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 2024-12-09.

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% 4 9% 5% 4 4 4 5% 7% 30%	101367
	Night	15% 13% 5% 10% 5% 4 4 3 5% 6% 30%	98351
Transparency	Day	51% 7% 12% 19% 10% 1	105043
	Night	53% 6% 11% 18% 10% 1	102814
Seeing	Day	23% 19% 29% 18% 9% 3	14666
	Night	22% 16% 28% 23% 10% 1	24450

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	16% 23% 11% 15% 4 3 4 2 4 4 14%	2836
	11th to 20th	Day	12% 24% 8% 16% 8% 6% 4 2 4 4 12%	2840
	21st to end	Day	23% 21% 12% 17% 7% 3 3 2 2 2 7%	3124
February	1st to 10th	Day	15% 21% 7% 13% 8% 5% 3 2 4 4 18%	1710
	11th to 20th	Day	14% 16% 8% 15% 9% 5% 4 3 4 4 18%	1691
	21st to end	Day	16% 18% 5% 13% 5% 4 5% 3 4 5% 20%	1570
March	1st to 10th	Day	20% 13% 7% 9% 5% 4 3 3 5% 6% 24%	3373
	11th to 20th	Day	12% 16% 6% 12% 5% 4 4 3 3 5% 31%	3480
	21st to end	Day	20% 13% 5% 10% 5% 4 3 3 4 5% 28%	3815
April	1st to 10th	Day	14% 15% 4 8% 4 5% 4 3 5% 6% 31%	2144
	11th to 20th	Day	19% 12% 3 8% 4 4 4 4 5% 6% 32%	2394
	21st to end	Day	19% 14% 3 7% 3 4 3 3 4 6% 34%	2470
May	1st to 10th	Day	19% 9% 3 9% 5% 5% 5% 4 4 7% 31%	4082
	11th to 20th	Day	19% 9% 4 7% 4 4 4 4 6% 7% 33%	4081
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	4635
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 6% 8% 36%	2660
	11th to 20th	Day	13% 7% 2 7% 5% 4 5% 4 6% 8% 38%	2660
	21st to end	Day	12% 7% 3 5% 4 4 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 4 7% 9% 39%	4219
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 7% 10% 43%	4120
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	4466
August	1st to 10th	Day	14% 7% 2 7% 4 5% 5% 5% 7% 9% 35%	4104
	11th to 20th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 34%	4044
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	4441
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 41%	2207
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 6% 8% 42%	2090
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 36%	2090
October	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 3 7% 7% 37%	1948
	11th to 20th	Day	22% 10% 3 8% 4 5% 4 2 5% 6% 31%	1900
	21st to end	Day	22% 14% 3 8% 5% 4 4 3 5% 6% 25%	2090
November	1st to 10th	Day	12% 13% 4 9% 6% 4 4 4 5% 6% 34%	1771
	11th to 20th	Day	17% 20% 5% 8% 5% 4 4 3 4 6% 24%	1724
	21st to end	Day	17% 22% 7% 12% 5% 3 5% 3 4 6% 16%	1600
December	1st to 10th	Day	15% 24% 10% 14% 7% 4 3 3 4 4 13%	1584
	11th to 20th	Day	16% 21% 8% 17% 6% 4 4 3 4 5% 13%	1910
	21st to end	Day	22% 26% 9% 12% 5% 4 3 2 4 3 9%	2834
January	1st to 10th	Night	14% 24% 12% 18% 6% 4 3 2 3 4 10%	4109
	11th to 20th	Night	13% 20% 10% 21% 6% 4 5% 4 3 3 10%	4120
	21st to end	Night	23% 21% 9% 16% 7% 3 3 2 2 3 11%	4532
February	1st to 10th	Night	18% 17% 5% 15% 7% 5% 4 3 4 4 17%	2470
	11th to 20th	Night	15% 15% 7% 15% 7% 5% 4 4 4 4 19%	2371
	21st to end	Night	19% 13% 6% 13% 7% 5% 4 2 4 4 23%	1884
March	1st to 10th	Night	20% 16% 4 8% 4 3 3 3 3 4 31%	3478
	11th to 20th	Night	14% 12% 8% 10% 5% 4 4 3 4 5% 31%	3480
	21st to end	Night	18% 13% 5% 10% 5% 4 3 3 4 4 32%	3817
April	1st to 10th	Night	17% 15% 4 8% 5% 4 3 4 4 6% 30%	2035
	11th to 20th	Night	16% 11% 2 7% 5% 4 5% 4 6% 6% 33%	1750
	21st to end	Night	15% 11% 3 7% 5% 4 5% 4 5% 6% 35%	1710
May	1st to 10th	Night	23% 10% 3 6% 4 4 5% 3 5% 7% 29%	2813
	11th to 20th	Night	14% 11% 3 7% 4 4 5% 3 6% 8% 35%	2812
	21st to end	Night	12% 5% 2 4 3 4 5% 3 7% 11% 44%	2902
June	1st to 10th	Night	12% 7% 3 6% 5% 5% 6% 4 5% 7% 41%	1520
	11th to 20th	Night	9% 6% 3 8% 4 6% 5% 4 6% 8% 42%	1520
	21st to end	Night	10% 6% 2 8% 3 5% 5% 4 7% 11% 40%	1520
July	1st to 10th	Night	10% 7% 2 6% 3 4 4 3 5% 9% 47%	2735
	11th to 20th	Night	10% 5% 2 5% 4 3 4 4 6% 7% 49%	2840
	21st to end	Night	8% 7% 2 6% 5% 5% 5% 4 6% 8% 45%	3087
August	1st to 10th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 7% 40%	2825
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 6% 41%	2785
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 6% 7% 7% 43%	3183
September	1st to 10th	Night	12% 7% 2 5% 5% 4 5% 4 5% 7% 45%	1970
	11th to 20th	Night	10% 5% 1 5% 4 4 3 4 5% 6% 51%	2090
	21st to end	Night	17% 8% 2 7% 6% 4 4 4 4 5% 38%	2090
October	1st to 10th	Night	14% 7% 3 6% 4 4 3 4 6% 7% 42%	2232
	11th to 20th	Night	18% 10% 3 8% 4 5% 4 4 5% 7% 33%	2280
	21st to end	Night	24% 10% 4 7% 4 4 4 3 5% 6% 28%	2508
November	1st to 10th	Night	14% 11% 3 8% 5% 4 4 3 5% 6% 37%	2526
	11th to 20th	Night	16% 19% 5% 11% 5% 4 4 2 5% 5% 24%	2674
	21st to end	Night	18% 20% 5% 10% 5% 4 5% 3 5% 5% 20%	2800
December	1st to 10th	Night	18% 23% 9% 11% 5% 4 4 3 5% 4 15%	2779
	11th to 20th	Night	16% 21% 7% 14% 6% 4 4 3 4 5% 17%	3285
	21st to end	Night	22% 24% 9% 12% 6% 4 3 2 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	74% 6% 10% 8% 2	2837
	11th to 20th	Day	76% 5% 9% 8% 2	2840
	21st to end	Day	85% 4 4 5% 2	3124
February	1st to 10th	Day	71% 4 9% 11% 5%	1710
	11th to 20th	Day	69% 6% 8% 11% 6%	1700
	21st to end	Day	64% 6% 11% 11% 6% 2	1570
March	1st to 10th	Day	58% 7% 11% 21% 4	3373
	11th to 20th	Day	55% 7% 15% 17% 6% 1	3480
	21st to end	Day	57% 7% 8% 17% 10% 1	3815
April	1st to 10th	Day	52% 6% 13% 16% 12% 1	2144
	11th to 20th	Day	52% 6% 10% 17% 13% 2	2394
	21st to end	Day	52% 6% 10% 20% 10% 2	2470
May	1st to 10th	Day	52% 7% 9% 22% 8% 1	4120
	11th to 20th	Day	48% 8% 12% 22% 9% 1	4085
	21st to end	Day	39% 10% 13% 18% 15% 5%	4649
June	1st to 10th	Day	44% 7% 12% 24% 12% 1	2660
	11th to 20th	Day	41% 9% 14% 20% 14% 2	2660
	21st to end	Day	39% 7% 13% 25% 14% 2	2660
July	1st to 10th	Day	39% 6% 13% 24% 15% 3	4154
	11th to 20th	Day	35% 7% 12% 27% 17% 1	4051
	21st to end	Day	39% 8% 15% 19% 19% 1	4324
August	1st to 10th	Day	44% 8% 14% 23% 11%	4091
	11th to 20th	Day	42% 9% 12% 29% 7% 1	4083
	21st to end	Day	38% 7% 15% 28% 11% 1	4441
September	1st to 10th	Day	41% 7% 15% 20% 15% 3	2207
	11th to 20th	Day	38% 6% 16% 22% 16% 1	2090
	21st to end	Day	48% 5% 11% 22% 12% 1	2090
October	1st to 10th	Day	34% 7% 14% 26% 19% 1	3234
	11th to 20th	Day	48% 5% 10% 19% 15% 3	3140
	21st to end	Day	46% 5% 14% 23% 12% 1	3428
November	1st to 10th	Day	49% 6% 12% 19% 13% 1	1771
	11th to 20th	Day	61% 5% 11% 13% 7% 2	1724
	21st to end	Day	68% 7% 9% 12% 5%	1600
December	1st to 10th	Day	76% 5% 8% 9% 2	1584
	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	77% 6% 7% 8% 3	4109
	11th to 20th	Night	78% 7% 7% 7% 1	4120
	21st to end	Night	80% 3 6% 8% 2	4532
February	1st to 10th	Night	69% 5% 10% 12% 4	2470
	11th to 20th	Night	68% 5% 9% 13% 5%	2373
	21st to end	Night	65% 6% 9% 13% 7% 1	1884
March	1st to 10th	Night	55% 6% 12% 19% 8%	3478
	11th to 20th	Night	52% 8% 15% 16% 8% 1	3480
	21st to end	Night	52% 8% 9% 19% 10% 1	3817
April	1st to 10th	Night	55% 6% 11% 15% 11% 2	2035
	11th to 20th	Night	49% 6% 11% 19% 13% 2	1750
	21st to end	Night	47% 6% 12% 20% 12% 3	1710
May	1st to 10th	Night	53% 7% 10% 19% 9% 2	2840
	11th to 20th	Night	44% 8% 13% 21% 12% 2	2813
	21st to end	Night	31% 8% 16% 25% 17% 3	2909
June	1st to 10th	Night	40% 8% 11% 24% 15% 2	1520
	11th to 20th	Night	38% 8% 13% 21% 18% 2	1520
	21st to end	Night	37% 9% 12% 24% 17% 1	1520
July	1st to 10th	Night	37% 6% 12% 25% 17% 4	2690
	11th to 20th	Night	32% 7% 12% 27% 19% 2	2786
	21st to end	Night	36% 8% 13% 18% 23% 2	2988
August	1st to 10th	Night	42% 7% 12% 25% 13% 1	2814
	11th to 20th	Night	42% 6% 10% 29% 11% 1	2812
	21st to end	Night	35% 8% 11% 29% 14% 2	3183
September	1st to 10th	Night	36% 8% 13% 24% 16% 3	1970
	11th to 20th	Night	31% 6% 16% 25% 20% 2	2090
	21st to end	Night	47% 6% 10% 21% 16% 1	2090
October	1st to 10th	Night	35% 5% 10% 27% 22% 1	3726
	11th to 20th	Night	45% 7% 10% 19% 16% 3	3779
	21st to end	Night	43% 6% 13% 23% 14% 1	4120
November	1st to 10th	Night	46% 7% 13% 20% 11% 2	2526
	11th to 20th	Night	61% 5% 10% 15% 6% 3	2674
	21st to end	Night	65% 6% 10% 14% 4	2800
December	1st to 10th	Night	71% 6% 10% 8% 4	2779
	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	31% 34% 27% 6% 2	270
	21st to end	Day	29% 35% 27% 7% 2	297
February	1st to 10th	Day	37% 23% 33% 5% 1	270
	11th to 20th	Day	33% 35% 25% 6%	270
	21st to end	Day	40% 32% 25% 4	225
March	1st to 10th	Day	30% 25% 30% 12% 3	420
	11th to 20th	Day	30% 24% 32% 11% 3 1	460
	21st to end	Day	35% 25% 25% 13% 3	506
April	1st to 10th	Day	27% 26% 29% 13% 5% 1	460
	11th to 20th	Day	33% 21% 24% 12% 8% 1	452
	21st to end	Day	28% 23% 30% 13% 5% 1	460
May	1st to 10th	Day	27% 22% 29% 17% 4 2	463
	11th to 20th	Day	22% 15% 31% 22% 7% 2	487
	21st to end	Day	15% 11% 32% 25% 13% 4	529
June	1st to 10th	Day	16% 16% 36% 21% 10% 2	490
	11th to 20th	Day	16% 14% 30% 28% 11% 1	490
	21st to end	Day	15% 13% 36% 24% 9% 3	490
July	1st to 10th	Day	12% 8% 25% 29% 19% 7%	485
	11th to 20th	Day	14% 9% 30% 25% 17% 5%	484
	21st to end	Day	10% 9% 32% 28% 15% 6%	532
August	1st to 10th	Day	9% 11% 25% 29% 19% 6%	486
	11th to 20th	Day	13% 10% 30% 26% 14% 8%	479
	21st to end	Day	11% 9% 27% 28% 17% 8%	539
September	1st to 10th	Day	16% 10% 24% 26% 18% 6%	490
	11th to 20th	Day	10% 7% 23% 32% 20% 9%	490
	21st to end	Day	23% 15% 26% 20% 12% 4	490
October	1st to 10th	Day	15% 17% 29% 20% 16% 3	490
	11th to 20th	Day	26% 22% 31% 15% 5% 1	490
	21st to end	Day	34% 20% 29% 14% 3	539
November	1st to 10th	Day	26% 23% 29% 14% 6% 1	309
	11th to 20th	Day	35% 22% 28% 11% 3	262
	21st to end	Day	33% 25% 36% 6% 1	200
December	1st to 10th	Day	27% 34% 26% 10% 2 2	196
	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	760
	21st to end	Night	26% 26% 31% 15% 2	836
February	1st to 10th	Night	32% 22% 28% 16% 2	760
	11th to 20th	Night	29% 23% 32% 15% 1	743
	21st to end	Night	31% 24% 30% 12% 1	628
March	1st to 10th	Night	25% 22% 30% 19% 3	760
	11th to 20th	Night	26% 23% 28% 17% 6% 1	760
	21st to end	Night	26% 19% 30% 19% 5%	836
April	1st to 10th	Night	25% 18% 31% 21% 5%	760
	11th to 20th	Night	24% 16% 35% 17% 7% 1	626
	21st to end	Night	19% 15% 35% 25% 6%	570
May	1st to 10th	Night	23% 12% 28% 28% 9% 1	570
	11th to 20th	Night	19% 11% 28% 32% 9% 2	561
	21st to end	Night	11% 5% 29% 36% 18% 1	517
June	1st to 10th	Night	12% 6% 34% 37% 12% 1	380
	11th to 20th	Night	11% 7% 31% 39% 13% 1	380
	21st to end	Night	10% 9% 27% 39% 14%	380
July	1st to 10th	Night	8% 4 19% 39% 26% 4	489
	11th to 20th	Night	9% 5% 23% 36% 24% 3	552
	21st to end	Night	9% 6% 18% 41% 23% 2	624
August	1st to 10th	Night	10% 5% 18% 40% 24% 3	567
	11th to 20th	Night	11% 6% 17% 37% 26% 4	564
	21st to end	Night	12% 5% 21% 31% 28% 3	660
September	1st to 10th	Night	15% 8% 19% 31% 24% 4	760
	11th to 20th	Night	13% 3 21% 34% 25% 4	760
	21st to end	Night	19% 10% 24% 27% 16% 4	760
October	1st to 10th	Night	17% 10% 29% 25% 16% 2	760
	11th to 20th	Night	23% 19% 26% 23% 8% 1	760
	21st to end	Night	31% 17% 31% 18% 4	836
November	1st to 10th	Night	21% 18% 29% 24% 7% 1	780
	11th to 20th	Night	29% 24% 28% 14% 5% 1	800
	21st to end	Night	33% 22% 28% 13% 3	800
December	1st to 10th	Night	32% 25% 28% 11% 4	795
	11th to 20th	Night	29% 27% 29% 13% 2	760
	21st to end	Night	33% 23% 31% 12% 1	836

Page generated at 2024-12-06 03:54:46. Version ac07

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