Palomar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Palomar Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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 4 1 3 2 2 3 2 4 5% 71%	134619
	Night	3 5% 1 4 3 3 3 3 4 5% 66%	131693
Transparency	Day	17% 3 5% 14% 27% 35%	129715
	Night	21% 3 5% 13% 24% 34%	127574
Seeing	Day	9% 9% 29% 35% 14% 5%	16843
	Night	8% 3 13% 35% 32% 8%	27169

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	4 5% 1 3 2 3 2 2 3 5% 70%	4098
	11th to 20th	Day	4 4 1 3 2 2 3 2 4 6% 68%	4284
	21st to end	Day	8% 8% 2 4 3 4 4 4 5% 8% 49%	5016
February	1st to 10th	Day	9% 7% 2 4 3 3 4 3 4 5% 57%	2000
	11th to 20th	Day	7% 7% 2 5% 3 3 4 3 5% 7% 53%	1950
	21st to end	Day	8% 11% 3 7% 3 3 3 4 4 6% 49%	1650
March	1st to 10th	Day	7% 10% 2 5% 3 3 4 4 7% 8% 48%	4518
	11th to 20th	Day	6% 8% 2 5% 4 4 3 3 5% 7% 53%	4797
	21st to end	Day	5% 6% 2 4 2 3 3 3 5% 6% 60%	5739
April	1st to 10th	Day	7% 6% 2 4 4 3 3 3 4 7% 58%	2400
	11th to 20th	Day	2 4 2 3 2 3 4 3 4 6% 67%	2369
	21st to end	Day	4 5% 1 3 2 2 2 3 4 7% 66%	2400
May	1st to 10th	Day	2 5% 1 4 2 2 2 2 3 4 73%	5765
	11th to 20th	Day	1 1 1 2 1 2 3 2 3 5% 79%	5841
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 78%	6423
June	1st to 10th	Day	2 3 1 2 2 2 2 2 3 5% 77%	2600
	11th to 20th	Day	1 2 2 2 1 1 2 4 85%	2600
	21st to end	Day	1 1 1 2 1 2 1 1 2 3 85%	2600
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 89%	5879
	11th to 20th	Day	1 1 1 1 2 2 2 4 5% 80%	5880
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	6197
August	1st to 10th	Day	1 1 1 1 1 2 2 3 5% 82%	5346
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	5364
	21st to end	Day	3 1 1 2 2 2 3 2 3 4 78%	5981
September	1st to 10th	Day	2 2 1 3 2 2 3 2 3 6% 75%	2292
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 3 87%	2090
	21st to end	Day	1 2 1 2 2 1 2 2 3 4 79%	2090
October	1st to 10th	Day	2 3 1 2 2 2 2 2 3 5% 76%	3490
	11th to 20th	Day	2 2 2 2 2 2 1 2 5% 81%	3469
	21st to end	Day	2 2 1 2 2 2 3 3 5% 6% 72%	3258
November	1st to 10th	Day	4 4 1 2 2 2 2 3 4 6% 70%	1764
	11th to 20th	Day	3 3 1 3 4 2 3 2 4 6% 68%	1800
	21st to end	Day	6% 6% 2 4 3 3 3 3 6% 7% 58%	1800
December	1st to 10th	Day	4 4 1 3 2 2 3 3 5% 8% 65%	2950
	11th to 20th	Day	7% 5% 2 4 4 5% 5% 5% 7% 6% 52%	3409
	21st to end	Day	5% 7% 2 6% 5% 5% 6% 6% 7% 7% 44%	4510
January	1st to 10th	Night	3 4 1 4 3 3 3 3 4 6% 66%	5867
	11th to 20th	Night	3 5% 1 4 3 3 3 3 4 7% 64%	5687
	21st to end	Night	9% 11% 2 6% 4 4 5% 4 5% 5% 46%	5984
February	1st to 10th	Night	8% 9% 2 4 3 2 4 3 5% 5% 54%	2400
	11th to 20th	Night	7% 9% 2 5% 4 4 3 2 4 5% 54%	2338
	21st to end	Night	8% 11% 2 5% 3 3 4 3 4 5% 50%	1980
March	1st to 10th	Night	7% 20% 4 8% 4 4 3 3 5% 7% 37%	5376
	11th to 20th	Night	5% 13% 3 7% 4 4 4 3 4 6% 47%	5203
	21st to end	Night	3 12% 3 7% 4 4 5% 4 5% 7% 47%	5232
April	1st to 10th	Night	5% 8% 2 5% 4 3 3 3 5% 7% 53%	2000
	11th to 20th	Night	5% 6% 2 5% 3 3 3 3 4 5% 61%	1979
	21st to end	Night	3 7% 2 4 3 3 3 3 4 6% 60%	2000
May	1st to 10th	Night	3 5% 2 4 3 3 3 2 4 5% 67%	4203
	11th to 20th	Night	1 2 1 5% 3 3 3 3 4 6% 68%	4074
	21st to end	Night	1 2 1 4 3 3 3 3 4 6% 68%	4474
June	1st to 10th	Night	1 3 1 3 2 3 2 2 4 6% 74%	1800
	11th to 20th	Night	1 2 1 2 2 1 1 1 3 4 83%	1800
	21st to end	Night	1 2 1 1 1 1 2 2 2 4 84%	1800
July	1st to 10th	Night	1 1 1 1 2 2 93%	4100
	11th to 20th	Night	1 1 1 1 1 2 1 3 4 86%	4100
	21st to end	Night	1 1 1 1 1 1 1 2 3 3 86%	4668
August	1st to 10th	Night	1 1 1 1 1 1 3 4 86%	4486
	11th to 20th	Night	1 1 1 1 1 2 3 89%	4506
	21st to end	Night	2 1 1 1 1 2 1 3 3 84%	5015
September	1st to 10th	Night	1 1 2 1 2 2 2 3 4 81%	1954
	11th to 20th	Night	1 1 1 2 1 2 2 2 2 2 84%	2090
	21st to end	Night	1 2 1 3 2 2 2 1 2 3 81%	2090
October	1st to 10th	Night	2 2 1 2 1 2 2 2 2 4 80%	3490
	11th to 20th	Night	1 3 1 3 2 2 2 2 3 5% 77%	3469
	21st to end	Night	1 3 1 3 3 3 3 3 4 5% 72%	4274
November	1st to 10th	Night	3 4 2 4 2 2 3 2 4 6% 68%	2535
	11th to 20th	Night	2 5% 2 4 3 3 2 3 4 5% 66%	2600
	21st to end	Night	5% 6% 3 6% 4 4 4 3 6% 7% 53%	2600
December	1st to 10th	Night	4 4 1 3 3 2 3 3 5% 5% 66%	4240
	11th to 20th	Night	6% 7% 2 5% 3 4 5% 5% 7% 8% 48%	4811
	21st to end	Night	4 8% 2 6% 4 4 4 4 7% 10% 47%	6468

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	20% 2 3 16% 33% 27%	4099
	11th to 20th	Day	18% 3 7% 16% 19% 36%	4284
	21st to end	Day	32% 5% 5% 10% 25% 23%	5016
February	1st to 10th	Day	31% 3 5% 12% 22% 27%	2000
	11th to 20th	Day	30% 3 6% 14% 26% 21%	1960
	21st to end	Day	36% 4 6% 14% 20% 22%	1650
March	1st to 10th	Day	31% 5% 6% 18% 21% 19%	4518
	11th to 20th	Day	28% 7% 8% 22% 23% 12%	4797
	21st to end	Day	24% 4 8% 31% 22% 10%	5739
April	1st to 10th	Day	27% 4 7% 21% 27% 15%	2400
	11th to 20th	Day	20% 3 7% 18% 30% 23%	2372
	21st to end	Day	18% 3 8% 20% 37% 15%	2400
May	1st to 10th	Day	18% 3 5% 18% 34% 23%	5765
	11th to 20th	Day	10% 2 5% 19% 39% 25%	5844
	21st to end	Day	10% 2 6% 15% 41% 27%	6424
June	1st to 10th	Day	11% 3 5% 18% 35% 28%	2600
	11th to 20th	Day	7% 2 4 10% 26% 52%	2600
	21st to end	Day	8% 2 2 6% 18% 64%	2600
July	1st to 10th	Day	4 1 2 4 14% 74%	5816
	11th to 20th	Day	8% 3 4 9% 27% 49%	5811
	21st to end	Day	7% 3 4 14% 23% 49%	6018
August	1st to 10th	Day	10% 3 4 15% 35% 33%	3890
	11th to 20th	Day	8% 2 3 10% 23% 54%	3880
	21st to end	Day	8% 2 2 6% 22% 60%	4311
September	1st to 10th	Day	13% 4 4 9% 26% 44%	2292
	11th to 20th	Day	7% 1 2 8% 33% 50%	2090
	21st to end	Day	10% 2 4 10% 29% 45%	2090
October	1st to 10th	Day	12% 2 3 9% 17% 57%	3490
	11th to 20th	Day	9% 2 3 10% 31% 46%	3469
	21st to end	Day	14% 2 3 7% 32% 41%	3258
November	1st to 10th	Day	17% 2 4 13% 27% 36%	1764
	11th to 20th	Day	19% 2 5% 10% 27% 37%	1800
	21st to end	Day	26% 4 6% 11% 23% 31%	1800
December	1st to 10th	Day	18% 2 4 11% 24% 40%	2950
	11th to 20th	Day	29% 5% 6% 13% 26% 21%	3408
	21st to end	Day	35% 4 7% 19% 21% 14%	4510
January	1st to 10th	Night	19% 4 3 14% 29% 30%	5867
	11th to 20th	Night	21% 5% 6% 16% 20% 33%	5687
	21st to end	Night	38% 5% 4 7% 22% 24%	5984
February	1st to 10th	Night	31% 3 4 13% 21% 28%	2400
	11th to 20th	Night	35% 2 5% 11% 23% 24%	2339
	21st to end	Night	37% 3 3 13% 21% 24%	1980
March	1st to 10th	Night	46% 5% 5% 13% 14% 17%	5376
	11th to 20th	Night	33% 8% 7% 18% 24% 10%	5203
	21st to end	Night	33% 6% 8% 17% 23% 13%	5232
April	1st to 10th	Night	29% 4 7% 18% 23% 18%	2000
	11th to 20th	Night	25% 3 6% 19% 23% 24%	1979
	21st to end	Night	26% 3 7% 17% 31% 17%	2000
May	1st to 10th	Night	21% 4 5% 16% 34% 20%	4200
	11th to 20th	Night	15% 4 6% 21% 34% 19%	4075
	21st to end	Night	14% 3 7% 17% 32% 27%	4474
June	1st to 10th	Night	12% 3 7% 16% 32% 30%	1800
	11th to 20th	Night	8% 1 3 12% 21% 55%	1800
	21st to end	Night	9% 2 2 7% 17% 64%	1800
July	1st to 10th	Night	3 1 1 5% 12% 78%	4055
	11th to 20th	Night	5% 2 3 9% 24% 56%	4046
	21st to end	Night	6% 2 3 14% 25% 50%	4536
August	1st to 10th	Night	6% 3 3 17% 37% 34%	3256
	11th to 20th	Night	5% 2 3 14% 20% 55%	3256
	21st to end	Night	5% 1 2 6% 18% 68%	3608
September	1st to 10th	Night	9% 3 5% 12% 24% 47%	1954
	11th to 20th	Night	9% 2 3 9% 24% 53%	2090
	21st to end	Night	10% 2 3 8% 31% 45%	2090
October	1st to 10th	Night	12% 1 2 9% 16% 59%	3490
	11th to 20th	Night	13% 1 3 9% 24% 50%	3469
	21st to end	Night	15% 4 2 7% 28% 44%	4274
November	1st to 10th	Night	20% 3 4 10% 25% 38%	2535
	11th to 20th	Night	21% 2 3 11% 24% 39%	2600
	21st to end	Night	30% 3 6% 11% 18% 31%	2600
December	1st to 10th	Night	20% 3 3 8% 22% 43%	4240
	11th to 20th	Night	31% 4 6% 13% 22% 24%	4811
	21st to end	Night	30% 5% 6% 22% 25% 12%	6468

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	19% 7% 30% 33% 10% 1	300
	11th to 20th	Day	14% 11% 27% 33% 14% 2	380
	21st to end	Day	16% 12% 31% 30% 9% 1	550
February	1st to 10th	Day	17% 11% 32% 26% 10% 3	500
	11th to 20th	Day	17% 15% 33% 28% 7%	495
	21st to end	Day	19% 18% 35% 21% 7%	414
March	1st to 10th	Day	19% 17% 35% 19% 8% 1	500
	11th to 20th	Day	20% 13% 33% 24% 9% 1	500
	21st to end	Day	16% 18% 37% 21% 7% 1	550
April	1st to 10th	Day	13% 14% 31% 27% 11% 3	500
	11th to 20th	Day	10% 20% 35% 24% 9% 2	492
	21st to end	Day	10% 15% 31% 27% 12% 5%	500
May	1st to 10th	Day	14% 13% 32% 27% 11% 3	503
	11th to 20th	Day	9% 11% 35% 30% 12% 3	528
	21st to end	Day	8% 13% 34% 30% 13% 3	573
June	1st to 10th	Day	3 8% 26% 38% 17% 7%	530
	11th to 20th	Day	2 5% 33% 39% 15% 7%	530
	21st to end	Day	2 5% 32% 37% 18% 7%	530
July	1st to 10th	Day	2 2 19% 49% 18% 11%	526
	11th to 20th	Day	1 1 14% 51% 19% 14%	524
	21st to end	Day	1 2 14% 48% 22% 12%	575
August	1st to 10th	Day	2 1 22% 46% 18% 11%	527
	11th to 20th	Day	2 1 19% 47% 16% 15%	517
	21st to end	Day	2 2 19% 46% 20% 10%	583
September	1st to 10th	Day	2 3 28% 43% 17% 7%	503
	11th to 20th	Day	4 6% 32% 39% 17% 4	490
	21st to end	Day	3 9% 30% 37% 16% 5%	490
October	1st to 10th	Day	6% 8% 32% 34% 18% 2	490
	11th to 20th	Day	6% 8% 33% 36% 15% 1	490
	21st to end	Day	6% 7% 27% 37% 19% 4	423
November	1st to 10th	Day	10% 8% 21% 38% 19% 3	300
	11th to 20th	Day	6% 8% 25% 39% 18% 3	300
	21st to end	Day	15% 8% 33% 29% 11% 4	300
December	1st to 10th	Day	12% 6% 31% 34% 14% 3	300
	11th to 20th	Day	8% 8% 29% 38% 15% 3	300
	21st to end	Day	21% 12% 25% 29% 11% 2	330
January	1st to 10th	Night	12% 3 24% 35% 20% 6%	800
	11th to 20th	Night	10% 5% 20% 40% 20% 6%	800
	21st to end	Night	16% 5% 19% 39% 18% 3	880
February	1st to 10th	Night	14% 4 19% 35% 23% 5%	800
	11th to 20th	Night	13% 5% 21% 35% 21% 5%	784
	21st to end	Night	16% 6% 21% 35% 17% 4	660
March	1st to 10th	Night	16% 6% 19% 38% 18% 2	800
	11th to 20th	Night	16% 3 15% 38% 21% 7%	800
	21st to end	Night	11% 5% 18% 38% 23% 4	880
April	1st to 10th	Night	11% 6% 18% 39% 24% 3	800
	11th to 20th	Night	9% 7% 17% 39% 25% 4	791
	21st to end	Night	10% 5% 14% 36% 29% 6%	800
May	1st to 10th	Night	7% 4 15% 39% 30% 5%	651
	11th to 20th	Night	5% 4 15% 36% 34% 6%	591
	21st to end	Night	6% 5% 17% 38% 31% 4	650
June	1st to 10th	Night	1 3 13% 38% 39% 6%	600
	11th to 20th	Night	1 2 11% 44% 39% 3	600
	21st to end	Night	2 1 9% 39% 43% 7%	600
July	1st to 10th	Night	1 3 33% 53% 10%	585
	11th to 20th	Night	1 2 26% 63% 9%	582
	21st to end	Night	1 2 1 24% 57% 15%	739
August	1st to 10th	Night	1 1 1 31% 52% 15%	796
	11th to 20th	Night	1 1 19% 63% 16%	792
	21st to end	Night	2 1 20% 58% 19%	880
September	1st to 10th	Night	2 1 2 25% 56% 13%	772
	11th to 20th	Night	2 2 5% 33% 47% 11%	760
	21st to end	Night	2 2 8% 36% 40% 11%	760
October	1st to 10th	Night	6% 2 10% 42% 33% 7%	760
	11th to 20th	Night	5% 3 12% 37% 33% 10%	760
	21st to end	Night	6% 3 14% 35% 29% 14%	836
November	1st to 10th	Night	7% 3 12% 41% 28% 10%	780
	11th to 20th	Night	6% 2 15% 39% 28% 9%	800
	21st to end	Night	10% 4 20% 36% 23% 7%	800
December	1st to 10th	Night	11% 4 18% 40% 20% 6%	800
	11th to 20th	Night	15% 5% 17% 43% 18% 3	800
	21st to end	Night	13% 5% 21% 38% 18% 5%	880

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