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 2026-04-02.

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	15% 12% 5% 10% 5% 4 4 3 5% 6% 29%	131185
	Night	15% 14% 5% 11% 5% 4 4 3 5% 6% 28%	130624
Transparency	Day	54% 6% 11% 18% 10% 1	134817
	Night	55% 6% 10% 17% 10% 1	135062
Seeing	Day	24% 19% 29% 18% 9% 3	16312
	Night	23% 17% 27% 23% 10% 1	26188

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	14% 21% 8% 27% 6% 3 3 2 3 3 10%	5258
	11th to 20th	Day	9% 20% 8% 23% 11% 7% 5% 2 3 3 9%	5280
	21st to end	Day	17% 17% 9% 15% 10% 8% 4 2 3 3 12%	5780
February	1st to 10th	Day	15% 20% 7% 14% 8% 6% 3 2 4 4 19%	1890
	11th to 20th	Day	14% 16% 8% 16% 9% 5% 4 3 4 4 18%	1875
	21st to end	Day	15% 18% 5% 14% 5% 4 5% 4 4 5% 20%	1730
March	1st to 10th	Day	16% 11% 8% 12% 5% 4 3 3 5% 6% 27%	4953
	11th to 20th	Day	12% 13% 5% 12% 5% 4 4 3 4 5% 33%	5090
	21st to end	Day	19% 14% 6% 10% 5% 4 3 3 4 5% 27%	5586
April	1st to 10th	Day	16% 15% 4 8% 4 5% 4 3 5% 6% 30%	2268
	11th to 20th	Day	18% 13% 3 8% 4 4 4 4 5% 6% 32%	2522
	21st to end	Day	19% 14% 3 7% 3 4 3 3 4 6% 34%	2600
May	1st to 10th	Day	21% 10% 3 9% 5% 5% 5% 3 4 6% 30%	5862
	11th to 20th	Day	19% 10% 3 7% 4 4 4 4 6% 7% 32%	5861
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 6% 7% 34%	6662
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 5% 8% 36%	2800
	11th to 20th	Day	14% 7% 2 7% 5% 4 5% 4 6% 8% 37%	2800
	21st to end	Day	12% 7% 2 5% 4 4 5% 5% 7% 10% 40%	2800
July	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 9% 43%	6041
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 5% 7% 10% 38%	5900
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	6424
August	1st to 10th	Day	13% 7% 2 7% 4 5% 5% 5% 7% 9% 35%	4234
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 8% 34%	4174
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	4581
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 42%	2323
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 6% 8% 43%	2200
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 36%	2200
October	1st to 10th	Day	16% 7% 2 7% 4 3 5% 3 6% 7% 39%	2050
	11th to 20th	Day	21% 10% 3 7% 4 5% 4 2 5% 7% 32%	2000
	21st to end	Day	22% 14% 3 8% 5% 4 4 3 5% 6% 26%	2200
November	1st to 10th	Day	13% 13% 4 9% 6% 4 4 4 5% 6% 34%	1861
	11th to 20th	Day	17% 21% 5% 9% 5% 4 4 3 4 6% 24%	1810
	21st to end	Day	17% 22% 8% 11% 6% 3 5% 3 4 6% 15%	1680
December	1st to 10th	Day	16% 19% 11% 17% 6% 4 3 3 4 4 13%	2680
	11th to 20th	Day	21% 22% 8% 12% 7% 4 3 3 4 5% 11%	3070
	21st to end	Day	22% 26% 8% 15% 5% 4 3 2 3 3 8%	4140
January	1st to 10th	Night	14% 20% 10% 27% 6% 4 3 2 3 3 9%	7635
	11th to 20th	Night	9% 19% 10% 21% 11% 6% 5% 3 3 3 10%	7680
	21st to end	Night	16% 18% 8% 13% 7% 6% 5% 3 4 5% 16%	8401
February	1st to 10th	Night	17% 17% 5% 16% 7% 5% 4 3 4 4 18%	2730
	11th to 20th	Night	15% 15% 7% 15% 8% 6% 4 4 4 4 19%	2627
	21st to end	Night	18% 13% 6% 13% 7% 5% 4 3 4 4 23%	2076
March	1st to 10th	Night	19% 16% 5% 9% 5% 4 3 3 4 4 30%	5118
	11th to 20th	Night	12% 11% 7% 9% 4 4 4 3 5% 6% 36%	5090
	21st to end	Night	18% 13% 5% 10% 5% 4 4 3 4 4 31%	5588
April	1st to 10th	Night	18% 15% 5% 8% 5% 3 3 4 4 5% 29%	2154
	11th to 20th	Night	15% 11% 3 7% 5% 4 5% 4 6% 6% 33%	1843
	21st to end	Night	15% 11% 3 7% 5% 4 5% 4 5% 7% 36%	1800
May	1st to 10th	Night	24% 11% 3 6% 4 4 5% 3 5% 6% 27%	4033
	11th to 20th	Night	16% 11% 3 7% 4 4 4 3 5% 8% 36%	4032
	21st to end	Night	18% 8% 2 5% 3 4 4 3 6% 9% 38%	4174
June	1st to 10th	Night	12% 7% 3 6% 5% 5% 6% 4 6% 6% 41%	1600
	11th to 20th	Night	9% 6% 3 8% 4 6% 5% 4 6% 8% 41%	1600
	21st to end	Night	9% 6% 2 7% 3 5% 5% 4 7% 10% 41%	1600
July	1st to 10th	Night	9% 6% 2 6% 3 4 4 4 5% 9% 45%	3911
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	4057
	21st to end	Night	8% 6% 2 6% 4 4 4 3 5% 8% 49%	4427
August	1st to 10th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 8% 40%	2915
	11th to 20th	Night	15% 8% 3 7% 4 4 4 3 5% 6% 42%	2875
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 6% 7% 7% 43%	3284
September	1st to 10th	Night	12% 6% 2 5% 5% 4 4 4 5% 7% 46%	2074
	11th to 20th	Night	9% 5% 1 5% 4 4 3 4 5% 6% 53%	2200
	21st to end	Night	17% 8% 2 7% 6% 4 4 4 5% 5% 38%	2200
October	1st to 10th	Night	13% 7% 3 6% 4 4 3 4 5% 7% 43%	2350
	11th to 20th	Night	17% 10% 3 8% 4 5% 4 4 5% 7% 34%	2400
	21st to end	Night	24% 10% 4 7% 4 4 4 3 5% 6% 29%	2640
November	1st to 10th	Night	14% 11% 3 8% 5% 4 4 3 5% 6% 37%	2656
	11th to 20th	Night	15% 19% 5% 11% 4 4 4 3 5% 6% 24%	2808
	21st to end	Night	18% 20% 6% 9% 5% 4 5% 3 5% 5% 20%	2940
December	1st to 10th	Night	14% 20% 9% 15% 6% 5% 4 3 4 4 15%	4720
	11th to 20th	Night	19% 22% 9% 12% 6% 3 3 3 3 4 15%	5345
	21st to end	Night	20% 24% 8% 13% 5% 4 4 2 4 4 11%	7041

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	81% 4 7% 5% 2 1	5259
	11th to 20th	Day	81% 4 6% 6% 3	5280
	21st to end	Day	78% 4 5% 10% 3	5780
February	1st to 10th	Day	70% 4 8% 12% 6%	1890
	11th to 20th	Day	69% 6% 8% 11% 5%	1884
	21st to end	Day	64% 6% 11% 11% 6% 2	1730
March	1st to 10th	Day	57% 6% 12% 19% 6%	4953
	11th to 20th	Day	52% 6% 15% 20% 6% 1	5090
	21st to end	Day	59% 6% 7% 17% 10% 1	5586
April	1st to 10th	Day	53% 6% 12% 16% 12% 1	2268
	11th to 20th	Day	53% 6% 10% 17% 12% 2	2522
	21st to end	Day	52% 5% 10% 20% 11% 2	2600
May	1st to 10th	Day	54% 7% 8% 19% 9% 3	5897
	11th to 20th	Day	50% 7% 10% 21% 10% 1	5865
	21st to end	Day	46% 8% 12% 19% 12% 3	6677
June	1st to 10th	Day	44% 7% 12% 24% 13% 1	2800
	11th to 20th	Day	42% 9% 14% 20% 14% 1	2800
	21st to end	Day	38% 8% 13% 24% 15% 2	2800
July	1st to 10th	Day	37% 5% 12% 28% 16% 2	5976
	11th to 20th	Day	39% 8% 13% 25% 15% 1	5831
	21st to end	Day	37% 7% 12% 22% 20% 1	6240
August	1st to 10th	Day	43% 8% 15% 23% 11%	4221
	11th to 20th	Day	42% 8% 12% 29% 8% 1	4213
	21st to end	Day	37% 7% 15% 28% 11% 1	4581
September	1st to 10th	Day	40% 7% 14% 21% 15% 3	2323
	11th to 20th	Day	36% 6% 16% 25% 16% 1	2200
	21st to end	Day	48% 5% 11% 22% 12% 1	2200
October	1st to 10th	Day	33% 7% 14% 26% 19% 1	3336
	11th to 20th	Day	47% 5% 11% 19% 16% 3	3240
	21st to end	Day	46% 5% 13% 23% 12% 1	3538
November	1st to 10th	Day	49% 6% 11% 20% 12% 1	1861
	11th to 20th	Day	62% 5% 11% 13% 7% 2	1810
	21st to end	Day	69% 6% 9% 12% 5%	1680
December	1st to 10th	Day	74% 7% 9% 8% 2	2680
	11th to 20th	Day	75% 5% 7% 10% 2	3066
	21st to end	Day	82% 4 6% 6% 1	4140
January	1st to 10th	Night	81% 5% 5% 7% 3	7636
	11th to 20th	Night	79% 6% 6% 6% 2 1	7680
	21st to end	Night	71% 4 8% 12% 5%	8401
February	1st to 10th	Night	69% 5% 9% 13% 4	2730
	11th to 20th	Night	68% 5% 9% 13% 5%	2629
	21st to end	Night	64% 5% 9% 13% 7% 1	2076
March	1st to 10th	Night	57% 5% 13% 17% 8%	5118
	11th to 20th	Night	47% 6% 14% 22% 9% 1	5090
	21st to end	Night	54% 6% 9% 17% 12% 1	5588
April	1st to 10th	Night	56% 6% 11% 15% 11% 2	2154
	11th to 20th	Night	49% 7% 11% 19% 13% 2	1843
	21st to end	Night	47% 6% 12% 20% 13% 3	1800
May	1st to 10th	Night	57% 6% 9% 14% 9% 5%	4060
	11th to 20th	Night	45% 7% 11% 22% 13% 2	4033
	21st to end	Night	41% 7% 13% 22% 14% 2	4181
June	1st to 10th	Night	40% 8% 11% 23% 15% 2	1600
	11th to 20th	Night	39% 8% 13% 21% 17% 2	1600
	21st to end	Night	36% 9% 12% 24% 17% 1	1600
July	1st to 10th	Night	36% 6% 12% 25% 18% 3	3868
	11th to 20th	Night	36% 7% 13% 26% 16% 2	4006
	21st to end	Night	34% 7% 12% 20% 26% 2	4297
August	1st to 10th	Night	41% 7% 12% 25% 14% 1	2904
	11th to 20th	Night	42% 6% 10% 29% 11% 1	2902
	21st to end	Night	35% 8% 12% 29% 14% 2	3284
September	1st to 10th	Night	36% 7% 13% 26% 16% 3	2074
	11th to 20th	Night	30% 6% 16% 26% 20% 2	2200
	21st to end	Night	47% 6% 10% 20% 16% 1	2200
October	1st to 10th	Night	34% 5% 10% 27% 22% 1	3844
	11th to 20th	Night	45% 7% 10% 19% 17% 2	3899
	21st to end	Night	43% 5% 12% 23% 15% 1	4252
November	1st to 10th	Night	46% 7% 13% 21% 11% 2	2656
	11th to 20th	Night	61% 5% 11% 14% 6% 3	2808
	21st to end	Night	66% 6% 10% 14% 4	2940
December	1st to 10th	Night	70% 7% 11% 9% 3	4720
	11th to 20th	Night	73% 5% 9% 12% 2	5348
	21st to end	Night	76% 5% 9% 9% 1	7041

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	34% 30% 27% 7% 2	330
	11th to 20th	Day	35% 34% 25% 5% 2	330
	21st to end	Day	32% 33% 27% 6% 1	363
February	1st to 10th	Day	35% 25% 33% 6% 1	330
	11th to 20th	Day	35% 32% 26% 6%	330
	21st to end	Day	38% 32% 26% 3	273
March	1st to 10th	Day	30% 27% 29% 10% 3	496
	11th to 20th	Day	32% 26% 29% 10% 2 1	540
	21st to end	Day	35% 25% 26% 11% 3	594
April	1st to 10th	Day	30% 25% 28% 12% 4 1	504
	11th to 20th	Day	34% 21% 24% 13% 7% 1	492
	21st to end	Day	28% 23% 30% 14% 4 1	500
May	1st to 10th	Day	28% 21% 30% 16% 4 2	503
	11th to 20th	Day	23% 15% 31% 22% 7% 2	527
	21st to end	Day	18% 12% 31% 23% 12% 4	573
June	1st to 10th	Day	17% 16% 36% 20% 9% 2	530
	11th to 20th	Day	18% 14% 29% 27% 10% 1	530
	21st to end	Day	14% 13% 34% 26% 10% 3	530
July	1st to 10th	Day	12% 8% 25% 30% 19% 6%	525
	11th to 20th	Day	14% 10% 30% 26% 16% 5%	524
	21st to end	Day	10% 9% 31% 28% 15% 7%	576
August	1st to 10th	Day	8% 11% 25% 31% 19% 6%	526
	11th to 20th	Day	13% 10% 29% 26% 15% 8%	519
	21st to end	Day	11% 9% 28% 28% 16% 8%	583
September	1st to 10th	Day	16% 11% 23% 25% 19% 6%	530
	11th to 20th	Day	9% 7% 22% 32% 20% 10%	530
	21st to end	Day	23% 14% 26% 21% 12% 4	530
October	1st to 10th	Day	15% 16% 28% 22% 17% 3	530
	11th to 20th	Day	25% 22% 33% 15% 4 1	530
	21st to end	Day	34% 20% 29% 14% 3	583
November	1st to 10th	Day	28% 23% 29% 13% 6% 1	339
	11th to 20th	Day	37% 21% 27% 11% 3	288
	21st to end	Day	35% 25% 35% 5%	220
December	1st to 10th	Day	28% 33% 26% 10% 1 1	220
	11th to 20th	Day	36% 27% 29% 7% 1	220
	21st to end	Day	35% 30% 27% 7%	264
January	1st to 10th	Night	28% 29% 31% 10% 3	840
	11th to 20th	Night	33% 26% 28% 11% 2	840
	21st to end	Night	27% 27% 30% 15% 2	924
February	1st to 10th	Night	32% 22% 28% 17% 2	840
	11th to 20th	Night	30% 23% 31% 14% 1	823
	21st to end	Night	31% 25% 31% 12% 1	692
March	1st to 10th	Night	26% 23% 31% 18% 3	840
	11th to 20th	Night	27% 23% 28% 16% 5% 1	840
	21st to end	Night	26% 20% 30% 19% 5%	924
April	1st to 10th	Night	27% 18% 31% 20% 5%	805
	11th to 20th	Night	24% 17% 34% 18% 7% 1	659
	21st to end	Night	19% 16% 35% 25% 6%	600
May	1st to 10th	Night	24% 11% 29% 27% 8% 1	600
	11th to 20th	Night	19% 11% 29% 31% 8% 2	591
	21st to end	Night	12% 6% 28% 35% 17% 1	545
June	1st to 10th	Night	12% 6% 34% 36% 12% 1	400
	11th to 20th	Night	11% 7% 31% 38% 13% 1	400
	21st to end	Night	10% 9% 27% 39% 15% 1	400
July	1st to 10th	Night	7% 5% 18% 40% 26% 4	516
	11th to 20th	Night	9% 4 23% 36% 24% 3	582
	21st to end	Night	10% 6% 18% 41% 23% 2	657
August	1st to 10th	Night	10% 5% 18% 41% 24% 3	597
	11th to 20th	Night	11% 6% 16% 36% 27% 3	594
	21st to end	Night	12% 5% 21% 32% 27% 3	695
September	1st to 10th	Night	14% 7% 20% 32% 24% 3	800
	11th to 20th	Night	12% 3 20% 34% 26% 4	800
	21st to end	Night	19% 10% 23% 28% 16% 3	800
October	1st to 10th	Night	17% 10% 28% 26% 17% 2	800
	11th to 20th	Night	23% 19% 27% 23% 8% 1	800
	21st to end	Night	31% 17% 31% 18% 3	880
November	1st to 10th	Night	22% 18% 29% 23% 7% 1	820
	11th to 20th	Night	29% 23% 28% 15% 5% 1	840
	21st to end	Night	34% 22% 28% 13% 3	840
December	1st to 10th	Night	32% 25% 28% 12% 4	840
	11th to 20th	Night	30% 28% 28% 13% 2	840
	21st to end	Night	33% 23% 31% 12% 1	924

Page generated at 2026-03-30 02:18:45. Version ac07

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