Rankin Science Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rankin Science Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	13% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	131339
	Night	13% 7% 2 6% 4 4 4 4 7% 8% 41%	129837
Transparency	Day	40% 8% 13% 20% 15% 5%	135011
	Night	37% 7% 13% 21% 16% 6%	134235
Seeing	Day	23% 16% 24% 20% 12% 5%	16849
	Night	21% 13% 21% 25% 16% 3	27000

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	13% 6% 2 5% 4 3 3 3 8% 9% 44%	5258
	11th to 20th	Day	7% 6% 2 6% 5% 3 4 3 5% 7% 53%	5280
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 6% 45%	5780
February	1st to 10th	Day	11% 9% 3 7% 5% 5% 5% 4 6% 7% 38%	1890
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 5% 7% 37%	2048
	21st to end	Day	16% 7% 2 6% 5% 4 5% 4 6% 7% 38%	1730
March	1st to 10th	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 40%	4950
	11th to 20th	Day	11% 5% 3 5% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 42%	5465
April	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 4 7% 9% 41%	2200
	11th to 20th	Day	16% 6% 2 5% 4 4 4 3 6% 8% 41%	2389
	21st to end	Day	15% 7% 2 6% 5% 5% 4 4 5% 7% 39%	2600
May	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 5% 8% 9% 27%	5862
	11th to 20th	Day	20% 8% 3 8% 5% 6% 6% 5% 7% 8% 24%	5861
	21st to end	Day	14% 6% 2 6% 4 5% 6% 5% 8% 10% 33%	6433
June	1st to 10th	Day	13% 7% 2 6% 6% 6% 7% 5% 7% 9% 32%	2600
	11th to 20th	Day	9% 7% 3 8% 6% 6% 7% 6% 9% 8% 32%	2600
	21st to end	Day	9% 6% 2 7% 7% 7% 7% 5% 8% 10% 32%	2600
July	1st to 10th	Day	8% 6% 2 8% 7% 7% 7% 6% 7% 10% 34%	5900
	11th to 20th	Day	8% 7% 3 9% 8% 7% 8% 6% 8% 10% 25%	5900
	21st to end	Day	13% 9% 4 10% 7% 7% 8% 5% 8% 10% 19%	6424
August	1st to 10th	Day	15% 9% 3 8% 7% 7% 7% 6% 8% 10% 22%	4234
	11th to 20th	Day	10% 6% 3 7% 6% 8% 7% 6% 9% 10% 28%	4174
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	4433
September	1st to 10th	Day	14% 7% 2 7% 6% 6% 6% 4 7% 8% 35%	2200
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 5% 6% 8% 31%	2200
	21st to end	Day	21% 6% 3 6% 5% 5% 5% 4 6% 6% 33%	2200
October	1st to 10th	Day	17% 5% 1 5% 4 4 4 4 4 6% 46%	2110
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 51%	2000
	21st to end	Day	15% 6% 2 5% 3 3 3 3 5% 6% 49%	2200
November	1st to 10th	Day	12% 6% 2 4 3 3 4 3 4 7% 53%	2010
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 5% 8% 50%	1890
	21st to end	Day	14% 6% 2 4 4 4 3 3 5% 8% 46%	1890
December	1st to 10th	Day	10% 7% 3 7% 5% 5% 4 3 5% 7% 44%	3020
	11th to 20th	Day	14% 6% 3 6% 5% 5% 4 4 6% 8% 40%	3463
	21st to end	Day	17% 8% 2 5% 4 4 4 4 6% 9% 37%	4565
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 8% 9% 45%	7635
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 3 6% 7% 50%	7680
	21st to end	Night	14% 6% 3 5% 4 4 3 3 5% 7% 47%	8401
February	1st to 10th	Night	11% 7% 2 6% 4 4 4 5% 7% 8% 40%	2727
	11th to 20th	Night	16% 9% 2 5% 4 4 5% 4 6% 9% 36%	2458
	21st to end	Night	13% 8% 2 7% 5% 5% 5% 4 7% 9% 37%	2076
March	1st to 10th	Night	19% 8% 2 5% 5% 4 4 4 7% 6% 38%	4946
	11th to 20th	Night	11% 5% 2 4 4 4 4 4 6% 7% 50%	4980
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 8% 42%	5467
April	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 7% 9% 42%	2200
	11th to 20th	Night	14% 6% 2 4 4 3 4 4 6% 8% 44%	1974
	21st to end	Night	13% 5% 2 6% 4 3 4 4 5% 8% 44%	1800
May	1st to 10th	Night	15% 8% 3 7% 4 4 5% 4 9% 11% 32%	4033
	11th to 20th	Night	21% 8% 3 6% 5% 4 5% 4 7% 10% 28%	4032
	21st to end	Night	17% 6% 2 5% 4 4 5% 5% 8% 10% 34%	4421
June	1st to 10th	Night	10% 7% 2 7% 4 4 6% 5% 8% 11% 35%	1800
	11th to 20th	Night	9% 6% 2 5% 5% 5% 5% 4 8% 11% 41%	1800
	21st to end	Night	8% 5% 2 5% 6% 5% 5% 5% 9% 11% 39%	1800
July	1st to 10th	Night	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 37%	4058
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 6% 8% 13% 35%	4057
	21st to end	Night	11% 9% 3 10% 6% 6% 6% 5% 9% 10% 24%	4427
August	1st to 10th	Night	14% 8% 2 7% 5% 5% 5% 5% 7% 12% 29%	2915
	11th to 20th	Night	11% 7% 3 8% 5% 5% 5% 5% 7% 10% 34%	2875
	21st to end	Night	7% 5% 2 5% 3 4 4 4 7% 9% 50%	3415
September	1st to 10th	Night	12% 8% 3 6% 5% 4 4 4 6% 8% 41%	2200
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 7% 39%	2200
	21st to end	Night	21% 8% 2 6% 3 4 4 3 5% 5% 39%	2200
October	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 7% 45%	2290
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 6% 49%	2400
	21st to end	Night	13% 6% 2 4 3 3 3 3 3 7% 53%	2640
November	1st to 10th	Night	13% 8% 2 4 3 3 3 2 6% 7% 49%	2505
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 8% 50%	2730
	21st to end	Night	12% 6% 2 5% 4 3 4 4 7% 7% 46%	2730
December	1st to 10th	Night	11% 6% 2 7% 4 4 4 3 6% 7% 45%	4380
	11th to 20th	Night	14% 10% 4 8% 5% 5% 4 3 5% 6% 36%	4952
	21st to end	Night	19% 7% 2 6% 4 3 4 3 7% 9% 36%	6633

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	33% 4 12% 23% 25% 4	5259
	11th to 20th	Day	28% 4 8% 17% 30% 14%	5280
	21st to end	Day	40% 3 8% 23% 19% 7%	5780
February	1st to 10th	Day	40% 6% 14% 21% 15% 4	1890
	11th to 20th	Day	45% 4 13% 18% 16% 4	2058
	21st to end	Day	43% 6% 11% 19% 15% 5%	1730
March	1st to 10th	Day	40% 6% 9% 17% 17% 11%	4950
	11th to 20th	Day	32% 5% 7% 20% 20% 16%	4980
	21st to end	Day	41% 5% 10% 26% 15% 4	5465
April	1st to 10th	Day	36% 6% 13% 24% 17% 4	2200
	11th to 20th	Day	39% 6% 12% 20% 18% 5%	2389
	21st to end	Day	42% 8% 13% 23% 12% 2	2600
May	1st to 10th	Day	48% 11% 13% 18% 8% 1	5897
	11th to 20th	Day	53% 10% 13% 17% 6% 1	5865
	21st to end	Day	41% 9% 14% 18% 15% 2	6447
June	1st to 10th	Day	44% 12% 15% 19% 9% 1	2600
	11th to 20th	Day	43% 11% 16% 17% 12% 2	2600
	21st to end	Day	41% 12% 15% 18% 13% 1	2600
July	1st to 10th	Day	41% 13% 16% 19% 10% 1	5835
	11th to 20th	Day	47% 15% 20% 13% 4	5831
	21st to end	Day	54% 13% 15% 13% 4	6240
August	1st to 10th	Day	51% 12% 17% 17% 4	4221
	11th to 20th	Day	41% 13% 20% 16% 7% 3	4213
	21st to end	Day	33% 10% 17% 31% 7% 2	4433
September	1st to 10th	Day	43% 8% 13% 17% 15% 3	2200
	11th to 20th	Day	46% 7% 12% 19% 14% 2	2200
	21st to end	Day	48% 8% 12% 16% 13% 3	2200
October	1st to 10th	Day	29% 5% 9% 20% 25% 13%	3435
	11th to 20th	Day	23% 3 7% 23% 22% 22%	3240
	21st to end	Day	27% 3 10% 27% 16% 16%	3538
November	1st to 10th	Day	31% 5% 11% 20% 26% 7%	2010
	11th to 20th	Day	32% 5% 10% 23% 23% 7%	1890
	21st to end	Day	36% 3 10% 25% 22% 4	1890
December	1st to 10th	Day	36% 6% 8% 18% 26% 5%	3020
	11th to 20th	Day	38% 6% 13% 18% 19% 6%	3460
	21st to end	Day	42% 5% 12% 26% 12% 3	4565
January	1st to 10th	Night	32% 3 13% 27% 16% 9%	7636
	11th to 20th	Night	31% 5% 7% 19% 26% 12%	7680
	21st to end	Night	37% 4 10% 19% 23% 7%	8401
February	1st to 10th	Night	37% 6% 14% 23% 16% 3	2727
	11th to 20th	Night	42% 5% 14% 20% 14% 5%	2459
	21st to end	Night	41% 8% 12% 19% 15% 6%	2076
March	1st to 10th	Night	39% 7% 8% 19% 17% 9%	4946
	11th to 20th	Night	29% 5% 8% 23% 20% 15%	4980
	21st to end	Night	38% 6% 11% 26% 13% 6%	5467
April	1st to 10th	Night	34% 8% 15% 23% 15% 5%	2200
	11th to 20th	Night	36% 6% 13% 22% 17% 6%	1974
	21st to end	Night	37% 7% 16% 24% 14% 2	1800
May	1st to 10th	Night	43% 13% 15% 21% 8% 1	4060
	11th to 20th	Night	49% 11% 15% 19% 6%	4033
	21st to end	Night	39% 10% 18% 21% 11% 1	4429
June	1st to 10th	Night	38% 13% 18% 21% 10% 1	1800
	11th to 20th	Night	34% 12% 21% 22% 10% 1	1800
	21st to end	Night	33% 13% 19% 21% 11% 3	1800
July	1st to 10th	Night	37% 12% 18% 24% 8% 1	4015
	11th to 20th	Night	38% 14% 27% 17% 3 1	4006
	21st to end	Night	48% 15% 18% 12% 6%	4297
August	1st to 10th	Night	45% 11% 23% 15% 6%	2904
	11th to 20th	Night	39% 13% 21% 17% 9% 1	2902
	21st to end	Night	27% 9% 19% 33% 8% 3	3415
September	1st to 10th	Night	39% 9% 15% 18% 15% 4	2200
	11th to 20th	Night	44% 8% 12% 21% 15% 1	2200
	21st to end	Night	46% 8% 10% 19% 14% 3	2200
October	1st to 10th	Night	29% 5% 9% 22% 21% 14%	3745
	11th to 20th	Night	22% 4 8% 21% 24% 21%	3899
	21st to end	Night	30% 3 7% 24% 21% 15%	4252
November	1st to 10th	Night	34% 6% 10% 22% 22% 7%	2505
	11th to 20th	Night	31% 5% 10% 23% 23% 8%	2730
	21st to end	Night	33% 4 10% 25% 22% 5%	2730
December	1st to 10th	Night	33% 7% 11% 20% 24% 4	4380
	11th to 20th	Night	45% 4 11% 19% 15% 5%	4954
	21st to end	Night	41% 5% 16% 21% 13% 3	6633

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	42% 24% 26% 7% 1	330
	11th to 20th	Day	40% 22% 29% 8% 1	330
	21st to end	Day	39% 28% 25% 7% 1	363
February	1st to 10th	Day	29% 30% 31% 9% 1	330
	11th to 20th	Day	48% 28% 19% 5%	330
	21st to end	Day	40% 25% 29% 5%	273
March	1st to 10th	Day	32% 22% 28% 13% 4	501
	11th to 20th	Day	31% 28% 25% 12% 2 1	500
	21st to end	Day	31% 25% 27% 12% 4	550
April	1st to 10th	Day	28% 24% 26% 15% 5% 1	500
	11th to 20th	Day	24% 23% 22% 20% 9% 2	492
	21st to end	Day	22% 19% 30% 16% 9% 4	500
May	1st to 10th	Day	22% 20% 32% 14% 8% 5%	503
	11th to 20th	Day	24% 13% 24% 23% 11% 5%	527
	21st to end	Day	22% 12% 20% 22% 15% 9%	573
June	1st to 10th	Day	16% 9% 28% 26% 14% 7%	530
	11th to 20th	Day	11% 8% 25% 29% 19% 8%	530
	21st to end	Day	11% 8% 28% 29% 15% 8%	530
July	1st to 10th	Day	10% 5% 17% 32% 25% 11%	525
	11th to 20th	Day	9% 8% 14% 33% 24% 12%	524
	21st to end	Day	12% 5% 16% 32% 22% 13%	576
August	1st to 10th	Day	20% 6% 22% 24% 16% 13%	526
	11th to 20th	Day	12% 8% 17% 27% 24% 12%	519
	21st to end	Day	16% 4 15% 29% 20% 16%	583
September	1st to 10th	Day	13% 5% 17% 29% 25% 11%	530
	11th to 20th	Day	20% 7% 15% 25% 25% 8%	530
	21st to end	Day	24% 11% 21% 24% 15% 4	530
October	1st to 10th	Day	22% 11% 20% 25% 18% 5%	530
	11th to 20th	Day	18% 18% 24% 23% 14% 2	530
	21st to end	Day	23% 17% 25% 23% 9% 2	583
November	1st to 10th	Day	23% 15% 31% 20% 9% 2	488
	11th to 20th	Day	25% 24% 29% 18% 3 1	330
	21st to end	Day	30% 18% 32% 16% 4 1	330
December	1st to 10th	Day	32% 29% 27% 11% 1	330
	11th to 20th	Day	30% 18% 38% 11% 3 1	330
	21st to end	Day	32% 22% 33% 9% 4 1	363
January	1st to 10th	Night	31% 21% 26% 19% 3	840
	11th to 20th	Night	29% 20% 28% 18% 4	840
	21st to end	Night	27% 22% 31% 18% 3	924
February	1st to 10th	Night	23% 19% 31% 25% 3	840
	11th to 20th	Night	31% 24% 28% 16% 1	823
	21st to end	Night	27% 21% 32% 18% 1	692
March	1st to 10th	Night	22% 20% 31% 22% 4	809
	11th to 20th	Night	27% 18% 28% 19% 7% 1	800
	21st to end	Night	25% 18% 29% 23% 6%	880
April	1st to 10th	Night	24% 19% 26% 25% 7%	800
	11th to 20th	Night	22% 17% 24% 25% 11% 1	757
	21st to end	Night	17% 14% 22% 35% 12% 1	600
May	1st to 10th	Night	18% 13% 25% 30% 13% 1	600
	11th to 20th	Night	18% 10% 20% 31% 18% 3	591
	21st to end	Night	20% 7% 14% 26% 26% 7%	650
June	1st to 10th	Night	13% 5% 17% 33% 28% 5%	600
	11th to 20th	Night	11% 4 18% 28% 33% 6%	600
	21st to end	Night	7% 5% 17% 32% 35% 4	600
July	1st to 10th	Night	11% 4 9% 31% 40% 5%	585
	11th to 20th	Night	9% 4 9% 29% 42% 7%	582
	21st to end	Night	9% 3 11% 29% 40% 8%	657
August	1st to 10th	Night	17% 3 11% 27% 35% 7%	597
	11th to 20th	Night	16% 3 11% 27% 35% 7%	594
	21st to end	Night	14% 3 11% 26% 38% 9%	755
September	1st to 10th	Night	15% 5% 11% 30% 33% 7%	800
	11th to 20th	Night	20% 5% 10% 26% 34% 5%	800
	21st to end	Night	25% 6% 14% 27% 25% 2	800
October	1st to 10th	Night	18% 11% 18% 23% 24% 6%	800
	11th to 20th	Night	20% 14% 22% 28% 14% 2	800
	21st to end	Night	23% 14% 20% 29% 12% 2	880
November	1st to 10th	Night	21% 12% 22% 30% 13% 3	820
	11th to 20th	Night	24% 13% 22% 31% 8% 2	840
	21st to end	Night	23% 15% 25% 27% 10% 1	840
December	1st to 10th	Night	27% 18% 26% 23% 6%	840
	11th to 20th	Night	24% 18% 26% 25% 6% 1	840
	21st to end	Night	31% 16% 24% 25% 5%	924

Page generated at 2026-02-28 01:40:20. Version ac07

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