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 2024-08-15.

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% 5% 2 5% 4 4 4 3 5% 7% 50%	95394
	Night	11% 5% 2 4 3 3 3 3 5% 7% 54%	93378
Transparency	Day	34% 6% 10% 24% 21% 6%	95170
	Night	31% 5% 10% 22% 24% 9%	93208
Seeing	Day	11% 5% 20% 27% 25% 10%	14918
	Night	12% 6% 27% 35% 18% 2	24744

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 5% 4 2 3 3 4 6% 52%	2847
	11th to 20th	Day	12% 5% 1 3 2 2 2 2 3 5% 62%	2850
	21st to end	Day	21% 8% 3 6% 3 2 2 2 3 3 47%	3135
February	1st to 10th	Day	17% 7% 3 6% 4 3 3 3 3 5% 45%	1786
	11th to 20th	Day	18% 6% 3 5% 3 4 3 2 4 5% 47%	1849
	21st to end	Day	22% 6% 2 4 2 2 2 2 3 4 51%	1613
March	1st to 10th	Day	15% 7% 2 5% 3 2 3 3 5% 6% 49%	3436
	11th to 20th	Day	18% 6% 3 6% 4 4 4 3 5% 6% 42%	3480
	21st to end	Day	15% 8% 2 6% 4 5% 5% 3 5% 6% 41%	3814
April	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 48%	2090
	11th to 20th	Day	14% 7% 2 6% 4 4 3 4 5% 6% 45%	2189
	21st to end	Day	13% 5% 2 5% 3 4 4 4 5% 6% 48%	2388
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 39%	4082
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 5% 6% 46%	4082
	21st to end	Day	16% 8% 2 7% 6% 5% 5% 4 5% 7% 35%	4477
June	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 7% 8% 50%	2470
	11th to 20th	Day	5% 4 1 4 5% 4 5% 5% 6% 9% 52%	2470
	21st to end	Day	5% 3 1 4 3 4 5% 5% 6% 10% 55%	2470
July	1st to 10th	Day	6% 5% 2 5% 5% 5% 5% 4 5% 7% 52%	4120
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 10% 62%	4120
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 45%	4465
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 10% 62%	2458
	11th to 20th	Day	7% 3 1 4 4 3 5% 5% 7% 9% 51%	2355
	21st to end	Day	4 3 1 6% 5% 5% 6% 5% 7% 10% 49%	2348
September	1st to 10th	Day	9% 4 2 4 3 4 4 3 6% 8% 54%	1980
	11th to 20th	Day	10% 4 2 7% 4 4 5% 4 5% 7% 48%	1980
	21st to end	Day	8% 3 1 4 3 3 4 4 5% 7% 59%	1980
October	1st to 10th	Day	6% 3 1 4 4 4 4 3 4 5% 62%	1910
	11th to 20th	Day	13% 4 1 3 2 2 2 2 4 6% 61%	1800
	21st to end	Day	15% 5% 2 5% 2 2 2 2 3 3 58%	1980
November	1st to 10th	Day	12% 5% 1 4 3 3 3 3 4 5% 57%	1858
	11th to 20th	Day	8% 4 2 3 4 4 3 3 3 6% 60%	1796
	21st to end	Day	15% 7% 2 4 3 2 2 2 3 5% 54%	1710
December	1st to 10th	Day	15% 6% 1 5% 3 4 4 3 4 5% 49%	1710
	11th to 20th	Day	16% 7% 3 6% 4 5% 4 3 4 5% 46%	2161
	21st to end	Day	18% 8% 2 4 3 3 3 3 4 6% 48%	3135
January	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 6% 54%	4108
	11th to 20th	Night	11% 5% 1 3 2 2 2 2 3 5% 63%	4120
	21st to end	Night	20% 7% 2 5% 3 3 2 2 4 4 48%	4532
February	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	2409
	11th to 20th	Night	15% 8% 2 4 3 3 4 3 4 6% 49%	2218
	21st to end	Night	19% 5% 2 3 3 3 3 2 3 4 53%	1838
March	1st to 10th	Night	15% 8% 3 6% 4 4 4 4 5% 5% 43%	3419
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 47%	3480
	21st to end	Night	14% 6% 2 6% 4 3 3 3 5% 5% 48%	3817
April	1st to 10th	Night	11% 4 2 5% 3 3 4 3 5% 8% 52%	2090
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 49%	1938
	21st to end	Night	11% 5% 2 5% 3 3 3 3 5% 7% 54%	1787
May	1st to 10th	Night	18% 9% 2 6% 4 4 4 4 5% 6% 38%	2823
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 7% 50%	2823
	21st to end	Night	10% 5% 2 5% 4 4 5% 5% 6% 9% 44%	3090
June	1st to 10th	Night	5% 3 1 4 3 4 3 4 6% 10% 56%	1710
	11th to 20th	Night	4 2 1 3 3 3 4 4 5% 8% 63%	1710
	21st to end	Night	2 2 1 3 3 3 4 4 6% 10% 61%	1710
July	1st to 10th	Night	5% 3 1 3 3 3 4 4 6% 10% 57%	2850
	11th to 20th	Night	3 2 1 3 2 2 2 2 5% 10% 67%	2850
	21st to end	Night	4 3 2 4 4 3 5% 4 7% 9% 55%	3098
August	1st to 10th	Night	2 2 1 2 1 2 3 3 5% 10% 69%	1701
	11th to 20th	Night	5% 4 1 4 3 3 3 3 6% 9% 58%	1645
	21st to end	Night	3 3 1 4 3 4 4 4 7% 11% 56%	2000
September	1st to 10th	Night	7% 4 2 4 3 3 3 3 5% 7% 59%	1980
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 53%	1980
	21st to end	Night	6% 3 2 2 3 3 3 3 4 7% 64%	1980
October	1st to 10th	Night	6% 3 1 4 3 4 2 3 5% 6% 64%	2029
	11th to 20th	Night	12% 4 2 3 2 2 2 2 3 5% 63%	2160
	21st to end	Night	15% 5% 2 3 2 2 2 2 3 4 61%	2376
November	1st to 10th	Night	9% 5% 2 4 3 3 3 2 4 4 61%	2220
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 6% 59%	2379
	21st to end	Night	15% 6% 2 4 3 3 3 3 4 5% 53%	2470
December	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 3 5% 54%	2470
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	3036
	21st to end	Night	16% 6% 2 4 3 2 2 2 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 7% 20% 32% 4	2848
	11th to 20th	Day	27% 3 9% 23% 31% 6%	2850
	21st to end	Day	45% 2 4 18% 22% 8%	3135
February	1st to 10th	Day	44% 3 6% 18% 20% 8%	1786
	11th to 20th	Day	41% 4 7% 21% 19% 8%	1859
	21st to end	Day	40% 3 4 16% 26% 11%	1613
March	1st to 10th	Day	35% 4 8% 17% 27% 10%	3436
	11th to 20th	Day	44% 5% 10% 21% 16% 4	3480
	21st to end	Day	43% 5% 10% 19% 19% 4	3814
April	1st to 10th	Day	37% 5% 9% 21% 18% 10%	2090
	11th to 20th	Day	40% 6% 8% 17% 21% 9%	2189
	21st to end	Day	36% 7% 9% 21% 24% 4	2388
May	1st to 10th	Day	46% 5% 8% 21% 15% 4	4120
	11th to 20th	Day	38% 5% 9% 25% 21% 1	4086
	21st to end	Day	46% 9% 11% 19% 11% 5%	4489
June	1st to 10th	Day	29% 9% 14% 28% 17% 3	2470
	11th to 20th	Day	28% 7% 13% 28% 19% 5%	2470
	21st to end	Day	24% 8% 12% 30% 23% 3	2470
July	1st to 10th	Day	29% 7% 15% 36% 12%	4056
	11th to 20th	Day	16% 7% 11% 41% 22% 2	4051
	21st to end	Day	30% 10% 14% 32% 13%	4325
August	1st to 10th	Day	16% 7% 16% 32% 25% 3	2444
	11th to 20th	Day	26% 9% 14% 29% 19% 3	2355
	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	31% 3 8% 19% 31% 8%	4108
	11th to 20th	Night	26% 3 9% 20% 30% 12%	4120
	21st to end	Night	41% 3 6% 15% 26% 9%	4532
February	1st to 10th	Night	38% 4 8% 18% 22% 11%	2409
	11th to 20th	Night	38% 4 7% 17% 25% 10%	2218
	21st to end	Night	37% 3 7% 15% 23% 15%	1838
March	1st to 10th	Night	42% 5% 6% 14% 21% 12%	3419
	11th to 20th	Night	38% 4 11% 20% 20% 7%	3480
	21st to end	Night	38% 5% 10% 18% 26% 4	3817
April	1st to 10th	Night	30% 5% 12% 21% 19% 14%	2090
	11th to 20th	Night	35% 5% 8% 20% 18% 13%	1938
	21st to end	Night	31% 4 10% 22% 26% 5%	1787
May	1st to 10th	Night	45% 8% 8% 19% 14% 7%	2850
	11th to 20th	Night	35% 5% 12% 21% 25% 2	2824
	21st to end	Night	34% 8% 14% 25% 13% 6%	3099
June	1st to 10th	Night	22% 7% 16% 31% 20% 4	1710
	11th to 20th	Night	19% 5% 13% 34% 21% 8%	1710
	21st to end	Night	19% 7% 12% 31% 26% 5%	1710
July	1st to 10th	Night	21% 7% 17% 38% 15% 2	2805
	11th to 20th	Night	14% 4 11% 43% 25% 3	2796
	21st to end	Night	23% 7% 16% 35% 17% 1	2999
August	1st to 10th	Night	11% 4 13% 35% 31% 5%	1691
	11th to 20th	Night	22% 6% 14% 30% 22% 6%	1645
	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% 23% 8% 2	270
	21st to end	Day	20% 8% 39% 22% 10% 1	297
February	1st to 10th	Day	20% 10% 39% 21% 9%	270
	11th to 20th	Day	18% 14% 42% 19% 6%	270
	21st to end	Day	30% 11% 39% 17% 3	225
March	1st to 10th	Day	19% 12% 33% 30% 6%	270
	11th to 20th	Day	18% 16% 37% 22% 5% 1	270
	21st to end	Day	13% 12% 44% 22% 7% 1	297
April	1st to 10th	Day	11% 12% 40% 27% 8% 2	270
	11th to 20th	Day	17% 12% 34% 28% 10%	387
	21st to end	Day	16% 10% 36% 27% 9% 2	568
May	1st to 10th	Day	15% 8% 31% 34% 11% 1	653
	11th to 20th	Day	16% 6% 25% 34% 16% 4	675
	21st to end	Day	14% 6% 24% 31% 21% 4	734
June	1st to 10th	Day	5% 2 11% 36% 35% 11%	680
	11th to 20th	Day	6% 2 14% 37% 32% 9%	680
	21st to end	Day	5% 1 9% 32% 37% 15%	680
July	1st to 10th	Day	6% 1 7% 27% 40% 19%	671
	11th to 20th	Day	3 1 3 26% 46% 21%	668
	21st to end	Day	4 1 4 23% 47% 22%	739
August	1st to 10th	Day	3 3 24% 50% 20%	676
	11th to 20th	Day	9% 1 4 26% 42% 18%	637
	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	15% 9% 37% 33% 6%	1045
February	1st to 10th	Night	19% 10% 37% 29% 6%	889
	11th to 20th	Night	18% 10% 40% 28% 3	744
	21st to end	Night	20% 14% 42% 20% 3	582
March	1st to 10th	Night	18% 10% 38% 28% 6%	570
	11th to 20th	Night	17% 9% 35% 32% 6%	570
	21st to end	Night	12% 13% 37% 32% 6%	627
April	1st to 10th	Night	12% 9% 44% 30% 5%	570
	11th to 20th	Night	15% 9% 41% 27% 7%	563
	21st to end	Night	13% 8% 40% 30% 9% 1	570
May	1st to 10th	Night	11% 7% 35% 37% 9% 1	570
	11th to 20th	Night	11% 3 32% 36% 16% 1	561
	21st to end	Night	8% 4 28% 42% 17% 2	617
June	1st to 10th	Night	5% 1 14% 43% 34% 4	570
	11th to 20th	Night	3 1 18% 53% 22% 1	570
	21st to end	Night	2 2 11% 43% 36% 5%	570
July	1st to 10th	Night	4 1 6% 42% 41% 6%	555
	11th to 20th	Night	3 4 38% 50% 4	552
	21st to end	Night	2 4 42% 46% 6%	624
August	1st to 10th	Night	2 2 42% 47% 6%	567
	11th to 20th	Night	6% 1 5% 35% 48% 5%	549
	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

Page generated at 2024-08-12 08:52:52. Version ac07

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