3724
	21st to end	Day	1 1 2 1 1 1 1 2 3 88%	4180
September	1st to 10th	Day	1 1 1 1 2 2 2 4 5% 81%	2280
	11th to 20th	Day	1 1 1 2 3 3 3 2 3 5% 74%	2242
	21st to end	Day	1 2 1 3 2 3 2 2 3 4 78%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	3247
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	3161
	21st to end	Day	1 1 1 3 3 3 4 3 6% 8% 69%	3355
November	1st to 10th	Day	2 4 1 4 4 5% 4 4 6% 7% 59%	1755
	11th to 20th	Day	2 3 1 3 3 4 4 5% 8% 9% 59%	1710
	21st to end	Day	1 4 2 5% 4 4 4 6% 6% 10% 54%	1658
December	1st to 10th	Day	2 5% 2 5% 4 5% 5% 6% 7% 10% 49%	1520
	11th to 20th	Day	2 9% 4 9% 5% 5% 6% 6% 9% 8% 35%	1928
	21st to end	Day	1 7% 3 8% 5% 5% 6% 5% 8% 13% 38%	2794
January	1st to 10th	Night	1 4 3 6% 4 4 5% 4 8% 12% 48%	4406
	11th to 20th	Night	1 6% 3 8% 6% 6% 6% 6% 9% 10% 39%	4358
	21st to end	Night	3 8% 4 9% 6% 6% 6% 5% 7% 8% 38%	4521
February	1st to 10th	Night	2 5% 2 6% 4 5% 5% 4 7% 8% 54%	2458
	11th to 20th	Night	1 5% 2 5% 3 4 4 5% 8% 10% 53%	2218
	21st to end	Night	1 4 2 6% 5% 4 4 5% 7% 9% 53%	1884
March	1st to 10th	Night	1 6% 4 8% 5% 5% 6% 5% 9% 10% 40%	3736
	11th to 20th	Night	1 4 2 6% 3 3 4 4 6% 7% 60%	3556
	21st to end	Night	2 9% 3 7% 5% 5% 5% 4 8% 9% 43%	3565
April	1st to 10th	Night	2 3 1 4 5% 3 5% 6% 8% 9% 53%	1900
	11th to 20th	Night	2 3 1 4 4 3 4 6% 7% 7% 60%	1879
	21st to end	Night	3 3 1 4 2 3 3 4 6% 7% 64%	1900
May	1st to 10th	Night	2 3 1 3 3 2 3 3 4 5% 71%	2969
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 6% 75%	2594
	21st to end	Night	3 3 2 4 2 3 3 3 5% 7% 67%	2762
June	1st to 10th	Night	4 4 1 4 3 3 3 3 4 6% 64%	1520
	11th to 20th	Night	1 1 2 1 2 3 3 3 4 79%	1520
	21st to end	Night	2 2 2 2 2 2 3 3 3 79%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	2540
	11th to 20th	Night	3 3 2 3 2 3 2 2 3 4 74%	2540
	21st to end	Night	2 2 1 2 2 2 2 2 3 5% 78%	2847
August	1st to 10th	Night	3 2 1 2 2 2 3 2 3 5% 74%	2906
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 86%	3126
	21st to end	Night	1 1 1 2 1 1 1 1 1 2 87%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 86%	1900
	11th to 20th	Night	1 2 1 3 2 2 2 1 4 5% 77%	1936
	21st to end	Night	1 2 1 3 2 2 2 2 3 5% 77%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 79%	3730
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 4 81%	3777
	21st to end	Night	1 2 1 3 3 2 3 3 6% 7% 69%	4209
November	1st to 10th	Night	2 3 2 3 3 3 4 4 6% 8% 62%	2528
	11th to 20th	Night	1 3 1 4 3 3 4 3 6% 10% 62%	2470
	21st to end	Night	2 4 1 4 3 4 5% 4 7% 10% 57%	2517
December	1st to 10th	Night	2 6% 2 5% 4 4 4 5% 6% 9% 53%	2660
	11th to 20th	Night	2 8% 3 8% 5% 5% 6% 6% 9% 9% 38%	3276
	21st to end	Night	1 6% 3 9% 7% 6% 7% 7% 8% 9% 36%	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	26% 14% 22% 25% 12%	2539
	11th to 20th	Day	30% 13% 17% 24% 12% 4	2586
	21st to end	Day	38% 12% 14% 22% 12% 2	3146
February	1st to 10th	Day	26% 9% 16% 26% 17% 6%	1720
	11th to 20th	Day	27% 8% 16% 27% 18% 5%	1860
	21st to end	Day	23% 11% 20% 25% 17% 4	1570
March	1st to 10th	Day	31% 17% 21% 16% 11% 3	3138
	11th to 20th	Day	17% 9% 15% 34% 23% 3	3422
	21st to end	Day	28% 12% 20% 23% 16% 2	4086
April	1st to 10th	Day	26% 10% 17% 23% 19% 5%	2280
	11th to 20th	Day	21% 8% 15% 28% 21% 6%	2252
	21st to end	Day	19% 9% 14% 29% 25% 4	2280
May	1st to 10th	Day	15% 7% 9% 31% 31% 7%	4003
	11th to 20th	Day	14% 4 8% 21% 40% 14%	4287
	21st to end	Day	17% 8% 10% 25% 34% 6%	4814
June	1st to 10th	Day	19% 7% 10% 22% 31% 11%	2660
	11th to 20th	Day	11% 4 7% 20% 35% 23%	2660
	21st to end	Day	11% 3 4 10% 30% 40%	2660
July	1st to 10th	Day	5% 1 2 5% 14% 72%	4351
	11th to 20th	Day	13% 3 3 9% 15% 56%	4345
	21st to end	Day	8% 3 4 8% 24% 53%	4554
August	1st to 10th	Day	12% 3 3 7% 23% 52%	4020
	11th to 20th	Day	6% 2 3 5% 20% 65%	3760
	21st to end	Day	6% 1 2 8% 38% 45%	4179
September	1st to 10th	Day	7% 3 4 8% 32% 46%	2280
	11th to 20th	Day	12% 4 6% 18% 35% 25%	2242
	21st to end	Day	12% 3 5% 17% 37% 27%	2090
October	1st to 10th	Day	12% 3 5% 12% 35% 33%	3247
	11th to 20th	Day	12% 2 6% 20% 47% 13%	3161
	21st to end	Day	12% 5% 9% 28% 34% 12%	3355
November	1st to 10th	Day	21% 8% 14% 23% 26% 9%	1755
	11th to 20th	Day	17% 8% 19% 24% 27% 5%	1710
	21st to end	Day	23% 9% 14% 24% 25% 6%	1658
December	1st to 10th	Day	26% 11% 15% 25% 19% 5%	1520
	11th to 20th	Day	38% 14% 13% 18% 13% 4	1928
	21st to end	Day	31% 12% 20% 22% 14% 2	2794
January	1st to 10th	Night	25% 12% 23% 27% 12% 1	4406
	11th to 20th	Night	33% 13% 22% 18% 11% 4	4358
	21st to end	Night	37% 12% 16% 21% 11% 2	4521
February	1st to 10th	Night	26% 10% 15% 24% 18% 6%	2458
	11th to 20th	Night	22% 9% 18% 29% 19% 3	2219
	21st to end	Night	24% 9% 19% 27% 16% 5%	1884
March	1st to 10th	Night	27% 16% 20% 21% 14% 3	3736
	11th to 20th	Night	18% 11% 13% 40% 15% 3	3556
	21st to end	Night	29% 15% 22% 22% 10% 2	3565
April	1st to 10th	Night	22% 12% 18% 28% 16% 4	1900
	11th to 20th	Night	19% 10% 17% 31% 17% 6%	1879
	21st to end	Night	18% 9% 17% 32% 21% 3	1900
May	1st to 10th	Night	12% 7% 11% 34% 30% 5%	2969
	11th to 20th	Night	11% 6% 9% 23% 38% 13%	2594
	21st to end	Night	15% 7% 11% 34% 30% 3	2762
June	1st to 10th	Night	20% 7% 10% 24% 29% 10%	1520
	11th to 20th	Night	8% 5% 6% 22% 37% 22%	1520
	21st to end	Night	11% 2 6% 14% 33% 34%	1520
July	1st to 10th	Night	4 1 2 5% 18% 70%	2500
	11th to 20th	Night	15% 3 4 8% 16% 55%	2492
	21st to end	Night	10% 3 4 7% 25% 52%	2758
August	1st to 10th	Night	12% 4 4 9% 28% 43%	2897
	11th to 20th	Night	6% 2 3 6% 24% 59%	3156
	21st to end	Night	7% 1 2 10% 40% 40%	3498
September	1st to 10th	Night	5% 2 4 8% 37% 43%	1900
	11th to 20th	Night	11% 4 7% 20% 34% 24%	1936
	21st to end	Night	12% 3 5% 19% 35% 26%	2090
October	1st to 10th	Night	11% 3 5% 14% 33% 34%	3730
	11th to 20th	Night	8% 3 6% 21% 48% 14%	3777
	21st to end	Night	12% 6% 8% 27% 35% 12%	4209
November	1st to 10th	Night	18% 9% 14% 25% 23% 11%	2528
	11th to 20th	Night	17% 8% 14% 27% 27% 6%	2470
	21st to end	Night	21% 9% 13% 27% 24% 7%	2517
December	1st to 10th	Night	27% 11% 14% 24% 19% 5%	2660
	11th to 20th	Night	32% 15% 15% 19% 16% 3	3276
	21st to end	Night	33% 13% 20% 18% 13% 3	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% 15% 30% 21% 13% 2	180
	11th to 20th	Day	10% 15% 31% 22% 17% 5%	213
	21st to end	Day	8% 13% 30% 28% 17% 4	407
February	1st to 10th	Day	11% 14% 30% 26% 16% 4	375
	11th to 20th	Day	9% 21% 26% 30% 13% 2	455
	21st to end	Day	9% 20% 26% 31% 14% 1	382
March	1st to 10th	Day	12% 23% 32% 22% 10% 2	460
	11th to 20th	Day	10% 22% 29% 21% 14% 3	460
	21st to end	Day	13% 24% 29% 24% 10%	506
April	1st to 10th	Day	11% 24% 29% 19% 12% 4	460
	11th to 20th	Day	14% 25% 27% 19% 13% 3	452
	21st to end	Day	10% 21% 30% 22% 14% 3	460
May	1st to 10th	Day	13% 22% 28% 24% 10% 3	463
	11th to 20th	Day	11% 22% 25% 22% 13% 7%	488
	21st to end	Day	10% 17% 22% 25% 20% 7%	529
June	1st to 10th	Day	7% 17% 27% 22% 21% 6%	490
	11th to 20th	Day	9% 19% 29% 24% 14% 5%	490
	21st to end	Day	3 12% 27% 30% 22% 6%	490
July	1st to 10th	Day	1 12% 30% 28% 22% 6%	486
	11th to 20th	Day	2 10% 27% 31% 22% 8%	484
	21st to end	Day	3 7% 27% 30% 25% 7%	531
August	1st to 10th	Day	4 10% 26% 28% 25% 7%	487
	11th to 20th	Day	2 11% 24% 27% 25% 11%	477
	21st to end	Day	3 9% 28% 30% 25% 6%	539
September	1st to 10th	Day	3 7% 29% 29% 24% 8%	490
	11th to 20th	Day	6% 15% 24% 28% 22% 6%	490
	21st to end	Day	4 12% 27% 34% 18% 5%	490
October	1st to 10th	Day	5% 14% 27% 26% 22% 6%	490
	11th to 20th	Day	7% 12% 23% 28% 24% 6%	490
	21st to end	Day	6% 14% 25% 28% 22% 4	482
November	1st to 10th	Day	10% 12% 24% 23% 22% 9%	297
	11th to 20th	Day	4 14% 21% 30% 21% 9%	270
	21st to end	Day	5% 13% 27% 24% 23% 7%	246
December	1st to 10th	Day	10% 14% 23% 23% 22% 8%	180
	11th to 20th	Day	6% 18% 32% 22% 15% 8%	180
	21st to end	Day	16% 16% 21% 28% 16% 4	198
January	1st to 10th	Night	6% 13% 29% 26% 19% 8%	760
	11th to 20th	Night	5% 12% 24% 37% 17% 5%	760
	21st to end	Night	6% 7% 29% 34% 18% 6%	836
February	1st to 10th	Night	7% 11% 26% 34% 20% 4	760
	11th to 20th	Night	4 12% 27% 34% 18% 5%	744
	21st to end	Night	6% 15% 26% 36% 14% 3	628
March	1st to 10th	Night	7% 14% 34% 28% 14% 3	760
	11th to 20th	Night	4 16% 26% 26% 25% 3	760
	21st to end	Night	5% 19% 30% 30% 14% 1	836
April	1st to 10th	Night	5% 13% 29% 32% 20% 1	760
	11th to 20th	Night	8% 15% 25% 32% 17% 4	751
	21st to end	Night	6% 11% 25% 33% 20% 4	760
May	1st to 10th	Night	6% 13% 26% 31% 20% 3	640
	11th to 20th	Night	3 8% 23% 41% 22% 3	426
	21st to end	Night	6% 6% 17% 40% 27% 3	411
June	1st to 10th	Night	6% 4 22% 38% 27% 4	380
	11th to 20th	Night	1 6% 22% 42% 28% 2	380
	21st to end	Night	2 3 18% 45% 30% 2	380
July	1st to 10th	Night	1 1 12% 51% 33% 2	370
	11th to 20th	Night	1 16% 53% 28% 1	368
	21st to end	Night	3 1 10% 44% 36% 5%	466
August	1st to 10th	Night	4 1 12% 43% 36% 4	599
	11th to 20th	Night	3 3 16% 45% 29% 4	752
	21st to end	Night	2 2 17% 45% 30% 3	836
September	1st to 10th	Night	2 2 17% 39% 36% 6%	760
	11th to 20th	Night	2 5% 19% 32% 35% 7%	760
	21st to end	Night	3 5% 21% 34% 29% 8%	760
October	1st to 10th	Night	4 5% 19% 36% 29% 6%	760
	11th to 20th	Night	5% 8% 14% 40% 28% 7%	760
	21st to end	Night	4 8% 20% 32% 27% 10%	836
November	1st to 10th	Night	4 11% 21% 28% 28% 7%	778
	11th to 20th	Night	4 10% 26% 36% 19% 5%	760
	21st to end	Night	6% 9% 26% 31% 23% 5%	760
December	1st to 10th	Night	8% 11% 24% 34% 16% 6%	760
	11th to 20th	Night	6% 13% 26% 33% 16% 5%	760
	21st to end	Night	8% 12% 32% 32% 14% 3	836

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