Merk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Merk Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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	7% 7% 3 6% 4 4 4 3 5% 6% 51%	105374
	Night	8% 8% 3 7% 4 4 4 4 5% 7% 45%	102382
Transparency	Day	33% 5% 11% 22% 20% 10%	105143
	Night	37% 6% 11% 21% 17% 8%	102259
Seeing	Day	10% 7% 20% 27% 24% 13%	14344
	Night	12% 5% 22% 36% 22% 3	23370

By Month

Percentage Cloud Cover NW (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% 20% 7% 12% 5% 5% 4 3 5% 5% 20%	2539
	11th to 20th	Day	19% 17% 5% 10% 5% 4 4 3 6% 6% 21%	2516
	21st to end	Day	23% 20% 7% 13% 4 4 3 3 3 4 15%	2794
February	1st to 10th	Day	13% 14% 4 10% 6% 5% 4 4 6% 7% 27%	1624
	11th to 20th	Day	10% 12% 4 10% 5% 4 4 4 5% 6% 36%	1715
	21st to end	Day	9% 12% 3 9% 6% 5% 5% 4 6% 6% 37%	1570
March	1st to 10th	Day	7% 8% 3 6% 5% 5% 5% 5% 8% 8% 40%	3138
	11th to 20th	Day	5% 6% 2 6% 4 4 4 3 5% 7% 55%	3299
	21st to end	Day	7% 9% 3 7% 6% 6% 6% 5% 7% 8% 35%	4151
April	1st to 10th	Day	8% 9% 3 8% 5% 5% 5% 4 6% 8% 39%	2280
	11th to 20th	Day	6% 7% 2 6% 5% 5% 4 4 7% 9% 45%	2252
	21st to end	Day	6% 6% 3 6% 5% 5% 5% 4 6% 8% 48%	2473
May	1st to 10th	Day	8% 5% 1 3 3 3 4 3 5% 9% 56%	4420
	11th to 20th	Day	4 3 2 4 4 3 4 3 5% 9% 60%	4377
	21st to end	Day	5% 7% 3 7% 5% 5% 5% 3 7% 7% 45%	4813
June	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 8% 46%	2660
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 5% 8% 53%	2660
	21st to end	Day	3 3 2 4 3 3 3 3 5% 6% 64%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 83%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 2 4 82%	4420
	21st to end	Day	1 1 2 1 2 2 2 3 4 81%	4790
August	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 75%	4401
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 6% 72%	4130
	21st to end	Day	4 4 2 5% 3 3 3 3 4 6% 65%	4180
September	1st to 10th	Day	3 2 1 2 2 2 3 2 5% 7% 71%	2280
	11th to 20th	Day	3 4 2 5% 4 3 4 3 6% 8% 56%	2237
	21st to end	Day	5% 5% 1 5% 3 4 4 3 6% 7% 56%	2090
October	1st to 10th	Day	5% 4 1 3 2 3 3 2 6% 9% 62%	3215
	11th to 20th	Day	5% 3 2 6% 6% 6% 6% 5% 7% 8% 48%	3106
	21st to end	Day	9% 7% 3 8% 3 4 5% 5% 6% 7% 45%	3101
November	1st to 10th	Day	12% 10% 3 8% 6% 5% 5% 4 5% 6% 35%	1755
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 4 3 5% 6% 30%	1549
	21st to end	Day	12% 15% 5% 10% 6% 5% 5% 4 5% 6% 25%	1520
December	1st to 10th	Day	11% 16% 6% 13% 7% 5% 5% 3 4 6% 24%	1520
	11th to 20th	Day	17% 20% 9% 15% 6% 5% 4 3 4 4 13%	1925
	21st to end	Day	14% 18% 6% 13% 7% 6% 5% 4 5% 5% 17%	2794
January	1st to 10th	Night	17% 17% 5% 11% 6% 6% 5% 3 5% 6% 17%	4406
	11th to 20th	Night	16% 13% 6% 10% 5% 5% 4 4 5% 8% 25%	4382
	21st to end	Night	17% 19% 9% 14% 6% 4 4 3 4 5% 15%	4862
February	1st to 10th	Night	11% 15% 4 9% 6% 5% 4 4 5% 6% 32%	2554
	11th to 20th	Night	10% 9% 4 8% 5% 5% 5% 4 7% 7% 36%	2351
	21st to end	Night	7% 11% 4 7% 6% 6% 5% 5% 7% 9% 33%	1884
March	1st to 10th	Night	7% 8% 3 7% 4 4 5% 5% 7% 10% 40%	3736
	11th to 20th	Night	6% 6% 2 4 4 3 3 2 5% 6% 59%	3677
	21st to end	Night	6% 8% 3 8% 7% 7% 7% 6% 8% 10% 31%	3502
April	1st to 10th	Night	8% 11% 3 8% 5% 5% 5% 4 7% 7% 39%	1900
	11th to 20th	Night	7% 7% 3 6% 5% 5% 4 3 6% 6% 48%	1879
	21st to end	Night	6% 6% 3 7% 5% 5% 4 5% 8% 8% 44%	1703
May	1st to 10th	Night	5% 6% 2 6% 3 3 4 3 5% 8% 55%	2540
	11th to 20th	Night	4 4 1 4 3 3 3 4 6% 7% 61%	2516
	21st to end	Night	5% 4 1 5% 5% 4 4 4 6% 9% 52%	2762
June	1st to 10th	Night	8% 5% 2 5% 5% 3 5% 4 6% 7% 49%	1520
	11th to 20th	Night	6% 5% 2 5% 4 3 5% 4 5% 7% 55%	1520
	21st to end	Night	4 3 2 4 3 4 4 4 5% 8% 61%	1520
July	1st to 10th	Night	1 1 2 1 1 2 2 4 4 83%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 82%	2540
	21st to end	Night	1 1 1 1 2 1 2 2 3 3 83%	2762
August	1st to 10th	Night	3 2 1 3 1 2 2 2 3 5% 77%	2529
	11th to 20th	Night	2 3 1 3 3 2 4 3 5% 5% 68%	2701
	21st to end	Night	2 3 1 4 3 3 3 3 4 7% 67%	3498
September	1st to 10th	Night	2 2 1 3 2 2 3 2 5% 5% 73%	1900
	11th to 20th	Night	4 4 1 5% 3 3 5% 4 6% 6% 59%	1943
	21st to end	Night	5% 4 1 4 2 3 3 3 6% 7% 60%	2090
October	1st to 10th	Night	5% 3 1 3 2 2 3 4 5% 8% 63%	3763
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 6% 8% 52%	3829
	21st to end	Night	10% 7% 2 6% 4 5% 5% 4 5% 7% 45%	4457
November	1st to 10th	Night	13% 8% 2 7% 5% 5% 5% 3 6% 8% 37%	2528
	11th to 20th	Night	13% 9% 3 7% 6% 5% 5% 4 6% 6% 35%	2626
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 4 5% 7% 32%	2660
December	1st to 10th	Night	13% 13% 5% 9% 6% 5% 5% 4 6% 7% 26%	2660
	11th to 20th	Night	16% 16% 9% 14% 6% 5% 4 4 5% 5% 15%	3280
	21st to end	Night	15% 19% 5% 9% 6% 5% 5% 4 5% 6% 21%	4862

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	64% 8% 10% 12% 6%	2539
	11th to 20th	Day	63% 6% 14% 13% 4	2540
	21st to end	Day	73% 6% 12% 6% 2	2794
February	1st to 10th	Day	56% 8% 16% 15% 5%	1624
	11th to 20th	Day	49% 7% 16% 18% 10%	1724
	21st to end	Day	46% 7% 15% 21% 9% 1	1570
March	1st to 10th	Day	37% 7% 13% 24% 18% 1	3138
	11th to 20th	Day	30% 4 12% 42% 11%	3299
	21st to end	Day	42% 9% 17% 19% 12% 2	4151
April	1st to 10th	Day	42% 6% 14% 23% 13% 2	2280
	11th to 20th	Day	35% 6% 15% 22% 17% 5%	2252
	21st to end	Day	34% 5% 13% 25% 19% 4	2473
May	1st to 10th	Day	25% 3 8% 25% 31% 7%	4420
	11th to 20th	Day	22% 4 9% 31% 27% 7%	4380
	21st to end	Day	35% 6% 14% 27% 14% 3	4814
June	1st to 10th	Day	36% 5% 9% 24% 21% 5%	2660
	11th to 20th	Day	28% 5% 11% 28% 21% 6%	2660
	21st to end	Day	21% 4 8% 29% 27% 10%	2660
July	1st to 10th	Day	9% 2 4 14% 29% 43%	4351
	11th to 20th	Day	9% 2 4 10% 25% 50%	4345
	21st to end	Day	10% 2 4 17% 36% 31%	4639
August	1st to 10th	Day	13% 2 5% 14% 37% 28%	4384
	11th to 20th	Day	13% 4 6% 35% 32% 10%	4172
	21st to end	Day	21% 3 9% 26% 33% 9%	4179
September	1st to 10th	Day	14% 4 9% 28% 31% 15%	2280
	11th to 20th	Day	25% 6% 12% 25% 25% 8%	2237
	21st to end	Day	26% 6% 14% 32% 17% 4	2090
October	1st to 10th	Day	20% 4 11% 38% 26% 2	3215
	11th to 20th	Day	33% 6% 18% 27% 16%	3106
	21st to end	Day	37% 6% 14% 27% 12% 4	3101
November	1st to 10th	Day	49% 8% 13% 20% 9% 1	1755
	11th to 20th	Day	55% 7% 14% 15% 7% 1	1549
	21st to end	Day	59% 7% 15% 12% 7% 1	1520
December	1st to 10th	Day	62% 7% 11% 13% 7%	1520
	11th to 20th	Day	76% 7% 8% 6% 3	1928
	21st to end	Day	66% 10% 11% 9% 4	2794
January	1st to 10th	Night	65% 9% 12% 10% 4	4406
	11th to 20th	Night	56% 7% 16% 16% 4	4420
	21st to end	Night	69% 9% 11% 7% 2 1	4862
February	1st to 10th	Night	53% 6% 17% 16% 7% 1	2554
	11th to 20th	Night	44% 8% 16% 20% 11%	2353
	21st to end	Night	45% 8% 15% 20% 11% 1	1884
March	1st to 10th	Night	36% 6% 12% 29% 16% 2	3736
	11th to 20th	Night	27% 4 9% 41% 17% 1	3678
	21st to end	Night	43% 10% 15% 25% 7%	3502
April	1st to 10th	Night	42% 6% 12% 25% 13% 2	1900
	11th to 20th	Night	36% 4 9% 27% 19% 5%	1879
	21st to end	Night	35% 6% 11% 24% 19% 5%	1703
May	1st to 10th	Night	27% 4 6% 20% 35% 7%	2540
	11th to 20th	Night	22% 4 8% 29% 32% 5%	2516
	21st to end	Night	26% 7% 16% 36% 12% 2	2762
June	1st to 10th	Night	32% 5% 9% 24% 25% 5%	1520
	11th to 20th	Night	29% 5% 8% 29% 23% 5%	1520
	21st to end	Night	22% 5% 8% 24% 30% 12%	1520
July	1st to 10th	Night	8% 2 3 14% 26% 48%	2500
	11th to 20th	Night	7% 2 3 10% 20% 58%	2492
	21st to end	Night	8% 1 2 14% 35% 40%	2674
August	1st to 10th	Night	12% 2 3 8% 39% 35%	2521
	11th to 20th	Night	18% 4 5% 26% 35% 13%	2725
	21st to end	Night	17% 4 6% 26% 30% 17%	3498
September	1st to 10th	Night	13% 4 7% 26% 28% 22%	1900
	11th to 20th	Night	22% 6% 10% 23% 27% 12%	1943
	21st to end	Night	23% 4 12% 31% 21% 9%	2090
October	1st to 10th	Night	20% 3 8% 36% 32% 2	3763
	11th to 20th	Night	29% 5% 15% 26% 22% 3	3829
	21st to end	Night	38% 5% 9% 31% 16% 2	4457
November	1st to 10th	Night	44% 8% 14% 22% 9% 2	2528
	11th to 20th	Night	48% 8% 16% 19% 8% 1	2626
	21st to end	Night	51% 8% 16% 17% 7% 1	2660
December	1st to 10th	Night	55% 9% 14% 17% 5%	2660
	11th to 20th	Night	69% 9% 11% 8% 3	3276
	21st to end	Night	61% 10% 12% 11% 6% 1	4862

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	25% 11% 17% 24% 19% 3	180
	11th to 20th	Day	27% 7% 16% 28% 16% 7%	180
	21st to end	Day	23% 6% 22% 24% 19% 6%	198
February	1st to 10th	Day	19% 9% 23% 27% 16% 6%	284
	11th to 20th	Day	18% 9% 27% 25% 15% 6%	389
	21st to end	Day	11% 13% 26% 26% 17% 5%	382
March	1st to 10th	Day	17% 8% 29% 25% 15% 7%	460
	11th to 20th	Day	14% 10% 20% 24% 22% 10%	460
	21st to end	Day	13% 12% 25% 29% 16% 4	506
April	1st to 10th	Day	13% 10% 28% 24% 17% 8%	460
	11th to 20th	Day	9% 13% 29% 26% 15% 8%	452
	21st to end	Day	6% 10% 26% 30% 19% 10%	460
May	1st to 10th	Day	8% 8% 20% 32% 23% 9%	463
	11th to 20th	Day	11% 6% 16% 28% 25% 13%	488
	21st to end	Day	9% 4 16% 26% 28% 17%	529
June	1st to 10th	Day	10% 7% 18% 23% 26% 16%	490
	11th to 20th	Day	6% 8% 26% 30% 21% 9%	490
	21st to end	Day	3 3 17% 27% 29% 20%	490
July	1st to 10th	Day	2 2 13% 32% 34% 17%	486
	11th to 20th	Day	1 2 15% 28% 36% 18%	484
	21st to end	Day	1 2 11% 25% 37% 24%	531
August	1st to 10th	Day	4 3 10% 23% 35% 24%	487
	11th to 20th	Day	2 2 9% 26% 33% 28%	477
	21st to end	Day	4 4 17% 25% 29% 21%	539
September	1st to 10th	Day	4 3 14% 29% 30% 20%	490
	11th to 20th	Day	6% 4 17% 24% 31% 18%	490
	21st to end	Day	8% 9% 17% 27% 27% 12%	490
October	1st to 10th	Day	8% 5% 23% 26% 24% 14%	490
	11th to 20th	Day	10% 7% 19% 23% 28% 13%	460
	21st to end	Day	15% 8% 22% 32% 16% 7%	330
November	1st to 10th	Day	17% 10% 20% 31% 20% 3	297
	11th to 20th	Day	20% 11% 22% 23% 16% 7%	194
	21st to end	Day	17% 6% 23% 27% 20% 7%	180
December	1st to 10th	Day	19% 6% 23% 28% 15% 9%	180
	11th to 20th	Day	21% 4 29% 27% 15% 4	180
	21st to end	Day	21% 7% 23% 29% 18% 2	198
January	1st to 10th	Night	25% 8% 23% 29% 13% 2	760
	11th to 20th	Night	20% 4 22% 34% 17% 3	760
	21st to end	Night	17% 3 20% 36% 20% 3	836
February	1st to 10th	Night	19% 5% 26% 33% 15% 2	760
	11th to 20th	Night	13% 5% 26% 36% 18% 1	744
	21st to end	Night	13% 9% 24% 35% 16% 2	628
March	1st to 10th	Night	13% 7% 25% 37% 16% 2	760
	11th to 20th	Night	11% 6% 24% 36% 22% 2	760
	21st to end	Night	12% 7% 28% 35% 16% 2	836
April	1st to 10th	Night	13% 6% 24% 34% 20% 2	760
	11th to 20th	Night	9% 8% 31% 31% 17% 4	751
	21st to end	Night	7% 7% 27% 37% 20% 2	563
May	1st to 10th	Night	7% 6% 29% 36% 19% 3	380
	11th to 20th	Night	8% 6% 20% 33% 30% 4	374
	21st to end	Night	6% 4 20% 38% 26% 5%	411
June	1st to 10th	Night	10% 3 20% 38% 27% 1	380
	11th to 20th	Night	7% 9% 26% 37% 19% 2	380
	21st to end	Night	6% 2 23% 41% 24% 4	380
July	1st to 10th	Night	3 2 17% 43% 33% 2	370
	11th to 20th	Night	3 3 18% 40% 33% 2	368
	21st to end	Night	1 3 12% 42% 39% 4	416
August	1st to 10th	Night	4 2 14% 35% 40% 5%	378
	11th to 20th	Night	3 1 13% 34% 39% 9%	489
	21st to end	Night	3 3 17% 34% 37% 6%	836
September	1st to 10th	Night	3 2 16% 41% 32% 6%	760
	11th to 20th	Night	5% 3 15% 32% 37% 8%	760
	21st to end	Night	6% 4 20% 34% 30% 5%	760
October	1st to 10th	Night	11% 3 18% 38% 27% 3	760
	11th to 20th	Night	10% 5% 22% 34% 25% 5%	760
	21st to end	Night	13% 5% 24% 36% 18% 3	836
November	1st to 10th	Night	17% 6% 20% 42% 13% 1	778
	11th to 20th	Night	16% 7% 26% 32% 16% 3	760
	21st to end	Night	16% 5% 20% 36% 21% 3	760
December	1st to 10th	Night	17% 6% 19% 37% 18% 3	760
	11th to 20th	Night	22% 7% 19% 37% 14% 2	760
	21st to end	Night	24% 7% 21% 36% 11% 1	836

Page generated at 2024-11-07 03:55:45. Version ac07

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