Occidental Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Occidental 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	9% 5% 2 4 3 3 3 3 4 6% 59%	105088
	Night	11% 6% 2 6% 4 3 4 3 4 6% 52%	102715
Transparency	Day	27% 5% 9% 20% 21% 18%	104849
	Night	32% 5% 8% 18% 20% 16%	102544
Seeing	Day	11% 11% 24% 29% 18% 6%	15402
	Night	12% 6% 16% 31% 29% 6%	24640

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	17% 7% 2 8% 5% 3 3 3 5% 6% 41%	2738
	11th to 20th	Day	23% 8% 4 8% 5% 4 3 3 4 5% 32%	2860
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 5% 7% 31%	3146
February	1st to 10th	Day	18% 5% 2 5% 5% 4 3 4 4 5% 45%	1710
	11th to 20th	Day	14% 7% 2 7% 3 4 4 4 5% 8% 42%	1688
	21st to end	Day	16% 6% 1 5% 4 3 4 3 4 7% 47%	1575
March	1st to 10th	Day	19% 13% 3 8% 5% 4 4 3 4 6% 31%	3442
	11th to 20th	Day	11% 4 1 5% 3 4 4 3 5% 7% 53%	3490
	21st to end	Day	20% 7% 2 5% 4 4 4 4 6% 8% 36%	3825
April	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 6% 8% 42%	2280
	11th to 20th	Day	8% 5% 2 5% 3 4 4 4 4 7% 52%	2249
	21st to end	Day	6% 4 2 4 3 3 4 3 6% 9% 58%	2280
May	1st to 10th	Day	6% 3 1 2 1 2 2 2 4 6% 72%	4074
	11th to 20th	Day	4 4 2 3 2 3 2 3 4 6% 69%	4117
	21st to end	Day	3 4 1 3 2 2 2 2 3 6% 73%	4813
June	1st to 10th	Day	6% 6% 2 4 3 2 3 3 5% 9% 59%	2660
	11th to 20th	Day	2 4 1 2 2 2 2 3 4 6% 73%	2660
	21st to end	Day	2 4 2 3 2 2 2 2 3 5% 74%	2660
July	1st to 10th	Day	1 3 1 3 2 2 2 1 2 4 79%	4396
	11th to 20th	Day	1 3 1 3 2 2 2 2 4 6% 73%	4110
	21st to end	Day	1 4 2 4 2 2 2 2 3 5% 74%	4455
August	1st to 10th	Day	1 3 2 6% 3 3 4 3 4 7% 63%	4091
	11th to 20th	Day	1 3 1 2 2 2 2 2 3 6% 77%	3934
	21st to end	Day	1 5% 2 5% 2 2 3 3 3 6% 68%	4180
September	1st to 10th	Day	1 4 1 3 2 3 2 1 3 7% 73%	2280
	11th to 20th	Day	4 4 2 6% 3 3 3 3 4 7% 62%	2254
	21st to end	Day	3 3 1 3 1 2 2 2 4 5% 73%	2010
October	1st to 10th	Day	4 2 1 3 2 2 3 2 4 6% 70%	3180
	11th to 20th	Day	9% 4 2 4 2 2 2 2 4 5% 63%	3161
	21st to end	Day	7% 4 2 5% 3 3 4 3 6% 8% 56%	3448
November	1st to 10th	Day	10% 4 1 4 4 4 4 3 6% 7% 53%	1898
	11th to 20th	Day	12% 5% 3 6% 4 4 4 3 5% 6% 48%	1662
	21st to end	Day	11% 5% 2 3 3 4 4 3 5% 6% 53%	1520
December	1st to 10th	Day	19% 6% 2 5% 3 3 3 4 5% 6% 44%	1520
	11th to 20th	Day	32% 9% 3 6% 3 3 3 3 4 5% 29%	1928
	21st to end	Day	24% 9% 2 6% 4 4 3 3 5% 7% 36%	2794
January	1st to 10th	Night	16% 8% 2 5% 4 4 5% 4 5% 7% 39%	4211
	11th to 20th	Night	23% 9% 3 8% 5% 5% 5% 3 4 5% 31%	4110
	21st to end	Night	25% 9% 3 7% 4 4 4 3 5% 6% 31%	4521
February	1st to 10th	Night	17% 6% 2 5% 4 2 4 4 4 6% 47%	2470
	11th to 20th	Night	13% 8% 3 6% 4 4 4 3 5% 8% 42%	2378
	21st to end	Night	14% 6% 2 4 2 2 3 3 5% 6% 53%	1875
March	1st to 10th	Night	20% 12% 3 7% 4 4 4 4 4 5% 34%	3432
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 6% 53%	3490
	21st to end	Night	16% 9% 3 7% 4 4 4 4 5% 6% 40%	3828
April	1st to 10th	Night	10% 6% 3 6% 3 3 4 3 5% 6% 50%	1900
	11th to 20th	Night	8% 6% 2 5% 5% 4 4 2 3 7% 54%	1879
	21st to end	Night	6% 4 2 4 3 2 3 3 5% 7% 61%	1900
May	1st to 10th	Night	4 4 2 3 2 2 2 1 3 5% 71%	2894
	11th to 20th	Night	2 5% 4 11% 4 3 4 3 4 6% 54%	2783
	21st to end	Night	2 4 1 3 3 1 3 2 3 5% 73%	2762
June	1st to 10th	Night	5% 5% 2 5% 3 2 4 4 5% 9% 56%	1520
	11th to 20th	Night	2 3 2 3 3 3 3 2 3 6% 70%	1520
	21st to end	Night	3 4 2 5% 2 3 2 2 3 6% 69%	1520
July	1st to 10th	Night	1 2 2 4 2 3 3 2 3 5% 73%	2561
	11th to 20th	Night	1 3 2 6% 3 3 3 2 3 6% 67%	2860
	21st to end	Night	1 4 5% 8% 4 3 3 4 5% 8% 55%	3109
August	1st to 10th	Night	2 5% 3 16% 6% 5% 4 3 5% 7% 43%	2848
	11th to 20th	Night	3 2 5% 2 2 3 3 3 7% 71%	2904
	21st to end	Night	1 4 2 8% 5% 4 4 3 4 6% 59%	3498
September	1st to 10th	Night	4 2 5% 3 4 3 2 4 8% 65%	1900
	11th to 20th	Night	5% 7% 2 6% 3 3 4 3 4 7% 56%	1923
	21st to end	Night	3 5% 3 5% 2 3 3 2 3 5% 66%	2168
October	1st to 10th	Night	3 3 2 4 3 3 3 2 4 6% 69%	3800
	11th to 20th	Night	6% 7% 3 7% 4 3 3 2 4 5% 56%	3777
	21st to end	Night	7% 4 2 5% 3 4 4 4 5% 6% 57%	4121
November	1st to 10th	Night	9% 5% 2 5% 4 4 3 3 5% 8% 52%	2279
	11th to 20th	Night	12% 6% 3 5% 3 3 4 4 6% 7% 47%	2516
	21st to end	Night	10% 4 1 5% 4 4 4 3 4 7% 54%	2660
December	1st to 10th	Night	21% 6% 2 6% 3 3 3 3 5% 5% 43%	2660
	11th to 20th	Night	32% 7% 2 5% 3 3 4 3 5% 6% 31%	3276
	21st to end	Night	24% 8% 2 6% 4 4 4 3 4 6% 35%	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% 7% 13% 27% 7% 3	2738
	11th to 20th	Day	55% 7% 10% 12% 9% 8%	2860
	21st to end	Day	52% 6% 10% 17% 10% 5%	3146
February	1st to 10th	Day	41% 5% 8% 19% 19% 8%	1710
	11th to 20th	Day	40% 6% 10% 19% 18% 6%	1697
	21st to end	Day	38% 5% 10% 27% 14% 6%	1575
March	1st to 10th	Day	53% 8% 9% 15% 12% 2	3442
	11th to 20th	Day	30% 6% 12% 27% 13% 11%	3490
	21st to end	Day	43% 9% 15% 20% 10% 2	3825
April	1st to 10th	Day	40% 6% 14% 22% 15% 4	2280
	11th to 20th	Day	31% 6% 16% 23% 19% 5%	2252
	21st to end	Day	23% 6% 15% 29% 23% 4	2280
May	1st to 10th	Day	16% 5% 13% 30% 22% 15%	4074
	11th to 20th	Day	16% 4 9% 31% 28% 13%	4120
	21st to end	Day	15% 4 13% 36% 29% 3	4814
June	1st to 10th	Day	24% 5% 11% 28% 25% 6%	2660
	11th to 20th	Day	14% 5% 10% 19% 37% 15%	2660
	21st to end	Day	16% 3 6% 17% 33% 26%	2660
July	1st to 10th	Day	11% 3 3 9% 15% 59%	4327
	11th to 20th	Day	12% 3 4 11% 18% 51%	4041
	21st to end	Day	14% 3 3 10% 27% 42%	4315
August	1st to 10th	Day	20% 4 5% 13% 24% 35%	4076
	11th to 20th	Day	11% 2 4 16% 39% 29%	3973
	21st to end	Day	18% 3 5% 17% 31% 25%	4179
September	1st to 10th	Day	13% 3 5% 14% 28% 37%	2280
	11th to 20th	Day	23% 4 8% 26% 29% 10%	2254
	21st to end	Day	15% 3 7% 24% 29% 22%	2010
October	1st to 10th	Day	15% 3 4 13% 24% 41%	3180
	11th to 20th	Day	25% 4 7% 22% 25% 18%	3161
	21st to end	Day	27% 5% 9% 24% 24% 11%	3448
November	1st to 10th	Day	30% 6% 10% 28% 18% 9%	1898
	11th to 20th	Day	36% 6% 11% 23% 15% 8%	1662
	21st to end	Day	32% 5% 13% 19% 19% 12%	1520
December	1st to 10th	Day	40% 7% 10% 16% 18% 9%	1520
	11th to 20th	Day	58% 5% 7% 13% 13% 4	1928
	21st to end	Day	49% 6% 12% 21% 9% 4	2794
January	1st to 10th	Night	41% 8% 18% 21% 8% 3	4211
	11th to 20th	Night	54% 8% 9% 12% 9% 7%	4110
	21st to end	Night	54% 7% 10% 12% 12% 6%	4521
February	1st to 10th	Night	38% 5% 10% 19% 17% 10%	2470
	11th to 20th	Night	40% 6% 9% 19% 19% 7%	2380
	21st to end	Night	32% 6% 12% 26% 20% 5%	1875
March	1st to 10th	Night	50% 11% 10% 15% 13% 2	3432
	11th to 20th	Night	31% 5% 10% 23% 22% 10%	3490
	21st to end	Night	43% 8% 13% 23% 9% 3	3828
April	1st to 10th	Night	34% 7% 14% 21% 19% 4	1900
	11th to 20th	Night	31% 6% 13% 25% 18% 8%	1879
	21st to end	Night	23% 5% 10% 31% 24% 6%	1900
May	1st to 10th	Night	19% 3 6% 28% 28% 15%	2894
	11th to 20th	Night	22% 8% 8% 20% 24% 18%	2784
	21st to end	Night	15% 3 9% 37% 31% 4	2762
June	1st to 10th	Night	26% 5% 10% 21% 31% 7%	1520
	11th to 20th	Night	17% 3 7% 21% 33% 18%	1520
	21st to end	Night	17% 4 4 15% 27% 32%	1520
July	1st to 10th	Night	13% 4 3 7% 18% 55%	2521
	11th to 20th	Night	18% 3 4 11% 17% 48%	2806
	21st to end	Night	26% 3 4 8% 18% 41%	3011
August	1st to 10th	Night	37% 6% 4 11% 19% 24%	2839
	11th to 20th	Night	15% 2 3 13% 35% 32%	2931
	21st to end	Night	23% 6% 4 15% 29% 23%	3498
September	1st to 10th	Night	17% 3 5% 14% 28% 34%	1900
	11th to 20th	Night	28% 4 6% 21% 28% 13%	1923
	21st to end	Night	20% 3 5% 21% 26% 24%	2168
October	1st to 10th	Night	17% 3 5% 10% 21% 43%	3800
	11th to 20th	Night	31% 3 6% 19% 20% 20%	3777
	21st to end	Night	28% 4 6% 21% 26% 16%	4121
November	1st to 10th	Night	32% 6% 10% 25% 18% 9%	2279
	11th to 20th	Night	35% 6% 13% 23% 16% 8%	2516
	21st to end	Night	31% 5% 9% 21% 21% 12%	2660
December	1st to 10th	Night	43% 6% 7% 18% 17% 10%	2660
	11th to 20th	Night	53% 6% 9% 14% 14% 4	3276
	21st to end	Night	49% 8% 12% 17% 12% 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	30% 14% 24% 17% 11% 4	301
	11th to 20th	Day	23% 11% 25% 24% 15% 3	370
	21st to end	Day	16% 13% 24% 29% 16% 2	407
February	1st to 10th	Day	19% 13% 21% 24% 18% 4	370
	11th to 20th	Day	20% 18% 26% 24% 12% 1	376
	21st to end	Day	18% 16% 25% 26% 13% 2	382
March	1st to 10th	Day	26% 18% 27% 20% 7% 2	460
	11th to 20th	Day	22% 16% 27% 19% 10% 4	460
	21st to end	Day	18% 19% 34% 21% 7% 1	506
April	1st to 10th	Day	17% 20% 28% 23% 9% 3	460
	11th to 20th	Day	15% 19% 29% 24% 8% 4	452
	21st to end	Day	9% 20% 32% 23% 11% 5%	460
May	1st to 10th	Day	11% 18% 31% 26% 12% 2	463
	11th to 20th	Day	10% 16% 30% 25% 15% 4	488
	21st to end	Day	11% 12% 26% 27% 19% 5%	529
June	1st to 10th	Day	8% 14% 28% 30% 14% 7%	490
	11th to 20th	Day	6% 13% 30% 34% 13% 4	490
	21st to end	Day	2 4 29% 34% 24% 6%	490
July	1st to 10th	Day	4 5% 21% 43% 23% 5%	486
	11th to 20th	Day	1 1 20% 42% 28% 7%	484
	21st to end	Day	2 2 14% 48% 27% 7%	531
August	1st to 10th	Day	3 4 22% 44% 21% 7%	487
	11th to 20th	Day	4 4 22% 36% 26% 8%	477
	21st to end	Day	5% 3 20% 39% 26% 7%	539
September	1st to 10th	Day	2 5% 19% 38% 26% 10%	490
	11th to 20th	Day	6% 9% 24% 31% 23% 8%	490
	21st to end	Day	5% 9% 21% 32% 25% 8%	490
October	1st to 10th	Day	6% 9% 26% 25% 23% 12%	490
	11th to 20th	Day	9% 9% 19% 27% 26% 10%	490
	21st to end	Day	13% 9% 19% 28% 25% 7%	539
November	1st to 10th	Day	11% 11% 19% 26% 26% 6%	470
	11th to 20th	Day	11% 10% 15% 24% 25% 15%	247
	21st to end	Day	10% 9% 23% 28% 18% 12%	180
December	1st to 10th	Day	20% 16% 13% 23% 18% 10%	180
	11th to 20th	Day	26% 11% 21% 18% 16% 8%	180
	21st to end	Day	22% 13% 20% 25% 16% 4	198
January	1st to 10th	Night	21% 8% 15% 29% 23% 5%	760
	11th to 20th	Night	21% 7% 14% 32% 21% 5%	760
	21st to end	Night	20% 6% 14% 33% 24% 3	836
February	1st to 10th	Night	19% 7% 15% 33% 23% 3	760
	11th to 20th	Night	17% 9% 18% 31% 21% 4	744
	21st to end	Night	16% 10% 21% 32% 18% 2	628
March	1st to 10th	Night	22% 12% 17% 27% 19% 3	760
	11th to 20th	Night	19% 8% 18% 31% 21% 3	760
	21st to end	Night	17% 10% 26% 29% 16% 3	836
April	1st to 10th	Night	13% 10% 29% 27% 18% 3	760
	11th to 20th	Night	11% 13% 24% 28% 20% 5%	751
	21st to end	Night	8% 13% 28% 24% 22% 5%	760
May	1st to 10th	Night	9% 8% 24% 29% 26% 4	591
	11th to 20th	Night	6% 3 23% 31% 34% 4	537
	21st to end	Night	7% 10% 22% 31% 27% 4	411
June	1st to 10th	Night	7% 6% 24% 34% 27% 2	380
	11th to 20th	Night	4 7% 21% 42% 24% 2	380
	21st to end	Night	3 1 17% 37% 39% 3	380
July	1st to 10th	Night	3 1 14% 35% 41% 6%	378
	11th to 20th	Night	3 1 7% 32% 47% 11%	552
	21st to end	Night	4 6% 33% 46% 11%	624
August	1st to 10th	Night	5% 1 12% 33% 40% 9%	567
	11th to 20th	Night	2 10% 35% 44% 9%	617
	21st to end	Night	4 1 8% 34% 45% 8%	836
September	1st to 10th	Night	2 1 9% 31% 47% 9%	760
	11th to 20th	Night	7% 3 12% 35% 35% 9%	760
	21st to end	Night	5% 3 14% 30% 36% 12%	760
October	1st to 10th	Night	7% 4 14% 30% 35% 10%	760
	11th to 20th	Night	10% 4 12% 28% 39% 8%	760
	21st to end	Night	11% 4 12% 27% 35% 11%	836
November	1st to 10th	Night	11% 5% 17% 26% 33% 9%	760
	11th to 20th	Night	14% 4 12% 34% 30% 6%	760
	21st to end	Night	11% 6% 16% 34% 26% 6%	760
December	1st to 10th	Night	23% 5% 15% 29% 23% 6%	760
	11th to 20th	Night	26% 7% 14% 29% 20% 4	760
	21st to end	Night	24% 7% 19% 30% 17% 2	836

Page generated at 2024-11-02 23:11:13. Version ac07

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