Z-Field Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Z-Field Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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	11% 5% 2 4 4 3 4 4 5% 7% 51%	75017
	Night	10% 4 2 4 4 2 3 4 5% 7% 56%	74940
Transparency	Day	33% 6% 10% 23% 22% 7%	74742
	Night	29% 5% 10% 22% 24% 10%	74735
Seeing	Day	11% 5% 20% 27% 25% 10%	14063
	Night	12% 6% 27% 35% 18% 2	24075

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	12% 6% 3 4 4 2 2 3 3 7% 54%	1707
	11th to 20th	Day	15% 5% 2 3 3 2 3 3 4 5% 55%	1710
	21st to end	Day	15% 5% 2 4 3 2 2 4 4 5% 53%	1791
February	1st to 10th	Day	17% 6% 4 5% 5% 2 3 3 3 5% 46%	1692
	11th to 20th	Day	18% 6% 3 5% 4 2 3 3 5% 5% 45%	1749
	21st to end	Day	23% 5% 3 4 2 2 2 3 2 4 50%	1520
March	1st to 10th	Day	18% 6% 2 5% 4 2 3 4 5% 6% 47%	1980
	11th to 20th	Day	18% 6% 2 4 4 2 4 3 4 5% 46%	1980
	21st to end	Day	15% 6% 3 5% 4 3 4 3 5% 5% 47%	2178
April	1st to 10th	Day	13% 6% 2 4 4 3 4 4 5% 6% 48%	1980
	11th to 20th	Day	14% 7% 2 5% 5% 3 3 4 5% 6% 46%	2072
	21st to end	Day	13% 5% 2 4 4 3 5% 5% 5% 7% 49%	2262
May	1st to 10th	Day	13% 5% 3 5% 4 3 5% 4 5% 6% 47%	2340
	11th to 20th	Day	12% 6% 3 6% 5% 3 5% 5% 6% 8% 43%	2302
	21st to end	Day	11% 6% 3 6% 6% 4 7% 6% 6% 7% 37%	2531
June	1st to 10th	Day	5% 3 2 4 5% 3 6% 6% 7% 8% 50%	2340
	11th to 20th	Day	5% 4 2 4 5% 3 5% 6% 6% 9% 52%	2340
	21st to end	Day	5% 3 1 3 4 3 5% 5% 6% 10% 54%	2340
July	1st to 10th	Day	7% 4 3 4 5% 5% 6% 7% 7% 10% 42%	2340
	11th to 20th	Day	3 2 1 2 3 2 4 5% 6% 10% 61%	2340
	21st to end	Day	3 3 2 4 4 3 4 6% 8% 12% 51%	2546
August	1st to 10th	Day	2 1 1 3 4 2 4 5% 6% 10% 61%	2328
	11th to 20th	Day	7% 4 1 4 4 2 4 6% 6% 8% 51%	2301
	21st to end	Day	4 3 2 5% 5% 3 6% 6% 7% 10% 49%	2348
September	1st to 10th	Day	9% 4 2 3 4 3 4 4 6% 8% 54%	1980
	11th to 20th	Day	10% 4 2 6% 5% 3 4 5% 5% 7% 48%	1980
	21st to end	Day	8% 3 1 3 3 2 3 4 5% 7% 59%	1980
October	1st to 10th	Day	6% 3 1 3 5% 3 5% 3 4 5% 62%	1910
	11th to 20th	Day	13% 4 1 3 3 2 2 3 4 6% 61%	1800
	21st to end	Day	15% 5% 2 4 3 2 3 3 3 3 58%	1980
November	1st to 10th	Day	12% 4 1 3 3 2 3 3 3 6% 57%	1858
	11th to 20th	Day	8% 4 2 3 4 3 3 4 3 6% 60%	1796
	21st to end	Day	15% 7% 2 4 3 2 3 3 3 5% 54%	1710
December	1st to 10th	Day	15% 6% 1 5% 4 2 4 4 5% 5% 50%	1710
	11th to 20th	Day	16% 6% 3 5% 5% 4 4 4 4 5% 46%	2161
	21st to end	Day	18% 7% 2 4 3 2 3 4 3 6% 48%	3135
January	1st to 10th	Night	11% 5% 1 4 4 2 3 4 4 6% 57%	2458
	11th to 20th	Night	14% 5% 2 3 3 2 3 3 3 5% 56%	2470
	21st to end	Night	13% 5% 2 4 4 2 3 3 4 5% 56%	2600
February	1st to 10th	Night	15% 6% 2 4 4 2 4 3 3 5% 51%	2282
	11th to 20th	Night	15% 7% 3 4 4 3 4 3 4 6% 49%	2098
	21st to end	Night	20% 5% 2 3 3 2 2 3 3 4 52%	1733
March	1st to 10th	Night	16% 6% 2 4 3 2 3 4 5% 7% 47%	1980
	11th to 20th	Night	15% 5% 2 5% 4 2 4 4 5% 6% 49%	1980
	21st to end	Night	11% 5% 2 5% 4 2 3 4 4 6% 53%	2178
April	1st to 10th	Night	10% 3 2 4 3 2 4 4 5% 9% 53%	1980
	11th to 20th	Night	13% 5% 3 4 4 2 4 4 4 7% 50%	1835
	21st to end	Night	11% 5% 2 4 4 2 3 4 5% 7% 54%	1693
May	1st to 10th	Night	10% 6% 3 4 4 3 4 4 5% 7% 51%	1620
	11th to 20th	Night	10% 5% 2 4 4 3 3 4 5% 9% 50%	1593
	21st to end	Night	8% 4 2 3 4 2 5% 5% 7% 11% 49%	1745
June	1st to 10th	Night	5% 3 1 4 4 3 3 4 6% 10% 57%	1620
	11th to 20th	Night	4 2 1 2 4 2 4 5% 5% 8% 63%	1620
	21st to end	Night	3 2 1 3 3 2 4 4 6% 11% 61%	1620
July	1st to 10th	Night	5% 3 2 4 4 2 4 6% 7% 11% 52%	1620
	11th to 20th	Night	2 2 1 2 3 2 2 3 5% 10% 69%	1620
	21st to end	Night	2 1 1 3 3 2 5% 5% 7% 11% 61%	1772
August	1st to 10th	Night	2 2 1 2 2 1 2 4 5% 10% 69%	1611
	11th to 20th	Night	5% 4 1 4 3 3 3 4 6% 9% 58%	1600
	21st to end	Night	3 3 1 3 4 3 4 5% 7% 11% 56%	2000
September	1st to 10th	Night	7% 3 2 4 4 2 3 3 5% 7% 59%	1980
	11th to 20th	Night	10% 5% 2 4 4 2 4 4 5% 8% 53%	1980
	21st to end	Night	6% 3 1 2 3 2 4 3 4 7% 64%	1980
October	1st to 10th	Night	6% 3 1 3 3 3 3 3 4 6% 64%	2029
	11th to 20th	Night	12% 4 2 3 3 1 2 3 3 5% 63%	2160
	21st to end	Night	15% 5% 2 3 3 1 2 2 3 4 61%	2376
November	1st to 10th	Night	9% 4 2 4 4 2 3 3 4 4 62%	2220
	11th to 20th	Night	7% 5% 2 3 3 2 3 4 5% 7% 59%	2379
	21st to end	Night	15% 5% 2 4 3 2 3 3 4 5% 53%	2470
December	1st to 10th	Night	13% 5% 2 4 4 2 3 4 4 5% 54%	2470
	11th to 20th	Night	14% 6% 2 5% 5% 2 4 3 4 6% 50%	3036
	21st to end	Night	16% 6% 2 4 4 2 2 3 3 6% 53%	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	33% 3 8% 20% 30% 6%	1708
	11th to 20th	Day	34% 4 7% 24% 26% 6%	1710
	21st to end	Day	34% 3 7% 18% 28% 9%	1791
February	1st to 10th	Day	43% 3 6% 18% 20% 8%	1692
	11th to 20th	Day	42% 4 7% 21% 18% 8%	1759
	21st to end	Day	40% 3 4 16% 24% 12%	1520
March	1st to 10th	Day	39% 4 7% 18% 19% 12%	1980
	11th to 20th	Day	41% 5% 8% 18% 22% 5%	1980
	21st to end	Day	40% 4 10% 19% 21% 7%	2178
April	1st to 10th	Day	37% 5% 9% 22% 18% 9%	1980
	11th to 20th	Day	39% 6% 8% 17% 21% 9%	2072
	21st to end	Day	35% 7% 9% 21% 24% 4	2262
May	1st to 10th	Day	37% 6% 9% 22% 19% 6%	2340
	11th to 20th	Day	39% 7% 12% 23% 17% 2	2306
	21st to end	Day	42% 10% 11% 21% 14% 2	2531
June	1st to 10th	Day	29% 9% 14% 28% 17% 3	2340
	11th to 20th	Day	29% 7% 13% 29% 18% 4	2340
	21st to end	Day	25% 9% 12% 28% 23% 3	2340
July	1st to 10th	Day	34% 9% 17% 26% 13%	2276
	11th to 20th	Day	17% 8% 15% 35% 22% 3	2271
	21st to end	Day	22% 9% 17% 36% 15%	2405
August	1st to 10th	Day	16% 7% 17% 32% 24% 3	2314
	11th to 20th	Day	27% 9% 14% 28% 19% 3	2301
	21st to end	Day	27% 8% 15% 30% 16% 4	2348
September	1st to 10th	Day	29% 6% 11% 23% 21% 10%	1980
	11th to 20th	Day	34% 9% 10% 18% 21% 8%	1980
	21st to end	Day	24% 5% 11% 21% 29% 11%	1980
October	1st to 10th	Day	26% 4 11% 23% 20% 16%	1910
	11th to 20th	Day	27% 4 8% 16% 30% 16%	1800
	21st to end	Day	32% 3 7% 17% 26% 15%	1980
November	1st to 10th	Day	30% 5% 7% 24% 26% 9%	1858
	11th to 20th	Day	28% 4 9% 20% 29% 10%	1796
	21st to end	Day	35% 3 5% 20% 27% 10%	1710
December	1st to 10th	Day	37% 4 7% 21% 24% 6%	1710
	11th to 20th	Day	42% 4 5% 16% 25% 9%	2159
	21st to end	Day	38% 3 6% 22% 23% 7%	3135
January	1st to 10th	Night	30% 4 7% 20% 28% 11%	2458
	11th to 20th	Night	32% 3 8% 20% 29% 9%	2470
	21st to end	Night	33% 3 5% 17% 29% 14%	2600
February	1st to 10th	Night	37% 4 8% 19% 22% 11%	2282
	11th to 20th	Night	38% 4 8% 17% 24% 10%	2098
	21st to end	Night	38% 3 7% 15% 21% 16%	1733
March	1st to 10th	Night	37% 5% 7% 17% 20% 14%	1980
	11th to 20th	Night	37% 4 10% 15% 24% 9%	1980
	21st to end	Night	34% 5% 11% 19% 24% 7%	2178
April	1st to 10th	Night	29% 5% 12% 22% 19% 13%	1980
	11th to 20th	Night	35% 5% 8% 20% 18% 14%	1835
	21st to end	Night	31% 5% 10% 23% 27% 5%	1693
May	1st to 10th	Night	33% 7% 9% 22% 21% 8%	1620
	11th to 20th	Night	32% 6% 16% 23% 20% 3	1594
	21st to end	Night	28% 8% 17% 27% 18% 3	1746
June	1st to 10th	Night	22% 7% 16% 31% 20% 4	1620
	11th to 20th	Night	19% 5% 13% 35% 20% 7%	1620
	21st to end	Night	19% 7% 12% 30% 27% 6%	1620
July	1st to 10th	Night	23% 8% 18% 31% 17% 2	1575
	11th to 20th	Night	12% 4 14% 37% 28% 5%	1566
	21st to end	Night	15% 8% 17% 38% 21% 2	1673
August	1st to 10th	Night	11% 4 14% 36% 29% 5%	1601
	11th to 20th	Night	23% 6% 13% 29% 23% 6%	1600
	21st to end	Night	21% 7% 14% 32% 22% 4	2000
September	1st to 10th	Night	25% 5% 11% 24% 25% 10%	1980
	11th to 20th	Night	30% 6% 12% 19% 23% 11%	1980
	21st to end	Night	21% 4 8% 23% 28% 16%	1980
October	1st to 10th	Night	21% 5% 10% 24% 22% 19%	2029
	11th to 20th	Night	26% 3 8% 15% 28% 21%	2160
	21st to end	Night	31% 3 5% 17% 26% 19%	2376
November	1st to 10th	Night	29% 3 7% 21% 26% 14%	2220
	11th to 20th	Night	27% 4 7% 20% 28% 14%	2379
	21st to end	Night	35% 3 8% 16% 27% 11%	2470
December	1st to 10th	Night	33% 4 8% 21% 23% 10%	2470
	11th to 20th	Night	37% 3 6% 16% 27% 11%	3037
	21st to end	Night	35% 3 7% 18% 27% 10%	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	16% 11% 34% 28% 10%	270
	11th to 20th	Day	16% 13% 38% 24% 8% 2	270
	21st to end	Day	18% 8% 42% 20% 10% 1	267
February	1st to 10th	Day	21% 10% 37% 23% 9%	240
	11th to 20th	Day	20% 15% 42% 19% 5%	240
	21st to end	Day	32% 11% 36% 17% 4	198
March	1st to 10th	Day	19% 13% 33% 28% 6%	240
	11th to 20th	Day	19% 15% 36% 22% 6% 2	240
	21st to end	Day	13% 13% 45% 22% 6% 1	264
April	1st to 10th	Day	11% 11% 41% 27% 8% 2	240
	11th to 20th	Day	16% 11% 33% 29% 11%	350
	21st to end	Day	16% 10% 38% 26% 9% 2	522
May	1st to 10th	Day	16% 7% 31% 34% 11% 1	603
	11th to 20th	Day	16% 5% 25% 33% 16% 4	625
	21st to end	Day	13% 5% 23% 32% 22% 4	679
June	1st to 10th	Day	5% 2 11% 36% 35% 11%	630
	11th to 20th	Day	6% 2 14% 38% 31% 10%	630
	21st to end	Day	5% 1 10% 33% 36% 15%	630
July	1st to 10th	Day	6% 2 7% 27% 40% 19%	621
	11th to 20th	Day	3 1 3 26% 46% 22%	618
	21st to end	Day	3 1 4 21% 48% 23%	684
August	1st to 10th	Day	3 3 25% 50% 18%	626
	11th to 20th	Day	10% 1 4 26% 42% 18%	615
	21st to end	Day	4 1 3 22% 38% 32%	467
September	1st to 10th	Day	6% 1 1 12% 41% 39%	270
	11th to 20th	Day	3 2 4 23% 34% 33%	270
	21st to end	Day	10% 3 7% 18% 40% 22%	270
October	1st to 10th	Day	3 1 13% 31% 41% 12%	270
	11th to 20th	Day	11% 7% 21% 32% 24% 6%	270
	21st to end	Day	22% 10% 20% 26% 18% 4	297
November	1st to 10th	Day	18% 5% 28% 27% 16% 6%	270
	11th to 20th	Day	13% 7% 29% 30% 17% 4	270
	21st to end	Day	17% 13% 32% 25% 11% 1	270
December	1st to 10th	Day	18% 6% 31% 31% 11% 3	270
	11th to 20th	Day	20% 8% 36% 27% 10%	270
	21st to end	Day	20% 9% 36% 26% 9% 1	297
January	1st to 10th	Night	12% 9% 38% 32% 8%	950
	11th to 20th	Night	16% 13% 35% 31% 5% 1	950
	21st to end	Night	14% 9% 38% 33% 6%	1000
February	1st to 10th	Night	19% 10% 37% 29% 6%	842
	11th to 20th	Night	18% 10% 40% 28% 3	704
	21st to end	Night	21% 14% 41% 21% 3	549
March	1st to 10th	Night	17% 11% 38% 28% 6%	540
	11th to 20th	Night	17% 9% 35% 32% 6%	540
	21st to end	Night	12% 12% 37% 32% 6%	594
April	1st to 10th	Night	11% 9% 44% 30% 6%	540
	11th to 20th	Night	15% 9% 40% 29% 8%	533
	21st to end	Night	12% 8% 40% 29% 10% 1	540
May	1st to 10th	Night	11% 7% 36% 36% 9% 1	540
	11th to 20th	Night	11% 3 33% 35% 17% 1	531
	21st to end	Night	7% 3 27% 42% 18% 2	584
June	1st to 10th	Night	5% 1 13% 43% 34% 4	540
	11th to 20th	Night	4 1 19% 53% 22% 1	540
	21st to end	Night	3 2 12% 44% 35% 5%	540
July	1st to 10th	Night	5% 1 6% 40% 42% 6%	525
	11th to 20th	Night	3 4 38% 51% 4	522
	21st to end	Night	2 3 41% 48% 6%	591
August	1st to 10th	Night	2 3 43% 46% 5%	537
	11th to 20th	Night	6% 1 6% 35% 47% 5%	534
	21st to end	Night	4 1 4 36% 49% 7%	594
September	1st to 10th	Night	8% 1 6% 36% 44% 5%	540
	11th to 20th	Night	12% 2 6% 36% 41% 4	540
	21st to end	Night	7% 1 12% 41% 35% 5%	540
October	1st to 10th	Night	7% 1 17% 44% 27% 4	589
	11th to 20th	Night	14% 4 24% 39% 18% 2	720
	21st to end	Night	17% 7% 28% 31% 15% 1	792
November	1st to 10th	Night	10% 3 29% 40% 17% 1	740
	11th to 20th	Night	9% 9% 29% 38% 13% 1	859
	21st to end	Night	17% 9% 34% 30% 10% 1	950
December	1st to 10th	Night	15% 8% 33% 35% 8%	950
	11th to 20th	Night	19% 8% 33% 34% 7%	950
	21st to end	Night	18% 8% 37% 31% 6%	1045

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