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 2026-05-02.

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% 11% 4 10% 5% 5% 5% 4 5% 7% 30%	131444
	Night	15% 12% 4 10% 5% 5% 4 3 5% 6% 30%	131026
Transparency	Day	52% 6% 11% 20% 10% 1	135076
	Night	53% 5% 11% 19% 11% 1	135464
Seeing	Day	21% 14% 28% 20% 12% 5%	16504
	Night	20% 11% 28% 28% 12% 1	26430

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	15% 20% 6% 22% 6% 4 4 3 4 4 12%	5258
	11th to 20th	Day	10% 21% 9% 18% 9% 6% 4 2 3 4 13%	5280
	21st to end	Day	17% 17% 7% 13% 10% 7% 4 2 4 4 14%	5780
February	1st to 10th	Day	17% 17% 6% 12% 7% 4 3 2 4 5% 22%	1890
	11th to 20th	Day	16% 13% 8% 14% 7% 4 4 4 5% 5% 21%	1880
	21st to end	Day	17% 15% 5% 11% 6% 5% 3 4 4 6% 24%	1730
March	1st to 10th	Day	17% 12% 6% 12% 5% 4 4 3 5% 5% 27%	4990
	11th to 20th	Day	11% 10% 3 11% 6% 5% 5% 3 4 6% 36%	5090
	21st to end	Day	19% 12% 5% 9% 5% 4 4 3 5% 6% 28%	5586
April	1st to 10th	Day	15% 13% 4 8% 6% 4 4 3 5% 7% 30%	2369
	11th to 20th	Day	17% 11% 4 8% 4 4 4 4 5% 6% 33%	2624
	21st to end	Day	18% 11% 3 7% 5% 4 4 4 5% 7% 33%	2730
May	1st to 10th	Day	20% 10% 4 8% 5% 5% 5% 4 5% 6% 28%	5875
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	17% 8% 3 6% 5% 5% 5% 4 6% 8% 33%	6565
June	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 4 6% 9% 35%	2800
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 7% 8% 35%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 10% 38%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	5909
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 5% 7% 10% 35%	5900
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	6424
August	1st to 10th	Day	13% 9% 3 7% 5% 5% 5% 5% 7% 9% 32%	4234
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	4174
	21st to end	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 10% 40%	4552
September	1st to 10th	Day	14% 6% 3 5% 5% 4 4 3 5% 7% 45%	2324
	11th to 20th	Day	10% 6% 3 6% 4 5% 5% 4 5% 8% 44%	2200
	21st to end	Day	19% 9% 3 7% 4 4 4 4 4 6% 36%	2200
October	1st to 10th	Day	15% 6% 2 7% 4 4 5% 3 5% 8% 40%	2047
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	2000
	21st to end	Day	21% 11% 3 7% 4 5% 4 3 5% 7% 30%	2200
November	1st to 10th	Day	11% 9% 3 9% 5% 4 5% 4 5% 6% 40%	1887
	11th to 20th	Day	15% 16% 5% 8% 5% 4 4 4 5% 6% 28%	1837
	21st to end	Day	15% 17% 8% 9% 5% 5% 5% 4 5% 5% 21%	1680
December	1st to 10th	Day	18% 15% 8% 15% 6% 4 4 3 4 4 19%	2680
	11th to 20th	Day	21% 19% 7% 12% 4 4 3 3 4 5% 18%	3070
	21st to end	Day	23% 23% 6% 13% 5% 4 4 3 3 5% 11%	4218
January	1st to 10th	Night	14% 19% 7% 22% 8% 5% 4 3 4 4 11%	7635
	11th to 20th	Night	10% 18% 9% 18% 9% 7% 6% 3 4 4 13%	7680
	21st to end	Night	17% 16% 6% 12% 7% 7% 5% 3 3 5% 19%	8401
February	1st to 10th	Night	19% 15% 6% 11% 6% 5% 4 3 4 5% 22%	2730
	11th to 20th	Night	17% 13% 6% 13% 6% 4 5% 4 5% 5% 22%	2621
	21st to end	Night	19% 14% 5% 11% 6% 4 5% 3 3 5% 24%	2076
March	1st to 10th	Night	21% 13% 5% 9% 4 3 3 2 3 4 31%	5081
	11th to 20th	Night	14% 10% 4 7% 4 4 5% 4 5% 6% 37%	5090
	21st to end	Night	16% 12% 4 9% 5% 4 4 3 4 5% 34%	5588
April	1st to 10th	Night	18% 12% 3 7% 5% 5% 4 3 5% 6% 32%	2249
	11th to 20th	Night	15% 10% 3 8% 6% 6% 5% 4 5% 7% 33%	1962
	21st to end	Night	15% 9% 3 7% 4 4 5% 4 5% 7% 36%	1890
May	1st to 10th	Night	25% 11% 3 7% 5% 4 4 3 5% 6% 26%	4042
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 3 5% 8% 37%	4032
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 10% 37%	4284
June	1st to 10th	Night	9% 6% 3 7% 6% 5% 4 3 5% 8% 44%	1600
	11th to 20th	Night	9% 7% 3 7% 4 5% 5% 4 6% 9% 42%	1600
	21st to end	Night	9% 6% 3 6% 4 4 6% 5% 7% 8% 43%	1600
July	1st to 10th	Night	9% 7% 3 6% 5% 5% 4 4 5% 9% 42%	4044
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	4057
	21st to end	Night	8% 6% 3 7% 4 5% 5% 4 6% 8% 46%	4427
August	1st to 10th	Night	11% 8% 3 8% 5% 5% 4 4 6% 8% 38%	2915
	11th to 20th	Night	12% 9% 3 6% 3 4 3 3 6% 7% 44%	2875
	21st to end	Night	8% 4 2 7% 5% 6% 6% 4 6% 9% 44%	3312
September	1st to 10th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 51%	2072
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 8% 52%	2200
	21st to end	Night	16% 8% 2 6% 5% 3 4 4 5% 7% 40%	2200
October	1st to 10th	Night	13% 5% 3 6% 4 3 4 4 5% 8% 45%	2351
	11th to 20th	Night	16% 8% 4 8% 5% 4 5% 3 5% 6% 37%	2400
	21st to end	Night	21% 10% 2 6% 4 3 4 4 4 6% 35%	2640
November	1st to 10th	Night	14% 9% 2 6% 4 5% 3 4 5% 7% 42%	2626
	11th to 20th	Night	16% 13% 4 9% 5% 4 5% 4 4 7% 30%	2779
	21st to end	Night	19% 15% 5% 9% 4 4 4 3 5% 7% 25%	2940
December	1st to 10th	Night	15% 16% 7% 12% 6% 6% 5% 4 4 5% 20%	4720
	11th to 20th	Night	19% 20% 7% 12% 5% 3 3 3 4 5% 19%	5345
	21st to end	Night	21% 21% 7% 12% 5% 4 4 3 4 5% 15%	6962

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	76% 4 8% 8% 3	5259
	11th to 20th	Day	76% 3 7% 8% 5%	5280
	21st to end	Day	76% 3 6% 11% 3	5780
February	1st to 10th	Day	66% 4 8% 14% 7%	1890
	11th to 20th	Day	65% 6% 8% 15% 7%	1889
	21st to end	Day	60% 5% 11% 16% 7% 2	1730
March	1st to 10th	Day	58% 4 10% 21% 6%	4990
	11th to 20th	Day	48% 6% 12% 25% 8% 2	5090
	21st to end	Day	56% 4 8% 18% 12% 1	5586
April	1st to 10th	Day	53% 6% 12% 17% 12% 1	2369
	11th to 20th	Day	50% 7% 11% 17% 13% 2	2624
	21st to end	Day	51% 6% 11% 21% 10% 2	2730
May	1st to 10th	Day	55% 7% 10% 16% 8% 4	5910
	11th to 20th	Day	50% 7% 11% 21% 10% 1	5865
	21st to end	Day	46% 8% 12% 20% 11% 3	6580
June	1st to 10th	Day	44% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	44% 8% 13% 22% 12% 1	2800
	21st to end	Day	39% 9% 14% 24% 14% 1	2800
July	1st to 10th	Day	37% 6% 12% 27% 15% 2	5844
	11th to 20th	Day	41% 7% 14% 25% 13% 1	5831
	21st to end	Day	40% 7% 12% 22% 19% 2	6240
August	1st to 10th	Day	46% 7% 14% 22% 10%	4221
	11th to 20th	Day	42% 8% 12% 28% 9% 1	4213
	21st to end	Day	37% 8% 13% 29% 11% 2	4552
September	1st to 10th	Day	39% 6% 12% 22% 19% 3	2324
	11th to 20th	Day	36% 6% 14% 24% 17% 2	2200
	21st to end	Day	48% 6% 11% 21% 14% 1	2200
October	1st to 10th	Day	32% 5% 11% 30% 20% 1	3333
	11th to 20th	Day	47% 5% 9% 20% 14% 4	3240
	21st to end	Day	41% 4 12% 26% 16% 1	3538
November	1st to 10th	Day	44% 5% 10% 23% 15% 2	1887
	11th to 20th	Day	56% 5% 9% 19% 8% 2	1837
	21st to end	Day	63% 5% 10% 15% 5% 1	1680
December	1st to 10th	Day	69% 4 11% 12% 3	2680
	11th to 20th	Day	68% 4 9% 13% 5%	3066
	21st to end	Day	77% 4 7% 8% 3	4218
January	1st to 10th	Night	76% 4 7% 9% 3	7636
	11th to 20th	Night	75% 4 7% 9% 4	7680
	21st to end	Night	70% 3 8% 13% 6% 1	8401
February	1st to 10th	Night	65% 4 9% 16% 6%	2730
	11th to 20th	Night	65% 5% 8% 15% 7%	2623
	21st to end	Night	63% 5% 8% 14% 8% 1	2076
March	1st to 10th	Night	57% 4 10% 20% 9% 1	5081
	11th to 20th	Night	46% 6% 14% 24% 8% 2	5090
	21st to end	Night	51% 5% 10% 18% 14% 1	5588
April	1st to 10th	Night	53% 5% 10% 18% 12% 2	2249
	11th to 20th	Night	49% 6% 12% 17% 14% 2	1962
	21st to end	Night	45% 6% 12% 19% 16% 2	1890
May	1st to 10th	Night	57% 7% 9% 14% 9% 4	4069
	11th to 20th	Night	46% 7% 10% 23% 14% 1	4033
	21st to end	Night	42% 7% 14% 24% 11% 2	4291
June	1st to 10th	Night	39% 7% 11% 24% 16% 3	1600
	11th to 20th	Night	38% 7% 15% 22% 16% 2	1600
	21st to end	Night	36% 9% 14% 23% 18% 1	1600
July	1st to 10th	Night	38% 7% 11% 24% 18% 2	4001
	11th to 20th	Night	38% 8% 17% 25% 12% 1	4006
	21st to end	Night	36% 7% 12% 19% 24% 2	4297
August	1st to 10th	Night	42% 7% 13% 23% 12% 1	2904
	11th to 20th	Night	39% 6% 11% 28% 15% 1	2902
	21st to end	Night	36% 7% 13% 30% 13% 2	3312
September	1st to 10th	Night	33% 6% 11% 25% 19% 4	2072
	11th to 20th	Night	30% 6% 13% 28% 22% 2	2200
	21st to end	Night	43% 5% 12% 22% 18% 1	2200
October	1st to 10th	Night	32% 5% 9% 28% 25% 1	3845
	11th to 20th	Night	41% 6% 11% 21% 15% 6%	3899
	21st to end	Night	37% 4 9% 29% 18% 2	4252
November	1st to 10th	Night	42% 5% 12% 23% 15% 3	2626
	11th to 20th	Night	53% 6% 10% 19% 9% 3	2779
	21st to end	Night	59% 6% 12% 18% 5% 1	2940
December	1st to 10th	Night	66% 6% 11% 14% 4	4720
	11th to 20th	Night	68% 4 10% 15% 2	5348
	21st to end	Night	72% 5% 9% 12% 2	6962

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% 35% 9% 3 1	330
	11th to 20th	Day	32% 28% 32% 6% 2	330
	21st to end	Day	31% 27% 31% 9% 2 1	363
February	1st to 10th	Day	30% 22% 35% 12% 1 1	330
	11th to 20th	Day	30% 28% 30% 10% 1	330
	21st to end	Day	26% 30% 35% 9%	273
March	1st to 10th	Day	25% 19% 36% 16% 5%	509
	11th to 20th	Day	31% 16% 33% 15% 6% 1	540
	21st to end	Day	31% 19% 29% 17% 4 1	594
April	1st to 10th	Day	25% 21% 29% 16% 7% 1	540
	11th to 20th	Day	27% 17% 28% 16% 8% 4	532
	21st to end	Day	24% 15% 33% 18% 8% 2	540
May	1st to 10th	Day	27% 17% 28% 17% 9% 3	507
	11th to 20th	Day	19% 11% 34% 20% 11% 4	527
	21st to end	Day	15% 8% 25% 27% 20% 6%	573
June	1st to 10th	Day	15% 9% 28% 30% 15% 4	530
	11th to 20th	Day	17% 9% 25% 28% 16% 5%	530
	21st to end	Day	14% 9% 26% 28% 17% 6%	530
July	1st to 10th	Day	13% 5% 18% 28% 23% 13%	525
	11th to 20th	Day	10% 7% 24% 29% 21% 9%	524
	21st to end	Day	11% 4 26% 27% 22% 10%	576
August	1st to 10th	Day	13% 5% 20% 28% 23% 10%	526
	11th to 20th	Day	14% 5% 21% 27% 21% 11%	519
	21st to end	Day	9% 7% 20% 29% 24% 11%	583
September	1st to 10th	Day	14% 8% 20% 23% 24% 11%	530
	11th to 20th	Day	8% 4 16% 29% 28% 15%	530
	21st to end	Day	23% 12% 22% 23% 13% 7%	530
October	1st to 10th	Day	13% 10% 26% 25% 20% 6%	530
	11th to 20th	Day	21% 14% 35% 19% 9% 2	530
	21st to end	Day	28% 16% 29% 20% 7%	583
November	1st to 10th	Day	17% 23% 33% 16% 9% 2	365
	11th to 20th	Day	27% 22% 30% 13% 7% 1	302
	21st to end	Day	24% 21% 40% 14% 1	220
December	1st to 10th	Day	27% 25% 33% 8% 5% 1	220
	11th to 20th	Day	27% 22% 35% 13% 3	220
	21st to end	Day	33% 23% 29% 13% 2	283
January	1st to 10th	Night	25% 20% 35% 17% 3	840
	11th to 20th	Night	27% 20% 33% 16% 4	840
	21st to end	Night	26% 19% 31% 21% 3	924
February	1st to 10th	Night	29% 14% 31% 23% 3	840
	11th to 20th	Night	25% 17% 37% 18% 2	823
	21st to end	Night	28% 15% 37% 17% 2	692
March	1st to 10th	Night	24% 14% 35% 23% 5%	840
	11th to 20th	Night	24% 15% 31% 23% 6%	840
	21st to end	Night	23% 13% 35% 23% 5% 1	924
April	1st to 10th	Night	23% 14% 32% 24% 6%	840
	11th to 20th	Night	21% 12% 35% 23% 8% 1	718
	21st to end	Night	19% 9% 35% 29% 7%	630
May	1st to 10th	Night	22% 7% 27% 34% 10% 1	603
	11th to 20th	Night	18% 5% 25% 38% 12% 2	591
	21st to end	Night	12% 4 20% 41% 21% 3	590
June	1st to 10th	Night	9% 4 29% 42% 16% 1	400
	11th to 20th	Night	11% 6% 23% 43% 17% 1	400
	21st to end	Night	10% 4 24% 39% 23% 1	400
July	1st to 10th	Night	9% 2 16% 39% 29% 5%	571
	11th to 20th	Night	9% 3 17% 39% 27% 4	582
	21st to end	Night	9% 4 14% 41% 29% 2	657
August	1st to 10th	Night	9% 3 13% 44% 29% 3	597
	11th to 20th	Night	12% 2 13% 39% 29% 5%	594
	21st to end	Night	11% 2 16% 36% 30% 5%	710
September	1st to 10th	Night	14% 4 19% 31% 28% 5%	800
	11th to 20th	Night	11% 3 12% 38% 32% 5%	800
	21st to end	Night	19% 6% 23% 29% 19% 4	800
October	1st to 10th	Night	16% 6% 24% 31% 20% 4	800
	11th to 20th	Night	21% 9% 32% 27% 10% 2	800
	21st to end	Night	26% 10% 29% 28% 6%	880
November	1st to 10th	Night	19% 11% 31% 27% 11%	820
	11th to 20th	Night	23% 15% 32% 22% 6% 1	840
	21st to end	Night	28% 16% 31% 20% 5% 1	840
December	1st to 10th	Night	29% 16% 33% 19% 4	840
	11th to 20th	Night	25% 17% 34% 20% 3	840
	21st to end	Night	26% 17% 35% 20% 2	924

Page generated at 2026-04-29 02:21:43. Version ac07

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