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 2024-01-12.

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 2 3 3 4 7% 56%	74667
	Night	11% 6% 2 5% 4 2 4 4 4 7% 51%	75059
Transparency	Day	29% 5% 10% 21% 22% 14%	74388
	Night	33% 5% 9% 19% 20% 13%	74860
Seeing	Day	11% 11% 24% 29% 19% 6%	14249
	Night	12% 6% 16% 31% 29% 6%	23606

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	22% 7% 2 6% 5% 2 3 4 5% 6% 38%	1640
	11th to 20th	Day	16% 6% 3 5% 6% 2 3 4 4 6% 46%	1630
	21st to end	Day	13% 7% 3 4 5% 3 5% 4 5% 7% 42%	1782
February	1st to 10th	Day	18% 5% 2 5% 5% 3 3 4 4 4 46%	1620
	11th to 20th	Day	12% 6% 3 6% 4 3 4 4 5% 8% 45%	1597
	21st to end	Day	17% 6% 1 4 5% 2 4 3 4 7% 48%	1484
March	1st to 10th	Day	18% 8% 3 6% 5% 3 4 4 5% 7% 36%	1980
	11th to 20th	Day	18% 6% 2 5% 5% 3 4 5% 5% 8% 38%	1980
	21st to end	Day	15% 5% 3 5% 4 3 4 4 5% 7% 46%	2178
April	1st to 10th	Day	14% 7% 3 6% 6% 2 4 4 5% 8% 41%	2160
	11th to 20th	Day	8% 5% 3 4 4 3 4 4 4 7% 53%	2129
	21st to end	Day	7% 3 2 3 3 2 3 4 5% 9% 58%	2160
May	1st to 10th	Day	7% 5% 2 3 2 2 3 3 5% 7% 61%	2317
	11th to 20th	Day	7% 4 2 2 3 2 3 3 5% 7% 61%	2321
	21st to end	Day	6% 6% 2 4 3 2 3 3 4 7% 59%	2723
June	1st to 10th	Day	7% 5% 2 3 3 2 3 3 4 9% 58%	2520
	11th to 20th	Day	2 4 1 2 2 2 2 3 3 6% 73%	2520
	21st to end	Day	2 4 2 3 2 2 2 2 3 5% 74%	2520
July	1st to 10th	Day	2 5% 2 3 2 1 2 2 3 5% 73%	2509
	11th to 20th	Day	1 5% 1 3 2 2 2 3 4 7% 69%	2340
	21st to end	Day	1 5% 2 4 3 2 3 3 3 6% 70%	2546
August	1st to 10th	Day	1 5% 2 4 3 2 3 2 4 6% 67%	2328
	11th to 20th	Day	1 5% 2 3 3 2 3 2 4 6% 69%	2245
	21st to end	Day	2 7% 3 5% 2 2 2 3 3 6% 64%	2376
September	1st to 10th	Day	1 4 1 3 2 2 2 2 3 7% 73%	2160
	11th to 20th	Day	5% 4 2 5% 3 2 3 3 4 7% 62%	2136
	21st to end	Day	4 3 1 2 2 1 2 3 4 6% 73%	1905
October	1st to 10th	Day	7% 3 1 3 3 3 3 3 4 5% 67%	1800
	11th to 20th	Day	8% 4 2 4 3 2 2 3 4 5% 61%	1800
	21st to end	Day	11% 4 2 4 2 2 3 3 4 7% 58%	1980
November	1st to 10th	Day	10% 4 2 3 4 3 4 4 6% 7% 53%	1857
	11th to 20th	Day	12% 5% 3 5% 5% 3 4 4 4 6% 49%	1662
	21st to end	Day	12% 4 2 3 3 3 5% 3 5% 6% 53%	1520
December	1st to 10th	Day	19% 6% 3 4 4 2 3 5% 5% 6% 44%	1520
	11th to 20th	Day	33% 8% 3 5% 4 2 3 4 4 5% 29%	1928
	21st to end	Day	24% 8% 2 5% 5% 3 3 3 5% 7% 36%	2794
January	1st to 10th	Night	19% 8% 3 4 5% 3 4 5% 6% 8% 36%	2523
	11th to 20th	Night	18% 6% 2 5% 5% 2 4 4 4 6% 43%	2358
	21st to end	Night	16% 5% 2 4 4 3 4 4 5% 7% 45%	2574
February	1st to 10th	Night	17% 5% 2 5% 4 2 3 5% 4 6% 48%	2340
	11th to 20th	Night	12% 6% 3 6% 5% 2 5% 4 5% 8% 44%	2249
	21st to end	Night	14% 5% 2 3 3 2 3 4 5% 6% 53%	1768
March	1st to 10th	Night	18% 7% 4 6% 4 3 5% 6% 4 5% 37%	1980
	11th to 20th	Night	16% 8% 3 5% 4 3 4 5% 6% 8% 40%	1980
	21st to end	Night	16% 5% 3 4 3 2 3 3 4 5% 51%	2178
April	1st to 10th	Night	11% 5% 3 5% 4 2 4 5% 5% 6% 49%	1800
	11th to 20th	Night	8% 6% 2 4 5% 3 4 3 3 7% 55%	1779
	21st to end	Night	6% 4 2 4 3 2 3 4 4 7% 61%	1800
May	1st to 10th	Night	5% 6% 3 4 3 2 3 2 5% 7% 59%	1640
	11th to 20th	Night	4 4 2 4 4 3 3 5% 4 8% 59%	1571
	21st to end	Night	5% 6% 3 4 5% 1 3 3 4 8% 59%	1552
June	1st to 10th	Night	5% 5% 3 4 4 1 4 4 5% 9% 56%	1440
	11th to 20th	Night	2 3 2 3 3 2 3 3 3 6% 70%	1440
	21st to end	Night	3 4 2 4 2 2 2 3 3 6% 69%	1440
July	1st to 10th	Night	1 4 2 4 3 2 3 3 3 7% 69%	1447
	11th to 20th	Night	1 4 2 5% 3 3 3 4 4 8% 61%	1620
	21st to end	Night	1 6% 4 8% 5% 3 4 4 6% 9% 51%	1773
August	1st to 10th	Night	2 7% 2 7% 4 3 3 3 4 7% 57%	1611
	11th to 20th	Night	5% 3 7% 5% 3 4 6% 4 9% 54%	1652
	21st to end	Night	1 7% 4 6% 5% 3 3 4 4 8% 57%	1980
September	1st to 10th	Night	1 4 2 4 3 2 3 3 3 8% 67%	1800
	11th to 20th	Night	5% 7% 3 5% 4 2 4 4 4 7% 56%	1821
	21st to end	Night	3 5% 2 5% 3 2 2 3 3 5% 67%	2053
October	1st to 10th	Night	5% 5% 2 4 4 2 4 3 4 5% 62%	2160
	11th to 20th	Night	8% 4 3 6% 4 2 3 4 5% 6% 56%	2160
	21st to end	Night	9% 4 2 4 4 2 3 4 4 6% 57%	2376
November	1st to 10th	Night	10% 5% 2 4 5% 3 3 4 5% 8% 51%	2220
	11th to 20th	Night	13% 6% 3 4 3 2 4 5% 6% 6% 48%	2516
	21st to end	Night	10% 4 2 4 4 3 4 4 4 7% 54%	2660
December	1st to 10th	Night	21% 6% 3 5% 4 2 4 3 5% 5% 44%	2660
	11th to 20th	Night	32% 6% 2 4 4 2 4 4 5% 6% 31%	3276
	21st to end	Night	24% 8% 2 5% 5% 3 5% 4 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	48% 6% 11% 20% 8% 6%	1640
	11th to 20th	Day	42% 4 9% 16% 15% 13%	1630
	21st to end	Day	42% 6% 10% 16% 18% 8%	1782
February	1st to 10th	Day	42% 5% 7% 18% 19% 8%	1620
	11th to 20th	Day	38% 6% 10% 20% 19% 6%	1606
	21st to end	Day	38% 5% 9% 26% 15% 7%	1484
March	1st to 10th	Day	47% 7% 12% 16% 15% 3	1980
	11th to 20th	Day	45% 6% 10% 21% 14% 5%	1980
	21st to end	Day	38% 7% 15% 21% 16% 3	2178
April	1st to 10th	Day	42% 6% 13% 21% 15% 4	2160
	11th to 20th	Day	30% 6% 16% 23% 20% 5%	2132
	21st to end	Day	24% 6% 15% 30% 22% 4	2160
May	1st to 10th	Day	24% 6% 10% 27% 26% 7%	2317
	11th to 20th	Day	22% 5% 12% 31% 22% 8%	2324
	21st to end	Day	27% 5% 18% 27% 20% 3	2724
June	1st to 10th	Day	25% 6% 12% 27% 25% 6%	2520
	11th to 20th	Day	14% 5% 11% 19% 35% 16%	2520
	21st to end	Day	16% 3 6% 18% 33% 24%	2520
July	1st to 10th	Day	16% 3 5% 16% 27% 34%	2440
	11th to 20th	Day	16% 3 7% 19% 24% 31%	2271
	21st to end	Day	18% 3 5% 15% 23% 36%	2405
August	1st to 10th	Day	20% 3 7% 20% 27% 23%	2312
	11th to 20th	Day	17% 2 6% 17% 32% 26%	2245
	21st to end	Day	22% 3 6% 17% 29% 23%	2376
September	1st to 10th	Day	14% 3 5% 14% 29% 36%	2160
	11th to 20th	Day	23% 4 8% 25% 29% 11%	2136
	21st to end	Day	15% 3 7% 25% 28% 21%	1905
October	1st to 10th	Day	21% 3 7% 18% 29% 22%	1800
	11th to 20th	Day	25% 5% 10% 19% 24% 18%	1800
	21st to end	Day	28% 5% 8% 21% 23% 15%	1980
November	1st to 10th	Day	30% 6% 10% 27% 18% 9%	1857
	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	45% 7% 13% 19% 11% 5%	2523
	11th to 20th	Night	43% 5% 10% 17% 13% 13%	2358
	21st to end	Night	38% 6% 11% 15% 20% 10%	2574
February	1st to 10th	Night	38% 5% 9% 19% 18% 11%	2340
	11th to 20th	Night	39% 6% 9% 19% 20% 7%	2251
	21st to end	Night	32% 6% 13% 24% 20% 5%	1768
March	1st to 10th	Night	47% 7% 11% 15% 16% 4	1980
	11th to 20th	Night	42% 5% 9% 21% 17% 5%	1980
	21st to end	Night	37% 5% 13% 23% 16% 6%	2178
April	1st to 10th	Night	35% 7% 14% 20% 20% 5%	1800
	11th to 20th	Night	30% 5% 12% 24% 19% 9%	1779
	21st to end	Night	24% 5% 10% 31% 24% 6%	1800
May	1st to 10th	Night	26% 5% 8% 21% 30% 9%	1640
	11th to 20th	Night	24% 6% 10% 27% 25% 8%	1572
	21st to end	Night	26% 5% 14% 29% 22% 4	1552
June	1st to 10th	Night	27% 5% 10% 21% 30% 7%	1440
	11th to 20th	Night	18% 4 7% 22% 31% 19%	1440
	21st to end	Night	18% 4 4 16% 28% 30%	1440
July	1st to 10th	Night	17% 2 4 12% 28% 36%	1407
	11th to 20th	Night	21% 2 6% 17% 23% 30%	1566
	21st to end	Night	32% 2 5% 11% 18% 33%	1674
August	1st to 10th	Night	29% 2 6% 15% 22% 26%	1602
	11th to 20th	Night	25% 4 6% 16% 23% 27%	1652
	21st to end	Night	26% 3 6% 17% 22% 27%	1980
September	1st to 10th	Night	17% 2 5% 15% 28% 33%	1800
	11th to 20th	Night	28% 4 7% 21% 27% 14%	1821
	21st to end	Night	20% 3 5% 22% 25% 24%	2053
October	1st to 10th	Night	22% 4 8% 17% 26% 22%	2160
	11th to 20th	Night	29% 4 7% 19% 20% 21%	2160
	21st to end	Night	28% 5% 7% 19% 23% 18%	2376
November	1st to 10th	Night	32% 6% 10% 25% 18% 9%	2220
	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% 13% 24% 17% 11% 4	301
	11th to 20th	Day	22% 11% 24% 25% 15% 3	344
	21st to end	Day	15% 14% 23% 29% 17% 3	374
February	1st to 10th	Day	19% 12% 21% 24% 19% 5%	340
	11th to 20th	Day	18% 18% 26% 24% 13% 1	345
	21st to end	Day	19% 16% 26% 25% 13% 1	346
March	1st to 10th	Day	25% 18% 28% 20% 8% 2	420
	11th to 20th	Day	24% 15% 28% 19% 10% 4	420
	21st to end	Day	16% 19% 34% 22% 8% 1	462
April	1st to 10th	Day	17% 20% 29% 21% 10% 2	420
	11th to 20th	Day	16% 20% 29% 23% 8% 4	412
	21st to end	Day	8% 21% 31% 24% 11% 5%	420
May	1st to 10th	Day	12% 18% 30% 26% 12% 3	423
	11th to 20th	Day	11% 17% 32% 24% 13% 4	448
	21st to end	Day	12% 13% 25% 27% 19% 5%	485
June	1st to 10th	Day	9% 14% 27% 30% 13% 7%	450
	11th to 20th	Day	6% 13% 30% 34% 13% 3	450
	21st to end	Day	2 5% 28% 35% 25% 6%	450
July	1st to 10th	Day	5% 5% 22% 42% 22% 4	446
	11th to 20th	Day	2 2 18% 43% 28% 7%	444
	21st to end	Day	2 2 13% 48% 28% 7%	487
August	1st to 10th	Day	3 4 21% 44% 21% 7%	447
	11th to 20th	Day	4 3 20% 38% 27% 9%	437
	21st to end	Day	5% 2 20% 39% 27% 6%	495
September	1st to 10th	Day	2 5% 18% 39% 26% 10%	450
	11th to 20th	Day	5% 9% 24% 32% 23% 8%	450
	21st to end	Day	5% 9% 21% 32% 25% 8%	450
October	1st to 10th	Day	6% 10% 25% 25% 23% 11%	450
	11th to 20th	Day	9% 9% 18% 27% 27% 10%	450
	21st to end	Day	14% 8% 18% 28% 25% 8%	495
November	1st to 10th	Day	12% 11% 18% 26% 27% 6%	453
	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% 7% 15% 29% 23% 5%	760
	11th to 20th	Night	21% 6% 14% 32% 21% 5%	726
	21st to end	Night	19% 5% 14% 33% 25% 4	792
February	1st to 10th	Night	19% 7% 14% 33% 24% 3	720
	11th to 20th	Night	16% 9% 18% 31% 21% 4	704
	21st to end	Night	17% 10% 22% 31% 18% 2	592
March	1st to 10th	Night	22% 12% 17% 27% 20% 4	720
	11th to 20th	Night	19% 8% 18% 32% 21% 3	720
	21st to end	Night	17% 8% 26% 29% 16% 3	792
April	1st to 10th	Night	13% 9% 29% 28% 18% 3	720
	11th to 20th	Night	11% 13% 24% 28% 19% 5%	711
	21st to end	Night	8% 12% 27% 24% 23% 5%	720
May	1st to 10th	Night	9% 7% 24% 29% 27% 4	560
	11th to 20th	Night	6% 3 23% 31% 32% 4	509
	21st to end	Night	7% 11% 21% 30% 26% 4	389
June	1st to 10th	Night	8% 6% 23% 34% 27% 2	360
	11th to 20th	Night	4 7% 21% 42% 24% 3	360
	21st to end	Night	4 1 17% 37% 39% 2	360
July	1st to 10th	Night	3 1 15% 34% 41% 6%	357
	11th to 20th	Night	3 1 7% 31% 48% 11%	522
	21st to end	Night	4 6% 34% 45% 11%	591
August	1st to 10th	Night	4 1 13% 33% 39% 10%	537
	11th to 20th	Night	2 1 9% 35% 45% 9%	584
	21st to end	Night	5% 1 7% 34% 46% 7%	792
September	1st to 10th	Night	2 1 9% 30% 48% 10%	720
	11th to 20th	Night	7% 3 11% 34% 35% 9%	720
	21st to end	Night	5% 4 14% 30% 36% 12%	720
October	1st to 10th	Night	7% 4 15% 30% 33% 11%	720
	11th to 20th	Night	10% 3 12% 28% 40% 8%	720
	21st to end	Night	11% 4 11% 26% 35% 12%	792
November	1st to 10th	Night	11% 4 16% 26% 33% 9%	740
	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 2024-01-09 04:14:39. Version ac07

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