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 2024-01-12.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	75009
	Night	12% 9% 4 7% 6% 3 4 4 5% 7% 39%	74707
Transparency	Day	45% 7% 14% 23% 10% 1	74711
	Night	45% 6% 13% 23% 12% 1	74526
Seeing	Day	16% 9% 30% 27% 14% 4	14846
	Night	16% 8% 32% 32% 11% 1	24865

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% 10% 7% 9% 10% 5% 6% 5% 4 7% 27%	1517
	11th to 20th	Day	11% 15% 7% 11% 8% 3 6% 5% 4 6% 23%	1462
	21st to end	Day	10% 12% 8% 13% 7% 5% 5% 6% 6% 7% 22%	1782
February	1st to 10th	Day	8% 12% 5% 9% 8% 4 5% 5% 6% 5% 33%	1620
	11th to 20th	Day	9% 11% 6% 10% 7% 4 5% 5% 5% 6% 33%	1574
	21st to end	Day	11% 12% 5% 10% 7% 4 4 5% 5% 7% 30%	1375
March	1st to 10th	Day	13% 12% 6% 7% 6% 4 4 4 5% 7% 31%	1940
	11th to 20th	Day	14% 9% 3 6% 6% 4 4 4 5% 7% 37%	1980
	21st to end	Day	19% 11% 5% 6% 6% 4 5% 5% 5% 6% 30%	2194
April	1st to 10th	Day	17% 8% 4 7% 6% 3 4 4 5% 7% 35%	2160
	11th to 20th	Day	20% 11% 4 6% 6% 3 4 4 4 5% 32%	2159
	21st to end	Day	21% 10% 4 6% 6% 4 4 4 5% 5% 31%	2340
May	1st to 10th	Day	16% 8% 4 7% 6% 3 5% 5% 6% 6% 35%	2434
	11th to 20th	Day	17% 8% 3 6% 5% 4 4 4 5% 7% 38%	2479
	21st to end	Day	14% 7% 2 7% 6% 4 5% 5% 5% 8% 37%	2725
June	1st to 10th	Day	12% 5% 2 5% 4 3 4 5% 6% 9% 45%	2520
	11th to 20th	Day	12% 6% 3 6% 7% 3 5% 5% 6% 9% 38%	2555
	21st to end	Day	9% 4 2 6% 5% 3 5% 6% 7% 9% 42%	2700
July	1st to 10th	Day	7% 4 2 4 4 3 5% 6% 8% 10% 49%	2606
	11th to 20th	Day	8% 5% 2 4 4 2 4 5% 6% 9% 51%	2520
	21st to end	Day	6% 4 2 5% 5% 3 6% 6% 7% 10% 48%	2716
August	1st to 10th	Day	9% 5% 2 6% 6% 3 6% 6% 6% 10% 40%	2328
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 5% 6% 8% 44%	2301
	21st to end	Day	9% 4 2 5% 5% 2 4 4 6% 8% 52%	2476
September	1st to 10th	Day	11% 4 2 5% 5% 3 4 5% 7% 7% 47%	2160
	11th to 20th	Day	12% 6% 2 4 5% 3 4 5% 5% 8% 46%	2124
	21st to end	Day	15% 6% 2 6% 4 3 3 5% 5% 6% 44%	1900
October	1st to 10th	Day	18% 6% 3 5% 6% 3 5% 4 5% 5% 40%	1800
	11th to 20th	Day	15% 11% 3 6% 5% 2 3 5% 6% 6% 38%	1800
	21st to end	Day	17% 11% 5% 6% 5% 3 3 4 3 6% 38%	1960
November	1st to 10th	Day	11% 12% 5% 7% 5% 3 5% 4 5% 6% 38%	1541
	11th to 20th	Day	11% 13% 7% 10% 8% 3 4 4 4 6% 29%	1520
	21st to end	Day	14% 11% 4 7% 5% 3 5% 5% 4 8% 34%	1520
December	1st to 10th	Day	11% 13% 6% 9% 8% 5% 6% 4 5% 7% 26%	1520
	11th to 20th	Day	11% 13% 8% 10% 8% 3 6% 4 4 6% 28%	1918
	21st to end	Day	21% 16% 7% 15% 6% 3 4 2 3 4 19%	2783
January	1st to 10th	Night	9% 9% 6% 10% 8% 4 6% 5% 5% 7% 31%	2648
	11th to 20th	Night	11% 12% 7% 10% 7% 3 5% 4 6% 7% 28%	2521
	21st to end	Night	11% 15% 6% 12% 7% 4 4 5% 5% 5% 26%	2574
February	1st to 10th	Night	7% 11% 5% 9% 7% 4 5% 4 6% 6% 36%	2340
	11th to 20th	Night	6% 9% 4 7% 6% 5% 5% 5% 6% 7% 39%	2273
	21st to end	Night	12% 10% 4 8% 7% 4 4 5% 4 6% 36%	1880
March	1st to 10th	Night	13% 11% 4 7% 6% 3 5% 5% 5% 6% 36%	2034
	11th to 20th	Night	13% 10% 4 6% 6% 3 4 4 4 6% 40%	1980
	21st to end	Night	18% 9% 4 6% 5% 3 5% 4 5% 6% 36%	2161
April	1st to 10th	Night	16% 9% 4 5% 5% 3 4 4 5% 7% 38%	1800
	11th to 20th	Night	20% 10% 4 6% 5% 2 4 4 5% 6% 34%	1748
	21st to end	Night	20% 11% 2 5% 4 3 5% 4 5% 6% 35%	1620
May	1st to 10th	Night	16% 8% 3 7% 5% 2 3 5% 5% 6% 40%	1524
	11th to 20th	Night	14% 6% 3 6% 5% 3 4 5% 5% 7% 42%	1416
	21st to end	Night	11% 6% 2 7% 4 3 5% 5% 5% 8% 43%	1551
June	1st to 10th	Night	10% 4 2 4 4 3 5% 5% 6% 8% 49%	1440
	11th to 20th	Night	8% 4 3 6% 5% 3 5% 5% 7% 8% 47%	1397
	21st to end	Night	6% 4 2 4 4 3 4 5% 6% 9% 53%	1260
July	1st to 10th	Night	5% 3 2 3 3 2 4 5% 7% 9% 58%	1345
	11th to 20th	Night	5% 3 1 4 4 3 4 4 5% 10% 56%	1440
	21st to end	Night	4 3 2 4 4 2 4 4 6% 9% 59%	1600
August	1st to 10th	Night	5% 5% 2 6% 5% 3 4 4 5% 8% 52%	1611
	11th to 20th	Night	11% 4 2 4 4 3 4 4 5% 8% 52%	1600
	21st to end	Night	7% 5% 2 4 5% 2 4 5% 5% 6% 56%	1880
September	1st to 10th	Night	9% 3 2 5% 5% 3 4 5% 5% 7% 53%	1800
	11th to 20th	Night	10% 4 2 3 5% 2 4 4 6% 8% 53%	1833
	21st to end	Night	13% 5% 2 4 4 3 4 4 5% 6% 49%	2042
October	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 4 7% 48%	2160
	11th to 20th	Night	16% 11% 2 5% 5% 2 4 3 6% 7% 39%	2160
	21st to end	Night	17% 10% 4 6% 4 3 4 4 5% 5% 37%	2394
November	1st to 10th	Night	13% 9% 4 5% 5% 3 5% 5% 5% 6% 41%	2532
	11th to 20th	Night	14% 11% 5% 11% 6% 3 5% 3 4 5% 34%	2660
	21st to end	Night	13% 9% 4 7% 7% 4 4 4 5% 7% 37%	2660
December	1st to 10th	Night	11% 11% 5% 11% 8% 4 5% 4 5% 6% 31%	2660
	11th to 20th	Night	11% 14% 6% 10% 7% 4 5% 5% 5% 6% 27%	3279
	21st to end	Night	21% 15% 5% 12% 6% 3 4 3 3 5% 23%	4884

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	55% 7% 12% 19% 6%	1518
	11th to 20th	Day	61% 7% 11% 13% 7%	1462
	21st to end	Day	59% 7% 12% 14% 7%	1782
February	1st to 10th	Day	51% 5% 16% 20% 7% 1	1620
	11th to 20th	Day	50% 7% 12% 23% 8% 1	1583
	21st to end	Day	54% 5% 10% 20% 10%	1375
March	1st to 10th	Day	52% 6% 14% 19% 8% 1	1940
	11th to 20th	Day	47% 6% 15% 23% 8% 1	1980
	21st to end	Day	54% 5% 11% 22% 6% 1	2194
April	1st to 10th	Day	48% 6% 12% 20% 12% 1	2160
	11th to 20th	Day	54% 5% 9% 19% 11% 2	2159
	21st to end	Day	54% 6% 12% 19% 9%	2340
May	1st to 10th	Day	48% 7% 13% 20% 10% 2	2434
	11th to 20th	Day	45% 6% 14% 23% 12% 1	2483
	21st to end	Day	45% 7% 11% 22% 13% 1	2725
June	1st to 10th	Day	35% 9% 14% 31% 11% 1	2520
	11th to 20th	Day	41% 8% 14% 23% 13% 1	2555
	21st to end	Day	34% 8% 18% 29% 9% 2	2700
July	1st to 10th	Day	27% 9% 17% 32% 14% 1	2537
	11th to 20th	Day	29% 9% 15% 27% 18% 3	2445
	21st to end	Day	29% 9% 17% 27% 16% 2	2565
August	1st to 10th	Day	37% 8% 15% 28% 9% 3	2314
	11th to 20th	Day	36% 9% 14% 30% 11%	2301
	21st to end	Day	29% 7% 15% 31% 16% 1	2476
September	1st to 10th	Day	35% 6% 15% 32% 11%	2160
	11th to 20th	Day	36% 8% 16% 32% 8%	2124
	21st to end	Day	40% 7% 12% 33% 8%	1900
October	1st to 10th	Day	46% 6% 11% 25% 12%	1800
	11th to 20th	Day	45% 6% 14% 25% 10% 1	1800
	21st to end	Day	49% 5% 13% 23% 9%	1960
November	1st to 10th	Day	48% 6% 12% 19% 12% 2	1541
	11th to 20th	Day	56% 6% 13% 15% 9%	1520
	21st to end	Day	49% 7% 16% 21% 6% 2	1520
December	1st to 10th	Day	57% 8% 16% 14% 5%	1520
	11th to 20th	Day	57% 5% 18% 16% 4	1915
	21st to end	Day	71% 4 9% 10% 6%	2783
January	1st to 10th	Night	52% 5% 14% 19% 9% 1	2648
	11th to 20th	Night	55% 5% 15% 17% 7% 1	2521
	21st to end	Night	59% 5% 12% 15% 8% 1	2574
February	1st to 10th	Night	48% 5% 14% 22% 10% 1	2340
	11th to 20th	Night	43% 6% 15% 25% 9% 2	2274
	21st to end	Night	49% 5% 11% 21% 13% 1	1880
March	1st to 10th	Night	48% 7% 15% 22% 8%	2034
	11th to 20th	Night	46% 5% 13% 22% 14% 1	1980
	21st to end	Night	50% 5% 11% 24% 8% 1	2161
April	1st to 10th	Night	47% 5% 10% 23% 15% 1	1800
	11th to 20th	Night	50% 5% 9% 20% 14% 2	1748
	21st to end	Night	50% 6% 9% 20% 13% 2	1620
May	1st to 10th	Night	44% 6% 13% 22% 14% 1	1524
	11th to 20th	Night	40% 8% 13% 23% 14% 2	1417
	21st to end	Night	39% 7% 13% 22% 17% 2	1552
June	1st to 10th	Night	31% 8% 15% 28% 17% 2	1440
	11th to 20th	Night	33% 8% 14% 25% 17% 2	1397
	21st to end	Night	26% 7% 17% 31% 16% 3	1260
July	1st to 10th	Night	21% 7% 16% 35% 20% 1	1306
	11th to 20th	Night	24% 6% 17% 32% 17% 4	1392
	21st to end	Night	22% 6% 16% 33% 18% 5%	1511
August	1st to 10th	Night	30% 7% 13% 31% 15% 4	1601
	11th to 20th	Night	31% 6% 16% 30% 15% 2	1600
	21st to end	Night	28% 8% 12% 31% 18% 3	1880
September	1st to 10th	Night	29% 6% 14% 33% 17%	1800
	11th to 20th	Night	29% 7% 17% 31% 17%	1833
	21st to end	Night	35% 6% 12% 31% 14% 1	2042
October	1st to 10th	Night	37% 5% 11% 30% 15% 2	2160
	11th to 20th	Night	44% 6% 14% 23% 13% 1	2160
	21st to end	Night	48% 6% 12% 22% 11% 1	2394
November	1st to 10th	Night	43% 6% 14% 23% 14% 2	2532
	11th to 20th	Night	53% 6% 12% 18% 10% 1	2660
	21st to end	Night	48% 6% 16% 23% 6% 1	2660
December	1st to 10th	Night	54% 6% 14% 20% 6%	2660
	11th to 20th	Night	56% 7% 13% 17% 5% 2	3281
	21st to end	Night	65% 5% 11% 15% 4	4884

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	14% 9% 35% 27% 12% 3	270
	11th to 20th	Day	17% 7% 33% 35% 6% 1	243
	21st to end	Day	19% 6% 39% 25% 8% 3	264
February	1st to 10th	Day	14% 17% 43% 21% 5%	240
	11th to 20th	Day	17% 15% 40% 20% 5% 3	240
	21st to end	Day	18% 11% 42% 24% 4	198
March	1st to 10th	Day	18% 14% 35% 25% 7% 1	240
	11th to 20th	Day	19% 8% 39% 23% 10% 3	240
	21st to end	Day	24% 16% 25% 22% 11% 3	280
April	1st to 10th	Day	24% 13% 32% 24% 7%	420
	11th to 20th	Day	29% 12% 24% 23% 11% 1	437
	21st to end	Day	25% 12% 31% 20% 9% 3	600
May	1st to 10th	Day	19% 14% 30% 26% 10% 1	603
	11th to 20th	Day	19% 11% 30% 23% 14% 4	625
	21st to end	Day	17% 8% 28% 26% 17% 3	679
June	1st to 10th	Day	10% 6% 29% 29% 20% 6%	630
	11th to 20th	Day	13% 7% 35% 32% 11% 3	630
	21st to end	Day	11% 7% 28% 31% 17% 6%	630
July	1st to 10th	Day	9% 5% 19% 32% 26% 10%	621
	11th to 20th	Day	9% 5% 27% 31% 19% 8%	618
	21st to end	Day	6% 5% 23% 36% 24% 7%	684
August	1st to 10th	Day	10% 4 22% 34% 24% 6%	626
	11th to 20th	Day	12% 5% 20% 31% 25% 7%	615
	21st to end	Day	9% 7% 23% 32% 23% 6%	595
September	1st to 10th	Day	14% 6% 29% 28% 19% 4	450
	11th to 20th	Day	13% 6% 26% 31% 18% 6%	414
	21st to end	Day	17% 12% 30% 24% 11% 6%	270
October	1st to 10th	Day	19% 9% 27% 24% 14% 7%	270
	11th to 20th	Day	15% 18% 34% 23% 8% 2	270
	21st to end	Day	23% 14% 39% 16% 6% 1	297
November	1st to 10th	Day	18% 12% 36% 23% 9% 1	270
	11th to 20th	Day	17% 17% 41% 22% 4	270
	21st to end	Day	19% 13% 43% 20% 5%	270
December	1st to 10th	Day	13% 12% 41% 23% 11%	270
	11th to 20th	Day	19% 13% 37% 27% 4	270
	21st to end	Day	28% 15% 34% 20% 2 1	297
January	1st to 10th	Night	11% 11% 37% 31% 9% 1	950
	11th to 20th	Night	17% 8% 36% 30% 7% 1	906
	21st to end	Night	18% 10% 36% 28% 7% 1	990
February	1st to 10th	Night	11% 11% 39% 31% 7% 1	900
	11th to 20th	Night	12% 11% 42% 27% 7%	879
	21st to end	Night	17% 10% 35% 29% 9% 1	696
March	1st to 10th	Night	17% 8% 36% 29% 9% 1	594
	11th to 20th	Night	17% 8% 40% 28% 7%	540
	21st to end	Night	21% 11% 28% 26% 11% 2	594
April	1st to 10th	Night	19% 11% 34% 28% 7%	540
	11th to 20th	Night	23% 10% 30% 28% 8% 1	533
	21st to end	Night	24% 10% 29% 28% 9% 1	540
May	1st to 10th	Night	19% 10% 32% 33% 6% 1	540
	11th to 20th	Night	16% 8% 31% 32% 12% 1	531
	21st to end	Night	13% 8% 25% 37% 16% 1	584
June	1st to 10th	Night	11% 5% 25% 43% 16% 1	540
	11th to 20th	Night	9% 3 31% 46% 11%	540
	21st to end	Night	8% 5% 22% 46% 18% 1	540
July	1st to 10th	Night	5% 2 20% 44% 27% 2	525
	11th to 20th	Night	7% 2 24% 47% 18% 2	522
	21st to end	Night	5% 3 24% 45% 21% 2	591
August	1st to 10th	Night	7% 4 18% 50% 20% 2	537
	11th to 20th	Night	13% 2 20% 36% 27% 2	534
	21st to end	Night	8% 4 24% 37% 25% 3	594
September	1st to 10th	Night	10% 4 24% 39% 20% 3	540
	11th to 20th	Night	11% 5% 29% 34% 18% 3	540
	21st to end	Night	15% 9% 29% 32% 13% 1	602
October	1st to 10th	Night	18% 8% 29% 30% 14% 1	720
	11th to 20th	Night	22% 10% 33% 28% 7% 1	720
	21st to end	Night	24% 9% 28% 31% 7%	792
November	1st to 10th	Night	18% 10% 31% 32% 8% 1	866
	11th to 20th	Night	18% 12% 35% 27% 8%	950
	21st to end	Night	18% 12% 36% 25% 8%	950
December	1st to 10th	Night	16% 9% 37% 30% 7% 1	950
	11th to 20th	Night	18% 8% 39% 27% 7% 1	950
	21st to end	Night	25% 10% 34% 26% 5%	1045

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