Ramsey Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ramsey Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 5% 4 5% 7% 31%	131014
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	130479
Transparency	Day	52% 5% 10% 20% 11% 1	134633
	Night	53% 5% 10% 19% 11% 2	134930
Seeing	Day	23% 15% 28% 19% 11% 4	16363
	Night	23% 12% 26% 25% 12% 1	26264

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	17% 19% 7% 18% 5% 4 4 3 3 5% 14%	5258
	11th to 20th	Day	12% 19% 9% 18% 8% 6% 4 2 3 4 16%	5280
	21st to end	Day	18% 17% 6% 12% 10% 7% 4 3 4 4 16%	5780
February	1st to 10th	Day	18% 17% 6% 9% 7% 4 3 2 4 5% 24%	1890
	11th to 20th	Day	17% 15% 6% 14% 5% 4 4 4 5% 5% 22%	1855
	21st to end	Day	17% 16% 5% 11% 4 4 4 3 5% 5% 27%	1730
March	1st to 10th	Day	17% 14% 5% 10% 5% 4 4 3 4 6% 28%	5042
	11th to 20th	Day	12% 9% 4 9% 5% 5% 4 3 5% 6% 37%	5057
	21st to end	Day	20% 10% 4 8% 5% 4 4 3 5% 6% 30%	5465
April	1st to 10th	Day	17% 13% 3 8% 5% 4 4 3 5% 8% 30%	2275
	11th to 20th	Day	16% 12% 3 7% 6% 4 4 3 5% 6% 34%	2457
	21st to end	Day	20% 10% 3 6% 5% 5% 5% 3 5% 6% 34%	2600
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 27%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 4 6% 8% 31%	5861
	21st to end	Day	19% 8% 2 6% 4 4 5% 5% 6% 9% 31%	6652
June	1st to 10th	Day	14% 7% 3 7% 5% 6% 5% 4 6% 8% 35%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	2800
	21st to end	Day	12% 5% 3 6% 4 5% 5% 5% 6% 11% 37%	2800
July	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 4 7% 9% 42%	5995
	11th to 20th	Day	11% 6% 3 8% 6% 5% 5% 5% 7% 10% 34%	5900
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	6424
August	1st to 10th	Day	13% 7% 3 8% 6% 6% 6% 5% 6% 8% 32%	4234
	11th to 20th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 10% 34%	4174
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 9% 41%	4510
September	1st to 10th	Day	15% 6% 2 5% 3 4 4 3 4 7% 46%	2355
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 45%	2200
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 5% 6% 35%	2200
October	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 6% 43%	2028
	11th to 20th	Day	18% 10% 2 8% 4 4 4 3 6% 5% 36%	2000
	21st to end	Day	22% 10% 3 6% 4 5% 4 3 4 7% 32%	2200
November	1st to 10th	Day	12% 8% 3 6% 5% 5% 5% 4 5% 7% 42%	1927
	11th to 20th	Day	17% 14% 4 8% 5% 4 4 4 4 5% 31%	1807
	21st to end	Day	17% 15% 6% 9% 5% 3 5% 3 5% 5% 25%	1680
December	1st to 10th	Day	19% 14% 6% 14% 5% 4 4 3 4 4 22%	2680
	11th to 20th	Day	23% 17% 6% 11% 4 3 3 2 4 4 22%	3070
	21st to end	Day	24% 21% 6% 12% 5% 5% 3 3 4 4 14%	4166
January	1st to 10th	Night	15% 19% 7% 21% 6% 4 5% 3 4 4 13%	7635
	11th to 20th	Night	13% 18% 9% 16% 9% 6% 4 3 4 4 15%	7680
	21st to end	Night	19% 16% 6% 11% 7% 6% 4 2 3 4 21%	8401
February	1st to 10th	Night	19% 14% 5% 11% 6% 4 5% 3 5% 5% 24%	2730
	11th to 20th	Night	18% 14% 5% 13% 5% 5% 5% 3 4 5% 23%	2647
	21st to end	Night	20% 14% 5% 12% 6% 3 4 3 5% 4 23%	2076
March	1st to 10th	Night	23% 12% 5% 9% 4 3 3 2 3 5% 32%	5029
	11th to 20th	Night	13% 11% 4 7% 4 4 4 3 5% 7% 38%	5058
	21st to end	Night	19% 11% 3 8% 5% 4 4 3 4 5% 34%	5467
April	1st to 10th	Night	18% 12% 3 6% 6% 4 4 3 5% 7% 32%	2124
	11th to 20th	Night	16% 10% 3 8% 4 4 4 4 5% 7% 35%	1907
	21st to end	Night	16% 10% 3 7% 4 4 4 3 6% 7% 37%	1800
May	1st to 10th	Night	26% 11% 3 7% 4 4 4 3 6% 6% 26%	4033
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 5% 8% 37%	4032
	21st to end	Night	18% 7% 2 5% 4 4 4 4 6% 8% 37%	4197
June	1st to 10th	Night	9% 7% 2 7% 5% 4 4 3 6% 8% 45%	1600
	11th to 20th	Night	11% 7% 2 7% 5% 4 3 3 6% 9% 43%	1600
	21st to end	Night	10% 4 2 6% 5% 5% 6% 5% 6% 8% 44%	1600
July	1st to 10th	Night	10% 8% 3 7% 4 4 4 4 5% 8% 43%	3956
	11th to 20th	Night	11% 6% 2 7% 5% 4 5% 4 7% 10% 39%	4057
	21st to end	Night	8% 6% 3 7% 5% 4 4 4 6% 8% 44%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	2915
	11th to 20th	Night	14% 7% 3 6% 4 3 3 4 5% 7% 44%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 5% 6% 9% 45%	3353
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 6% 51%	2040
	11th to 20th	Night	9% 4 1 5% 4 4 4 3 5% 9% 53%	2200
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 6% 41%	2200
October	1st to 10th	Night	15% 5% 2 6% 4 3 4 3 5% 8% 46%	2371
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 3 5% 7% 38%	2400
	21st to end	Night	23% 8% 2 6% 4 3 4 3 4 7% 37%	2640
November	1st to 10th	Night	12% 9% 2 5% 6% 4 5% 3 4 6% 44%	2588
	11th to 20th	Night	18% 13% 3 8% 4 4 4 3 5% 6% 32%	2809
	21st to end	Night	20% 14% 5% 8% 4 4 4 3 4 6% 27%	2940
December	1st to 10th	Night	18% 16% 9% 14% 5% 4 4 3 5% 5% 18%	4720
	11th to 20th	Night	21% 19% 8% 9% 4 3 3 3 4 5% 21%	5345
	21st to end	Night	24% 19% 6% 10% 5% 4 4 3 4 6% 16%	7027

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	74% 4 9% 10% 3	5259
	11th to 20th	Day	75% 2 6% 9% 6% 1	5280
	21st to end	Day	73% 3 7% 13% 4	5780
February	1st to 10th	Day	64% 3 9% 15% 8% 1	1890
	11th to 20th	Day	65% 4 9% 15% 7%	1864
	21st to end	Day	60% 4 10% 18% 7% 2	1730
March	1st to 10th	Day	58% 4 9% 22% 7% 1	5042
	11th to 20th	Day	47% 5% 11% 25% 9% 2	5057
	21st to end	Day	55% 4 8% 19% 13% 1	5465
April	1st to 10th	Day	53% 5% 11% 18% 12% 1	2275
	11th to 20th	Day	51% 5% 10% 18% 14% 2	2457
	21st to end	Day	51% 6% 9% 22% 11% 2	2600
May	1st to 10th	Day	55% 6% 9% 16% 9% 4	5897
	11th to 20th	Day	51% 7% 10% 21% 10% 1	5865
	21st to end	Day	47% 7% 12% 21% 10% 3	6667
June	1st to 10th	Day	45% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	45% 7% 13% 22% 12% 1	2800
	21st to end	Day	40% 8% 12% 25% 14% 1	2800
July	1st to 10th	Day	38% 7% 12% 27% 15% 2	5930
	11th to 20th	Day	42% 7% 13% 24% 13% 1	5831
	21st to end	Day	41% 6% 11% 22% 20% 1	6240
August	1st to 10th	Day	47% 7% 13% 22% 10%	4221
	11th to 20th	Day	42% 7% 10% 30% 10% 1	4213
	21st to end	Day	36% 8% 13% 29% 12% 2	4510
September	1st to 10th	Day	38% 5% 10% 23% 20% 3	2355
	11th to 20th	Day	35% 5% 15% 27% 17% 2	2200
	21st to end	Day	50% 5% 10% 22% 13% 1	2200
October	1st to 10th	Day	32% 5% 9% 31% 21% 1	3301
	11th to 20th	Day	47% 5% 9% 20% 15% 4	3240
	21st to end	Day	41% 4 12% 27% 17% 1	3538
November	1st to 10th	Day	42% 6% 11% 23% 16% 2	1927
	11th to 20th	Day	56% 4 9% 19% 10% 2	1807
	21st to end	Day	61% 4 11% 16% 6% 1	1680
December	1st to 10th	Day	67% 4 12% 13% 4	2680
	11th to 20th	Day	67% 3 10% 14% 5%	3066
	21st to end	Day	75% 4 8% 10% 3	4166
January	1st to 10th	Night	76% 4 8% 10% 3	7636
	11th to 20th	Night	74% 4 8% 10% 5% 1	7680
	21st to end	Night	70% 3 7% 14% 6% 1	8401
February	1st to 10th	Night	64% 4 9% 17% 6%	2730
	11th to 20th	Night	65% 4 8% 16% 7%	2649
	21st to end	Night	64% 5% 8% 14% 9% 1	2076
March	1st to 10th	Night	58% 3 9% 20% 9% 1	5029
	11th to 20th	Night	46% 4 13% 26% 8% 3	5058
	21st to end	Night	52% 4 10% 18% 15% 1	5467
April	1st to 10th	Night	52% 5% 9% 18% 13% 2	2124
	11th to 20th	Night	48% 6% 11% 17% 15% 3	1907
	21st to end	Night	46% 5% 10% 21% 16% 2	1800
May	1st to 10th	Night	58% 6% 8% 15% 10% 3	4060
	11th to 20th	Night	46% 7% 9% 24% 13% 1	4033
	21st to end	Night	43% 6% 12% 25% 11% 2	4204
June	1st to 10th	Night	37% 7% 12% 24% 17% 3	1600
	11th to 20th	Night	37% 6% 15% 24% 16% 2	1600
	21st to end	Night	36% 9% 13% 24% 17% 1	1600
July	1st to 10th	Night	40% 6% 9% 25% 18% 2	3913
	11th to 20th	Night	39% 8% 17% 25% 12% 1	4006
	21st to end	Night	37% 6% 11% 19% 24% 2	4297
August	1st to 10th	Night	44% 7% 12% 24% 12% 1	2904
	11th to 20th	Night	40% 5% 11% 27% 16% 1	2902
	21st to end	Night	35% 6% 12% 31% 13% 2	3353
September	1st to 10th	Night	33% 7% 10% 24% 21% 5%	2040
	11th to 20th	Night	29% 6% 13% 28% 22% 2	2200
	21st to end	Night	43% 5% 12% 21% 18% 1	2200
October	1st to 10th	Night	33% 4 9% 28% 25% 2	3878
	11th to 20th	Night	39% 5% 11% 23% 15% 7%	3899
	21st to end	Night	37% 4 8% 29% 20% 2	4252
November	1st to 10th	Night	41% 5% 11% 24% 17% 3	2588
	11th to 20th	Night	54% 4 10% 20% 9% 3	2809
	21st to end	Night	59% 4 11% 20% 5% 1	2940
December	1st to 10th	Night	69% 3 10% 14% 3	4720
	11th to 20th	Night	67% 4 12% 14% 3	5348
	21st to end	Night	71% 5% 9% 13% 2	7027

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	35% 24% 27% 9% 4	330
	11th to 20th	Day	38% 29% 27% 5% 1	330
	21st to end	Day	33% 31% 26% 7% 2	363
February	1st to 10th	Day	34% 22% 34% 8% 2	330
	11th to 20th	Day	39% 27% 28% 6%	330
	21st to end	Day	37% 25% 30% 8%	273
March	1st to 10th	Day	29% 25% 30% 13% 3	537
	11th to 20th	Day	33% 21% 29% 11% 5% 1	528
	21st to end	Day	31% 23% 28% 15% 3 1	550
April	1st to 10th	Day	32% 20% 27% 13% 6% 1	500
	11th to 20th	Day	29% 19% 28% 15% 8% 2	492
	21st to end	Day	29% 16% 32% 15% 7% 1	500
May	1st to 10th	Day	29% 18% 28% 16% 6% 3	503
	11th to 20th	Day	23% 13% 31% 21% 7% 4	527
	21st to end	Day	19% 10% 26% 25% 15% 5%	573
June	1st to 10th	Day	15% 12% 33% 25% 12% 4	530
	11th to 20th	Day	18% 10% 26% 27% 15% 3	530
	21st to end	Day	14% 8% 32% 27% 15% 3	530
July	1st to 10th	Day	14% 5% 23% 29% 21% 9%	525
	11th to 20th	Day	11% 7% 27% 29% 17% 8%	524
	21st to end	Day	13% 6% 24% 31% 18% 8%	576
August	1st to 10th	Day	13% 6% 23% 30% 19% 9%	526
	11th to 20th	Day	15% 6% 21% 28% 20% 9%	519
	21st to end	Day	12% 5% 28% 26% 20% 10%	583
September	1st to 10th	Day	16% 9% 20% 26% 19% 10%	530
	11th to 20th	Day	12% 4 19% 28% 24% 13%	530
	21st to end	Day	23% 11% 25% 21% 15% 4	530
October	1st to 10th	Day	15% 11% 27% 24% 19% 5%	530
	11th to 20th	Day	22% 14% 32% 21% 9% 2	530
	21st to end	Day	31% 16% 31% 16% 5%	583
November	1st to 10th	Day	20% 20% 35% 14% 9% 2	405
	11th to 20th	Day	33% 16% 30% 13% 6% 1	286
	21st to end	Day	29% 25% 35% 9% 1	220
December	1st to 10th	Day	31% 28% 29% 8% 2 1	220
	11th to 20th	Day	34% 23% 31% 10% 2	220
	21st to end	Day	40% 21% 26% 11% 1	270
January	1st to 10th	Night	30% 20% 34% 13% 3	840
	11th to 20th	Night	32% 21% 29% 15% 3	840
	21st to end	Night	31% 21% 28% 17% 3	924
February	1st to 10th	Night	30% 18% 30% 21% 2	840
	11th to 20th	Night	30% 21% 34% 14% 2	823
	21st to end	Night	30% 20% 32% 16% 2	692
March	1st to 10th	Night	27% 19% 31% 20% 3	840
	11th to 20th	Night	29% 17% 27% 22% 5% 1	829
	21st to end	Night	27% 16% 32% 20% 5% 1	880
April	1st to 10th	Night	26% 16% 30% 22% 6%	800
	11th to 20th	Night	24% 13% 34% 20% 8% 1	723
	21st to end	Night	21% 10% 35% 28% 6%	600
May	1st to 10th	Night	25% 9% 25% 31% 9% 1	600
	11th to 20th	Night	20% 9% 26% 30% 13% 1	591
	21st to end	Night	12% 5% 24% 36% 21% 3	555
June	1st to 10th	Night	12% 6% 25% 40% 17%	400
	11th to 20th	Night	13% 5% 25% 40% 17% 1	400
	21st to end	Night	9% 3 28% 35% 25% 1	400
July	1st to 10th	Night	7% 4 16% 41% 26% 7%	535
	11th to 20th	Night	9% 2 21% 35% 30% 4	582
	21st to end	Night	12% 3 16% 39% 28% 3	657
August	1st to 10th	Night	10% 3 14% 41% 29% 3	597
	11th to 20th	Night	14% 3 12% 38% 29% 4	594
	21st to end	Night	10% 4 21% 31% 28% 6%	738
September	1st to 10th	Night	14% 4 20% 31% 26% 5%	800
	11th to 20th	Night	9% 4 15% 37% 31% 4	800
	21st to end	Night	20% 8% 22% 26% 19% 4	800
October	1st to 10th	Night	17% 9% 22% 28% 23% 3	800
	11th to 20th	Night	20% 13% 30% 24% 12% 1	800
	21st to end	Night	29% 12% 28% 24% 7%	880
November	1st to 10th	Night	19% 13% 27% 30% 11%	820
	11th to 20th	Night	28% 16% 27% 21% 7% 1	840
	21st to end	Night	31% 15% 29% 20% 6%	840
December	1st to 10th	Night	32% 18% 28% 17% 5%	840
	11th to 20th	Night	31% 18% 28% 19% 3	840
	21st to end	Night	32% 17% 29% 19% 2	924

Page generated at 2026-03-15 02:23:53. Version ac07

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