Pastor's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pastor's Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	12% 6% 3 6% 6% 4 5% 5% 6% 8% 40%	74970
	Night	13% 6% 2 5% 4 3 4 4 5% 8% 47%	74807
Transparency	Day	41% 5% 11% 19% 17% 7%	74695
	Night	36% 5% 12% 20% 18% 9%	74602
Seeing	Day	14% 8% 21% 27% 22% 8%	13774
	Night	16% 7% 24% 32% 18% 2	23746

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	18% 7% 2 5% 4 3 4 3 4 6% 44%	1707
	11th to 20th	Day	20% 6% 3 5% 4 2 3 4 4 5% 44%	1647
	21st to end	Day	18% 8% 2 5% 4 2 3 4 4 6% 43%	1782
February	1st to 10th	Day	22% 8% 3 7% 5% 2 3 3 3 5% 38%	1683
	11th to 20th	Day	22% 8% 3 5% 6% 3 4 3 3 5% 39%	1763
	21st to end	Day	22% 7% 3 5% 5% 3 4 4 4 5% 38%	1628
March	1st to 10th	Day	22% 9% 3 6% 4 3 4 4 4 6% 36%	1980
	11th to 20th	Day	18% 7% 4 6% 5% 3 4 4 5% 6% 38%	1980
	21st to end	Day	16% 7% 3 6% 5% 2 5% 4 4 6% 42%	2178
April	1st to 10th	Day	16% 9% 3 6% 6% 4 5% 4 5% 6% 38%	1980
	11th to 20th	Day	17% 7% 3 5% 6% 3 4 4 5% 7% 38%	2106
	21st to end	Day	11% 7% 3 5% 6% 3 5% 4 5% 6% 43%	2160
May	1st to 10th	Day	11% 6% 5% 6% 6% 4 7% 6% 6% 7% 37%	2213
	11th to 20th	Day	10% 6% 3 6% 6% 4 5% 5% 6% 10% 39%	2302
	21st to end	Day	10% 6% 4 7% 7% 4 6% 7% 8% 10% 31%	2531
June	1st to 10th	Day	6% 4 2 6% 6% 5% 7% 9% 8% 8% 39%	2340
	11th to 20th	Day	6% 3 2 5% 6% 4 8% 7% 8% 11% 39%	2340
	21st to end	Day	6% 6% 3 7% 9% 7% 8% 8% 7% 10% 29%	2340
July	1st to 10th	Day	9% 5% 3 6% 7% 6% 7% 8% 8% 10% 32%	2340
	11th to 20th	Day	5% 4 3 6% 7% 5% 8% 7% 9% 11% 36%	2340
	21st to end	Day	5% 3 2 6% 7% 5% 8% 9% 8% 12% 35%	2546
August	1st to 10th	Day	3 3 2 4 6% 4 7% 8% 9% 13% 42%	2328
	11th to 20th	Day	5% 4 2 5% 6% 4 7% 7% 8% 11% 40%	2218
	21st to end	Day	7% 5% 2 7% 7% 5% 8% 8% 7% 10% 36%	2361
September	1st to 10th	Day	6% 5% 2 6% 7% 5% 6% 7% 6% 10% 40%	1980
	11th to 20th	Day	12% 7% 3 6% 7% 5% 6% 6% 5% 8% 36%	1980
	21st to end	Day	7% 5% 2 5% 5% 4 6% 5% 5% 8% 48%	1980
October	1st to 10th	Day	7% 3 2 5% 5% 3 4 4 5% 5% 57%	1864
	11th to 20th	Day	11% 5% 2 4 4 3 5% 4 5% 5% 52%	1800
	21st to end	Day	14% 5% 2 4 3 3 3 4 4 5% 54%	1980
November	1st to 10th	Day	11% 4 2 5% 5% 4 5% 4 3 8% 48%	1858
	11th to 20th	Day	12% 6% 2 6% 4 3 4 4 5% 6% 48%	1900
	21st to end	Day	18% 6% 2 6% 6% 3 4 3 4 5% 43%	1829
December	1st to 10th	Day	16% 8% 3 5% 4 3 5% 3 4 6% 42%	1710
	11th to 20th	Day	16% 8% 3 6% 6% 3 4 3 4 6% 41%	2161
	21st to end	Day	18% 8% 3 5% 5% 2 3 4 4 5% 44%	3135
January	1st to 10th	Night	18% 7% 3 5% 3 2 3 3 4 6% 46%	2458
	11th to 20th	Night	20% 6% 3 3 3 2 2 3 4 5% 49%	2386
	21st to end	Night	18% 7% 3 5% 4 3 4 3 3 6% 44%	2574
February	1st to 10th	Night	20% 8% 4 5% 4 2 4 4 4 5% 40%	2291
	11th to 20th	Night	22% 8% 3 4 3 2 3 3 4 5% 42%	2081
	21st to end	Night	21% 8% 3 5% 5% 3 4 4 4 5% 39%	1628
March	1st to 10th	Night	19% 8% 3 5% 4 2 3 4 4 6% 40%	1980
	11th to 20th	Night	17% 7% 3 6% 5% 2 4 4 5% 6% 40%	1980
	21st to end	Night	14% 7% 3 4 4 3 4 3 4 6% 47%	2178
April	1st to 10th	Night	14% 8% 3 5% 5% 3 4 5% 5% 8% 40%	1980
	11th to 20th	Night	15% 7% 3 6% 4 3 3 3 5% 7% 44%	1803
	21st to end	Night	14% 6% 2 5% 4 3 4 4 4 8% 46%	1800
May	1st to 10th	Night	13% 7% 3 6% 6% 3 4 4 6% 9% 41%	1742
	11th to 20th	Night	13% 5% 2 4 4 3 3 5% 5% 8% 46%	1593
	21st to end	Night	9% 5% 2 4 5% 3 5% 5% 6% 11% 46%	1745
June	1st to 10th	Night	5% 3 1 4 4 3 4 5% 6% 11% 54%	1620
	11th to 20th	Night	4 3 1 3 3 2 4 5% 8% 11% 56%	1620
	21st to end	Night	5% 4 3 5% 5% 2 6% 6% 7% 13% 45%	1620
July	1st to 10th	Night	5% 4 2 5% 4 3 6% 7% 7% 13% 44%	1620
	11th to 20th	Night	3 3 1 3 4 2 4 5% 6% 12% 56%	1620
	21st to end	Night	3 2 2 4 4 3 5% 6% 7% 14% 50%	1772
August	1st to 10th	Night	2 2 1 3 3 1 3 4 6% 13% 62%	1611
	11th to 20th	Night	4 2 2 3 4 3 4 5% 9% 13% 51%	1679
	21st to end	Night	6% 3 2 4 5% 3 6% 7% 7% 13% 45%	1988
September	1st to 10th	Night	4 5% 2 5% 5% 3 4 5% 7% 9% 50%	1980
	11th to 20th	Night	11% 6% 2 6% 5% 3 5% 5% 6% 9% 42%	1980
	21st to end	Night	9% 5% 2 4 3 3 4 4 5% 9% 53%	1980
October	1st to 10th	Night	5% 4 1 3 3 2 4 5% 5% 9% 59%	2078
	11th to 20th	Night	10% 5% 1 3 3 2 3 3 4 6% 59%	2160
	21st to end	Night	13% 5% 2 4 4 1 3 2 4 4 58%	2376
November	1st to 10th	Night	11% 5% 1 4 5% 3 4 4 5% 6% 54%	2220
	11th to 20th	Night	11% 6% 3 5% 4 3 3 4 5% 6% 50%	2280
	21st to end	Night	17% 6% 3 4 4 3 4 4 4 7% 44%	2346
December	1st to 10th	Night	18% 8% 3 5% 5% 3 4 4 4 5% 42%	2470
	11th to 20th	Night	17% 9% 3 5% 5% 3 3 3 4 5% 42%	3036
	21st to end	Night	17% 7% 3 5% 4 2 3 3 4 5% 47%	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	42% 3 6% 17% 23% 10%	1708
	11th to 20th	Day	43% 3 5% 16% 22% 11%	1647
	21st to end	Day	44% 3 5% 15% 21% 13%	1782
February	1st to 10th	Day	51% 3 5% 14% 16% 12%	1683
	11th to 20th	Day	51% 2 6% 14% 17% 10%	1773
	21st to end	Day	49% 3 8% 14% 15% 11%	1628
March	1st to 10th	Day	52% 3 8% 12% 18% 7%	1980
	11th to 20th	Day	47% 4 7% 13% 20% 9%	1980
	21st to end	Day	45% 4 8% 15% 22% 7%	2178
April	1st to 10th	Day	49% 4 5% 18% 16% 8%	1980
	11th to 20th	Day	46% 3 8% 16% 17% 10%	2106
	21st to end	Day	41% 5% 8% 19% 22% 6%	2160
May	1st to 10th	Day	45% 6% 9% 18% 16% 7%	2213
	11th to 20th	Day	39% 5% 11% 23% 19% 3	2306
	21st to end	Day	46% 6% 13% 23% 12% 1	2531
June	1st to 10th	Day	38% 7% 15% 23% 16% 1	2340
	11th to 20th	Day	35% 8% 16% 25% 14% 2	2340
	21st to end	Day	47% 8% 15% 20% 9% 1	2340
July	1st to 10th	Day	43% 9% 19% 22% 6% 1	2276
	11th to 20th	Day	37% 10% 21% 22% 10%	2271
	21st to end	Day	35% 10% 21% 25% 7% 1	2405
August	1st to 10th	Day	30% 8% 21% 29% 12% 1	2314
	11th to 20th	Day	33% 9% 21% 24% 10% 2	2218
	21st to end	Day	40% 9% 18% 21% 9% 3	2361
September	1st to 10th	Day	36% 10% 14% 19% 15% 6%	1980
	11th to 20th	Day	44% 7% 13% 17% 15% 4	1980
	21st to end	Day	34% 6% 12% 20% 18% 11%	1980
October	1st to 10th	Day	29% 4 9% 23% 22% 13%	1864
	11th to 20th	Day	34% 5% 5% 19% 21% 16%	1800
	21st to end	Day	35% 3 5% 14% 26% 17%	1980
November	1st to 10th	Day	36% 4 10% 18% 22% 11%	1858
	11th to 20th	Day	38% 3 8% 16% 23% 13%	1900
	21st to end	Day	45% 3 5% 18% 18% 10%	1829
December	1st to 10th	Day	46% 3 8% 18% 18% 8%	1710
	11th to 20th	Day	45% 3 6% 15% 21% 10%	2159
	21st to end	Day	43% 3 5% 14% 25% 10%	3135
January	1st to 10th	Night	41% 3 6% 18% 21% 12%	2458
	11th to 20th	Night	39% 3 6% 18% 21% 12%	2386
	21st to end	Night	43% 4 4 15% 19% 14%	2574
February	1st to 10th	Night	47% 3 5% 13% 17% 13%	2291
	11th to 20th	Night	46% 3 6% 15% 18% 11%	2081
	21st to end	Night	50% 3 6% 13% 14% 14%	1628
March	1st to 10th	Night	46% 4 7% 16% 17% 10%	1980
	11th to 20th	Night	45% 4 10% 12% 19% 9%	1980
	21st to end	Night	40% 3 11% 16% 21% 9%	2178
April	1st to 10th	Night	43% 5% 10% 17% 16% 10%	1980
	11th to 20th	Night	40% 4 9% 18% 16% 12%	1803
	21st to end	Night	39% 4 8% 22% 21% 7%	1800
May	1st to 10th	Night	41% 5% 10% 19% 17% 7%	1742
	11th to 20th	Night	37% 5% 11% 24% 18% 4	1594
	21st to end	Night	32% 6% 18% 29% 12% 2	1746
June	1st to 10th	Night	24% 7% 19% 32% 18%	1620
	11th to 20th	Night	21% 7% 20% 30% 18% 3	1620
	21st to end	Night	30% 9% 20% 30% 11% 1	1620
July	1st to 10th	Night	30% 9% 26% 28% 7%	1575
	11th to 20th	Night	19% 8% 32% 32% 10%	1566
	21st to end	Night	21% 11% 31% 30% 7%	1673
August	1st to 10th	Night	15% 6% 29% 38% 11% 1	1601
	11th to 20th	Night	21% 9% 30% 29% 9% 1	1679
	21st to end	Night	29% 10% 25% 25% 8% 3	1988
September	1st to 10th	Night	29% 8% 24% 21% 13% 6%	1980
	11th to 20th	Night	38% 7% 15% 19% 14% 6%	1980
	21st to end	Night	29% 5% 14% 21% 21% 10%	1980
October	1st to 10th	Night	24% 4 10% 28% 19% 15%	2078
	11th to 20th	Night	27% 4 7% 22% 25% 15%	2160
	21st to end	Night	32% 3 5% 16% 24% 20%	2376
November	1st to 10th	Night	32% 4 11% 20% 21% 12%	2220
	11th to 20th	Night	35% 3 9% 19% 21% 12%	2280
	21st to end	Night	42% 3 8% 17% 19% 11%	2346
December	1st to 10th	Night	45% 4 7% 19% 15% 9%	2470
	11th to 20th	Night	46% 2 6% 12% 20% 14%	3037
	21st to end	Night	41% 3 5% 14% 28% 8%	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	27% 15% 38% 13% 6% 1	270
	11th to 20th	Day	29% 10% 33% 22% 6% 1	249
	21st to end	Day	25% 20% 36% 17% 3	264
February	1st to 10th	Day	30% 18% 33% 15% 3 1	240
	11th to 20th	Day	32% 18% 32% 16% 1	240
	21st to end	Day	31% 23% 36% 9% 1 1	198
March	1st to 10th	Day	29% 18% 35% 14% 3	240
	11th to 20th	Day	20% 26% 35% 15% 3	240
	21st to end	Day	25% 14% 38% 18% 4	264
April	1st to 10th	Day	19% 18% 41% 19% 3	240
	11th to 20th	Day	29% 15% 23% 24% 9%	384
	21st to end	Day	17% 12% 38% 23% 9% 1	420
May	1st to 10th	Day	18% 10% 36% 26% 9%	476
	11th to 20th	Day	14% 8% 32% 31% 14% 2	625
	21st to end	Day	12% 7% 28% 34% 16% 3	679
June	1st to 10th	Day	8% 2 16% 36% 29% 10%	630
	11th to 20th	Day	8% 2 19% 35% 28% 8%	630
	21st to end	Day	9% 3 11% 31% 33% 13%	630
July	1st to 10th	Day	7% 3 8% 32% 35% 16%	621
	11th to 20th	Day	4 2 4 33% 39% 18%	618
	21st to end	Day	4 2 4 28% 44% 18%	684
August	1st to 10th	Day	4 2 3 28% 47% 16%	626
	11th to 20th	Day	5% 1 2 24% 49% 19%	532
	21st to end	Day	11% 2 4 18% 39% 26%	480
September	1st to 10th	Day	6% 2 4 29% 38% 20%	270
	11th to 20th	Day	14% 1 9% 30% 34% 12%	270
	21st to end	Day	10% 1 9% 34% 37% 8%	270
October	1st to 10th	Day	6% 3 19% 40% 28% 3	270
	11th to 20th	Day	14% 5% 25% 31% 20% 6%	270
	21st to end	Day	26% 11% 21% 27% 12% 3	297
November	1st to 10th	Day	14% 6% 27% 37% 16% 1	270
	11th to 20th	Day	20% 8% 33% 30% 7% 1	270
	21st to end	Day	21% 20% 31% 19% 7% 2	270
December	1st to 10th	Day	21% 11% 36% 19% 7% 4	270
	11th to 20th	Day	24% 15% 38% 20% 2 1	270
	21st to end	Day	25% 15% 33% 20% 5% 1	297
January	1st to 10th	Night	24% 12% 32% 25% 7%	950
	11th to 20th	Night	24% 14% 28% 27% 8% 1	918
	21st to end	Night	23% 12% 34% 26% 5%	990
February	1st to 10th	Night	26% 13% 32% 25% 3	851
	11th to 20th	Night	29% 13% 31% 24% 4	687
	21st to end	Night	30% 14% 33% 21% 2	444
March	1st to 10th	Night	26% 11% 34% 25% 4	540
	11th to 20th	Night	20% 10% 36% 29% 6%	540
	21st to end	Night	19% 10% 40% 23% 7%	594
April	1st to 10th	Night	18% 10% 40% 26% 6%	540
	11th to 20th	Night	20% 11% 33% 28% 8%	533
	21st to end	Night	19% 9% 36% 26% 9%	540
May	1st to 10th	Night	18% 9% 34% 31% 8%	540
	11th to 20th	Night	16% 6% 29% 33% 15% 2	531
	21st to end	Night	9% 5% 27% 38% 19% 2	584
June	1st to 10th	Night	7% 2 15% 41% 31% 4	540
	11th to 20th	Night	6% 2 21% 45% 23% 3	540
	21st to end	Night	6% 2 12% 42% 32% 6%	540
July	1st to 10th	Night	5% 2 7% 41% 38% 7%	525
	11th to 20th	Night	4 1 7% 43% 39% 7%	522
	21st to end	Night	4 1 6% 44% 40% 5%	591
August	1st to 10th	Night	2 1 5% 46% 41% 5%	537
	11th to 20th	Night	5% 1 3 44% 40% 6%	534
	21st to end	Night	7% 3 5% 32% 44% 9%	594
September	1st to 10th	Night	6% 1 6% 31% 48% 9%	540
	11th to 20th	Night	14% 1 5% 31% 41% 7%	540
	21st to end	Night	11% 1 11% 35% 36% 6%	540
October	1st to 10th	Night	6% 2 14% 39% 33% 6%	638
	11th to 20th	Night	13% 5% 22% 38% 20% 2	720
	21st to end	Night	17% 6% 28% 28% 19% 2	792
November	1st to 10th	Night	11% 5% 24% 36% 22% 1	740
	11th to 20th	Night	14% 9% 27% 35% 14% 1	760
	21st to end	Night	21% 11% 28% 28% 12% 1	826
December	1st to 10th	Night	23% 9% 27% 29% 11% 1	950
	11th to 20th	Night	27% 10% 30% 27% 6% 1	950
	21st to end	Night	24% 12% 29% 28% 6%	1045

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