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 2024-07-03.

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	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	89704
	Night	14% 7% 2 6% 4 4 4 4 6% 8% 40%	89462
Transparency	Day	42% 8% 12% 20% 14% 3	89478
	Night	38% 8% 14% 22% 14% 4	89292
Seeing	Day	22% 15% 24% 21% 13% 6%	14642
	Night	20% 13% 21% 26% 17% 3	24752

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	14% 6% 2 5% 5% 3 2 3 6% 10% 42%	2836
	11th to 20th	Day	11% 8% 3 7% 5% 4 4 4 5% 7% 42%	2840
	21st to end	Day	20% 8% 2 7% 4 3 3 3 5% 6% 40%	3124
February	1st to 10th	Day	12% 9% 3 7% 5% 5% 5% 4 6% 7% 38%	1710
	11th to 20th	Day	16% 10% 4 7% 4 4 4 3 4 7% 38%	1848
	21st to end	Day	17% 7% 2 7% 5% 4 4 3 5% 7% 37%	1570
March	1st to 10th	Day	21% 6% 2 6% 4 4 4 3 5% 6% 38%	3428
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 5% 6% 49%	3480
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 38%	3815
April	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	2090
	11th to 20th	Day	17% 6% 3 5% 4 4 4 3 6% 8% 40%	2268
	21st to end	Day	15% 7% 2 6% 5% 5% 4 3 5% 7% 39%	2470
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 6% 4 7% 8% 31%	4082
	11th to 20th	Day	18% 8% 4 9% 5% 5% 6% 4 7% 8% 27%	4081
	21st to end	Day	11% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	4476
June	1st to 10th	Day	14% 7% 2 6% 6% 6% 7% 5% 7% 9% 31%	2470
	11th to 20th	Day	10% 7% 3 8% 6% 5% 7% 6% 9% 8% 32%	2470
	21st to end	Day	10% 6% 2 7% 7% 7% 7% 5% 8% 10% 32%	2470
July	1st to 10th	Day	11% 8% 3 9% 8% 6% 6% 5% 8% 10% 26%	2366
	11th to 20th	Day	10% 9% 3 9% 8% 6% 7% 5% 8% 9% 25%	2340
	21st to end	Day	11% 8% 3 10% 7% 7% 8% 5% 8% 8% 25%	2547
August	1st to 10th	Day	14% 8% 3 8% 7% 7% 7% 5% 8% 10% 23%	2327
	11th to 20th	Day	13% 8% 3 9% 8% 8% 7% 6% 7% 8% 23%	2303
	21st to end	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 9% 30%	2451
September	1st to 10th	Day	15% 7% 2 7% 6% 6% 6% 3 7% 8% 33%	1980
	11th to 20th	Day	18% 8% 3 7% 5% 5% 5% 5% 6% 8% 31%	1980
	21st to end	Day	20% 6% 2 6% 5% 4 4 4 6% 6% 35%	1980
October	1st to 10th	Day	18% 6% 2 5% 4 4 3 4 4 6% 45%	1900
	11th to 20th	Day	14% 6% 2 5% 3 4 3 3 5% 7% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 3 3 3 5% 6% 48%	1980
November	1st to 10th	Day	12% 6% 2 4 4 3 3 3 3 6% 54%	1815
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 4 7% 50%	1710
	21st to end	Day	14% 6% 3 4 4 3 3 3 5% 8% 46%	1710
December	1st to 10th	Day	15% 9% 3 9% 6% 5% 5% 3 5% 7% 33%	1710
	11th to 20th	Day	13% 5% 2 5% 4 3 3 4 6% 9% 46%	2153
	21st to end	Day	18% 6% 2 6% 4 5% 5% 4 6% 7% 38%	3124
January	1st to 10th	Night	13% 6% 2 5% 5% 4 4 4 7% 8% 41%	4109
	11th to 20th	Night	10% 6% 2 8% 6% 4 5% 3 6% 7% 41%	4120
	21st to end	Night	19% 8% 2 5% 5% 4 3 3 5% 6% 39%	4532
February	1st to 10th	Night	12% 7% 2 6% 4 4 4 5% 7% 8% 41%	2468
	11th to 20th	Night	16% 9% 2 5% 4 4 5% 3 6% 9% 37%	2218
	21st to end	Night	13% 8% 2 7% 5% 5% 5% 4 7% 8% 36%	1884
March	1st to 10th	Night	22% 10% 3 7% 5% 4 4 3 5% 6% 31%	3423
	11th to 20th	Night	9% 6% 2 5% 5% 5% 4 4 5% 7% 47%	3480
	21st to end	Night	16% 7% 1 5% 4 4 4 4 5% 9% 42%	3817
April	1st to 10th	Night	10% 7% 2 6% 5% 3 4 4 7% 9% 42%	2090
	11th to 20th	Night	15% 6% 2 4 4 3 4 4 6% 8% 43%	1875
	21st to end	Night	13% 5% 2 6% 4 4 4 4 5% 8% 45%	1710
May	1st to 10th	Night	15% 8% 2 7% 4 4 5% 4 8% 9% 33%	2813
	11th to 20th	Night	19% 8% 3 6% 5% 3 5% 3 6% 9% 33%	2812
	21st to end	Night	10% 6% 2 6% 4 4 5% 5% 7% 11% 40%	3079
June	1st to 10th	Night	10% 7% 2 7% 4 4 6% 5% 7% 12% 36%	1710
	11th to 20th	Night	9% 6% 2 5% 5% 4 5% 4 8% 11% 41%	1710
	21st to end	Night	7% 5% 2 5% 6% 5% 5% 5% 9% 12% 39%	1710
July	1st to 10th	Night	10% 6% 2 5% 6% 4 5% 5% 8% 11% 37%	1638
	11th to 20th	Night	9% 5% 2 6% 4 6% 6% 5% 8% 13% 37%	1620
	21st to end	Night	6% 5% 2 7% 6% 6% 7% 5% 8% 12% 35%	1771
August	1st to 10th	Night	14% 6% 2 6% 5% 5% 5% 4 6% 11% 35%	1611
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 5% 6% 9% 34%	1598
	21st to end	Night	11% 7% 2 6% 4 5% 5% 5% 7% 10% 38%	1884
September	1st to 10th	Night	12% 8% 3 6% 5% 4 4 4 6% 8% 41%	1980
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 5% 7% 39%	1980
	21st to end	Night	19% 8% 2 6% 3 4 4 3 5% 5% 42%	1980
October	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 5% 7% 45%	2060
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	2160
	21st to end	Night	13% 6% 2 4 3 3 3 3 3 7% 52%	2376
November	1st to 10th	Night	13% 8% 2 4 3 3 3 2 6% 7% 49%	2260
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 5% 7% 50%	2470
	21st to end	Night	13% 6% 2 5% 4 3 4 4 6% 7% 46%	2470
December	1st to 10th	Night	15% 8% 3 8% 4 4 5% 3 5% 8% 36%	2470
	11th to 20th	Night	13% 7% 2 7% 5% 5% 4 3 5% 7% 41%	3042
	21st to end	Night	20% 9% 3 7% 4 4 4 4 6% 8% 32%	4532

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% 3 12% 29% 19% 4	2837
	11th to 20th	Day	39% 5% 12% 21% 19% 4	2840
	21st to end	Day	44% 4 8% 24% 17% 4	3124
February	1st to 10th	Day	41% 5% 15% 20% 15% 3	1710
	11th to 20th	Day	45% 5% 14% 17% 16% 4	1858
	21st to end	Day	45% 6% 11% 19% 14% 5%	1570
March	1st to 10th	Day	42% 8% 11% 19% 16% 4	3428
	11th to 20th	Day	34% 6% 9% 24% 18% 10%	3480
	21st to end	Day	44% 5% 10% 23% 13% 4	3815
April	1st to 10th	Day	36% 7% 13% 24% 17% 4	2090
	11th to 20th	Day	40% 6% 12% 19% 18% 5%	2268
	21st to end	Day	42% 8% 13% 23% 12% 2	2470
May	1st to 10th	Day	47% 11% 12% 20% 9% 1	4120
	11th to 20th	Day	50% 9% 13% 21% 6% 1	4085
	21st to end	Day	36% 11% 15% 20% 15% 2	4489
June	1st to 10th	Day	44% 12% 15% 19% 9% 1	2470
	11th to 20th	Day	43% 11% 16% 18% 12% 2	2470
	21st to end	Day	42% 12% 14% 17% 13% 1	2470
July	1st to 10th	Day	46% 14% 15% 15% 9% 1	2301
	11th to 20th	Day	48% 14% 16% 11% 10% 1	2271
	21st to end	Day	48% 14% 16% 16% 6% 1	2405
August	1st to 10th	Day	51% 12% 15% 16% 6%	2314
	11th to 20th	Day	52% 13% 13% 14% 6% 1	2303
	21st to end	Day	45% 11% 16% 19% 7% 3	2451
September	1st to 10th	Day	44% 8% 14% 18% 13% 3	1980
	11th to 20th	Day	46% 7% 12% 19% 14% 2	1980
	21st to end	Day	46% 8% 12% 17% 14% 3	1980
October	1st to 10th	Day	40% 5% 10% 20% 20% 5%	1900
	11th to 20th	Day	34% 5% 11% 24% 19% 6%	1800
	21st to end	Day	34% 4 9% 26% 19% 7%	1980
November	1st to 10th	Day	31% 4 11% 21% 27% 7%	1815
	11th to 20th	Day	33% 5% 10% 22% 23% 7%	1710
	21st to end	Day	36% 3 9% 24% 23% 4	1710
December	1st to 10th	Day	48% 6% 10% 18% 14% 4	1710
	11th to 20th	Day	32% 5% 13% 22% 20% 7%	2150
	21st to end	Day	43% 6% 9% 25% 15% 2	3124
January	1st to 10th	Night	36% 5% 14% 27% 13% 6%	4109
	11th to 20th	Night	37% 6% 11% 18% 20% 8%	4120
	21st to end	Night	45% 4 11% 19% 15% 6%	4532
February	1st to 10th	Night	38% 6% 14% 23% 16% 3	2468
	11th to 20th	Night	42% 5% 14% 20% 13% 6%	2219
	21st to end	Night	42% 8% 12% 19% 13% 5%	1884
March	1st to 10th	Night	47% 10% 10% 17% 12% 4	3423
	11th to 20th	Night	31% 7% 10% 26% 17% 10%	3480
	21st to end	Night	38% 7% 11% 26% 12% 7%	3817
April	1st to 10th	Night	34% 8% 15% 24% 15% 5%	2090
	11th to 20th	Night	36% 6% 13% 22% 17% 6%	1875
	21st to end	Night	36% 7% 16% 25% 15% 2	1710
May	1st to 10th	Night	43% 12% 15% 23% 7% 1	2840
	11th to 20th	Night	44% 12% 14% 21% 7%	2813
	21st to end	Night	32% 13% 20% 23% 11% 1	3087
June	1st to 10th	Night	38% 13% 18% 21% 10% 1	1710
	11th to 20th	Night	33% 12% 22% 23% 10% 1	1710
	21st to end	Night	33% 13% 19% 20% 11% 3	1710
July	1st to 10th	Night	36% 14% 18% 18% 11% 2	1593
	11th to 20th	Night	36% 14% 23% 17% 8% 2	1566
	21st to end	Night	35% 15% 22% 20% 7% 1	1672
August	1st to 10th	Night	41% 13% 18% 17% 10%	1600
	11th to 20th	Night	43% 13% 15% 18% 9% 1	1598
	21st to end	Night	38% 11% 18% 22% 9% 2	1884
September	1st to 10th	Night	40% 9% 14% 20% 14% 3	1980
	11th to 20th	Night	43% 8% 12% 20% 16% 1	1980
	21st to end	Night	43% 8% 10% 20% 16% 3	1980
October	1st to 10th	Night	38% 6% 12% 21% 21% 3	2060
	11th to 20th	Night	35% 5% 10% 25% 19% 5%	2160
	21st to end	Night	32% 4 9% 24% 22% 8%	2376
November	1st to 10th	Night	33% 6% 10% 21% 23% 7%	2260
	11th to 20th	Night	32% 4 10% 22% 22% 9%	2470
	21st to end	Night	34% 4 11% 26% 21% 5%	2470
December	1st to 10th	Night	43% 7% 12% 21% 13% 4	2470
	11th to 20th	Night	38% 4 12% 21% 20% 5%	3044
	21st to end	Night	47% 6% 14% 20% 12% 1	4532

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	37% 26% 27% 8% 1	270
	11th to 20th	Day	36% 22% 32% 9% 1	270
	21st to end	Day	34% 31% 26% 7% 1	297
February	1st to 10th	Day	27% 27% 33% 11% 1	270
	11th to 20th	Day	45% 30% 20% 5%	270
	21st to end	Day	40% 26% 29% 5%	225
March	1st to 10th	Day	29% 23% 28% 15% 5%	453
	11th to 20th	Day	30% 27% 26% 12% 3 1	460
	21st to end	Day	29% 25% 28% 13% 5%	506
April	1st to 10th	Day	27% 23% 27% 16% 5% 1	460
	11th to 20th	Day	24% 22% 22% 20% 10% 2	452
	21st to end	Day	23% 19% 28% 16% 9% 4	460
May	1st to 10th	Day	22% 19% 32% 14% 8% 5%	463
	11th to 20th	Day	22% 13% 24% 24% 12% 6%	487
	21st to end	Day	19% 11% 20% 23% 16% 10%	529
June	1st to 10th	Day	16% 8% 27% 28% 15% 7%	490
	11th to 20th	Day	11% 8% 24% 29% 20% 9%	490
	21st to end	Day	12% 9% 29% 28% 14% 9%	490
July	1st to 10th	Day	11% 4 17% 29% 26% 12%	453
	11th to 20th	Day	9% 7% 14% 33% 24% 12%	444
	21st to end	Day	12% 4 14% 34% 23% 13%	488
August	1st to 10th	Day	17% 5% 21% 24% 17% 15%	446
	11th to 20th	Day	12% 6% 15% 27% 26% 14%	439
	21st to end	Day	17% 4 12% 28% 22% 17%	495
September	1st to 10th	Day	14% 3 14% 29% 27% 12%	450
	11th to 20th	Day	20% 6% 14% 25% 27% 8%	450
	21st to end	Day	22% 11% 20% 25% 17% 5%	450
October	1st to 10th	Day	26% 10% 18% 23% 18% 5%	450
	11th to 20th	Day	19% 18% 24% 22% 15% 2	450
	21st to end	Day	21% 17% 26% 24% 9% 3	495
November	1st to 10th	Day	22% 13% 30% 22% 11% 3	413
	11th to 20th	Day	23% 23% 28% 21% 4 1	270
	21st to end	Day	23% 19% 33% 19% 5% 1	270
December	1st to 10th	Day	30% 27% 28% 13% 1	270
	11th to 20th	Day	27% 18% 38% 13% 3 1	270
	21st to end	Day	30% 23% 33% 10% 4 1	297
January	1st to 10th	Night	28% 21% 27% 20% 4	760
	11th to 20th	Night	27% 19% 29% 20% 5%	760
	21st to end	Night	25% 21% 31% 19% 3	836
February	1st to 10th	Night	21% 19% 31% 26% 3	760
	11th to 20th	Night	30% 24% 28% 16% 1	743
	21st to end	Night	26% 21% 32% 19% 1	628
March	1st to 10th	Night	21% 20% 31% 23% 4	760
	11th to 20th	Night	27% 19% 28% 19% 7% 1	760
	21st to end	Night	24% 18% 29% 23% 7%	836
April	1st to 10th	Night	22% 19% 27% 25% 7%	760
	11th to 20th	Night	22% 17% 23% 25% 12% 1	719
	21st to end	Night	17% 14% 22% 35% 12% 1	570
May	1st to 10th	Night	19% 12% 24% 30% 13% 1	570
	11th to 20th	Night	17% 9% 21% 31% 19% 3	561
	21st to end	Night	17% 7% 14% 26% 28% 8%	617
June	1st to 10th	Night	12% 5% 16% 34% 28% 5%	570
	11th to 20th	Night	11% 3 18% 27% 33% 7%	570
	21st to end	Night	8% 5% 18% 32% 35% 4	570
July	1st to 10th	Night	11% 3 9% 31% 40% 6%	531
	11th to 20th	Night	9% 4 8% 28% 43% 7%	522
	21st to end	Night	8% 2 11% 29% 42% 9%	591
August	1st to 10th	Night	15% 3 10% 27% 37% 7%	537
	11th to 20th	Night	16% 3 9% 27% 37% 8%	534
	21st to end	Night	14% 3 10% 27% 39% 9%	679
September	1st to 10th	Night	15% 5% 10% 28% 34% 8%	720
	11th to 20th	Night	20% 5% 10% 25% 34% 5%	720
	21st to end	Night	23% 7% 14% 27% 27% 3	720
October	1st to 10th	Night	19% 11% 18% 22% 24% 6%	720
	11th to 20th	Night	20% 14% 22% 28% 14% 3	720
	21st to end	Night	21% 15% 20% 29% 12% 2	792
November	1st to 10th	Night	20% 12% 21% 30% 14% 3	740
	11th to 20th	Night	24% 12% 21% 33% 9% 2	760
	21st to end	Night	21% 15% 24% 28% 11% 1	760
December	1st to 10th	Night	28% 17% 24% 24% 7%	760
	11th to 20th	Night	22% 18% 26% 26% 6% 1	760
	21st to end	Night	30% 16% 24% 25% 5%	836

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