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 2024-07-31.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	89802
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	89950
Transparency	Day	53% 7% 11% 19% 10% 1	89575
	Night	53% 6% 11% 19% 10% 1	89781
Seeing	Day	20% 13% 28% 21% 13% 5%	14264
	Night	20% 10% 28% 28% 12% 2	24087

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% 14% 5% 4 4 3 4 5% 15%	2836
	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	11% 6% 3 8% 5% 5% 5% 4 7% 9% 39%	2479
	11th to 20th	Day	11% 6% 3 6% 5% 6% 5% 4 6% 10% 38%	2470
	21st to end	Day	12% 7% 2 7% 5% 6% 5% 4 7% 9% 36%	2679
August	1st to 10th	Day	10% 6% 2 6% 6% 5% 6% 6% 7% 9% 35%	2327
	11th to 20th	Day	11% 7% 3 7% 6% 5% 6% 5% 8% 9% 33%	2303
	21st to end	Day	11% 6% 2 7% 6% 5% 6% 5% 7% 8% 39%	2511
September	1st to 10th	Day	15% 6% 3 5% 5% 4 4 3 5% 7% 43%	2092
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 4 5% 8% 42%	1980
	21st to end	Day	18% 8% 3 7% 4 4 4 4 4 7% 38%	1980
October	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 3 5% 8% 37%	1843
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 3 5% 6% 33%	1800
	21st to end	Day	22% 12% 4 8% 5% 5% 3 3 4 6% 28%	1980
November	1st to 10th	Day	10% 10% 3 9% 5% 4 4 4 5% 6% 40%	1704
	11th to 20th	Day	15% 16% 4 8% 5% 4 4 3 5% 6% 29%	1663
	21st to end	Day	16% 16% 8% 9% 5% 5% 5% 4 5% 5% 23%	1520
December	1st to 10th	Day	19% 20% 8% 11% 7% 4 3 3 4 4 17%	1520
	11th to 20th	Day	17% 17% 8% 14% 5% 5% 4 3 6% 6% 17%	1910
	21st to end	Day	24% 24% 7% 11% 5% 3 3 3 3 5% 11%	2882
January	1st to 10th	Night	15% 21% 7% 15% 6% 4 4 3 4 5% 16%	4109
	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	8% 5% 3 6% 5% 5% 4 4 6% 9% 44%	1696
	11th to 20th	Night	8% 4 2 4 5% 4 4 5% 7% 10% 47%	1710
	21st to end	Night	10% 6% 2 7% 4 5% 5% 5% 6% 8% 42%	1870
August	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 7% 10% 45%	1611
	11th to 20th	Night	9% 7% 2 6% 3 4 4 3 7% 9% 46%	1598
	21st to end	Night	10% 4 2 5% 4 4 5% 4 7% 7% 47%	1826
September	1st to 10th	Night	12% 6% 3 5% 4 4 3 3 5% 7% 48%	1864
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 5% 8% 51%	1980
	21st to end	Night	15% 7% 2 5% 5% 4 4 4 5% 8% 42%	1980
October	1st to 10th	Night	14% 5% 3 6% 4 3 4 4 6% 8% 43%	2115
	11th to 20th	Night	17% 8% 4 8% 5% 5% 5% 3 5% 6% 35%	2160
	21st to end	Night	23% 11% 3 6% 4 4 4 4 4 6% 32%	2376
November	1st to 10th	Night	15% 9% 2 6% 4 4 3 3 5% 7% 41%	2370
	11th to 20th	Night	16% 13% 4 9% 5% 4 4 3 5% 7% 30%	2513
	21st to end	Night	18% 15% 5% 8% 4 4 4 3 5% 7% 26%	2660
December	1st to 10th	Night	20% 17% 6% 11% 5% 5% 4 4 3 5% 20%	2660
	11th to 20th	Night	17% 19% 6% 11% 6% 4 3 3 4 5% 22%	3285
	21st to end	Night	24% 21% 8% 10% 5% 4 4 3 4 5% 13%	4756

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	71% 6% 8% 12% 3	2837
	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	41% 8% 13% 23% 13% 3	2414
	11th to 20th	Day	40% 7% 13% 25% 14% 1	2401
	21st to end	Day	41% 8% 14% 22% 13% 1	2537
August	1st to 10th	Day	42% 8% 15% 25% 9%	2314
	11th to 20th	Day	43% 9% 15% 22% 9% 1	2303
	21st to end	Day	40% 9% 13% 22% 12% 4	2511
September	1st to 10th	Day	41% 6% 13% 20% 17% 3	2092
	11th to 20th	Day	38% 6% 15% 24% 15% 1	1980
	21st to end	Day	45% 6% 11% 23% 14% 1	1980
October	1st to 10th	Day	44% 7% 11% 22% 15% 1	1843
	11th to 20th	Day	51% 6% 11% 21% 10% 1	1800
	21st to end	Day	56% 5% 12% 16% 9% 1	1980
November	1st to 10th	Day	44% 5% 10% 23% 15% 2	1704
	11th to 20th	Day	55% 6% 9% 19% 8% 3	1663
	21st to end	Day	62% 5% 10% 16% 6% 1	1520
December	1st to 10th	Day	71% 5% 9% 11% 4	1520
	11th to 20th	Day	68% 5% 8% 15% 4	1906
	21st to end	Day	78% 5% 9% 7% 2	2882
January	1st to 10th	Night	70% 5% 9% 11% 3	4109
	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	35% 9% 13% 23% 18% 3	1651
	11th to 20th	Night	29% 8% 17% 26% 18% 2	1656
	21st to end	Night	38% 9% 16% 20% 15% 3	1771
August	1st to 10th	Night	32% 9% 15% 28% 16% 1	1600
	11th to 20th	Night	35% 7% 16% 26% 14% 2	1598
	21st to end	Night	33% 8% 13% 25% 17% 4	1826
September	1st to 10th	Night	35% 7% 12% 23% 18% 5%	1864
	11th to 20th	Night	31% 7% 13% 27% 21% 2	1980
	21st to end	Night	40% 5% 13% 23% 18% 1	1980
October	1st to 10th	Night	38% 7% 13% 23% 18% 1	2115
	11th to 20th	Night	49% 7% 10% 22% 12% 1	2160
	21st to end	Night	52% 6% 10% 20% 10% 2	2376
November	1st to 10th	Night	42% 5% 11% 23% 15% 3	2370
	11th to 20th	Night	53% 6% 10% 19% 10% 3	2513
	21st to end	Night	58% 6% 12% 18% 6% 1	2660
December	1st to 10th	Night	67% 5% 10% 12% 5%	2660
	11th to 20th	Night	65% 6% 11% 16% 3	3288
	21st to end	Night	74% 6% 10% 9% 2	4756

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	29% 21% 34% 10% 4 1	270
	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	11% 4 26% 26% 23% 10%	529
August	1st to 10th	Day	14% 4 19% 28% 24% 12%	446
	11th to 20th	Day	15% 5% 21% 25% 22% 12%	439
	21st to end	Day	10% 6% 19% 28% 25% 12%	495
September	1st to 10th	Day	15% 6% 20% 24% 23% 11%	450
	11th to 20th	Day	10% 4 17% 29% 27% 13%	450
	21st to end	Day	21% 12% 22% 23% 14% 8%	450
October	1st to 10th	Day	14% 10% 26% 25% 18% 7%	450
	11th to 20th	Day	21% 14% 36% 21% 8% 1	450
	21st to end	Day	31% 15% 28% 19% 6%	495
November	1st to 10th	Day	17% 23% 31% 17% 11% 2	302
	11th to 20th	Day	25% 23% 29% 14% 8% 2	248
	21st to end	Day	20% 21% 43% 14% 2	180
December	1st to 10th	Day	27% 26% 31% 9% 6% 2	180
	11th to 20th	Day	25% 21% 39% 13% 2	180
	21st to end	Day	31% 24% 30% 13% 3	231
January	1st to 10th	Night	25% 20% 35% 17% 3	760
	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	9% 2 12% 43% 31% 3	537
	11th to 20th	Night	12% 2 13% 39% 29% 5%	534
	21st to end	Night	11% 2 17% 35% 30% 5%	638
September	1st to 10th	Night	15% 3 18% 31% 28% 5%	720
	11th to 20th	Night	12% 3 12% 36% 31% 5%	720
	21st to end	Night	17% 5% 24% 29% 20% 4	720
October	1st to 10th	Night	16% 6% 25% 31% 18% 4	720
	11th to 20th	Night	22% 9% 31% 27% 10% 1	720
	21st to end	Night	28% 10% 29% 27% 6%	792
November	1st to 10th	Night	20% 10% 29% 28% 12% 1	740
	11th to 20th	Night	23% 14% 33% 22% 7% 1	760
	21st to end	Night	26% 16% 31% 21% 6% 1	760
December	1st to 10th	Night	30% 15% 32% 19% 4	760
	11th to 20th	Night	25% 16% 34% 21% 4	760
	21st to end	Night	26% 16% 35% 20% 2	836

Page generated at 2024-07-28 09:52:11. Version ac07

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