Clarke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clarke Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	104673
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	104282
Transparency	Day	51% 6% 11% 20% 10% 1	108349
	Night	52% 6% 11% 20% 11% 1	108745
Seeing	Day	20% 13% 28% 21% 13% 5%	14797
	Night	20% 10% 28% 28% 12% 2	24696

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	18% 22% 7% 15% 5% 4 4 3 3 5% 14%	2890
	11th to 20th	Day	13% 23% 9% 13% 6% 6% 3 3 3 5% 15%	2840
	21st to end	Day	25% 21% 10% 13% 6% 4 3 2 3 3 9%	3124
February	1st to 10th	Day	18% 18% 6% 12% 8% 4 3 2 4 4 21%	1710
	11th to 20th	Day	16% 13% 7% 14% 7% 4 4 4 5% 5% 21%	1696
	21st to end	Day	18% 15% 5% 11% 6% 4 3 4 4 6% 24%	1570
March	1st to 10th	Day	21% 12% 6% 9% 5% 4 3 3 4 6% 25%	3395
	11th to 20th	Day	12% 12% 4 11% 6% 5% 5% 3 4 5% 33%	3480
	21st to end	Day	20% 11% 4 8% 5% 4 4 3 5% 5% 30%	3815
April	1st to 10th	Day	14% 13% 3 8% 6% 4 4 4 5% 7% 31%	2143
	11th to 20th	Day	18% 11% 4 8% 4 4 4 4 5% 6% 33%	2371
	21st to end	Day	19% 11% 3 7% 5% 4 4 4 5% 6% 32%	2470
May	1st to 10th	Day	21% 10% 4 7% 5% 5% 5% 4 5% 6% 28%	4082
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 4 5% 7% 31%	4081
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 4 7% 9% 39%	4566
June	1st to 10th	Day	13% 8% 2 7% 5% 5% 6% 4 6% 9% 35%	2660
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 37%	2660
July	1st to 10th	Day	9% 5% 3 7% 5% 5% 5% 5% 7% 10% 39%	4129
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 4 7% 11% 41%	4120
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 5% 7% 9% 36%	4466
August	1st to 10th	Day	13% 9% 3 7% 6% 5% 5% 5% 7% 9% 31%	4104
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 9% 33%	4044
	21st to end	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 10% 40%	4413
September	1st to 10th	Day	14% 6% 3 5% 5% 4 4 3 5% 7% 44%	2208
	11th to 20th	Day	11% 6% 3 6% 4 4 5% 4 5% 8% 43%	2090
	21st to end	Day	19% 8% 3 7% 4 4 4 4 4 6% 37%	2090
October	1st to 10th	Day	15% 7% 2 7% 4 4 5% 4 6% 8% 39%	1945
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 3 5% 5% 34%	1900
	21st to end	Day	21% 12% 3 7% 5% 5% 3 3 4 7% 30%	2090
November	1st to 10th	Day	11% 9% 3 9% 5% 4 5% 4 4 6% 40%	1795
	11th to 20th	Day	15% 16% 4 8% 5% 4 4 3 5% 6% 29%	1750
	21st to end	Day	15% 17% 8% 9% 5% 5% 5% 4 5% 5% 22%	1600
December	1st to 10th	Day	19% 15% 8% 15% 6% 4 3 3 4 4 19%	2600
	11th to 20th	Day	22% 19% 7% 12% 4 4 3 2 4 5% 18%	2990
	21st to end	Day	23% 23% 6% 13% 6% 4 4 3 3 5% 11%	4126
January	1st to 10th	Night	15% 21% 7% 15% 6% 4 4 3 4 5% 16%	4188
	11th to 20th	Night	14% 19% 8% 17% 6% 5% 6% 3 4 4 13%	4120
	21st to end	Night	26% 18% 8% 13% 7% 4 3 2 2 3 14%	4532
February	1st to 10th	Night	20% 15% 5% 11% 6% 5% 4 3 4 6% 22%	2470
	11th to 20th	Night	17% 14% 5% 12% 6% 4 5% 4 5% 5% 23%	2365
	21st to end	Night	20% 14% 5% 11% 7% 4 5% 3 4 5% 24%	1884
March	1st to 10th	Night	22% 14% 4 6% 4 3 3 3 4 5% 33%	3456
	11th to 20th	Night	17% 11% 5% 8% 4 3 4 4 5% 6% 33%	3480
	21st to end	Night	17% 12% 4 8% 5% 3 3 3 4 5% 37%	3817
April	1st to 10th	Night	16% 13% 3 7% 5% 5% 3 4 5% 7% 33%	2035
	11th to 20th	Night	15% 9% 3 7% 6% 5% 5% 3 5% 7% 34%	1773
	21st to end	Night	16% 9% 3 7% 4 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	25% 10% 3 7% 5% 4 4 3 5% 6% 27%	2813
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 3 5% 7% 37%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 7% 11% 43%	2984
June	1st to 10th	Night	9% 6% 2 8% 6% 5% 4 3 5% 8% 43%	1520
	11th to 20th	Night	9% 6% 3 7% 4 5% 5% 4 6% 9% 43%	1520
	21st to end	Night	9% 6% 3 6% 4 4 5% 5% 7% 8% 43%	1520
July	1st to 10th	Night	9% 7% 3 6% 5% 5% 4 4 5% 9% 42%	2826
	11th to 20th	Night	10% 5% 2 4 5% 4 4 4 7% 9% 47%	2840
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 5% 5% 8% 43%	3087
August	1st to 10th	Night	11% 8% 3 8% 5% 5% 4 4 7% 8% 37%	2825
	11th to 20th	Night	12% 9% 3 6% 3 4 4 3 6% 7% 43%	2785
	21st to end	Night	8% 4 2 7% 5% 6% 6% 4 7% 8% 44%	3210
September	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 5% 6% 49%	1968
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 8% 52%	2090
	21st to end	Night	16% 8% 2 5% 4 3 4 3 5% 7% 40%	2090
October	1st to 10th	Night	14% 5% 3 6% 4 3 4 4 5% 8% 44%	2233
	11th to 20th	Night	16% 8% 4 8% 5% 5% 5% 3 5% 6% 36%	2280
	21st to end	Night	21% 10% 2 6% 4 4 4 4 5% 6% 34%	2508
November	1st to 10th	Night	14% 9% 2 6% 4 5% 3 4 5% 7% 42%	2498
	11th to 20th	Night	16% 13% 4 9% 5% 3 4 4 4 7% 30%	2646
	21st to end	Night	19% 15% 5% 9% 4 4 4 3 5% 7% 25%	2800
December	1st to 10th	Night	16% 16% 7% 12% 6% 6% 5% 4 4 5% 20%	4580
	11th to 20th	Night	19% 20% 7% 11% 5% 3 3 3 4 5% 19%	5205
	21st to end	Night	21% 21% 7% 12% 6% 4 4 3 4 5% 14%	6812

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	72% 6% 8% 11% 3	2891
	11th to 20th	Day	73% 3 10% 11% 3	2840
	21st to end	Day	80% 4 7% 6% 2	3124
February	1st to 10th	Day	67% 4 8% 14% 6% 1	1710
	11th to 20th	Day	64% 6% 8% 15% 7%	1705
	21st to end	Day	61% 4 11% 15% 7% 2	1570
March	1st to 10th	Day	60% 5% 10% 20% 5%	3395
	11th to 20th	Day	51% 7% 12% 22% 7% 1	3480
	21st to end	Day	55% 5% 10% 17% 13% 1	3815
April	1st to 10th	Day	52% 6% 12% 18% 12% 1	2143
	11th to 20th	Day	50% 7% 11% 17% 13% 2	2371
	21st to end	Day	52% 6% 11% 20% 9% 2	2470
May	1st to 10th	Day	55% 7% 11% 19% 8% 1	4120
	11th to 20th	Day	50% 8% 11% 20% 9% 1	4085
	21st to end	Day	39% 9% 14% 21% 14% 4	4580
June	1st to 10th	Day	44% 7% 13% 22% 12% 1	2660
	11th to 20th	Day	43% 8% 13% 22% 13% 1	2660
	21st to end	Day	39% 9% 13% 24% 13% 2	2660
July	1st to 10th	Day	39% 7% 13% 23% 15% 3	4064
	11th to 20th	Day	36% 6% 13% 27% 17% 1	4051
	21st to end	Day	42% 7% 14% 18% 17% 2	4324
August	1st to 10th	Day	47% 7% 14% 22% 10%	4091
	11th to 20th	Day	42% 8% 12% 28% 9% 1	4083
	21st to end	Day	37% 8% 14% 29% 11% 2	4413
September	1st to 10th	Day	39% 6% 13% 21% 18% 3	2208
	11th to 20th	Day	37% 6% 14% 24% 17% 2	2090
	21st to end	Day	47% 6% 11% 22% 13%	2090
October	1st to 10th	Day	33% 5% 12% 30% 19% 1	3231
	11th to 20th	Day	47% 5% 9% 20% 14% 4	3140
	21st to end	Day	41% 4 13% 26% 15% 1	3428
November	1st to 10th	Day	44% 5% 10% 24% 16% 2	1795
	11th to 20th	Day	56% 6% 9% 19% 9% 3	1750
	21st to end	Day	63% 5% 10% 15% 5% 1	1600
December	1st to 10th	Day	69% 5% 11% 12% 3	2600
	11th to 20th	Day	69% 4 9% 13% 5%	2986
	21st to end	Day	77% 4 7% 8% 3	4126
January	1st to 10th	Night	71% 5% 9% 11% 3	4188
	11th to 20th	Night	75% 4 8% 10% 2	4120
	21st to end	Night	77% 3 6% 11% 2	4532
February	1st to 10th	Night	65% 5% 9% 15% 6%	2470
	11th to 20th	Night	63% 5% 8% 16% 7% 1	2367
	21st to end	Night	63% 5% 8% 14% 8% 2	1884
March	1st to 10th	Night	55% 4 9% 21% 9%	3456
	11th to 20th	Night	50% 7% 14% 20% 7% 1	3480
	21st to end	Night	49% 5% 11% 22% 12% 1	3817
April	1st to 10th	Night	51% 5% 11% 18% 13% 2	2035
	11th to 20th	Night	48% 6% 12% 17% 15% 3	1773
	21st to end	Night	46% 6% 12% 19% 16% 2	1710
May	1st to 10th	Night	56% 7% 9% 18% 9% 2	2840
	11th to 20th	Night	46% 7% 11% 23% 11% 2	2813
	21st to end	Night	32% 8% 16% 26% 14% 3	2991
June	1st to 10th	Night	38% 8% 11% 25% 16% 2	1520
	11th to 20th	Night	36% 7% 15% 23% 16% 2	1520
	21st to end	Night	35% 9% 14% 23% 17% 1	1520
July	1st to 10th	Night	38% 7% 12% 23% 17% 3	2781
	11th to 20th	Night	32% 7% 15% 29% 16% 1	2786
	21st to end	Night	38% 8% 13% 17% 22% 3	2988
August	1st to 10th	Night	43% 7% 13% 23% 12% 1	2814
	11th to 20th	Night	39% 6% 12% 27% 15% 1	2812
	21st to end	Night	36% 7% 13% 29% 13% 2	3210
September	1st to 10th	Night	34% 7% 12% 24% 19% 4	1968
	11th to 20th	Night	30% 6% 13% 26% 22% 2	2090
	21st to end	Night	42% 5% 12% 22% 17% 1	2090
October	1st to 10th	Night	32% 5% 9% 29% 24% 1	3727
	11th to 20th	Night	41% 6% 11% 21% 14% 6%	3779
	21st to end	Night	37% 4 10% 29% 18% 1	4120
November	1st to 10th	Night	42% 5% 11% 23% 15% 3	2498
	11th to 20th	Night	53% 5% 10% 18% 9% 3	2646
	21st to end	Night	59% 6% 12% 18% 5% 1	2800
December	1st to 10th	Night	66% 6% 11% 14% 3	4580
	11th to 20th	Night	68% 4 11% 14% 2	5208
	21st to end	Night	72% 5% 9% 11% 2	6812

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	31% 21% 34% 10% 4 1	288
	11th to 20th	Day	29% 25% 36% 7% 3	270
	21st to end	Day	29% 26% 33% 9% 2 1	297
February	1st to 10th	Day	32% 20% 36% 11% 1	270
	11th to 20th	Day	28% 29% 32% 10% 1	270
	21st to end	Day	27% 32% 31% 10%	225
March	1st to 10th	Day	24% 19% 34% 18% 5%	431
	11th to 20th	Day	29% 15% 33% 16% 6% 1	460
	21st to end	Day	32% 17% 28% 18% 4 1	506
April	1st to 10th	Day	23% 21% 29% 17% 8% 2	460
	11th to 20th	Day	27% 15% 29% 15% 9% 4	452
	21st to end	Day	24% 15% 34% 18% 7% 2	460
May	1st to 10th	Day	26% 16% 27% 18% 10% 3	463
	11th to 20th	Day	19% 11% 34% 20% 12% 5%	487
	21st to end	Day	12% 7% 25% 28% 21% 6%	529
June	1st to 10th	Day	15% 8% 27% 30% 16% 4	490
	11th to 20th	Day	16% 9% 24% 28% 17% 6%	490
	21st to end	Day	14% 9% 28% 27% 16% 6%	490
July	1st to 10th	Day	14% 4 18% 27% 23% 14%	485
	11th to 20th	Day	10% 7% 24% 28% 22% 10%	484
	21st to end	Day	10% 4 26% 26% 23% 10%	532
August	1st to 10th	Day	14% 5% 19% 27% 22% 11%	486
	11th to 20th	Day	15% 5% 22% 26% 21% 11%	479
	21st to end	Day	9% 6% 19% 29% 25% 12%	539
September	1st to 10th	Day	13% 8% 20% 25% 23% 11%	490
	11th to 20th	Day	9% 4 17% 30% 28% 13%	490
	21st to end	Day	23% 12% 22% 22% 13% 8%	490
October	1st to 10th	Day	13% 10% 27% 24% 19% 6%	490
	11th to 20th	Day	21% 14% 33% 20% 9% 2	490
	21st to end	Day	28% 16% 28% 20% 7%	539
November	1st to 10th	Day	17% 23% 33% 17% 10% 2	333
	11th to 20th	Day	26% 21% 31% 13% 7% 1	275
	21st to end	Day	22% 21% 42% 14% 2	200
December	1st to 10th	Day	28% 25% 33% 8% 6% 2	200
	11th to 20th	Day	26% 21% 37% 13% 4	200
	21st to end	Day	31% 23% 30% 13% 2	257
January	1st to 10th	Night	25% 20% 34% 17% 3	785
	11th to 20th	Night	27% 18% 33% 18% 4	760
	21st to end	Night	26% 18% 31% 22% 4	836
February	1st to 10th	Night	30% 14% 31% 22% 3	760
	11th to 20th	Night	25% 17% 37% 19% 2	743
	21st to end	Night	28% 15% 37% 17% 3	628
March	1st to 10th	Night	23% 14% 35% 24% 5%	760
	11th to 20th	Night	23% 15% 31% 24% 6% 1	760
	21st to end	Night	23% 12% 34% 24% 6% 1	836
April	1st to 10th	Night	22% 13% 33% 26% 6%	760
	11th to 20th	Night	21% 11% 34% 23% 9% 1	649
	21st to end	Night	19% 9% 36% 29% 7%	570
May	1st to 10th	Night	21% 7% 27% 34% 10% 1	570
	11th to 20th	Night	18% 4 26% 37% 13% 2	561
	21st to end	Night	10% 4 20% 42% 22% 3	560
June	1st to 10th	Night	9% 3 29% 42% 17% 1	380
	11th to 20th	Night	11% 5% 22% 43% 18% 1	380
	21st to end	Night	10% 4 24% 39% 22% 1	380
July	1st to 10th	Night	9% 3 15% 38% 29% 5%	541
	11th to 20th	Night	10% 3 17% 40% 26% 5%	552
	21st to end	Night	10% 4 14% 42% 28% 2	624
August	1st to 10th	Night	10% 3 12% 43% 30% 3	567
	11th to 20th	Night	12% 2 14% 38% 28% 5%	564
	21st to end	Night	11% 2 16% 36% 30% 5%	674
September	1st to 10th	Night	15% 4 19% 31% 27% 5%	760
	11th to 20th	Night	11% 3 12% 37% 31% 5%	760
	21st to end	Night	19% 6% 24% 29% 19% 4	760
October	1st to 10th	Night	16% 6% 25% 31% 18% 4	760
	11th to 20th	Night	21% 9% 31% 26% 11% 2	760
	21st to end	Night	26% 10% 29% 28% 6%	836
November	1st to 10th	Night	19% 10% 31% 28% 12% 1	780
	11th to 20th	Night	23% 14% 33% 22% 7% 1	800
	21st to end	Night	27% 16% 31% 20% 6% 1	800
December	1st to 10th	Night	29% 15% 32% 19% 4	800
	11th to 20th	Night	26% 17% 33% 21% 4	800
	21st to end	Night	26% 16% 35% 20% 2	880

Page generated at 2025-01-04 03:23:59. Version ac07

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