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 2025-01-22.

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	12% 5% 2 5% 4 3 4 3 5% 7% 51%	105420
	Night	11% 5% 2 4 3 3 3 3 4 7% 55%	104069
Transparency	Day	32% 5% 10% 24% 22% 7%	109150
	Night	29% 4 9% 23% 25% 10%	108466
Seeing	Day	11% 5% 20% 27% 25% 10%	15416
	Night	12% 6% 27% 35% 18% 2	25396

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 2 4 6% 52%	2937
	11th to 20th	Day	11% 5% 2 3 2 2 3 2 3 5% 62%	2940
	21st to end	Day	21% 8% 3 6% 3 2 2 2 3 3 47%	3144
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 1 1 2 3 3 3 3 5% 8% 69%	4104
	11th to 20th	Day	4 2 1 3 3 3 4 4 6% 10% 59%	4041
	21st to end	Day	3 2 1 4 4 4 5% 4 7% 10% 56%	4110
September	1st to 10th	Day	9% 4 2 4 4 4 4 3 5% 7% 54%	2090
	11th to 20th	Day	10% 4 2 7% 4 4 5% 4 5% 7% 49%	2090
	21st to end	Day	7% 3 2 4 3 3 4 4 4 7% 60%	2090
October	1st to 10th	Day	6% 3 1 4 4 4 4 2 4 5% 64%	2016
	11th to 20th	Day	12% 4 1 3 2 2 2 2 4 6% 62%	1900
	21st to end	Day	14% 5% 2 5% 2 2 2 2 3 3 59%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 5% 56%	1958
	11th to 20th	Day	8% 4 1 3 4 4 3 3 3 6% 61%	1890
	21st to end	Day	14% 7% 2 4 3 2 2 2 3 5% 55%	1800
December	1st to 10th	Day	17% 5% 2 5% 3 3 3 2 3 5% 52%	2940
	11th to 20th	Day	19% 7% 2 5% 3 4 3 3 4 4 46%	3391
	21st to end	Day	20% 8% 2 5% 2 2 2 3 3 5% 47%	4488
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 6% 54%	4238
	11th to 20th	Night	11% 4 1 3 2 2 2 2 3 5% 63%	4250
	21st to end	Night	20% 7% 2 5% 3 3 2 2 4 4 48%	4546
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	1 2 2 1 2 2 3 5% 8% 74%	2836
	11th to 20th	Night	3 2 1 2 2 2 3 3 6% 11% 66%	2798
	21st to end	Night	2 2 1 3 2 4 4 3 6% 10% 63%	3540
September	1st to 10th	Night	7% 4 2 4 3 3 4 3 5% 7% 58%	2090
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 54%	2090
	21st to end	Night	6% 3 2 3 3 3 3 2 4 7% 64%	2090
October	1st to 10th	Night	5% 3 1 3 3 3 2 2 4 6% 66%	2142
	11th to 20th	Night	12% 4 1 3 2 2 2 2 3 5% 64%	2280
	21st to end	Night	15% 5% 2 3 2 2 2 2 3 4 61%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 4 60%	2340
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 6% 60%	2505
	21st to end	Night	14% 6% 2 4 3 3 2 3 4 5% 54%	2600
December	1st to 10th	Night	15% 6% 1 5% 3 3 3 2 3 5% 54%	4250
	11th to 20th	Night	18% 6% 2 6% 3 3 3 2 3 5% 50%	4816
	21st to end	Night	16% 7% 2 5% 3 3 2 2 3 5% 52%	6490

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% 21% 31% 4	2938
	11th to 20th	Day	27% 3 9% 23% 32% 7%	2940
	21st to end	Day	45% 2 4 18% 22% 8%	3144
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	13% 5% 13% 28% 34% 7%	4090
	11th to 20th	Day	19% 6% 15% 40% 17% 2	4080
	21st to end	Day	21% 9% 19% 35% 13% 2	4109
September	1st to 10th	Day	30% 6% 11% 22% 20% 12%	2090
	11th to 20th	Day	33% 8% 10% 19% 22% 7%	2090
	21st to end	Day	24% 5% 10% 20% 29% 11%	2090
October	1st to 10th	Day	15% 2 6% 14% 33% 30%	3354
	11th to 20th	Day	17% 3 5% 20% 34% 21%	3140
	21st to end	Day	25% 4 6% 29% 27% 9%	3428
November	1st to 10th	Day	31% 5% 7% 23% 25% 9%	1958
	11th to 20th	Day	27% 4 9% 19% 29% 12%	1890
	21st to end	Day	35% 3 5% 20% 28% 10%	1800
December	1st to 10th	Day	37% 3 5% 28% 23% 4	2940
	11th to 20th	Day	43% 3 4 13% 26% 10%	3389
	21st to end	Day	41% 2 5% 23% 21% 7%	4488
January	1st to 10th	Night	31% 3 8% 19% 30% 8%	4238
	11th to 20th	Night	25% 3 9% 20% 31% 12%	4250
	21st to end	Night	42% 3 6% 15% 26% 9%	4546
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	9% 4 10% 31% 41% 6%	2826
	11th to 20th	Night	14% 4 14% 41% 22% 6%	2825
	21st to end	Night	17% 7% 20% 34% 20% 3	3540
September	1st to 10th	Night	26% 5% 11% 23% 24% 12%	2090
	11th to 20th	Night	29% 6% 11% 20% 23% 11%	2090
	21st to end	Night	21% 4 8% 23% 28% 16%	2090
October	1st to 10th	Night	12% 3 6% 14% 30% 35%	3571
	11th to 20th	Night	16% 2 5% 18% 34% 26%	3779
	21st to end	Night	25% 3 4 26% 26% 16%	4120
November	1st to 10th	Night	30% 3 6% 21% 25% 15%	2340
	11th to 20th	Night	26% 4 7% 19% 28% 16%	2505
	21st to end	Night	34% 3 8% 17% 28% 11%	2600
December	1st to 10th	Night	35% 3 6% 22% 27% 8%	4250
	11th to 20th	Night	40% 3 5% 14% 24% 14%	4817
	21st to end	Night	37% 3 6% 19% 26% 9%	6490

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	17% 11% 35% 27% 10%	300
	11th to 20th	Day	15% 13% 38% 23% 9% 2	300
	21st to end	Day	20% 8% 39% 21% 10% 1	300
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 3 26% 43% 18%	665
	21st to end	Day	4 1 4 22% 38% 31%	508
September	1st to 10th	Day	7% 1 2 14% 40% 36%	300
	11th to 20th	Day	3 2 4 25% 34% 32%	300
	21st to end	Day	9% 2 8% 20% 39% 21%	300
October	1st to 10th	Day	2 1 12% 30% 40% 15%	300
	11th to 20th	Day	10% 6% 21% 33% 25% 5%	300
	21st to end	Day	20% 9% 19% 28% 19% 5%	330
November	1st to 10th	Day	18% 6% 28% 27% 15% 5%	300
	11th to 20th	Day	12% 7% 29% 28% 19% 4	300
	21st to end	Day	16% 12% 33% 26% 11% 2	300
December	1st to 10th	Day	18% 7% 30% 31% 11% 2	300
	11th to 20th	Day	20% 8% 36% 26% 10% 1	300
	21st to end	Day	20% 8% 35% 28% 9% 1	330
January	1st to 10th	Night	13% 9% 38% 32% 8%	1000
	11th to 20th	Night	15% 13% 35% 31% 5% 1	1000
	21st to end	Night	15% 9% 37% 33% 6%	1051
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	5% 1 5% 37% 47% 5%	564
	21st to end	Night	4 1 4 37% 49% 6%	627
September	1st to 10th	Night	8% 1 6% 36% 43% 5%	570
	11th to 20th	Night	12% 2 6% 37% 40% 3	570
	21st to end	Night	7% 1 12% 41% 35% 5%	570
October	1st to 10th	Night	7% 1 16% 43% 29% 4	622
	11th to 20th	Night	13% 4 23% 40% 18% 1	760
	21st to end	Night	17% 7% 28% 32% 16% 1	836
November	1st to 10th	Night	11% 3 28% 40% 16% 1	780
	11th to 20th	Night	9% 9% 30% 39% 13% 1	905
	21st to end	Night	16% 9% 34% 30% 10% 1	1000
December	1st to 10th	Night	15% 8% 33% 36% 7%	1000
	11th to 20th	Night	19% 8% 33% 33% 7%	1000
	21st to end	Night	18% 8% 37% 31% 6%	1100

Page generated at 2025-01-19 02:06:43. Version ac07

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