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 2024-11-05.

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% 38%	101959
	Night	12% 10% 3 8% 5% 4 4 3 5% 6% 39%	97154
Transparency	Day	43% 7% 13% 25% 11% 1	105565
	Night	43% 6% 13% 24% 13% 1	101689
Seeing	Day	15% 10% 30% 27% 14% 4	16041
	Night	15% 8% 32% 32% 11% 1	25901

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	12% 16% 8% 16% 6% 5% 4 3 4 6% 21%	2527
	11th to 20th	Day	6% 11% 6% 14% 8% 7% 9% 7% 5% 5% 21%	2544
	21st to end	Day	8% 20% 8% 12% 5% 5% 4 4 5% 6% 24%	3135
February	1st to 10th	Day	7% 13% 5% 10% 7% 5% 5% 4 5% 5% 33%	1710
	11th to 20th	Day	8% 12% 5% 9% 6% 5% 5% 4 5% 6% 35%	1664
	21st to end	Day	10% 13% 5% 11% 6% 4 4 4 5% 7% 32%	1459
March	1st to 10th	Day	12% 11% 4 8% 5% 5% 4 3 5% 6% 38%	3371
	11th to 20th	Day	10% 10% 3 9% 5% 5% 4 3 5% 7% 41%	3480
	21st to end	Day	22% 12% 5% 10% 6% 5% 4 3 4 5% 23%	3843
April	1st to 10th	Day	17% 9% 4 7% 5% 4 4 4 5% 7% 35%	2280
	11th to 20th	Day	20% 11% 4 8% 4 5% 4 3 5% 5% 32%	2281
	21st to end	Day	21% 11% 3 7% 5% 4 4 3 5% 6% 31%	2470
May	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 41%	4260
	11th to 20th	Day	14% 6% 2 6% 3 4 4 3 5% 7% 45%	4399
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 5% 8% 35%	4824
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 45%	2660
	11th to 20th	Day	13% 6% 3 7% 6% 5% 4 4 7% 9% 37%	2697
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 9% 41%	2850
July	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 6% 8% 10% 38%	4578
	11th to 20th	Day	5% 3 1 4 3 3 3 4 6% 8% 60%	4440
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 9% 48%	4760
August	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 43%	4104
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 9% 43%	4041
	21st to end	Day	10% 6% 3 7% 5% 4 4 3 6% 7% 46%	4350
September	1st to 10th	Day	11% 4 2 6% 4 4 4 3 6% 7% 49%	2280
	11th to 20th	Day	11% 6% 2 5% 5% 5% 4 4 5% 8% 46%	2242
	21st to end	Day	14% 6% 2 7% 3 3 4 4 5% 6% 46%	2005
October	1st to 10th	Day	17% 6% 3 6% 5% 4 4 3 5% 5% 42%	1900
	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% 6% 37%	1576
	11th to 20th	Day	11% 14% 6% 12% 7% 3 4 3 5% 5% 29%	1520
	21st to end	Day	13% 12% 4 8% 5% 4 4 4 5% 8% 33%	1520
December	1st to 10th	Day	10% 14% 5% 11% 7% 7% 5% 3 5% 6% 26%	1520
	11th to 20th	Day	11% 14% 7% 11% 7% 5% 5% 3 4 5% 28%	1918
	21st to end	Day	21% 17% 7% 16% 4 4 3 2 3 4 19%	2783
January	1st to 10th	Night	11% 14% 7% 14% 6% 4 4 3 4 5% 28%	4428
	11th to 20th	Night	7% 11% 5% 13% 8% 7% 9% 6% 6% 6% 21%	4412
	21st to end	Night	9% 22% 7% 12% 5% 5% 4 3 4 4 26%	4532
February	1st to 10th	Night	7% 11% 5% 10% 6% 6% 5% 3 5% 6% 36%	2470
	11th to 20th	Night	6% 10% 4 8% 6% 6% 5% 4 6% 7% 39%	2403
	21st to end	Night	11% 10% 4 9% 6% 5% 4 3 5% 6% 37%	1994
March	1st to 10th	Night	9% 8% 3 6% 4 4 4 4 5% 7% 47%	3510
	11th to 20th	Night	8% 9% 3 7% 5% 5% 4 4 6% 8% 41%	3480
	21st to end	Night	26% 12% 3 10% 4 3 3 2 4 4 28%	3787
April	1st to 10th	Night	16% 10% 3 6% 5% 4 4 3 5% 7% 37%	1900
	11th to 20th	Night	19% 10% 4 7% 4 3 4 4 5% 5% 35%	1846
	21st to end	Night	20% 11% 2 6% 4 4 5% 3 5% 6% 35%	1710
May	1st to 10th	Night	13% 8% 2 7% 5% 4 4 3 5% 5% 43%	2643
	11th to 20th	Night	14% 5% 2 4 3 3 3 4 6% 7% 49%	2506
	21st to end	Night	14% 8% 3 7% 4 3 5% 4 5% 7% 41%	2743
June	1st to 10th	Night	11% 4 1 5% 4 4 5% 4 6% 7% 49%	1520
	11th to 20th	Night	8% 4 2 7% 4 4 5% 4 7% 8% 46%	1474
	21st to end	Night	7% 5% 1 4 4 4 4 4 6% 8% 53%	1330
July	1st to 10th	Night	11% 4 1 4 4 4 5% 4 7% 9% 46%	2370
	11th to 20th	Night	4 2 1 4 3 4 4 3 6% 9% 60%	2530
	21st to end	Night	5% 3 2 4 4 4 4 3 5% 8% 58%	2801
August	1st to 10th	Night	11% 6% 2 7% 4 3 4 3 4 7% 50%	2836
	11th to 20th	Night	13% 4 1 5% 4 3 4 4 5% 8% 49%	2798
	21st to end	Night	9% 6% 3 6% 4 4 5% 4 5% 7% 47%	3311
September	1st to 10th	Night	9% 4 1 5% 4 3 4 4 5% 7% 54%	1900
	11th to 20th	Night	9% 5% 1 5% 4 3 4 3 5% 8% 54%	1935
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 6% 51%	2156
October	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 4 7% 49%	2280
	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%	2598
	11th to 20th	Night	14% 12% 4 12% 5% 4 4 3 4 5% 33%	2660
	21st to end	Night	13% 10% 3 8% 6% 4 4 3 5% 7% 36%	2660
December	1st to 10th	Night	10% 12% 5% 11% 7% 5% 4 3 5% 6% 31%	2660
	11th to 20th	Night	10% 14% 5% 11% 6% 6% 5% 4 5% 6% 27%	3279
	21st to end	Night	21% 16% 5% 13% 5% 4 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	63% 6% 11% 16% 3	2528
	11th to 20th	Day	61% 6% 13% 15% 5%	2544
	21st to end	Day	60% 6% 13% 16% 5%	3135
February	1st to 10th	Day	51% 5% 15% 20% 8% 1	1710
	11th to 20th	Day	48% 7% 12% 24% 8% 1	1673
	21st to end	Day	52% 5% 12% 20% 10% 1	1459
March	1st to 10th	Day	45% 7% 14% 24% 10%	3371
	11th to 20th	Day	41% 6% 12% 32% 8% 1	3480
	21st to end	Day	62% 6% 9% 18% 4	3843
April	1st to 10th	Day	48% 6% 12% 20% 12% 1	2280
	11th to 20th	Day	53% 5% 9% 19% 12% 2	2281
	21st to end	Day	54% 6% 12% 20% 9%	2470
May	1st to 10th	Day	41% 7% 13% 24% 14% 2	4298
	11th to 20th	Day	38% 6% 12% 28% 14% 2	4403
	21st to end	Day	45% 8% 13% 24% 10% 1	4837
June	1st to 10th	Day	34% 8% 14% 31% 12% 2	2660
	11th to 20th	Day	43% 7% 13% 22% 13% 1	2697
	21st to end	Day	35% 8% 17% 28% 9% 2	2850
July	1st to 10th	Day	37% 10% 18% 23% 11% 1	4509
	11th to 20th	Day	21% 7% 16% 37% 17% 2	4365
	21st to end	Day	29% 9% 16% 28% 17% 1	4610
August	1st to 10th	Day	37% 6% 12% 31% 12% 2	4090
	11th to 20th	Day	35% 8% 13% 33% 10%	4080
	21st to end	Day	35% 7% 17% 27% 13% 1	4349
September	1st to 10th	Day	33% 6% 15% 33% 11%	2280
	11th to 20th	Day	35% 8% 16% 32% 8%	2242
	21st to end	Day	38% 7% 11% 33% 11%	2005
October	1st to 10th	Day	33% 4 8% 34% 21%	3160
	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	1576
	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	58% 5% 12% 18% 6% 1	4428
	11th to 20th	Night	60% 6% 13% 15% 5%	4412
	21st to end	Night	61% 5% 9% 19% 5% 1	4532
February	1st to 10th	Night	48% 5% 13% 22% 10% 1	2470
	11th to 20th	Night	43% 6% 15% 25% 9% 2	2404
	21st to end	Night	47% 5% 12% 22% 12% 1	1994
March	1st to 10th	Night	36% 6% 13% 26% 17% 1	3510
	11th to 20th	Night	38% 6% 12% 28% 16% 1	3480
	21st to end	Night	60% 4 8% 21% 6% 1	3787
April	1st to 10th	Night	47% 5% 10% 23% 15% 1	1900
	11th to 20th	Night	48% 5% 9% 20% 15% 2	1846
	21st to end	Night	49% 6% 9% 20% 13% 2	1710
May	1st to 10th	Night	41% 7% 11% 21% 18% 2	2670
	11th to 20th	Night	33% 6% 11% 26% 19% 4	2507
	21st to end	Night	40% 7% 13% 23% 15% 1	2751
June	1st to 10th	Night	31% 8% 14% 28% 17% 2	1520
	11th to 20th	Night	33% 8% 14% 25% 18% 2	1474
	21st to end	Night	27% 7% 16% 31% 16% 3	1330
July	1st to 10th	Night	31% 9% 20% 25% 14% 1	2331
	11th to 20th	Night	20% 6% 15% 38% 18% 2	2482
	21st to end	Night	24% 7% 15% 33% 18% 3	2712
August	1st to 10th	Night	35% 6% 11% 30% 14% 5%	2826
	11th to 20th	Night	32% 6% 14% 32% 15% 1	2825
	21st to end	Night	34% 8% 16% 28% 11% 2	3311
September	1st to 10th	Night	29% 6% 15% 33% 17%	1900
	11th to 20th	Night	29% 7% 17% 31% 17%	1935
	21st to end	Night	33% 6% 12% 32% 16% 1	2156
October	1st to 10th	Night	24% 3 7% 35% 27% 4	3800
	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% 14% 22% 13% 2	2598
	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	13% 9% 36% 27% 12% 3	270
	11th to 20th	Day	16% 9% 37% 33% 5% 1	270
	21st to end	Day	19% 7% 37% 25% 10% 2	297
February	1st to 10th	Day	14% 17% 41% 20% 7% 1	270
	11th to 20th	Day	16% 14% 40% 21% 6% 2	270
	21st to end	Day	17% 11% 44% 24% 4	225
March	1st to 10th	Day	18% 15% 35% 24% 7% 1	270
	11th to 20th	Day	18% 8% 40% 23% 9% 3	270
	21st to end	Day	24% 15% 26% 22% 10% 2	314
April	1st to 10th	Day	23% 13% 33% 23% 7%	460
	11th to 20th	Day	28% 15% 24% 22% 10% 1	479
	21st to end	Day	25% 12% 32% 20% 9% 2	650
May	1st to 10th	Day	19% 15% 30% 25% 10% 2	653
	11th to 20th	Day	17% 11% 30% 25% 14% 3	675
	21st to end	Day	17% 9% 28% 26% 17% 3	734
June	1st to 10th	Day	11% 6% 29% 29% 19% 5%	680
	11th to 20th	Day	14% 8% 35% 30% 10% 2	680
	21st to end	Day	12% 8% 28% 30% 16% 6%	680
July	1st to 10th	Day	9% 5% 19% 31% 25% 10%	671
	11th to 20th	Day	9% 5% 27% 31% 20% 9%	668
	21st to end	Day	6% 5% 23% 35% 24% 7%	739
August	1st to 10th	Day	10% 4 23% 33% 23% 6%	676
	11th to 20th	Day	12% 5% 20% 31% 25% 6%	665
	21st to end	Day	9% 8% 25% 32% 21% 5%	644
September	1st to 10th	Day	13% 6% 30% 29% 18% 4	490
	11th to 20th	Day	12% 7% 26% 32% 18% 5%	452
	21st to end	Day	16% 12% 29% 23% 13% 7%	300
October	1st to 10th	Day	18% 10% 27% 25% 13% 6%	300
	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	20% 12% 36% 22% 9% 1	282
	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% 38% 30% 7% 1	950
	21st to end	Night	18% 10% 35% 29% 7% 1	1045
February	1st to 10th	Night	12% 10% 38% 31% 7% 1	950
	11th to 20th	Night	11% 11% 43% 28% 7%	929
	21st to end	Night	16% 10% 36% 30% 9% 1	738
March	1st to 10th	Night	16% 9% 38% 28% 8% 1	627
	11th to 20th	Night	16% 9% 40% 28% 7%	570
	21st to end	Night	22% 11% 29% 26% 11% 2	627
April	1st to 10th	Night	19% 12% 34% 28% 7%	570
	11th to 20th	Night	23% 11% 30% 28% 8% 1	563
	21st to end	Night	23% 9% 29% 28% 9% 1	570
May	1st to 10th	Night	18% 11% 32% 33% 6% 1	570
	11th to 20th	Night	16% 8% 32% 33% 11% 1	561
	21st to end	Night	12% 8% 26% 37% 16% 1	617
June	1st to 10th	Night	11% 5% 26% 41% 15% 1	570
	11th to 20th	Night	10% 4 30% 46% 10%	570
	21st to end	Night	9% 5% 23% 45% 17% 1	570
July	1st to 10th	Night	6% 2 20% 44% 27% 2	555
	11th to 20th	Night	7% 2 23% 46% 19% 2	552
	21st to end	Night	5% 3 24% 44% 22% 2	624
August	1st to 10th	Night	8% 4 19% 49% 19% 2	567
	11th to 20th	Night	12% 2 20% 38% 26% 2	564
	21st to end	Night	8% 4 23% 38% 24% 2	627
September	1st to 10th	Night	10% 5% 24% 40% 19% 2	570
	11th to 20th	Night	10% 5% 29% 35% 18% 3	570
	21st to end	Night	15% 9% 28% 32% 15% 1	636
October	1st to 10th	Night	18% 8% 29% 30% 14% 1	760
	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	888
	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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