John's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John's Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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% 3 6% 6% 4 6% 6% 7% 9% 40%	74856
	Night	7% 5% 2 5% 6% 3 5% 5% 7% 10% 44%	74950
Transparency	Day	39% 9% 15% 18% 14% 5%	74579
	Night	34% 9% 17% 20% 15% 5%	74746
Seeing	Day	7% 4 16% 26% 27% 20%	14354
	Night	8% 3 17% 34% 28% 10%	24464

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	10% 6% 3 5% 5% 2 4 5% 6% 10% 44%	1706
	11th to 20th	Day	8% 4 2 4 5% 3 4 5% 6% 9% 50%	1665
	21st to end	Day	12% 5% 2 5% 5% 3 4 5% 6% 8% 45%	1949
February	1st to 10th	Day	11% 6% 2 5% 5% 3 5% 5% 6% 7% 45%	1800
	11th to 20th	Day	5% 4 2 4 4 2 4 4 5% 7% 56%	1748
	21st to end	Day	8% 4 2 5% 4 3 4 4 5% 9% 52%	1531
March	1st to 10th	Day	4 3 1 4 3 3 4 5% 5% 7% 60%	1980
	11th to 20th	Day	7% 3 2 5% 3 3 3 4 6% 8% 56%	1980
	21st to end	Day	7% 4 2 3 4 3 4 4 6% 8% 55%	2178
April	1st to 10th	Day	6% 4 2 4 5% 3 6% 5% 7% 9% 51%	1980
	11th to 20th	Day	8% 4 2 6% 5% 4 5% 6% 6% 9% 46%	1956
	21st to end	Day	4 3 2 4 4 3 5% 5% 6% 9% 55%	2124
May	1st to 10th	Day	4 2 1 3 3 2 4 4 7% 9% 61%	2340
	11th to 20th	Day	9% 5% 2 4 5% 4 6% 6% 7% 9% 43%	2301
	21st to end	Day	7% 5% 3 4 7% 4 7% 7% 7% 10% 40%	2531
June	1st to 10th	Day	11% 7% 3 7% 6% 5% 7% 8% 8% 10% 28%	2340
	11th to 20th	Day	6% 5% 3 6% 7% 5% 8% 9% 9% 12% 30%	2340
	21st to end	Day	7% 6% 4 6% 7% 5% 9% 8% 9% 10% 28%	2340
July	1st to 10th	Day	9% 4 3 6% 8% 5% 10% 9% 9% 11% 24%	2340
	11th to 20th	Day	6% 6% 3 8% 9% 6% 10% 9% 9% 11% 24%	2340
	21st to end	Day	7% 6% 3 7% 10% 5% 9% 9% 9% 11% 23%	2547
August	1st to 10th	Day	5% 5% 3 7% 9% 6% 10% 9% 10% 11% 24%	2291
	11th to 20th	Day	6% 6% 3 8% 9% 7% 9% 11% 10% 11% 21%	2117
	21st to end	Day	10% 8% 3 7% 7% 5% 8% 9% 9% 11% 22%	2178
September	1st to 10th	Day	8% 5% 3 7% 8% 7% 8% 8% 10% 11% 24%	1980
	11th to 20th	Day	9% 7% 4 8% 8% 6% 8% 8% 8% 10% 25%	1980
	21st to end	Day	7% 6% 3 8% 8% 4 7% 7% 8% 12% 31%	1980
October	1st to 10th	Day	7% 5% 3 8% 7% 5% 6% 6% 7% 9% 37%	1980
	11th to 20th	Day	7% 4 2 5% 6% 4 4 6% 6% 8% 49%	1847
	21st to end	Day	7% 5% 2 5% 5% 3 5% 4 6% 9% 48%	1980
November	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 6% 9% 50%	1857
	11th to 20th	Day	9% 6% 3 5% 5% 3 4 5% 6% 7% 47%	1900
	21st to end	Day	8% 6% 2 4 5% 3 5% 5% 6% 9% 47%	1763
December	1st to 10th	Day	7% 5% 2 5% 5% 3 5% 6% 6% 10% 46%	1710
	11th to 20th	Day	10% 6% 3 6% 5% 2 5% 5% 6% 7% 45%	2153
	21st to end	Day	11% 8% 4 9% 8% 4 6% 5% 5% 7% 35%	3124
January	1st to 10th	Night	10% 6% 2 5% 6% 3 5% 5% 6% 9% 43%	2459
	11th to 20th	Night	9% 6% 3 5% 5% 3 4 5% 6% 9% 46%	2417
	21st to end	Night	12% 6% 3 5% 6% 4 5% 4 6% 6% 44%	2403
February	1st to 10th	Night	11% 6% 3 5% 5% 3 4 4 5% 9% 45%	2160
	11th to 20th	Night	9% 5% 2 3 4 3 4 4 6% 7% 53%	2098
	21st to end	Night	6% 5% 2 4 5% 3 5% 5% 8% 8% 49%	1722
March	1st to 10th	Night	4 3 1 3 4 2 4 4 6% 9% 60%	1980
	11th to 20th	Night	5% 4 3 4 4 3 3 5% 7% 9% 53%	1980
	21st to end	Night	5% 3 2 5% 5% 3 4 4 5% 7% 57%	2178
April	1st to 10th	Night	5% 4 1 4 4 2 4 5% 6% 9% 55%	1980
	11th to 20th	Night	5% 5% 1 5% 5% 3 5% 5% 7% 9% 49%	1953
	21st to end	Night	4 3 2 3 5% 3 4 4 5% 10% 56%	1846
May	1st to 10th	Night	2 2 1 3 2 2 4 4 5% 9% 65%	1620
	11th to 20th	Night	7% 5% 2 4 7% 3 5% 6% 6% 9% 47%	1592
	21st to end	Night	5% 3 3 5% 5% 3 5% 5% 6% 8% 53%	1744
June	1st to 10th	Night	10% 5% 4 5% 5% 4 6% 6% 6% 10% 39%	1620
	11th to 20th	Night	5% 4 2 4 6% 4 5% 7% 8% 13% 41%	1620
	21st to end	Night	5% 6% 3 4 7% 4 7% 8% 9% 13% 35%	1620
July	1st to 10th	Night	5% 6% 2 7% 6% 5% 7% 7% 9% 13% 34%	1620
	11th to 20th	Night	5% 4 2 6% 8% 5% 8% 7% 10% 16% 30%	1620
	21st to end	Night	3 5% 3 7% 7% 4 8% 7% 10% 13% 31%	1771
August	1st to 10th	Night	5% 4 1 6% 6% 4 6% 7% 11% 14% 36%	1641
	11th to 20th	Night	6% 5% 2 5% 7% 5% 7% 8% 9% 14% 32%	1776
	21st to end	Night	9% 6% 3 7% 8% 5% 9% 7% 9% 11% 28%	2166
September	1st to 10th	Night	4 5% 3 7% 7% 6% 8% 7% 9% 12% 31%	1980
	11th to 20th	Night	9% 6% 3 8% 8% 5% 7% 6% 8% 10% 30%	1980
	21st to end	Night	7% 8% 3 7% 7% 5% 7% 6% 7% 10% 32%	1980
October	1st to 10th	Night	6% 5% 3 6% 6% 4 6% 6% 7% 11% 41%	1980
	11th to 20th	Night	6% 5% 3 5% 5% 3 4 5% 5% 9% 50%	2110
	21st to end	Night	8% 5% 2 5% 5% 3 4 4 6% 9% 49%	2376
November	1st to 10th	Night	6% 4 3 5% 5% 3 4 5% 6% 9% 52%	2221
	11th to 20th	Night	8% 5% 3 6% 5% 3 4 5% 5% 7% 49%	2280
	21st to end	Night	8% 5% 2 6% 5% 3 5% 4 6% 8% 47%	2413
December	1st to 10th	Night	7% 4 3 5% 5% 3 5% 5% 6% 8% 49%	2470
	11th to 20th	Night	11% 5% 2 5% 5% 3 4 4 5% 8% 47%	3042
	21st to end	Night	13% 8% 3 6% 6% 4 5% 5% 6% 8% 35%	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	35% 6% 10% 24% 18% 7%	1707
	11th to 20th	Day	31% 4 9% 28% 20% 8%	1665
	21st to end	Day	37% 5% 8% 19% 23% 8%	1949
February	1st to 10th	Day	37% 6% 9% 20% 17% 11%	1800
	11th to 20th	Day	26% 5% 11% 22% 26% 11%	1758
	21st to end	Day	31% 5% 12% 17% 26% 9%	1531
March	1st to 10th	Day	23% 5% 11% 26% 25% 10%	1980
	11th to 20th	Day	27% 6% 11% 27% 21% 9%	1980
	21st to end	Day	28% 4 11% 25% 24% 7%	2178
April	1st to 10th	Day	29% 7% 17% 22% 18% 7%	1980
	11th to 20th	Day	35% 7% 13% 17% 22% 6%	1956
	21st to end	Day	27% 6% 12% 23% 25% 7%	2124
May	1st to 10th	Day	20% 6% 13% 27% 24% 9%	2340
	11th to 20th	Day	37% 9% 16% 19% 17% 2	2305
	21st to end	Day	39% 8% 15% 19% 17% 2	2531
June	1st to 10th	Day	50% 10% 19% 16% 4	2340
	11th to 20th	Day	45% 13% 22% 18% 3	2340
	21st to end	Day	47% 14% 23% 13% 2 1	2340
July	1st to 10th	Day	51% 15% 25% 7% 2	2275
	11th to 20th	Day	51% 15% 26% 8% 1	2271
	21st to end	Day	48% 14% 25% 12% 1	2405
August	1st to 10th	Day	48% 15% 28% 8%	2278
	11th to 20th	Day	51% 16% 23% 10%	2117
	21st to end	Day	53% 14% 22% 10% 1	2178
September	1st to 10th	Day	49% 15% 22% 13% 1	1980
	11th to 20th	Day	51% 13% 20% 13% 3	1980
	21st to end	Day	46% 11% 15% 16% 10% 2	1980
October	1st to 10th	Day	42% 8% 13% 16% 12% 8%	1980
	11th to 20th	Day	32% 7% 10% 22% 23% 5%	1847
	21st to end	Day	33% 7% 10% 19% 20% 12%	1980
November	1st to 10th	Day	31% 6% 11% 22% 19% 11%	1857
	11th to 20th	Day	36% 5% 9% 22% 20% 8%	1900
	21st to end	Day	33% 6% 12% 21% 21% 8%	1763
December	1st to 10th	Day	32% 6% 14% 25% 18% 5%	1710
	11th to 20th	Day	36% 5% 9% 20% 18% 11%	2150
	21st to end	Day	48% 6% 10% 18% 16% 4	3124
January	1st to 10th	Night	37% 5% 11% 22% 18% 7%	2459
	11th to 20th	Night	34% 5% 10% 25% 19% 6%	2417
	21st to end	Night	39% 5% 11% 19% 19% 7%	2403
February	1st to 10th	Night	36% 5% 11% 19% 21% 8%	2160
	11th to 20th	Night	29% 4 12% 20% 25% 10%	2099
	21st to end	Night	30% 6% 13% 20% 25% 6%	1722
March	1st to 10th	Night	21% 5% 12% 31% 24% 8%	1980
	11th to 20th	Night	27% 6% 13% 29% 18% 7%	1980
	21st to end	Night	27% 5% 10% 27% 24% 7%	2178
April	1st to 10th	Night	24% 6% 20% 27% 16% 7%	1980
	11th to 20th	Night	29% 7% 16% 22% 21% 5%	1953
	21st to end	Night	24% 5% 14% 27% 25% 5%	1846
May	1st to 10th	Night	16% 6% 12% 34% 23% 10%	1620
	11th to 20th	Night	33% 9% 19% 22% 17% 1	1593
	21st to end	Night	28% 8% 18% 25% 19% 1	1745
June	1st to 10th	Night	39% 10% 23% 22% 6%	1620
	11th to 20th	Night	32% 12% 31% 21% 4	1620
	21st to end	Night	35% 17% 27% 15% 6%	1620
July	1st to 10th	Night	40% 16% 33% 10% 1	1575
	11th to 20th	Night	40% 18% 31% 10%	1566
	21st to end	Night	38% 17% 33% 12% 1	1672
August	1st to 10th	Night	34% 17% 37% 11% 1	1630
	11th to 20th	Night	38% 16% 36% 10%	1776
	21st to end	Night	47% 15% 23% 12% 2	2166
September	1st to 10th	Night	41% 16% 27% 14% 2	1980
	11th to 20th	Night	46% 13% 24% 14% 2	1980
	21st to end	Night	43% 12% 18% 16% 9% 1	1980
October	1st to 10th	Night	35% 10% 16% 19% 14% 7%	1980
	11th to 20th	Night	31% 7% 12% 24% 21% 5%	2110
	21st to end	Night	32% 7% 11% 20% 22% 9%	2376
November	1st to 10th	Night	30% 8% 11% 20% 20% 11%	2221
	11th to 20th	Night	33% 6% 11% 22% 20% 8%	2280
	21st to end	Night	34% 5% 10% 24% 19% 7%	2413
December	1st to 10th	Night	32% 6% 12% 28% 17% 4	2470
	11th to 20th	Night	35% 5% 10% 21% 17% 11%	3044
	21st to end	Night	45% 6% 13% 17% 15% 4	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	11% 9% 34% 33% 12% 2	270
	11th to 20th	Day	7% 12% 29% 31% 16% 4	255
	21st to end	Day	12% 13% 42% 26% 7%	264
February	1st to 10th	Day	8% 10% 32% 33% 15% 3	240
	11th to 20th	Day	4 7% 30% 35% 21% 4	240
	21st to end	Day	7% 6% 25% 37% 22% 3	249
March	1st to 10th	Day	4 5% 38% 38% 14% 1	420
	11th to 20th	Day	7% 7% 28% 28% 27% 2	420
	21st to end	Day	5% 6% 28% 36% 21% 5%	462
April	1st to 10th	Day	5% 6% 30% 34% 22% 2	420
	11th to 20th	Day	8% 7% 25% 36% 20% 4	412
	21st to end	Day	5% 2 21% 34% 29% 8%	420
May	1st to 10th	Day	5% 2 15% 37% 30% 11%	423
	11th to 20th	Day	10% 2 12% 29% 32% 15%	447
	21st to end	Day	6% 2 8% 22% 34% 28%	485
June	1st to 10th	Day	12% 2 8% 24% 31% 22%	450
	11th to 20th	Day	6% 3 6% 20% 32% 33%	450
	21st to end	Day	6% 1 3 12% 30% 49%	450
July	1st to 10th	Day	6% 2 1 8% 28% 55%	445
	11th to 20th	Day	5% 3 9% 35% 48%	444
	21st to end	Day	9% 1 5% 11% 30% 44%	488
August	1st to 10th	Day	7% 4 8% 33% 48%	446
	11th to 20th	Day	5% 2 6% 9% 28% 49%	439
	21st to end	Day	8% 1 7% 15% 30% 38%	495
September	1st to 10th	Day	6% 2 4 14% 30% 44%	450
	11th to 20th	Day	8% 1 4 22% 34% 30%	450
	21st to end	Day	6% 2 4 20% 36% 33%	450
October	1st to 10th	Day	7% 3 13% 33% 34% 10%	450
	11th to 20th	Day	7% 2 14% 30% 34% 13%	450
	21st to end	Day	8% 3 13% 35% 34% 7%	495
November	1st to 10th	Day	8% 3 19% 33% 30% 7%	455
	11th to 20th	Day	10% 6% 20% 39% 21% 4	460
	21st to end	Day	7% 5% 25% 35% 24% 4	323
December	1st to 10th	Day	5% 7% 28% 34% 19% 7%	270
	11th to 20th	Day	9% 6% 32% 31% 20% 3	270
	21st to end	Day	5% 7% 27% 35% 21% 4	297
January	1st to 10th	Night	10% 7% 28% 39% 15% 1	760
	11th to 20th	Night	11% 6% 24% 43% 15% 1	744
	21st to end	Night	12% 9% 30% 37% 12%	792
February	1st to 10th	Night	13% 6% 28% 39% 15%	720
	11th to 20th	Night	9% 6% 25% 41% 18% 1	703
	21st to end	Night	7% 5% 23% 48% 16%	592
March	1st to 10th	Night	5% 4 34% 46% 11% 1	720
	11th to 20th	Night	6% 5% 24% 45% 18% 1	720
	21st to end	Night	6% 3 25% 46% 18% 1	792
April	1st to 10th	Night	5% 3 26% 45% 21%	720
	11th to 20th	Night	7% 4 25% 45% 18% 1	711
	21st to end	Night	5% 2 24% 47% 20% 1	718
May	1st to 10th	Night	4 13% 48% 33% 3	540
	11th to 20th	Night	7% 1 13% 39% 35% 5%	531
	21st to end	Night	4 1 10% 32% 45% 8%	584
June	1st to 10th	Night	10% 1 4 26% 47% 12%	540
	11th to 20th	Night	5% 1 3 16% 48% 26%	540
	21st to end	Night	5% 2 3 12% 45% 32%	540
July	1st to 10th	Night	6% 2 9% 44% 39%	525
	11th to 20th	Night	4 1 2 9% 43% 42%	522
	21st to end	Night	4 1 3 10% 47% 36%	591
August	1st to 10th	Night	6% 1 2 8% 47% 37%	537
	11th to 20th	Night	6% 1 2 9% 37% 45%	534
	21st to end	Night	11% 1 6% 17% 41% 24%	780
September	1st to 10th	Night	7% 1 6% 16% 47% 23%	720
	11th to 20th	Night	10% 2 5% 24% 42% 16%	720
	21st to end	Night	12% 1 6% 29% 39% 13%	720
October	1st to 10th	Night	9% 2 14% 40% 31% 5%	720
	11th to 20th	Night	8% 3 14% 37% 36% 3	720
	21st to end	Night	9% 3 18% 40% 28% 3	792
November	1st to 10th	Night	7% 2 23% 42% 22% 3	740
	11th to 20th	Night	8% 4 22% 43% 20% 2	760
	21st to end	Night	7% 4 25% 46% 18% 1	760
December	1st to 10th	Night	9% 3 23% 39% 25% 1	760
	11th to 20th	Night	10% 5% 23% 41% 20% 2	760
	21st to end	Night	8% 5% 26% 40% 20% 1	836

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