Spencer's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spencer's Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	4 2 1 3 3 2 4 4 5% 6% 67%	74965
	Night	4 2 1 3 3 2 3 4 4 6% 68%	74820
Transparency	Day	18% 3 5% 11% 24% 38%	74692
	Night	18% 3 6% 12% 25% 36%	74615
Seeing	Day	5% 6% 21% 34% 24% 9%	13800
	Night	4 2 16% 43% 30% 4	24019

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	5% 2 2 4 3 2 4 4 3 7% 62%	1708
	11th to 20th	Day	5% 3 1 3 4 3 3 3 4 6% 66%	1642
	21st to end	Day	5% 4 1 3 3 2 3 4 4 6% 65%	1782
February	1st to 10th	Day	6% 3 1 2 2 2 2 2 4 6% 69%	1674
	11th to 20th	Day	7% 3 2 4 4 2 4 3 4 5% 62%	1750
	21st to end	Day	7% 3 2 3 3 1 2 3 4 5% 68%	1560
March	1st to 10th	Day	5% 2 1 3 3 2 4 3 4 8% 64%	1980
	11th to 20th	Day	4 3 1 2 3 2 3 3 3 5% 69%	1980
	21st to end	Day	3 2 1 3 2 2 3 3 4 4 74%	2178
April	1st to 10th	Day	2 1 1 1 2 2 3 3 4 6% 75%	1980
	11th to 20th	Day	3 2 1 2 1 1 1 3 2 4 80%	2080
	21st to end	Day	1 1 1 2 2 1 2 2 3 4 82%	2190
May	1st to 10th	Day	1 1 1 2 2 1 2 2 3 3 81%	2340
	11th to 20th	Day	1 1 2 2 1 3 2 3 4 80%	2301
	21st to end	Day	1 1 1 1 1 1 2 2 3 87%	2530
June	1st to 10th	Day	2 1 1 1 2 1 2 3 4 7% 77%	2340
	11th to 20th	Day	1 1 2 1 1 2 1 2 3 85%	2340
	21st to end	Day	2 2 1 3 2 2 4 4 5% 6% 69%	2340
July	1st to 10th	Day	3 2 2 4 5% 4 6% 6% 7% 8% 53%	2340
	11th to 20th	Day	3 4 2 6% 7% 4 6% 8% 9% 13% 38%	2340
	21st to end	Day	7% 5% 3 6% 7% 4 7% 7% 9% 11% 33%	2545
August	1st to 10th	Day	5% 4 2 5% 5% 3 6% 6% 7% 8% 49%	2328
	11th to 20th	Day	4 3 1 4 6% 3 7% 7% 9% 10% 48%	2299
	21st to end	Day	3 2 1 5% 6% 4 7% 5% 7% 8% 52%	2324
September	1st to 10th	Day	6% 4 1 4 5% 3 5% 5% 6% 7% 56%	1980
	11th to 20th	Day	2 2 1 2 3 2 4 3 4 6% 70%	1980
	21st to end	Day	3 2 1 2 2 2 3 3 4 6% 71%	1980
October	1st to 10th	Day	2 1 2 2 2 3 4 4 6% 74%	1913
	11th to 20th	Day	2 1 1 1 2 1 1 2 3 4 82%	1800
	21st to end	Day	1 1 1 2 2 1 2 2 4 5% 79%	1980
November	1st to 10th	Day	2 1 2 2 1 2 3 3 7% 77%	1859
	11th to 20th	Day	2 2 1 2 3 1 3 3 4 5% 74%	1867
	21st to end	Day	4 3 2 3 3 2 4 5% 4 6% 65%	1710
December	1st to 10th	Day	6% 3 1 2 4 2 4 4 5% 7% 62%	1710
	11th to 20th	Day	6% 4 2 6% 4 3 3 5% 7% 7% 54%	2169
	21st to end	Day	7% 3 2 4 4 2 3 3 5% 6% 61%	3146
January	1st to 10th	Night	5% 3 1 3 3 1 3 3 5% 6% 64%	2457
	11th to 20th	Night	6% 3 1 3 2 2 3 3 4 5% 68%	2379
	21st to end	Night	6% 3 2 4 4 3 4 4 4 5% 62%	2574
February	1st to 10th	Night	6% 2 1 3 3 1 2 3 4 5% 69%	2282
	11th to 20th	Night	6% 3 2 4 3 2 4 3 4 6% 64%	2098
	21st to end	Night	5% 3 1 2 3 2 4 3 4 6% 67%	1696
March	1st to 10th	Night	4 2 1 3 3 2 3 4 5% 7% 65%	1980
	11th to 20th	Night	4 3 1 2 2 2 3 3 4 5% 71%	1980
	21st to end	Night	3 3 1 3 3 2 3 4 3 5% 71%	2178
April	1st to 10th	Night	2 1 1 2 2 2 2 2 5% 6% 75%	1980
	11th to 20th	Night	2 2 1 2 2 2 2 3 3 4 79%	1827
	21st to end	Night	2 1 1 1 1 1 1 2 4 84%	1764
May	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 80%	1620
	11th to 20th	Night	1 1 1 1 2 1 3 2 4 5% 80%	1594
	21st to end	Night	1 1 1 1 2 2 2 3 88%	1746
June	1st to 10th	Night	2 1 1 1 2 1 3 3 3 5% 77%	1620
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 5% 84%	1620
	21st to end	Night	2 1 1 3 3 2 4 3 4 6% 71%	1620
July	1st to 10th	Night	3 3 2 4 4 3 5% 5% 6% 8% 57%	1620
	11th to 20th	Night	3 2 1 4 6% 4 6% 8% 10% 14% 42%	1620
	21st to end	Night	4 4 3 5% 6% 4 7% 10% 10% 16% 33%	1773
August	1st to 10th	Night	2 3 1 3 4 3 4 6% 8% 10% 56%	1611
	11th to 20th	Night	4 3 2 4 4 3 5% 5% 8% 10% 51%	1602
	21st to end	Night	4 3 1 4 4 3 5% 6% 8% 11% 52%	2028
September	1st to 10th	Night	5% 3 1 4 4 3 5% 5% 5% 7% 57%	1980
	11th to 20th	Night	1 3 1 3 4 2 3 3 4 5% 71%	1980
	21st to end	Night	2 2 1 3 3 2 3 3 4 4 73%	1980
October	1st to 10th	Night	2 2 1 2 3 2 3 3 4 5% 74%	2045
	11th to 20th	Night	2 1 1 1 2 1 2 2 3 4 81%	2160
	21st to end	Night	1 2 1 1 2 1 2 2 3 4 83%	2376
November	1st to 10th	Night	2 2 2 2 1 2 3 3 6% 77%	2220
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 6% 75%	2308
	21st to end	Night	6% 3 1 4 3 2 4 3 3 6% 65%	2470
December	1st to 10th	Night	6% 3 1 3 3 2 2 4 4 6% 67%	2470
	11th to 20th	Night	6% 4 2 5% 6% 3 3 3 4 6% 57%	3041
	21st to end	Night	7% 5% 2 3 4 2 4 4 5% 5% 59%	4521

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	22% 3 5% 14% 30% 27%	1709
	11th to 20th	Day	21% 3 5% 10% 29% 33%	1642
	21st to end	Day	22% 2 4 14% 28% 30%	1782
February	1st to 10th	Day	19% 1 4 11% 23% 41%	1674
	11th to 20th	Day	26% 2 4 13% 24% 31%	1760
	21st to end	Day	21% 2 3 10% 28% 36%	1560
March	1st to 10th	Day	20% 3 3 9% 30% 36%	1980
	11th to 20th	Day	19% 2 3 8% 25% 42%	1980
	21st to end	Day	16% 2 3 8% 33% 39%	2178
April	1st to 10th	Day	11% 1 2 5% 34% 46%	1980
	11th to 20th	Day	11% 1 2 5% 24% 57%	2083
	21st to end	Day	8% 2 2 6% 25% 57%	2190
May	1st to 10th	Day	10% 1 3 6% 19% 62%	2340
	11th to 20th	Day	10% 3 2 7% 21% 57%	2304
	21st to end	Day	6% 1 1 3 21% 68%	2530
June	1st to 10th	Day	10% 2 3 3 21% 62%	2340
	11th to 20th	Day	7% 1 2 2 13% 75%	2340
	21st to end	Day	16% 3 5% 7% 17% 53%	2340
July	1st to 10th	Day	25% 7% 9% 13% 18% 29%	2276
	11th to 20th	Day	31% 10% 17% 21% 15% 7%	2271
	21st to end	Day	38% 10% 16% 20% 11% 4	2404
August	1st to 10th	Day	29% 7% 12% 16% 17% 19%	2313
	11th to 20th	Day	27% 8% 14% 17% 20% 14%	2299
	21st to end	Day	27% 6% 12% 19% 19% 17%	2324
September	1st to 10th	Day	26% 5% 8% 18% 18% 25%	1980
	11th to 20th	Day	14% 4 6% 11% 25% 40%	1980
	21st to end	Day	15% 3 5% 9% 30% 38%	1980
October	1st to 10th	Day	11% 4 5% 13% 26% 40%	1913
	11th to 20th	Day	9% 1 2 9% 25% 54%	1800
	21st to end	Day	11% 2 3 8% 25% 51%	1980
November	1st to 10th	Day	9% 1 4 10% 37% 39%	1859
	11th to 20th	Day	14% 2 3 10% 28% 43%	1867
	21st to end	Day	20% 3 4 12% 26% 35%	1710
December	1st to 10th	Day	22% 3 5% 14% 29% 28%	1710
	11th to 20th	Day	29% 2 6% 16% 25% 21%	2168
	21st to end	Day	24% 2 5% 16% 33% 19%	3146
January	1st to 10th	Night	21% 2 6% 14% 32% 25%	2457
	11th to 20th	Night	19% 3 5% 12% 30% 32%	2379
	21st to end	Night	25% 4 5% 12% 31% 25%	2574
February	1st to 10th	Night	18% 3 4 12% 22% 40%	2282
	11th to 20th	Night	23% 3 5% 14% 24% 31%	2099
	21st to end	Night	20% 2 4 10% 31% 34%	1696
March	1st to 10th	Night	19% 3 4 12% 28% 35%	1980
	11th to 20th	Night	16% 3 3 11% 23% 44%	1980
	21st to end	Night	17% 2 4 10% 33% 34%	2178
April	1st to 10th	Night	12% 2 3 8% 33% 43%	1980
	11th to 20th	Night	12% 1 3 7% 24% 53%	1827
	21st to end	Night	8% 1 1 6% 26% 56%	1764
May	1st to 10th	Night	9% 1 3 7% 22% 57%	1620
	11th to 20th	Night	9% 2 3 8% 21% 57%	1595
	21st to end	Night	6% 1 2 5% 21% 66%	1746
June	1st to 10th	Night	11% 2 2 4 21% 60%	1620
	11th to 20th	Night	6% 1 2 3 15% 74%	1620
	21st to end	Night	15% 2 6% 5% 17% 56%	1620
July	1st to 10th	Night	22% 6% 12% 13% 14% 33%	1575
	11th to 20th	Night	26% 8% 19% 25% 14% 8%	1566
	21st to end	Night	32% 13% 21% 18% 12% 4	1674
August	1st to 10th	Night	20% 7% 14% 21% 19% 19%	1602
	11th to 20th	Night	25% 7% 15% 19% 19% 15%	1602
	21st to end	Night	23% 6% 14% 21% 20% 16%	2028
September	1st to 10th	Night	25% 5% 8% 22% 17% 22%	1980
	11th to 20th	Night	16% 4 5% 11% 28% 36%	1980
	21st to end	Night	15% 3 4 11% 29% 37%	1980
October	1st to 10th	Night	13% 3 6% 15% 24% 39%	2045
	11th to 20th	Night	9% 2 4 10% 25% 51%	2160
	21st to end	Night	8% 2 3 9% 27% 50%	2376
November	1st to 10th	Night	11% 2 3 11% 33% 41%	2220
	11th to 20th	Night	13% 2 3 11% 30% 41%	2308
	21st to end	Night	22% 2 4 11% 28% 33%	2470
December	1st to 10th	Night	20% 2 5% 18% 26% 29%	2470
	11th to 20th	Night	29% 3 5% 15% 27% 21%	3041
	21st to end	Night	25% 3 6% 14% 30% 21%	4521

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	9% 6% 29% 36% 19% 2	270
	11th to 20th	Day	11% 1 28% 37% 17% 6%	248
	21st to end	Day	4 6% 25% 37% 23% 4	264
February	1st to 10th	Day	5% 7% 29% 33% 19% 6%	294
	11th to 20th	Day	14% 12% 33% 29% 11% 1	415
	21st to end	Day	13% 14% 35% 26% 10% 1	346
March	1st to 10th	Day	8% 13% 37% 29% 10% 3	420
	11th to 20th	Day	9% 11% 31% 28% 13% 8%	420
	21st to end	Day	7% 15% 34% 28% 12% 4	462
April	1st to 10th	Day	3 13% 29% 29% 20% 6%	420
	11th to 20th	Day	6% 14% 32% 30% 16% 3	412
	21st to end	Day	4 14% 29% 31% 18% 5%	420
May	1st to 10th	Day	3 12% 27% 34% 17% 7%	423
	11th to 20th	Day	2 7% 26% 40% 19% 6%	448
	21st to end	Day	3 6% 27% 38% 21% 5%	485
June	1st to 10th	Day	4 4 20% 43% 22% 8%	450
	11th to 20th	Day	2 3 20% 42% 25% 8%	450
	21st to end	Day	3 1 17% 40% 29% 10%	450
July	1st to 10th	Day	3 1 8% 36% 37% 15%	446
	11th to 20th	Day	6% 1 3 32% 38% 20%	444
	21st to end	Day	7% 2 3 26% 38% 23%	487
August	1st to 10th	Day	5% 4 28% 43% 19%	447
	11th to 20th	Day	5% 1 4 26% 43% 21%	437
	21st to end	Day	3 1 4 29% 37% 26%	495
September	1st to 10th	Day	8% 3 10% 37% 31% 11%	450
	11th to 20th	Day	3 1 13% 38% 33% 12%	450
	21st to end	Day	6% 5% 18% 34% 27% 11%	450
October	1st to 10th	Day	2 5% 23% 35% 24% 10%	383
	11th to 20th	Day	3 3 20% 36% 27% 11%	270
	21st to end	Day	2 3 23% 32% 30% 10%	297
November	1st to 10th	Day	3 6% 23% 38% 24% 4	270
	11th to 20th	Day	3 4 21% 39% 27% 4	270
	21st to end	Day	4 6% 31% 37% 17% 4	270
December	1st to 10th	Day	8% 3 27% 33% 28% 2	270
	11th to 20th	Day	6% 6% 27% 43% 16% 2	270
	21st to end	Day	11% 8% 28% 31% 18% 4	297
January	1st to 10th	Night	7% 2 23% 45% 21% 3	760
	11th to 20th	Night	7% 2 20% 43% 25% 3	732
	21st to end	Night	8% 3 20% 47% 21% 2	792
February	1st to 10th	Night	6% 2 19% 46% 21% 6%	720
	11th to 20th	Night	8% 4 21% 49% 16% 2	704
	21st to end	Night	8% 3 27% 45% 16% 1	592
March	1st to 10th	Night	5% 4 28% 46% 17% 1	720
	11th to 20th	Night	5% 3 22% 41% 25% 5%	720
	21st to end	Night	4 3 21% 47% 23% 2	792
April	1st to 10th	Night	2 3 22% 51% 20% 2	720
	11th to 20th	Night	3 5% 26% 49% 17% 1	583
	21st to end	Night	3 3 23% 53% 17% 2	540
May	1st to 10th	Night	2 3 22% 51% 21% 1	540
	11th to 20th	Night	1 3 16% 52% 27% 1	531
	21st to end	Night	1 2 18% 52% 26%	584
June	1st to 10th	Night	2 2 10% 54% 31% 1	540
	11th to 20th	Night	1 1 12% 53% 32% 1	540
	21st to end	Night	1 1 8% 43% 46% 1	540
July	1st to 10th	Night	4 6% 33% 54% 3	525
	11th to 20th	Night	3 1 3 23% 64% 7%	522
	21st to end	Night	5% 1 3 24% 54% 13%	591
August	1st to 10th	Night	3 1 1 23% 60% 12%	537
	11th to 20th	Night	4 2 2 27% 58% 7%	534
	21st to end	Night	4 1 3 24% 58% 11%	652
September	1st to 10th	Night	6% 1 6% 29% 48% 10%	720
	11th to 20th	Night	3 1 8% 41% 42% 7%	720
	21st to end	Night	3 1 12% 43% 37% 4	720
October	1st to 10th	Night	3 1 15% 47% 31% 3	720
	11th to 20th	Night	2 2 15% 47% 31% 4	720
	21st to end	Night	2 1 14% 47% 28% 8%	792
November	1st to 10th	Night	2 2 16% 44% 31% 5%	740
	11th to 20th	Night	3 1 16% 45% 32% 2	760
	21st to end	Night	7% 2 22% 44% 20% 4	760
December	1st to 10th	Night	8% 2 17% 45% 24% 3	760
	11th to 20th	Night	8% 4 20% 48% 18% 3	760
	21st to end	Night	8% 4 20% 43% 20% 4	836

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