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 2025-10-04.

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	12% 9% 3 8% 5% 4 4 4 5% 7% 39%	127425
	Night	12% 9% 4 8% 5% 4 4 4 5% 6% 38%	122203
Transparency	Day	43% 6% 13% 25% 11% 1	130989
	Night	44% 6% 13% 24% 12% 1	126716
Seeing	Day	16% 10% 30% 27% 14% 4	17341
	Night	15% 9% 32% 32% 11% 1	27135

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	8% 13% 7% 17% 8% 5% 5% 3 4 6% 24%	3617
	11th to 20th	Day	6% 10% 7% 13% 7% 6% 9% 8% 8% 5% 21%	3648
	21st to end	Day	6% 16% 7% 12% 5% 5% 4 3 5% 6% 31%	4488
February	1st to 10th	Day	7% 13% 5% 11% 8% 5% 5% 4 5% 6% 32%	1800
	11th to 20th	Day	8% 11% 5% 10% 6% 5% 5% 4 6% 6% 35%	1754
	21st to end	Day	9% 12% 5% 11% 6% 4 4 4 5% 7% 33%	1534
March	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 6% 8% 44%	4843
	11th to 20th	Day	9% 10% 3 8% 5% 4 4 3 6% 7% 41%	4980
	21st to end	Day	21% 11% 4 9% 5% 5% 4 3 5% 6% 27%	5505
April	1st to 10th	Day	17% 9% 4 7% 5% 4 4 4 5% 7% 35%	2400
	11th to 20th	Day	19% 12% 4 8% 4 5% 4 3 4 5% 32%	2403
	21st to end	Day	21% 11% 3 7% 5% 4 4 3 5% 6% 31%	2600
May	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 5% 6% 50%	6110
	11th to 20th	Day	18% 7% 2 6% 3 4 3 3 5% 6% 43%	6319
	21st to end	Day	17% 7% 2 7% 4 4 4 4 5% 8% 37%	6935
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 8% 45%	2800
	11th to 20th	Day	14% 7% 3 7% 6% 5% 5% 4 6% 9% 36%	2839
	21st to end	Day	11% 5% 2 6% 5% 4 5% 5% 7% 9% 40%	3000
July	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 10% 45%	6563
	11th to 20th	Day	9% 4 1 4 3 3 3 3 5% 8% 57%	6360
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 47%	6843
August	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 6% 8% 43%	4234
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 9% 43%	4171
	21st to end	Day	9% 6% 2 7% 5% 4 4 3 6% 8% 47%	4487
September	1st to 10th	Day	11% 4 2 6% 4 4 4 3 7% 7% 47%	2400
	11th to 20th	Day	11% 6% 2 5% 5% 5% 4 4 6% 8% 45%	2360
	21st to end	Day	14% 7% 2 7% 3 3 4 4 5% 6% 47%	2110
October	1st to 10th	Day	17% 6% 3 6% 5% 4 4 3 5% 5% 42%	1930
	11th to 20th	Day	15% 11% 3 6% 4 3 4 3 6% 6% 39%	1900
	21st to end	Day	16% 12% 4 7% 4 3 4 3 3 6% 38%	2068
November	1st to 10th	Day	12% 13% 4 7% 5% 4 4 3 5% 5% 37%	1624
	11th to 20th	Day	11% 14% 6% 11% 7% 3 4 3 4 5% 29%	1600
	21st to end	Day	13% 13% 5% 8% 5% 4 4 3 4 8% 33%	1600
December	1st to 10th	Day	7% 12% 5% 13% 10% 8% 6% 4 6% 6% 23%	2610
	11th to 20th	Day	10% 16% 7% 10% 6% 4 4 3 5% 5% 29%	3008
	21st to end	Day	19% 21% 8% 19% 5% 4 3 2 3 3 14%	3982
January	1st to 10th	Night	8% 11% 6% 15% 8% 5% 4 3 4 6% 30%	6348
	11th to 20th	Night	7% 11% 5% 12% 7% 7% 10% 8% 7% 5% 21%	6318
	21st to end	Night	7% 18% 6% 10% 5% 5% 4 3 5% 5% 32%	6490
February	1st to 10th	Night	8% 11% 5% 10% 6% 6% 5% 3 5% 6% 36%	2600
	11th to 20th	Night	6% 10% 4 8% 6% 6% 5% 4 6% 7% 39%	2533
	21st to end	Night	11% 10% 4 9% 6% 5% 4 4 5% 6% 37%	2095
March	1st to 10th	Night	10% 7% 3 6% 4 4 4 3 5% 7% 48%	5052
	11th to 20th	Night	9% 7% 3 7% 5% 4 4 4 7% 10% 39%	4980
	21st to end	Night	24% 12% 4 10% 4 4 3 3 4 5% 28%	5423
April	1st to 10th	Night	16% 10% 3 6% 5% 4 4 3 5% 7% 37%	2000
	11th to 20th	Night	19% 10% 4 7% 4 3 4 3 5% 6% 35%	1944
	21st to end	Night	20% 11% 2 6% 4 4 5% 3 5% 6% 35%	1800
May	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 4 5% 52%	3789
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 7% 49%	3596
	21st to end	Night	16% 9% 3 7% 4 3 4 4 5% 7% 40%	3942
June	1st to 10th	Night	11% 5% 2 5% 3 4 5% 5% 6% 7% 48%	1600
	11th to 20th	Night	9% 4 3 7% 4 4 5% 4 7% 8% 45%	1551
	21st to end	Night	8% 5% 1 4 4 4 4 4 6% 8% 52%	1400
July	1st to 10th	Night	8% 3 1 4 4 4 5% 4 7% 9% 52%	3393
	11th to 20th	Night	6% 3 2 5% 3 4 4 3 6% 9% 55%	3617
	21st to end	Night	8% 5% 2 5% 4 4 5% 3 5% 7% 50%	4026
August	1st to 10th	Night	11% 6% 2 7% 4 3 4 3 4 7% 49%	2926
	11th to 20th	Night	13% 4 1 5% 4 3 4 4 5% 8% 49%	2888
	21st to end	Night	9% 6% 3 6% 4 3 5% 4 5% 7% 48%	3416
September	1st to 10th	Night	9% 4 2 6% 4 4 4 4 5% 7% 53%	2000
	11th to 20th	Night	9% 5% 1 5% 4 3 4 3 5% 8% 53%	2037
	21st to end	Night	12% 5% 1 4 3 3 4 3 5% 6% 52%	2270
October	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 4 7% 49%	2317
	11th to 20th	Night	15% 11% 2 6% 4 3 4 3 6% 6% 41%	2280
	21st to end	Night	17% 11% 4 6% 4 4 4 3 5% 5% 37%	2528
November	1st to 10th	Night	13% 10% 3 6% 5% 4 4 4 5% 6% 40%	2669
	11th to 20th	Night	14% 12% 4 12% 5% 4 4 3 4 5% 34%	2800
	21st to end	Night	13% 11% 4 8% 6% 5% 4 3 5% 6% 35%	2800
December	1st to 10th	Night	7% 9% 5% 14% 7% 7% 6% 4 6% 6% 30%	4580
	11th to 20th	Night	10% 16% 7% 13% 6% 5% 5% 4 4 5% 25%	5199
	21st to end	Night	20% 17% 8% 16% 5% 4 3 2 3 4 18%	6996

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	60% 6% 9% 18% 7%	3618
	11th to 20th	Day	58% 6% 10% 17% 9%	3648
	21st to end	Day	54% 6% 15% 17% 8%	4488
February	1st to 10th	Day	52% 5% 15% 19% 8% 2	1800
	11th to 20th	Day	48% 7% 11% 25% 8% 1	1763
	21st to end	Day	51% 5% 13% 21% 10% 1	1534
March	1st to 10th	Day	38% 6% 19% 25% 11% 2	4843
	11th to 20th	Day	41% 5% 14% 30% 9% 1	4980
	21st to end	Day	58% 5% 10% 19% 7% 1	5507
April	1st to 10th	Day	48% 6% 12% 20% 13% 1	2400
	11th to 20th	Day	54% 6% 9% 19% 11% 2	2403
	21st to end	Day	53% 6% 11% 20% 9%	2600
May	1st to 10th	Day	34% 5% 13% 27% 18% 2	6148
	11th to 20th	Day	41% 6% 10% 22% 15% 5%	6323
	21st to end	Day	43% 7% 10% 29% 10% 1	6949
June	1st to 10th	Day	34% 8% 14% 31% 12% 1	2800
	11th to 20th	Day	44% 8% 13% 21% 13% 1	2839
	21st to end	Day	36% 8% 17% 28% 10% 2	3000
July	1st to 10th	Day	31% 9% 17% 32% 10%	6494
	11th to 20th	Day	25% 6% 15% 38% 15% 1	6285
	21st to end	Day	31% 8% 15% 29% 16% 1	6648
August	1st to 10th	Day	37% 6% 13% 30% 12% 2	4220
	11th to 20th	Day	36% 8% 13% 33% 10%	4210
	21st to end	Day	34% 7% 17% 28% 13% 1	4486
September	1st to 10th	Day	34% 7% 15% 33% 11%	2400
	11th to 20th	Day	36% 8% 16% 31% 8%	2360
	21st to end	Day	38% 6% 11% 34% 11%	2110
October	1st to 10th	Day	33% 4 8% 34% 21%	3190
	11th to 20th	Day	38% 5% 12% 29% 14% 1	3140
	21st to end	Day	44% 4 15% 28% 9%	3382
November	1st to 10th	Day	48% 6% 13% 19% 12% 2	1624
	11th to 20th	Day	56% 6% 13% 15% 9%	1600
	21st to end	Day	50% 7% 15% 20% 6% 2	1600
December	1st to 10th	Day	59% 8% 15% 13% 4 1	2610
	11th to 20th	Day	57% 5% 13% 20% 6%	3005
	21st to end	Day	77% 4 8% 8% 4	3982
January	1st to 10th	Night	55% 5% 11% 20% 8%	6348
	11th to 20th	Night	59% 6% 11% 14% 10%	6318
	21st to end	Night	53% 6% 12% 22% 6%	6490
February	1st to 10th	Night	49% 5% 13% 21% 10% 1	2600
	11th to 20th	Night	44% 6% 15% 25% 9% 2	2534
	21st to end	Night	47% 5% 13% 22% 12% 1	2095
March	1st to 10th	Night	35% 6% 14% 26% 16% 3	5052
	11th to 20th	Night	37% 6% 17% 26% 14%	4980
	21st to end	Night	59% 4 9% 18% 9% 1	5423
April	1st to 10th	Night	47% 4 10% 23% 15% 1	2000
	11th to 20th	Night	48% 5% 9% 20% 15% 2	1944
	21st to end	Night	50% 6% 9% 20% 14% 2	1800
May	1st to 10th	Night	34% 6% 11% 27% 20% 3	3816
	11th to 20th	Night	35% 6% 11% 23% 18% 9%	3597
	21st to end	Night	43% 6% 11% 27% 13% 1	3950
June	1st to 10th	Night	32% 8% 14% 28% 17% 2	1600
	11th to 20th	Night	34% 8% 15% 24% 17% 2	1551
	21st to end	Night	28% 7% 16% 30% 16% 3	1400
July	1st to 10th	Night	27% 8% 19% 31% 15% 1	3356
	11th to 20th	Night	24% 6% 16% 37% 15% 1	3572
	21st to end	Night	32% 7% 14% 29% 15% 2	3910
August	1st to 10th	Night	35% 6% 11% 29% 14% 4	2916
	11th to 20th	Night	33% 6% 13% 32% 14% 1	2915
	21st to end	Night	34% 8% 17% 29% 11% 2	3416
September	1st to 10th	Night	29% 6% 14% 33% 17%	2000
	11th to 20th	Night	29% 7% 18% 30% 16%	2037
	21st to end	Night	33% 6% 12% 32% 16% 1	2270
October	1st to 10th	Night	24% 3 7% 35% 27% 4	3837
	11th to 20th	Night	34% 5% 13% 24% 21% 3	3779
	21st to end	Night	43% 5% 12% 27% 12%	4164
November	1st to 10th	Night	43% 6% 13% 23% 13% 1	2669
	11th to 20th	Night	53% 6% 12% 19% 11% 1	2800
	21st to end	Night	49% 6% 16% 22% 6% 1	2800
December	1st to 10th	Night	52% 8% 17% 18% 5%	4580
	11th to 20th	Night	61% 6% 10% 17% 5% 1	5201
	21st to end	Night	71% 4 9% 12% 4	6996

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	12% 10% 35% 26% 13% 3	300
	11th to 20th	Day	15% 10% 37% 33% 5% 1	300
	21st to end	Day	18% 8% 38% 25% 9% 2	330
February	1st to 10th	Day	16% 16% 41% 20% 6% 1	300
	11th to 20th	Day	15% 14% 41% 22% 6% 2	300
	21st to end	Day	17% 12% 44% 23% 4	249
March	1st to 10th	Day	18% 14% 38% 24% 6% 1	300
	11th to 20th	Day	18% 8% 41% 22% 8% 2	300
	21st to end	Day	24% 15% 28% 21% 9% 2	348
April	1st to 10th	Day	23% 15% 32% 24% 6%	500
	11th to 20th	Day	28% 16% 25% 21% 10% 1	521
	21st to end	Day	25% 13% 31% 20% 8% 2	700
May	1st to 10th	Day	18% 15% 29% 25% 11% 2	703
	11th to 20th	Day	19% 10% 29% 25% 13% 4	725
	21st to end	Day	17% 9% 29% 25% 16% 3	789
June	1st to 10th	Day	10% 7% 30% 29% 19% 5%	730
	11th to 20th	Day	15% 9% 34% 30% 10% 2	730
	21st to end	Day	13% 8% 30% 29% 15% 5%	730
July	1st to 10th	Day	9% 5% 20% 32% 25% 9%	721
	11th to 20th	Day	9% 5% 27% 31% 19% 8%	718
	21st to end	Day	5% 6% 24% 35% 23% 7%	794
August	1st to 10th	Day	10% 5% 24% 33% 23% 6%	726
	11th to 20th	Day	13% 6% 20% 30% 25% 6%	715
	21st to end	Day	9% 8% 24% 33% 21% 5%	693
September	1st to 10th	Day	13% 7% 30% 28% 17% 5%	530
	11th to 20th	Day	12% 7% 26% 32% 18% 5%	490
	21st to end	Day	15% 11% 28% 25% 15% 7%	330
October	1st to 10th	Day	18% 10% 27% 27% 13% 6%	309
	11th to 20th	Day	16% 16% 37% 22% 7% 2	300
	21st to end	Day	23% 14% 40% 16% 6% 1	330
November	1st to 10th	Day	19% 12% 37% 21% 9% 1	300
	11th to 20th	Day	19% 17% 39% 21% 4	300
	21st to end	Day	19% 14% 43% 19% 5%	300
December	1st to 10th	Day	14% 12% 42% 22% 10%	300
	11th to 20th	Day	20% 14% 37% 26% 4	300
	21st to end	Day	28% 15% 33% 20% 3 1	330
January	1st to 10th	Night	11% 11% 37% 32% 9% 1	1000
	11th to 20th	Night	17% 9% 38% 30% 7% 1	1000
	21st to end	Night	17% 11% 36% 28% 7% 1	1100
February	1st to 10th	Night	13% 11% 39% 30% 7% 1	1000
	11th to 20th	Night	11% 11% 43% 28% 7%	979
	21st to end	Night	15% 10% 37% 29% 8% 1	775
March	1st to 10th	Night	16% 8% 37% 29% 8% 1	660
	11th to 20th	Night	16% 10% 40% 27% 7%	600
	21st to end	Night	23% 10% 29% 25% 10% 2	660
April	1st to 10th	Night	19% 13% 32% 29% 7%	600
	11th to 20th	Night	23% 11% 30% 28% 7% 1	593
	21st to end	Night	24% 10% 29% 28% 9% 1	600
May	1st to 10th	Night	18% 10% 33% 32% 7% 1	600
	11th to 20th	Night	17% 8% 31% 33% 11% 1	591
	21st to end	Night	13% 8% 25% 37% 16% 1	650
June	1st to 10th	Night	11% 6% 27% 41% 15% 1	600
	11th to 20th	Night	10% 4 31% 46% 10%	600
	21st to end	Night	9% 6% 23% 44% 16% 1	600
July	1st to 10th	Night	6% 2 20% 44% 27% 2	585
	11th to 20th	Night	8% 3 24% 46% 19% 2	582
	21st to end	Night	5% 3 24% 44% 22% 2	657
August	1st to 10th	Night	8% 4 18% 48% 19% 2	597
	11th to 20th	Night	12% 2 20% 37% 26% 2	594
	21st to end	Night	7% 4 24% 38% 24% 2	660
September	1st to 10th	Night	10% 5% 25% 39% 19% 3	600
	11th to 20th	Night	10% 4 30% 35% 18% 3	600
	21st to end	Night	14% 9% 27% 33% 16% 1	670
October	1st to 10th	Night	18% 8% 29% 30% 14% 1	773
	11th to 20th	Night	21% 10% 34% 28% 7% 1	760
	21st to end	Night	24% 9% 29% 31% 7%	836
November	1st to 10th	Night	18% 10% 32% 31% 8% 1	913
	11th to 20th	Night	19% 12% 35% 26% 7%	1000
	21st to end	Night	18% 13% 36% 25% 8%	1000
December	1st to 10th	Night	16% 9% 38% 30% 7% 1	1000
	11th to 20th	Night	18% 9% 39% 27% 7% 1	1000
	21st to end	Night	25% 10% 33% 26% 5% 1	1100

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