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 2025-10-19.

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	8% 5% 2 4 3 3 3 3 4 6% 60%	135226
	Night	10% 6% 2 6% 4 3 4 3 5% 6% 51%	131949
Transparency	Day	26% 5% 9% 21% 23% 17%	130112
	Night	33% 6% 8% 18% 20% 15%	128053
Seeing	Day	11% 11% 24% 29% 18% 6%	16646
	Night	12% 6% 16% 31% 29% 6%	25868

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% 5% 2 6% 4 4 4 4 6% 7% 44%	3936
	11th to 20th	Day	16% 6% 3 6% 4 3 3 3 3 4 48%	4100
	21st to end	Day	18% 7% 3 6% 4 3 4 3 5% 6% 43%	4510
February	1st to 10th	Day	19% 5% 2 5% 4 4 3 4 4 4 45%	1800
	11th to 20th	Day	13% 7% 2 8% 3 4 4 4 5% 8% 41%	1780
	21st to end	Day	16% 6% 1 5% 4 3 4 3 5% 7% 47%	1655
March	1st to 10th	Day	16% 10% 3 7% 4 4 4 4 5% 7% 36%	4952
	11th to 20th	Day	14% 4 2 5% 3 4 4 4 6% 8% 47%	5000
	21st to end	Day	18% 7% 2 5% 4 4 4 4 7% 9% 35%	5486
April	1st to 10th	Day	12% 7% 3 6% 5% 4 4 4 6% 7% 43%	2400
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 4 7% 51%	2369
	21st to end	Day	6% 4 2 4 3 3 4 3 6% 9% 57%	2400
May	1st to 10th	Day	4 2 1 2 1 2 2 2 4 6% 75%	5831
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 5% 73%	5900
	21st to end	Day	2 3 1 2 1 1 2 2 3 5% 77%	6902
June	1st to 10th	Day	6% 5% 2 4 3 2 3 3 5% 8% 60%	2800
	11th to 20th	Day	2 3 1 2 1 2 2 2 4 6% 74%	2800
	21st to end	Day	2 3 2 3 2 2 2 2 3 4 75%	2800
July	1st to 10th	Day	1 3 1 3 2 2 2 1 2 4 79%	6282
	11th to 20th	Day	1 4 2 4 2 2 2 2 3 6% 73%	5880
	21st to end	Day	1 4 2 5% 2 2 2 2 3 5% 73%	6402
August	1st to 10th	Day	1 2 2 5% 3 2 3 2 3 6% 72%	5777
	11th to 20th	Day	2 1 2 2 2 1 1 3 5% 79%	5665
	21st to end	Day	1 4 2 6% 3 2 3 3 4 7% 66%	5984
September	1st to 10th	Day	1 4 1 3 2 3 2 2 4 7% 71%	2400
	11th to 20th	Day	4 4 2 5% 3 3 3 3 4 7% 62%	2372
	21st to end	Day	4 3 1 3 2 2 2 2 4 5% 72%	2116
October	1st to 10th	Day	4 2 1 3 2 2 3 2 4 6% 70%	3280
	11th to 20th	Day	9% 4 2 4 2 3 2 2 4 5% 63%	3242
	21st to end	Day	7% 4 2 5% 3 3 4 3 6% 8% 56%	3448
November	1st to 10th	Day	10% 4 1 3 4 3 4 3 6% 7% 54%	1956
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 5% 6% 48%	1749
	21st to end	Day	12% 5% 2 4 3 4 4 3 5% 6% 52%	1600
December	1st to 10th	Day	11% 4 2 4 3 3 4 4 8% 11% 46%	2620
	11th to 20th	Day	27% 8% 3 6% 4 4 4 4 7% 7% 26%	3028
	21st to end	Day	26% 9% 2 5% 4 4 4 4 6% 7% 30%	4004
January	1st to 10th	Night	12% 6% 2 5% 4 5% 6% 5% 7% 9% 40%	6020
	11th to 20th	Night	16% 6% 2 7% 4 4 5% 4 5% 6% 41%	5880
	21st to end	Night	18% 7% 3 7% 4 4 4 3 5% 6% 41%	6468
February	1st to 10th	Night	19% 5% 2 5% 4 3 4 4 4 5% 46%	2600
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	2506
	21st to end	Night	13% 6% 2 4 2 3 3 3 5% 6% 53%	1970
March	1st to 10th	Night	16% 10% 3 7% 4 4 5% 4 5% 7% 35%	4942
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 6% 46%	5000
	21st to end	Night	16% 9% 3 6% 3 3 4 4 5% 8% 39%	5489
April	1st to 10th	Night	10% 6% 3 6% 4 3 4 4 5% 6% 49%	2000
	11th to 20th	Night	8% 7% 2 6% 5% 4 4 2 3 7% 53%	1979
	21st to end	Night	5% 5% 2 5% 3 2 3 3 5% 7% 60%	2000
May	1st to 10th	Night	3 4 2 3 2 2 2 2 3 5% 71%	4148
	11th to 20th	Night	3 4 3 8% 3 3 4 3 3 6% 61%	3995
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 76%	3972
June	1st to 10th	Night	5% 6% 2 5% 3 2 4 4 5% 9% 56%	1600
	11th to 20th	Night	2 3 2 3 3 3 3 2 3 6% 71%	1600
	21st to end	Night	3 4 2 5% 2 3 2 2 3 6% 70%	1600
July	1st to 10th	Night	1 4 2 6% 3 3 3 2 3 5% 68%	3675
	11th to 20th	Night	1 8% 4 10% 4 3 3 3 4 6% 54%	4100
	21st to end	Night	1 7% 5% 9% 4 4 3 3 5% 7% 52%	4472
August	1st to 10th	Night	1 4 2 11% 5% 4 3 2 4 6% 56%	4035
	11th to 20th	Night	4 2 5% 2 2 3 2 3 6% 70%	4186
	21st to end	Night	1 3 3 11% 5% 4 4 3 4 7% 55%	5015
September	1st to 10th	Night	1 4 2 5% 3 4 3 2 4 8% 65%	2000
	11th to 20th	Night	5% 7% 2 6% 4 3 4 3 4 8% 55%	2024
	21st to end	Night	3 5% 2 5% 2 3 3 2 3 5% 65%	2282
October	1st to 10th	Night	3 3 2 4 3 2 3 2 4 6% 68%	3920
	11th to 20th	Night	6% 7% 3 7% 4 3 3 2 4 6% 55%	3873
	21st to end	Night	7% 4 2 5% 3 4 4 4 5% 6% 57%	4121
November	1st to 10th	Night	9% 5% 1 5% 4 4 3 3 5% 8% 52%	2340
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 6% 7% 48%	2649
	21st to end	Night	11% 5% 2 5% 4 4 4 3 4 7% 53%	2800
December	1st to 10th	Night	12% 4 2 5% 4 5% 5% 5% 7% 8% 43%	4560
	11th to 20th	Night	27% 7% 2 5% 4 4 6% 4 7% 7% 25%	5176
	21st to end	Night	33% 8% 2 5% 3 3 3 3 4 5% 31%	6952

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	38% 8% 15% 21% 11% 7%	3936
	11th to 20th	Day	40% 6% 7% 13% 18% 15%	4100
	21st to end	Day	42% 5% 7% 14% 13% 18%	4510
February	1st to 10th	Day	42% 5% 8% 19% 18% 8%	1800
	11th to 20th	Day	41% 6% 10% 19% 17% 6%	1789
	21st to end	Day	37% 5% 9% 26% 15% 7%	1655
March	1st to 10th	Day	45% 8% 7% 20% 17% 2	4952
	11th to 20th	Day	35% 6% 16% 24% 10% 8%	5000
	21st to end	Day	42% 10% 16% 22% 9% 1	5486
April	1st to 10th	Day	40% 6% 14% 22% 15% 3	2400
	11th to 20th	Day	32% 6% 15% 23% 19% 6%	2372
	21st to end	Day	24% 6% 15% 30% 22% 3	2400
May	1st to 10th	Day	13% 4 9% 27% 29% 18%	5831
	11th to 20th	Day	12% 4 8% 35% 31% 10%	5903
	21st to end	Day	12% 3 9% 34% 38% 3	6904
June	1st to 10th	Day	24% 5% 11% 28% 26% 6%	2800
	11th to 20th	Day	13% 5% 10% 19% 36% 16%	2800
	21st to end	Day	15% 3 5% 17% 34% 25%	2800
July	1st to 10th	Day	11% 3 2 9% 24% 50%	6214
	11th to 20th	Day	14% 3 4 8% 19% 53%	5811
	21st to end	Day	16% 3 6% 17% 27% 31%	6217
August	1st to 10th	Day	19% 4 5% 13% 25% 34%	4206
	11th to 20th	Day	10% 2 4 16% 39% 29%	4100
	21st to end	Day	19% 3 5% 17% 32% 25%	4311
September	1st to 10th	Day	14% 3 5% 14% 28% 36%	2400
	11th to 20th	Day	23% 4 8% 26% 29% 10%	2372
	21st to end	Day	16% 3 7% 24% 29% 22%	2116
October	1st to 10th	Day	16% 3 5% 13% 24% 40%	3280
	11th to 20th	Day	25% 4 7% 22% 25% 17%	3242
	21st to end	Day	27% 5% 9% 24% 24% 11%	3448
November	1st to 10th	Day	29% 6% 10% 28% 18% 9%	1956
	11th to 20th	Day	37% 6% 12% 23% 15% 7%	1749
	21st to end	Day	33% 5% 12% 19% 19% 13%	1600
December	1st to 10th	Day	29% 6% 9% 26% 22% 8%	2620
	11th to 20th	Day	55% 5% 8% 17% 12% 3	3028
	21st to end	Day	52% 7% 11% 20% 7% 3	4004
January	1st to 10th	Night	37% 9% 20% 18% 12% 4	6020
	11th to 20th	Night	42% 8% 8% 15% 15% 12%	5880
	21st to end	Night	44% 6% 8% 10% 18% 15%	6468
February	1st to 10th	Night	39% 5% 9% 19% 17% 11%	2600
	11th to 20th	Night	40% 6% 10% 19% 18% 6%	2508
	21st to end	Night	31% 6% 12% 25% 20% 6%	1970
March	1st to 10th	Night	45% 9% 9% 18% 16% 3	4942
	11th to 20th	Night	37% 5% 12% 21% 18% 8%	5000
	21st to end	Night	42% 8% 14% 23% 10% 2	5489
April	1st to 10th	Night	34% 7% 14% 22% 20% 4	2000
	11th to 20th	Night	32% 6% 12% 24% 18% 9%	1979
	21st to end	Night	24% 5% 10% 32% 23% 6%	2000
May	1st to 10th	Night	18% 3 5% 24% 32% 19%	4145
	11th to 20th	Night	20% 6% 6% 28% 27% 13%	3996
	21st to end	Night	13% 3 8% 31% 41% 5%	3972
June	1st to 10th	Night	26% 5% 9% 21% 31% 7%	1600
	11th to 20th	Night	16% 3 7% 21% 34% 20%	1600
	21st to end	Night	17% 4 3 15% 28% 32%	1600
July	1st to 10th	Night	16% 5% 3 8% 23% 45%	3635
	11th to 20th	Night	28% 5% 4 8% 15% 40%	4046
	21st to end	Night	29% 4 4 12% 20% 30%	4345
August	1st to 10th	Night	36% 6% 4 11% 20% 23%	2929
	11th to 20th	Night	15% 2 3 13% 35% 32%	3024
	21st to end	Night	23% 6% 4 15% 29% 23%	3608
September	1st to 10th	Night	18% 3 5% 15% 28% 33%	2000
	11th to 20th	Night	28% 4 6% 21% 28% 13%	2024
	21st to end	Night	22% 4 5% 21% 26% 24%	2282
October	1st to 10th	Night	18% 3 5% 11% 22% 42%	3920
	11th to 20th	Night	31% 3 6% 20% 20% 19%	3873
	21st to end	Night	28% 4 6% 21% 26% 16%	4121
November	1st to 10th	Night	31% 6% 10% 26% 18% 9%	2340
	11th to 20th	Night	34% 6% 13% 23% 16% 7%	2649
	21st to end	Night	32% 5% 9% 21% 21% 12%	2800
December	1st to 10th	Night	36% 6% 7% 22% 19% 10%	4560
	11th to 20th	Night	54% 6% 10% 15% 12% 3	5176
	21st to end	Night	56% 6% 9% 17% 10% 2	6952

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	28% 13% 24% 19% 13% 3	328
	11th to 20th	Day	21% 10% 26% 25% 16% 3	400
	21st to end	Day	15% 13% 24% 28% 16% 3	440
February	1st to 10th	Day	22% 13% 21% 23% 18% 4	400
	11th to 20th	Day	20% 18% 25% 24% 12% 1	408
	21st to end	Day	18% 15% 26% 26% 14% 1	414
March	1st to 10th	Day	25% 18% 27% 20% 7% 2	500
	11th to 20th	Day	23% 16% 27% 19% 10% 4	500
	21st to end	Day	19% 18% 33% 20% 7% 2	550
April	1st to 10th	Day	16% 20% 28% 22% 10% 3	500
	11th to 20th	Day	15% 19% 29% 23% 9% 5%	492
	21st to end	Day	8% 19% 32% 23% 12% 6%	500
May	1st to 10th	Day	11% 17% 31% 26% 12% 3	503
	11th to 20th	Day	11% 16% 30% 24% 15% 4	528
	21st to end	Day	10% 12% 27% 27% 18% 6%	573
June	1st to 10th	Day	8% 13% 29% 30% 14% 7%	530
	11th to 20th	Day	5% 13% 31% 33% 13% 4	530
	21st to end	Day	2 5% 28% 34% 24% 7%	530
July	1st to 10th	Day	4 5% 22% 41% 22% 5%	526
	11th to 20th	Day	1 1 20% 42% 28% 7%	524
	21st to end	Day	2 2 15% 47% 27% 7%	575
August	1st to 10th	Day	3 4 22% 42% 21% 8%	527
	11th to 20th	Day	4 3 23% 36% 25% 8%	517
	21st to end	Day	4 3 20% 39% 26% 8%	583
September	1st to 10th	Day	2 5% 21% 37% 26% 9%	530
	11th to 20th	Day	5% 9% 25% 31% 22% 9%	530
	21st to end	Day	6% 9% 22% 32% 24% 8%	530
October	1st to 10th	Day	7% 9% 25% 25% 22% 12%	530
	11th to 20th	Day	10% 10% 19% 27% 25% 11%	523
	21st to end	Day	13% 9% 19% 28% 25% 7%	539
November	1st to 10th	Day	11% 11% 19% 26% 26% 7%	492
	11th to 20th	Day	12% 11% 18% 23% 23% 13%	274
	21st to end	Day	13% 9% 23% 27% 19% 11%	200
December	1st to 10th	Day	18% 15% 13% 24% 20% 12%	200
	11th to 20th	Day	26% 11% 20% 20% 16% 8%	200
	21st to end	Day	25% 13% 20% 25% 15% 4	220
January	1st to 10th	Night	20% 8% 16% 30% 22% 5%	800
	11th to 20th	Night	20% 6% 15% 34% 21% 5%	800
	21st to end	Night	19% 6% 14% 32% 25% 4	880
February	1st to 10th	Night	21% 7% 15% 32% 23% 3	800
	11th to 20th	Night	17% 9% 18% 31% 20% 4	784
	21st to end	Night	15% 10% 21% 34% 18% 2	660
March	1st to 10th	Night	21% 12% 17% 27% 19% 3	800
	11th to 20th	Night	19% 9% 18% 31% 20% 3	800
	21st to end	Night	17% 9% 26% 28% 16% 3	880
April	1st to 10th	Night	12% 10% 29% 28% 19% 3	800
	11th to 20th	Night	11% 13% 24% 28% 20% 5%	791
	21st to end	Night	8% 12% 27% 25% 23% 5%	800
May	1st to 10th	Night	9% 7% 24% 28% 27% 4	622
	11th to 20th	Night	6% 3 23% 31% 33% 4	565
	21st to end	Night	6% 10% 22% 30% 27% 4	433
June	1st to 10th	Night	7% 6% 23% 36% 27% 2	400
	11th to 20th	Night	4 7% 21% 43% 24% 2	400
	21st to end	Night	3 2 16% 37% 39% 3	400
July	1st to 10th	Night	3 1 14% 36% 40% 6%	399
	11th to 20th	Night	3 1 6% 31% 49% 10%	582
	21st to end	Night	4 7% 33% 46% 11%	657
August	1st to 10th	Night	4 1 12% 32% 40% 10%	597
	11th to 20th	Night	2 10% 36% 43% 8%	650
	21st to end	Night	4 1 8% 34% 45% 8%	880
September	1st to 10th	Night	2 2 10% 31% 46% 9%	800
	11th to 20th	Night	7% 3 12% 34% 35% 10%	800
	21st to end	Night	6% 3 14% 30% 36% 12%	800
October	1st to 10th	Night	7% 4 14% 31% 34% 10%	800
	11th to 20th	Night	10% 4 12% 28% 39% 8%	792
	21st to end	Night	11% 4 12% 27% 35% 11%	836
November	1st to 10th	Night	11% 5% 17% 26% 33% 8%	780
	11th to 20th	Night	14% 5% 13% 34% 29% 6%	800
	21st to end	Night	12% 7% 16% 34% 26% 6%	800
December	1st to 10th	Night	22% 5% 14% 30% 23% 6%	800
	11th to 20th	Night	26% 7% 14% 29% 20% 4	800
	21st to end	Night	26% 7% 20% 29% 17% 1	880

Page generated at 2025-10-15 22:50:24. Version ac07

Copyright © 2025 Attilla Danko.