Zeppy's Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Zeppy's Astronomical 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	16% 11% 5% 8% 6% 3 5% 5% 5% 7% 30%	74631
	Night	15% 12% 5% 8% 6% 3 4 4 5% 6% 30%	75175
Transparency	Day	53% 7% 12% 17% 10% 1	74352
	Night	54% 7% 11% 17% 10% 1	74973
Seeing	Day	23% 18% 29% 18% 9% 3	13461
	Night	22% 16% 28% 23% 10% 1	23316

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	17% 17% 12% 12% 8% 4 4 4 4 4 16%	1706
	11th to 20th	Day	16% 20% 9% 14% 8% 3 4 3 4 4 13%	1665
	21st to end	Day	14% 19% 12% 17% 10% 3 3 4 4 4 10%	1782
February	1st to 10th	Day	16% 20% 8% 12% 10% 5% 4 2 3 4 17%	1620
	11th to 20th	Day	15% 15% 9% 13% 10% 5% 4 4 4 4 17%	1600
	21st to end	Day	17% 17% 6% 13% 6% 3 5% 5% 3 5% 20%	1480
March	1st to 10th	Day	18% 14% 7% 8% 6% 3 3 5% 5% 7% 25%	1942
	11th to 20th	Day	16% 15% 6% 10% 6% 3 4 4 3 6% 28%	1980
	21st to end	Day	21% 13% 6% 8% 6% 3 4 4 4 5% 27%	2178
April	1st to 10th	Day	14% 13% 6% 8% 4 3 4 4 5% 6% 32%	2031
	11th to 20th	Day	19% 12% 4 7% 5% 3 4 4 4 6% 32%	2266
	21st to end	Day	20% 13% 4 6% 4 3 4 3 4 5% 34%	2340
May	1st to 10th	Day	17% 10% 3 8% 6% 4 6% 6% 4 6% 31%	2340
	11th to 20th	Day	20% 7% 4 6% 4 3 5% 5% 6% 7% 33%	2301
	21st to end	Day	11% 6% 3 6% 6% 4 5% 5% 7% 8% 38%	2621
June	1st to 10th	Day	16% 6% 3 6% 5% 3 5% 6% 6% 8% 36%	2520
	11th to 20th	Day	14% 7% 3 6% 6% 3 5% 5% 6% 8% 38%	2520
	21st to end	Day	12% 6% 3 5% 5% 3 5% 5% 7% 10% 40%	2520
July	1st to 10th	Day	10% 5% 3 6% 6% 3 5% 5% 6% 8% 43%	2397
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 5% 7% 10% 39%	2340
	21st to end	Day	10% 5% 2 6% 6% 4 6% 5% 7% 9% 40%	2547
August	1st to 10th	Day	9% 6% 3 6% 6% 3 5% 6% 7% 9% 39%	2327
	11th to 20th	Day	9% 6% 3 6% 8% 5% 5% 7% 8% 9% 34%	2303
	21st to end	Day	12% 6% 3 6% 6% 3 5% 6% 7% 8% 38%	2525
September	1st to 10th	Day	16% 7% 2 6% 5% 3 4 5% 6% 7% 39%	2091
	11th to 20th	Day	12% 5% 3 6% 5% 4 4 5% 6% 8% 40%	1980
	21st to end	Day	19% 8% 3 5% 5% 2 4 4 5% 6% 37%	1980
October	1st to 10th	Day	17% 7% 3 6% 6% 2 6% 4 6% 7% 36%	1846
	11th to 20th	Day	22% 10% 4 7% 5% 4 4 3 5% 6% 30%	1800
	21st to end	Day	24% 13% 4 7% 6% 3 5% 4 5% 6% 24%	1980
November	1st to 10th	Day	12% 13% 4 7% 7% 3 4 4 5% 6% 34%	1681
	11th to 20th	Day	17% 19% 5% 7% 6% 3 4 3 4 7% 24%	1638
	21st to end	Day	18% 20% 8% 11% 6% 3 4 4 4 6% 17%	1520
December	1st to 10th	Day	16% 22% 12% 12% 8% 3 3 3 3 4 13%	1520
	11th to 20th	Day	16% 19% 8% 16% 6% 3 4 4 4 5% 14%	1910
	21st to end	Day	22% 25% 10% 11% 7% 3 3 3 4 3 9%	2834
January	1st to 10th	Night	17% 19% 9% 15% 8% 3 4 4 3 4 15%	2459
	11th to 20th	Night	18% 19% 8% 14% 8% 3 4 3 3 4 15%	2417
	21st to end	Night	14% 19% 9% 14% 10% 4 5% 3 4 4 15%	2574
February	1st to 10th	Night	19% 16% 6% 14% 9% 3 4 4 4 4 16%	2340
	11th to 20th	Night	16% 14% 8% 13% 9% 4 5% 5% 4 4 19%	2242
	21st to end	Night	20% 12% 7% 11% 9% 3 5% 3 4 4 23%	1776
March	1st to 10th	Night	15% 14% 6% 9% 7% 3 4 5% 4 4 29%	2016
	11th to 20th	Night	17% 11% 5% 8% 5% 2 5% 3 5% 6% 32%	1980
	21st to end	Night	18% 13% 6% 8% 7% 3 4 4 4 5% 30%	2178
April	1st to 10th	Night	16% 13% 6% 7% 6% 3 4 4 5% 6% 31%	1928
	11th to 20th	Night	16% 11% 3 6% 5% 3 5% 5% 6% 7% 34%	1657
	21st to end	Night	15% 10% 4 6% 6% 4 5% 5% 5% 7% 35%	1620
May	1st to 10th	Night	18% 9% 3 6% 6% 4 6% 5% 4 8% 30%	1620
	11th to 20th	Night	15% 9% 4 6% 5% 3 5% 4 6% 8% 35%	1592
	21st to end	Night	11% 5% 2 4 5% 3 6% 4 7% 10% 43%	1650
June	1st to 10th	Night	11% 6% 3 6% 5% 5% 5% 6% 5% 6% 42%	1440
	11th to 20th	Night	9% 5% 3 7% 5% 4 6% 5% 5% 9% 42%	1440
	21st to end	Night	10% 6% 2 6% 4 3 5% 5% 7% 10% 41%	1440
July	1st to 10th	Night	7% 5% 3 5% 4 3 5% 5% 5% 8% 50%	1557
	11th to 20th	Night	8% 5% 2 5% 5% 2 4 5% 7% 8% 47%	1620
	21st to end	Night	8% 6% 3 5% 6% 4 6% 6% 5% 8% 44%	1771
August	1st to 10th	Night	7% 5% 2 5% 6% 3 5% 6% 7% 8% 47%	1611
	11th to 20th	Night	9% 6% 3 6% 6% 2 5% 4 7% 8% 44%	1598
	21st to end	Night	9% 6% 2 6% 5% 3 4 6% 7% 7% 46%	1813
September	1st to 10th	Night	13% 6% 2 4 6% 3 5% 5% 5% 7% 44%	1866
	11th to 20th	Night	10% 5% 2 5% 4 3 4 5% 6% 6% 50%	1980
	21st to end	Night	16% 7% 2 7% 6% 3 4 5% 4 6% 40%	1980
October	1st to 10th	Night	14% 7% 3 6% 4 3 3 5% 5% 8% 41%	2114
	11th to 20th	Night	18% 9% 4 7% 6% 3 5% 5% 5% 7% 32%	2160
	21st to end	Night	26% 10% 4 6% 5% 4 4 4 5% 6% 27%	2376
November	1st to 10th	Night	15% 10% 4 7% 6% 3 4 4 4 7% 36%	2396
	11th to 20th	Night	16% 18% 5% 10% 5% 3 4 3 5% 6% 25%	2540
	21st to end	Night	18% 18% 6% 8% 7% 3 5% 5% 5% 5% 20%	2660
December	1st to 10th	Night	19% 21% 10% 9% 6% 3 4 4 4 4 16%	2660
	11th to 20th	Night	16% 19% 8% 12% 7% 3 4 4 4 5% 17%	3285
	21st to end	Night	22% 23% 10% 11% 7% 3 3 3 4 4 9%	4819

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	73% 5% 10% 8% 3	1707
	11th to 20th	Day	75% 5% 11% 7% 2	1665
	21st to end	Day	77% 5% 7% 8% 3	1782
February	1st to 10th	Day	73% 4 8% 10% 4	1620
	11th to 20th	Day	70% 6% 8% 10% 5%	1609
	21st to end	Day	66% 6% 10% 11% 6% 2	1480
March	1st to 10th	Day	58% 6% 13% 18% 5%	1942
	11th to 20th	Day	58% 5% 10% 15% 10% 2	1980
	21st to end	Day	59% 5% 8% 17% 9% 2	2178
April	1st to 10th	Day	52% 6% 13% 17% 12% 1	2031
	11th to 20th	Day	53% 6% 10% 16% 13% 2	2266
	21st to end	Day	53% 5% 10% 20% 10% 2	2340
May	1st to 10th	Day	53% 8% 10% 19% 9% 2	2340
	11th to 20th	Day	50% 7% 11% 17% 13% 2	2305
	21st to end	Day	41% 9% 14% 22% 13% 1	2621
June	1st to 10th	Day	44% 7% 12% 23% 12% 1	2520
	11th to 20th	Day	42% 9% 15% 20% 13% 1	2520
	21st to end	Day	39% 8% 13% 25% 14% 2	2520
July	1st to 10th	Day	39% 7% 12% 25% 16% 2	2332
	11th to 20th	Day	39% 8% 13% 24% 14% 1	2271
	21st to end	Day	37% 9% 16% 22% 15% 2	2405
August	1st to 10th	Day	38% 9% 15% 25% 13%	2314
	11th to 20th	Day	42% 11% 15% 20% 10% 2	2303
	21st to end	Day	40% 9% 14% 21% 14% 2	2525
September	1st to 10th	Day	42% 7% 15% 19% 14% 3	2091
	11th to 20th	Day	39% 7% 16% 22% 15% 1	1980
	21st to end	Day	47% 6% 11% 23% 13% 1	1980
October	1st to 10th	Day	45% 7% 13% 21% 13% 1	1846
	11th to 20th	Day	54% 6% 12% 18% 8% 1	1800
	21st to end	Day	61% 6% 12% 12% 8% 1	1980
November	1st to 10th	Day	49% 6% 12% 18% 13% 1	1681
	11th to 20th	Day	60% 5% 12% 14% 7% 3	1638
	21st to end	Day	67% 7% 9% 12% 5%	1520
December	1st to 10th	Day	76% 5% 8% 9% 3	1520
	11th to 20th	Day	71% 7% 8% 10% 3	1906
	21st to end	Day	81% 5% 7% 6% 1	2834
January	1st to 10th	Night	72% 6% 8% 10% 4	2459
	11th to 20th	Night	74% 5% 9% 9% 2 1	2417
	21st to end	Night	74% 5% 8% 10% 3 1	2574
February	1st to 10th	Night	71% 5% 9% 11% 3	2340
	11th to 20th	Night	68% 5% 9% 13% 5%	2244
	21st to end	Night	65% 5% 9% 13% 7% 1	1776
March	1st to 10th	Night	56% 6% 10% 19% 8%	2016
	11th to 20th	Night	53% 6% 11% 18% 11% 2	1980
	21st to end	Night	56% 5% 8% 17% 11% 2	2178
April	1st to 10th	Night	54% 6% 12% 16% 11% 2	1928
	11th to 20th	Night	49% 6% 11% 19% 13% 2	1657
	21st to end	Night	48% 6% 12% 19% 12% 3	1620
May	1st to 10th	Night	52% 6% 11% 17% 11% 3	1620
	11th to 20th	Night	46% 8% 11% 18% 14% 4	1593
	21st to end	Night	35% 8% 17% 25% 14% 2	1651
June	1st to 10th	Night	40% 8% 11% 24% 15% 2	1440
	11th to 20th	Night	38% 8% 13% 21% 18% 2	1440
	21st to end	Night	36% 9% 12% 24% 17% 1	1440
July	1st to 10th	Night	34% 8% 11% 24% 19% 4	1512
	11th to 20th	Night	31% 10% 14% 23% 19% 3	1566
	21st to end	Night	35% 9% 14% 22% 17% 2	1672
August	1st to 10th	Night	31% 9% 15% 27% 16% 1	1600
	11th to 20th	Night	36% 8% 15% 26% 12% 2	1598
	21st to end	Night	33% 8% 13% 25% 17% 3	1813
September	1st to 10th	Night	37% 8% 14% 23% 15% 3	1866
	11th to 20th	Night	32% 7% 16% 25% 19% 1	1980
	21st to end	Night	45% 6% 11% 21% 17% 1	1980
October	1st to 10th	Night	40% 7% 14% 20% 18% 1	2114
	11th to 20th	Night	50% 7% 12% 20% 10% 1	2160
	21st to end	Night	57% 7% 11% 16% 9% 1	2376
November	1st to 10th	Night	46% 7% 13% 20% 11% 3	2396
	11th to 20th	Night	60% 5% 11% 15% 6% 3	2540
	21st to end	Night	65% 6% 10% 14% 4	2660
December	1st to 10th	Night	71% 6% 10% 8% 4	2660
	11th to 20th	Night	69% 7% 11% 11% 2	3288
	21st to end	Night	78% 6% 9% 6% 1	4819

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	33% 29% 28% 8% 2	270
	11th to 20th	Day	30% 35% 27% 5% 2	255
	21st to end	Day	28% 37% 27% 6% 2	264
February	1st to 10th	Day	39% 24% 33% 4	240
	11th to 20th	Day	35% 35% 26% 4	240
	21st to end	Day	39% 33% 25% 3	198
March	1st to 10th	Day	30% 26% 30% 11% 3	382
	11th to 20th	Day	30% 24% 31% 12% 3 1	420
	21st to end	Day	35% 24% 25% 13% 3	462
April	1st to 10th	Day	27% 26% 28% 14% 5% 1	420
	11th to 20th	Day	32% 20% 25% 13% 8% 1	412
	21st to end	Day	30% 21% 30% 13% 5% 1	420
May	1st to 10th	Day	27% 22% 28% 17% 4 2	423
	11th to 20th	Day	23% 15% 31% 22% 7% 2	447
	21st to end	Day	15% 11% 33% 24% 14% 4	485
June	1st to 10th	Day	16% 14% 37% 22% 9% 2	450
	11th to 20th	Day	17% 14% 29% 27% 11% 2	450
	21st to end	Day	14% 13% 35% 25% 10% 3	450
July	1st to 10th	Day	12% 7% 23% 29% 20% 7%	445
	11th to 20th	Day	14% 9% 30% 26% 17% 5%	444
	21st to end	Day	10% 9% 33% 28% 14% 6%	488
August	1st to 10th	Day	8% 10% 25% 30% 20% 7%	446
	11th to 20th	Day	13% 9% 29% 27% 14% 8%	439
	21st to end	Day	11% 9% 27% 28% 16% 8%	495
September	1st to 10th	Day	17% 10% 24% 26% 18% 6%	450
	11th to 20th	Day	11% 8% 24% 30% 19% 9%	450
	21st to end	Day	21% 15% 26% 20% 13% 5%	450
October	1st to 10th	Day	15% 16% 29% 20% 16% 3	450
	11th to 20th	Day	27% 22% 33% 15% 3 1	450
	21st to end	Day	35% 21% 29% 13% 3	495
November	1st to 10th	Day	26% 23% 28% 15% 7% 1	279
	11th to 20th	Day	36% 21% 27% 13% 4	236
	21st to end	Day	31% 25% 37% 7% 1	180
December	1st to 10th	Day	27% 35% 24% 11% 2 2	180
	11th to 20th	Day	31% 31% 31% 6% 1	180
	21st to end	Day	32% 33% 28% 6%	216
January	1st to 10th	Night	28% 28% 31% 10% 3	760
	11th to 20th	Night	32% 24% 29% 12% 2	744
	21st to end	Night	25% 27% 31% 16% 2	792
February	1st to 10th	Night	33% 22% 29% 15% 1	720
	11th to 20th	Night	30% 22% 32% 15% 1	703
	21st to end	Night	32% 24% 31% 12% 1	592
March	1st to 10th	Night	25% 23% 31% 19% 3	720
	11th to 20th	Night	25% 22% 28% 17% 6% 1	720
	21st to end	Night	26% 19% 29% 20% 5% 1	792
April	1st to 10th	Night	24% 17% 31% 22% 5%	720
	11th to 20th	Night	23% 16% 35% 18% 7% 1	593
	21st to end	Night	19% 15% 36% 24% 6%	540
May	1st to 10th	Night	23% 11% 29% 27% 9% 1	540
	11th to 20th	Night	19% 10% 28% 32% 8% 2	531
	21st to end	Night	12% 5% 29% 35% 18% 1	490
June	1st to 10th	Night	11% 5% 34% 38% 12% 1	360
	11th to 20th	Night	11% 6% 31% 39% 13% 1	360
	21st to end	Night	11% 9% 26% 40% 14%	360
July	1st to 10th	Night	7% 4 18% 39% 27% 4	462
	11th to 20th	Night	9% 4 23% 36% 24% 3	522
	21st to end	Night	9% 6% 19% 41% 23% 2	591
August	1st to 10th	Night	9% 4 18% 41% 25% 3	537
	11th to 20th	Night	10% 5% 16% 38% 27% 4	534
	21st to end	Night	13% 5% 21% 31% 28% 3	625
September	1st to 10th	Night	15% 7% 19% 31% 24% 4	720
	11th to 20th	Night	13% 4 21% 33% 25% 4	720
	21st to end	Night	18% 10% 24% 28% 16% 4	720
October	1st to 10th	Night	17% 11% 29% 25% 16% 2	720
	11th to 20th	Night	23% 20% 27% 23% 7%	720
	21st to end	Night	32% 17% 31% 17% 3	792
November	1st to 10th	Night	21% 18% 28% 25% 8% 1	740
	11th to 20th	Night	28% 24% 28% 15% 5% 1	760
	21st to end	Night	32% 22% 29% 13% 3	760
December	1st to 10th	Night	33% 24% 28% 11% 4	760
	11th to 20th	Night	29% 27% 29% 13% 2	760
	21st to end	Night	33% 23% 31% 12% 1	836

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