O'Brien
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the O'Brien clear sky chart from 2005-11-06 to 2024-02-20.

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% 6% 3 6% 6% 4 6% 6% 6% 9% 40%	78809
	Night	9% 6% 3 5% 5% 3 5% 5% 6% 9% 44%	80383
Transparency	Day	39% 7% 14% 18% 15% 7%	78532
	Night	36% 6% 14% 19% 16% 8%	80179
Seeing	Day	10% 6% 19% 24% 23% 17%	14229
	Night	11% 5% 20% 34% 26% 6%	24360

By Month

Percentage Cloud Cover SE (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% 7% 3 5% 4 2 4 4 4 7% 46%	2836
	11th to 20th	Day	15% 8% 3 6% 5% 3 4 4 4 8% 39%	2840
	21st to end	Day	10% 6% 3 5% 4 3 5% 5% 6% 8% 44%	3323
February	1st to 10th	Day	16% 7% 3 5% 5% 2 3 3 5% 6% 44%	1900
	11th to 20th	Day	13% 6% 2 5% 5% 3 4 4 4 6% 49%	1838
	21st to end	Day	14% 6% 3 5% 4 4 4 4 4 5% 48%	1480
March	1st to 10th	Day	8% 3 2 4 4 2 4 5% 6% 9% 53%	1962
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 4 6% 8% 48%	1980
	21st to end	Day	8% 4 2 4 5% 3 5% 4 5% 7% 51%	2178
April	1st to 10th	Day	8% 5% 3 5% 4 3 4 5% 7% 8% 48%	1980
	11th to 20th	Day	10% 6% 3 6% 6% 3 6% 5% 5% 7% 45%	1956
	21st to end	Day	5% 3 2 4 5% 3 4 5% 7% 7% 55%	2302
May	1st to 10th	Day	5% 3 1 4 4 2 4 5% 5% 9% 59%	2340
	11th to 20th	Day	6% 4 3 4 7% 5% 6% 7% 7% 11% 41%	2301
	21st to end	Day	6% 5% 2 5% 6% 4 8% 7% 7% 9% 40%	2531
June	1st to 10th	Day	9% 6% 3 8% 7% 5% 8% 7% 8% 9% 29%	2340
	11th to 20th	Day	5% 4 3 6% 7% 5% 9% 9% 9% 13% 30%	2340
	21st to end	Day	7% 6% 3 6% 8% 5% 8% 8% 9% 11% 29%	2340
July	1st to 10th	Day	7% 6% 3 6% 7% 5% 8% 8% 9% 14% 27%	2340
	11th to 20th	Day	6% 6% 4 7% 10% 6% 9% 10% 10% 11% 22%	2340
	21st to end	Day	6% 5% 3 7% 8% 5% 9% 9% 8% 13% 26%	2547
August	1st to 10th	Day	9% 5% 2 6% 8% 6% 7% 8% 8% 13% 27%	2327
	11th to 20th	Day	9% 7% 3 8% 9% 6% 9% 9% 8% 11% 20%	2139
	21st to end	Day	10% 7% 3 9% 8% 5% 7% 8% 8% 10% 24%	2252
September	1st to 10th	Day	9% 6% 4 9% 9% 6% 8% 8% 9% 9% 24%	1980
	11th to 20th	Day	8% 6% 4 9% 9% 6% 6% 7% 8% 9% 27%	1980
	21st to end	Day	9% 7% 2 8% 7% 3 5% 5% 6% 7% 39%	1980
October	1st to 10th	Day	11% 5% 3 6% 7% 3 4 6% 6% 7% 42%	1980
	11th to 20th	Day	6% 3 2 4 6% 3 4 4 5% 8% 55%	1851
	21st to end	Day	7% 4 2 4 5% 3 5% 5% 6% 7% 51%	1980
November	1st to 10th	Day	8% 5% 2 4 4 2 4 5% 6% 8% 52%	1857
	11th to 20th	Day	10% 6% 2 5% 4 3 4 4 5% 6% 52%	1792
	21st to end	Day	10% 5% 3 5% 4 3 5% 4 6% 7% 48%	1710
December	1st to 10th	Day	9% 6% 3 5% 5% 4 4 5% 6% 8% 46%	1710
	11th to 20th	Day	15% 6% 3 5% 4 2 4 4 5% 7% 45%	2153
	21st to end	Day	17% 8% 3 6% 5% 3 4 4 4 5% 41%	3124
January	1st to 10th	Night	14% 9% 2 5% 4 2 4 4 5% 7% 44%	4109
	11th to 20th	Night	10% 7% 3 7% 7% 4 5% 5% 5% 8% 40%	4120
	21st to end	Night	13% 8% 3 6% 5% 3 4 5% 5% 7% 42%	4330
February	1st to 10th	Night	13% 6% 2 6% 5% 2 4 3 5% 6% 47%	2280
	11th to 20th	Night	12% 7% 3 5% 5% 3 4 3 4 6% 49%	2213
	21st to end	Night	11% 8% 3 6% 4 3 3 4 4 6% 48%	1776
March	1st to 10th	Night	9% 5% 2 4 3 2 4 4 6% 8% 53%	1995
	11th to 20th	Night	10% 5% 2 5% 5% 3 4 4 5% 9% 47%	1980
	21st to end	Night	8% 5% 2 4 4 2 5% 4 5% 6% 55%	2178
April	1st to 10th	Night	9% 5% 3 4 4 3 5% 4 5% 8% 50%	1980
	11th to 20th	Night	7% 5% 3 6% 4 3 4 5% 6% 7% 50%	1953
	21st to end	Night	5% 4 2 3 4 2 4 4 6% 9% 58%	1670
May	1st to 10th	Night	4 3 2 3 5% 2 3 4 5% 8% 61%	1620
	11th to 20th	Night	4 4 2 5% 5% 3 5% 6% 6% 11% 47%	1592
	21st to end	Night	5% 3 2 4 4 4 5% 5% 7% 11% 50%	1744
June	1st to 10th	Night	9% 5% 3 6% 6% 3 5% 7% 9% 11% 36%	1620
	11th to 20th	Night	5% 4 2 5% 5% 4 7% 8% 10% 14% 37%	1620
	21st to end	Night	7% 4 2 6% 6% 4 6% 6% 9% 13% 36%	1620
July	1st to 10th	Night	5% 4 3 5% 7% 4 7% 6% 10% 14% 33%	1620
	11th to 20th	Night	5% 5% 3 7% 6% 4 7% 7% 10% 14% 32%	1620
	21st to end	Night	4 5% 2 5% 6% 5% 6% 7% 9% 14% 36%	1771
August	1st to 10th	Night	6% 4 3 5% 6% 3 5% 8% 11% 13% 35%	1611
	11th to 20th	Night	5% 4 2 6% 6% 4 7% 8% 9% 14% 34%	1757
	21st to end	Night	9% 7% 3 7% 7% 5% 7% 7% 8% 9% 31%	2083
September	1st to 10th	Night	7% 6% 4 8% 8% 4 6% 8% 8% 11% 28%	1980
	11th to 20th	Night	9% 7% 3 7% 7% 5% 6% 6% 8% 11% 32%	1980
	21st to end	Night	8% 6% 3 6% 7% 3 5% 6% 6% 8% 41%	1980
October	1st to 10th	Night	8% 7% 2 6% 5% 3 5% 5% 6% 6% 47%	1980
	11th to 20th	Night	6% 4 2 4 5% 3 3 4 5% 8% 57%	2106
	21st to end	Night	7% 5% 2 5% 6% 3 5% 5% 5% 7% 51%	2376
November	1st to 10th	Night	7% 5% 2 4 5% 3 4 4 6% 7% 52%	2221
	11th to 20th	Night	10% 6% 2 5% 5% 3 4 3 4 6% 53%	2384
	21st to end	Night	10% 6% 3 5% 4 3 5% 5% 5% 7% 47%	2470
December	1st to 10th	Night	12% 5% 3 6% 5% 3 4 4 5% 8% 46%	2470
	11th to 20th	Night	14% 7% 3 4 4 2 4 4 5% 6% 47%	3042
	21st to end	Night	15% 8% 3 5% 5% 3 4 5% 5% 7% 40%	4532

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	39% 4 7% 17% 20% 14%	2837
	11th to 20th	Day	45% 3 6% 15% 17% 14%	2840
	21st to end	Day	38% 3 11% 15% 19% 12%	3323
February	1st to 10th	Day	41% 4 10% 14% 19% 12%	1900
	11th to 20th	Day	39% 3 6% 17% 24% 12%	1848
	21st to end	Day	38% 4 8% 17% 19% 13%	1480
March	1st to 10th	Day	27% 5% 10% 23% 25% 12%	1962
	11th to 20th	Day	34% 5% 13% 20% 17% 11%	1980
	21st to end	Day	32% 4 13% 22% 19% 10%	2178
April	1st to 10th	Day	34% 5% 12% 17% 19% 13%	1980
	11th to 20th	Day	39% 4 9% 21% 20% 9%	1956
	21st to end	Day	27% 5% 12% 25% 23% 9%	2302
May	1st to 10th	Day	23% 4 11% 26% 26% 10%	2340
	11th to 20th	Day	35% 7% 18% 20% 17% 4	2305
	21st to end	Day	38% 7% 11% 23% 18% 3	2531
June	1st to 10th	Day	47% 10% 18% 16% 9% 1	2340
	11th to 20th	Day	41% 10% 21% 21% 7%	2340
	21st to end	Day	45% 11% 22% 15% 6% 1	2340
July	1st to 10th	Day	42% 14% 27% 14% 1	2275
	11th to 20th	Day	48% 13% 26% 10% 3	2271
	21st to end	Day	45% 12% 25% 16% 2	2405
August	1st to 10th	Day	43% 13% 27% 15% 2	2314
	11th to 20th	Day	52% 12% 24% 10% 2	2139
	21st to end	Day	51% 12% 19% 13% 3 1	2252
September	1st to 10th	Day	51% 11% 18% 12% 6% 2	1980
	11th to 20th	Day	49% 9% 18% 14% 8% 1	1980
	21st to end	Day	42% 6% 13% 19% 15% 5%	1980
October	1st to 10th	Day	41% 6% 9% 17% 16% 11%	1980
	11th to 20th	Day	28% 3 9% 22% 29% 9%	1851
	21st to end	Day	31% 5% 9% 16% 24% 15%	1980
November	1st to 10th	Day	30% 4 10% 20% 20% 15%	1857
	11th to 20th	Day	34% 4 8% 18% 25% 12%	1792
	21st to end	Day	35% 4 9% 20% 23% 8%	1710
December	1st to 10th	Day	36% 4 13% 22% 18% 7%	1710
	11th to 20th	Day	40% 4 7% 20% 21% 9%	2150
	21st to end	Day	46% 3 5% 19% 19% 7%	3124
January	1st to 10th	Night	41% 3 8% 14% 19% 16%	4109
	11th to 20th	Night	43% 3 7% 15% 21% 11%	4120
	21st to end	Night	43% 3 11% 13% 19% 10%	4330
February	1st to 10th	Night	38% 4 8% 16% 20% 13%	2280
	11th to 20th	Night	38% 3 7% 19% 19% 14%	2214
	21st to end	Night	38% 4 9% 19% 17% 13%	1776
March	1st to 10th	Night	29% 4 11% 22% 21% 12%	1995
	11th to 20th	Night	34% 5% 14% 20% 16% 11%	1980
	21st to end	Night	31% 4 11% 24% 19% 11%	2178
April	1st to 10th	Night	34% 5% 13% 19% 18% 12%	1980
	11th to 20th	Night	33% 5% 9% 22% 22% 8%	1953
	21st to end	Night	24% 4 12% 30% 22% 8%	1670
May	1st to 10th	Night	22% 5% 11% 28% 23% 10%	1620
	11th to 20th	Night	29% 6% 23% 22% 15% 5%	1593
	21st to end	Night	27% 8% 18% 23% 21% 3	1745
June	1st to 10th	Night	37% 9% 23% 23% 7%	1620
	11th to 20th	Night	32% 12% 29% 22% 6%	1620
	21st to end	Night	35% 10% 27% 20% 7% 1	1620
July	1st to 10th	Night	38% 13% 29% 17% 3	1575
	11th to 20th	Night	39% 15% 30% 13% 3	1566
	21st to end	Night	33% 13% 34% 17% 3	1672
August	1st to 10th	Night	33% 14% 32% 17% 4	1600
	11th to 20th	Night	36% 14% 37% 12% 2	1757
	21st to end	Night	46% 12% 22% 16% 3 1	2083
September	1st to 10th	Night	44% 13% 21% 14% 6% 1	1980
	11th to 20th	Night	44% 10% 20% 17% 7% 1	1980
	21st to end	Night	38% 7% 17% 18% 15% 4	1980
October	1st to 10th	Night	36% 6% 13% 17% 16% 12%	1980
	11th to 20th	Night	27% 4 9% 25% 28% 8%	2106
	21st to end	Night	31% 5% 9% 17% 23% 15%	2376
November	1st to 10th	Night	31% 5% 9% 22% 19% 15%	2221
	11th to 20th	Night	34% 4 8% 18% 22% 14%	2384
	21st to end	Night	36% 4 10% 20% 20% 10%	2470
December	1st to 10th	Night	38% 4 12% 24% 16% 7%	2470
	11th to 20th	Night	39% 4 9% 16% 22% 10%	3044
	21st to end	Night	43% 4 9% 20% 19% 5%	4532

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	10% 13% 48% 21% 7% 1	270
	11th to 20th	Day	9% 15% 36% 28% 10% 3	270
	21st to end	Day	16% 19% 44% 16% 4	297
February	1st to 10th	Day	15% 16% 35% 24% 7% 2	270
	11th to 20th	Day	17% 12% 40% 20% 10%	267
	21st to end	Day	9% 14% 34% 30% 13%	198
March	1st to 10th	Day	10% 14% 31% 29% 16% 1	402
	11th to 20th	Day	14% 8% 33% 25% 17% 3	420
	21st to end	Day	10% 9% 31% 30% 18% 2	462
April	1st to 10th	Day	13% 11% 28% 24% 20% 4	420
	11th to 20th	Day	11% 9% 26% 29% 19% 5%	412
	21st to end	Day	6% 5% 22% 34% 24% 9%	420
May	1st to 10th	Day	7% 5% 21% 28% 26% 13%	423
	11th to 20th	Day	5% 3 14% 28% 31% 19%	447
	21st to end	Day	8% 4 10% 18% 33% 27%	485
June	1st to 10th	Day	13% 2 9% 21% 30% 25%	450
	11th to 20th	Day	7% 4 10% 18% 27% 34%	450
	21st to end	Day	6% 4 5% 17% 33% 35%	450
July	1st to 10th	Day	6% 1 5% 14% 25% 48%	445
	11th to 20th	Day	7% 2 2 10% 31% 49%	444
	21st to end	Day	7% 1 4 15% 37% 35%	488
August	1st to 10th	Day	12% 1 4 16% 27% 40%	446
	11th to 20th	Day	8% 2 4 12% 32% 42%	439
	21st to end	Day	10% 3 6% 19% 27% 35%	495
September	1st to 10th	Day	10% 3 8% 17% 32% 30%	450
	11th to 20th	Day	10% 4 8% 26% 33% 19%	450
	21st to end	Day	10% 2 9% 23% 35% 22%	450
October	1st to 10th	Day	14% 4 16% 32% 24% 9%	450
	11th to 20th	Day	4 4 17% 34% 31% 10%	450
	21st to end	Day	8% 6% 21% 38% 22% 5%	495
November	1st to 10th	Day	13% 4 22% 34% 21% 6%	455
	11th to 20th	Day	15% 7% 29% 33% 13% 4	352
	21st to end	Day	8% 10% 33% 33% 14% 1	270
December	1st to 10th	Day	10% 11% 34% 25% 14% 5%	270
	11th to 20th	Day	17% 9% 35% 28% 10% 1	270
	21st to end	Day	13% 11% 31% 30% 12% 2	297
January	1st to 10th	Night	16% 8% 33% 34% 8% 1	760
	11th to 20th	Night	12% 8% 28% 38% 14% 1	760
	21st to end	Night	17% 10% 34% 32% 7%	836
February	1st to 10th	Night	16% 8% 29% 34% 13%	760
	11th to 20th	Night	16% 9% 32% 35% 8%	742
	21st to end	Night	15% 6% 32% 41% 6%	592
March	1st to 10th	Night	10% 7% 32% 39% 12%	720
	11th to 20th	Night	13% 6% 28% 37% 15% 1	720
	21st to end	Night	10% 7% 28% 40% 15%	792
April	1st to 10th	Night	12% 6% 26% 41% 15% 1	720
	11th to 20th	Night	10% 6% 26% 41% 16% 1	711
	21st to end	Night	6% 3 24% 42% 23% 2	558
May	1st to 10th	Night	5% 3 17% 49% 24% 2	540
	11th to 20th	Night	5% 3 14% 42% 32% 4	531
	21st to end	Night	5% 2 9% 35% 43% 6%	584
June	1st to 10th	Night	10% 1 9% 32% 39% 8%	540
	11th to 20th	Night	6% 3 4 27% 42% 18%	540
	21st to end	Night	7% 2 4 22% 48% 17%	540
July	1st to 10th	Night	6% 2 3 18% 49% 23%	525
	11th to 20th	Night	6% 1 2 11% 52% 28%	522
	21st to end	Night	6% 1 2 15% 59% 17%	591
August	1st to 10th	Night	7% 1 3 14% 55% 20%	537
	11th to 20th	Night	5% 1 4 11% 55% 24%	534
	21st to end	Night	10% 2 5% 22% 43% 18%	697
September	1st to 10th	Night	9% 2 8% 21% 43% 16%	720
	11th to 20th	Night	12% 3 8% 29% 38% 9%	720
	21st to end	Night	10% 3 11% 31% 38% 6%	720
October	1st to 10th	Night	12% 3 18% 39% 26% 3	720
	11th to 20th	Night	7% 2 17% 43% 28% 2	720
	21st to end	Night	9% 4 20% 41% 24% 2	792
November	1st to 10th	Night	9% 4 25% 40% 20% 1	740
	11th to 20th	Night	12% 6% 25% 40% 16% 1	760
	21st to end	Night	12% 5% 27% 42% 14% 1	760
December	1st to 10th	Night	13% 6% 26% 37% 18% 1	760
	11th to 20th	Night	14% 6% 27% 35% 18%	760
	21st to end	Night	15% 6% 26% 37% 15%	836

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