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 2024-01-13.

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	8% 7% 3 6% 5% 3 4 4 5% 7% 48%	74792
	Night	9% 8% 4 6% 5% 3 5% 5% 5% 7% 43%	74956
Transparency	Day	36% 6% 11% 22% 18% 6%	74495
	Night	40% 6% 12% 20% 15% 6%	74771
Seeing	Day	10% 7% 20% 27% 24% 13%	13219
	Night	12% 5% 22% 36% 22% 3	22388

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	18% 19% 6% 9% 6% 3 5% 4 4 5% 20%	1519
	11th to 20th	Day	16% 19% 6% 12% 8% 4 3 5% 5% 5% 17%	1456
	21st to end	Day	13% 17% 5% 11% 7% 5% 5% 4 4 6% 22%	1584
February	1st to 10th	Day	14% 13% 5% 9% 7% 4 4 6% 6% 7% 28%	1539
	11th to 20th	Day	10% 11% 4 8% 6% 3 4 4 5% 6% 38%	1623
	21st to end	Day	9% 11% 4 7% 6% 4 5% 5% 6% 6% 37%	1480
March	1st to 10th	Day	11% 9% 4 6% 6% 3 5% 5% 6% 8% 38%	1800
	11th to 20th	Day	9% 8% 5% 8% 6% 4 5% 6% 5% 7% 37%	1867
	21st to end	Day	9% 9% 3 6% 7% 4 6% 7% 8% 8% 34%	2362
April	1st to 10th	Day	8% 8% 4 7% 6% 4 5% 5% 6% 8% 39%	2160
	11th to 20th	Day	6% 6% 2 6% 6% 4 5% 5% 6% 9% 45%	2132
	21st to end	Day	6% 5% 3 5% 6% 3 5% 5% 5% 8% 49%	2342
May	1st to 10th	Day	4 4 2 4 4 2 5% 5% 7% 9% 54%	2520
	11th to 20th	Day	7% 5% 3 5% 7% 2 6% 6% 7% 10% 43%	2477
	21st to end	Day	7% 6% 3 6% 5% 3 5% 4 6% 7% 47%	2723
June	1st to 10th	Day	9% 5% 3 5% 6% 3 5% 4 6% 8% 46%	2520
	11th to 20th	Day	6% 4 2 4 5% 3 5% 5% 5% 8% 52%	2520
	21st to end	Day	4 3 2 4 3 2 3 4 4 7% 64%	2520
July	1st to 10th	Day	2 2 1 2 2 2 3 3 4 6% 74%	2520
	11th to 20th	Day	1 1 1 3 3 2 3 3 3 6% 75%	2520
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	2741
August	1st to 10th	Day	2 3 1 2 2 2 2 3 3 4 76%	2507
	11th to 20th	Day	2 2 1 2 2 1 3 3 3 5% 75%	2366
	21st to end	Day	3 2 1 3 3 2 3 4 5% 7% 66%	2376
September	1st to 10th	Day	3 2 1 2 2 2 3 3 4 7% 71%	2160
	11th to 20th	Day	3 4 2 4 5% 2 5% 4 6% 7% 57%	2120
	21st to end	Day	5% 5% 1 4 4 3 4 5% 6% 7% 55%	1980
October	1st to 10th	Day	6% 5% 3 4 5% 3 3 4 6% 7% 54%	1822
	11th to 20th	Day	7% 3 2 5% 6% 4 5% 5% 6% 8% 50%	1772
	21st to end	Day	12% 8% 3 6% 5% 3 4 5% 5% 7% 42%	1782
November	1st to 10th	Day	13% 9% 4 8% 7% 4 6% 5% 5% 5% 35%	1674
	11th to 20th	Day	13% 13% 5% 9% 7% 4 4 4 5% 6% 30%	1549
	21st to end	Day	12% 14% 6% 9% 8% 4 5% 6% 5% 6% 26%	1520
December	1st to 10th	Day	12% 13% 7% 12% 8% 4 5% 4 4 6% 24%	1520
	11th to 20th	Day	17% 18% 10% 14% 8% 4 4 4 4 4 13%	1925
	21st to end	Day	14% 17% 7% 11% 9% 5% 5% 5% 5% 5% 17%	2794
January	1st to 10th	Night	20% 14% 5% 8% 7% 4 5% 4 6% 7% 20%	2646
	11th to 20th	Night	14% 13% 8% 11% 7% 4 5% 4 5% 6% 22%	2548
	21st to end	Night	11% 14% 6% 11% 7% 4 5% 5% 6% 6% 24%	2772
February	1st to 10th	Night	11% 14% 4 8% 6% 3 5% 5% 5% 6% 33%	2419
	11th to 20th	Night	10% 8% 4 7% 6% 4 6% 5% 6% 7% 37%	2223
	21st to end	Night	8% 10% 4 6% 7% 3 6% 6% 7% 9% 33%	1776
March	1st to 10th	Night	11% 9% 5% 7% 5% 3 4 5% 5% 9% 37%	2160
	11th to 20th	Night	9% 8% 4 6% 7% 3 5% 5% 6% 8% 39%	2089
	21st to end	Night	9% 8% 5% 7% 6% 5% 6% 7% 6% 9% 33%	1994
April	1st to 10th	Night	9% 9% 4 8% 5% 3 5% 5% 7% 7% 39%	1800
	11th to 20th	Night	7% 7% 3 5% 6% 3 5% 4 5% 6% 48%	1779
	21st to end	Night	6% 5% 3 6% 6% 3 5% 6% 8% 8% 43%	1614
May	1st to 10th	Night	5% 4 2 5% 5% 2 4 6% 6% 10% 52%	1440
	11th to 20th	Night	7% 6% 2 6% 5% 3 5% 6% 8% 8% 45%	1416
	21st to end	Night	6% 4 2 5% 5% 4 5% 6% 7% 9% 48%	1552
June	1st to 10th	Night	8% 5% 3 4 5% 2 5% 5% 6% 7% 49%	1440
	11th to 20th	Night	6% 4 2 4 5% 2 6% 5% 5% 7% 55%	1440
	21st to end	Night	4 3 2 3 3 2 4 5% 5% 8% 61%	1440
July	1st to 10th	Night	2 1 1 2 2 1 3 4 6% 7% 71%	1440
	11th to 20th	Night	2 2 1 1 2 2 3 3 3 6% 76%	1440
	21st to end	Night	1 1 1 2 1 2 4 3 3 82%	1576
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 78%	1432
	11th to 20th	Night	2 2 2 3 2 2 3 4 6% 74%	1530
	21st to end	Night	2 2 1 2 3 2 3 3 5% 7% 69%	1980
September	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 5% 73%	1800
	11th to 20th	Night	4 3 2 4 3 2 5% 5% 6% 6% 61%	1840
	21st to end	Night	5% 4 2 4 3 3 3 4 6% 8% 59%	1980
October	1st to 10th	Night	9% 5% 2 5% 4 3 4 5% 4 7% 53%	2136
	11th to 20th	Night	7% 5% 2 4 5% 3 5% 4 5% 7% 52%	2187
	21st to end	Night	10% 7% 3 6% 5% 4 5% 5% 5% 7% 44%	2574
November	1st to 10th	Night	14% 8% 3 6% 5% 4 5% 5% 6% 7% 36%	2405
	11th to 20th	Night	13% 8% 3 6% 7% 4 5% 5% 6% 6% 35%	2626
	21st to end	Night	13% 10% 5% 8% 7% 3 6% 5% 5% 7% 32%	2660
December	1st to 10th	Night	13% 11% 7% 8% 7% 4 5% 6% 6% 7% 26%	2660
	11th to 20th	Night	17% 15% 9% 13% 7% 3 5% 5% 5% 6% 15%	3280
	21st to end	Night	15% 18% 5% 8% 8% 4 5% 5% 5% 7% 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	67% 7% 11% 10% 4 1	1519
	11th to 20th	Day	71% 8% 11% 9% 2	1456
	21st to end	Day	63% 8% 15% 9% 4 1	1584
February	1st to 10th	Day	55% 8% 16% 15% 5% 1	1539
	11th to 20th	Day	47% 7% 16% 19% 10%	1632
	21st to end	Day	46% 7% 15% 21% 9% 1	1480
March	1st to 10th	Day	44% 7% 15% 23% 10% 1	1800
	11th to 20th	Day	45% 6% 11% 27% 10%	1867
	21st to end	Day	44% 8% 18% 20% 9% 1	2362
April	1st to 10th	Day	43% 6% 14% 23% 13% 2	2160
	11th to 20th	Day	36% 6% 15% 22% 17% 4	2132
	21st to end	Day	34% 6% 12% 25% 19% 4	2342
May	1st to 10th	Day	25% 5% 11% 33% 23% 3	2520
	11th to 20th	Day	35% 6% 13% 24% 18% 3	2480
	21st to end	Day	35% 6% 13% 27% 17% 3	2724
June	1st to 10th	Day	36% 5% 9% 25% 20% 5%	2520
	11th to 20th	Day	29% 5% 11% 29% 20% 5%	2520
	21st to end	Day	21% 4 8% 29% 27% 11%	2520
July	1st to 10th	Day	14% 3 6% 23% 34% 20%	2451
	11th to 20th	Day	13% 3 6% 17% 35% 26%	2445
	21st to end	Day	8% 2 4 15% 42% 28%	2589
August	1st to 10th	Day	14% 3 7% 20% 33% 23%	2490
	11th to 20th	Day	12% 3 6% 25% 37% 17%	2366
	21st to end	Day	17% 4 9% 28% 33% 10%	2376
September	1st to 10th	Day	14% 4 9% 28% 30% 15%	2160
	11th to 20th	Day	24% 6% 11% 24% 26% 9%	2120
	21st to end	Day	27% 6% 15% 31% 17% 4	1980
October	1st to 10th	Day	30% 5% 12% 34% 18% 2	1822
	11th to 20th	Day	32% 6% 18% 31% 12% 1	1772
	21st to end	Day	40% 7% 15% 24% 13% 1	1782
November	1st to 10th	Day	50% 8% 12% 19% 9% 1	1674
	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	63% 9% 13% 11% 3 1	2646
	11th to 20th	Night	62% 8% 14% 13% 3	2548
	21st to end	Night	59% 9% 16% 12% 3 1	2772
February	1st to 10th	Night	51% 6% 18% 16% 7% 1	2419
	11th to 20th	Night	44% 8% 17% 21% 10%	2225
	21st to end	Night	44% 8% 14% 20% 12% 1	1776
March	1st to 10th	Night	43% 6% 14% 25% 11% 1	2160
	11th to 20th	Night	42% 6% 12% 28% 12% 1	2090
	21st to end	Night	45% 8% 13% 25% 9%	1994
April	1st to 10th	Night	43% 6% 11% 25% 14% 2	1800
	11th to 20th	Night	36% 4 9% 27% 19% 4	1779
	21st to end	Night	36% 6% 11% 23% 19% 5%	1614
May	1st to 10th	Night	27% 6% 9% 29% 25% 4	1440
	11th to 20th	Night	34% 5% 11% 24% 21% 5%	1416
	21st to end	Night	30% 7% 13% 28% 18% 3	1552
June	1st to 10th	Night	33% 5% 9% 24% 25% 5%	1440
	11th to 20th	Night	30% 5% 8% 30% 23% 4	1440
	21st to end	Night	22% 5% 8% 25% 30% 12%	1440
July	1st to 10th	Night	13% 3 6% 21% 32% 25%	1400
	11th to 20th	Night	12% 3 5% 17% 32% 32%	1392
	21st to end	Night	7% 2 3 13% 38% 37%	1488
August	1st to 10th	Night	13% 2 5% 13% 36% 30%	1424
	11th to 20th	Night	13% 3 5% 19% 37% 22%	1530
	21st to end	Night	14% 4 6% 24% 35% 17%	1980
September	1st to 10th	Night	13% 4 7% 27% 29% 21%	1800
	11th to 20th	Night	22% 6% 10% 23% 28% 13%	1840
	21st to end	Night	24% 4 12% 31% 21% 9%	1980
October	1st to 10th	Night	31% 5% 9% 28% 26% 1	2136
	11th to 20th	Night	32% 5% 12% 29% 19% 3	2187
	21st to end	Night	39% 7% 11% 27% 15% 2	2574
November	1st to 10th	Night	45% 8% 14% 22% 9% 2	2405
	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% 20% 3	180
	11th to 20th	Day	27% 7% 15% 27% 16% 7%	164
	21st to end	Day	22% 6% 23% 25% 18% 6%	176
February	1st to 10th	Day	20% 9% 24% 27% 15% 5%	259
	11th to 20th	Day	18% 9% 27% 25% 16% 5%	357
	21st to end	Day	11% 12% 26% 29% 17% 5%	346
March	1st to 10th	Day	17% 8% 30% 24% 14% 6%	420
	11th to 20th	Day	15% 10% 21% 24% 23% 7%	420
	21st to end	Day	13% 13% 25% 29% 16% 4	462
April	1st to 10th	Day	13% 10% 27% 25% 16% 8%	420
	11th to 20th	Day	10% 13% 30% 25% 14% 8%	412
	21st to end	Day	6% 10% 26% 30% 19% 9%	420
May	1st to 10th	Day	7% 8% 20% 32% 24% 9%	423
	11th to 20th	Day	11% 6% 16% 28% 27% 13%	448
	21st to end	Day	9% 3 16% 25% 29% 18%	485
June	1st to 10th	Day	9% 6% 19% 23% 26% 16%	450
	11th to 20th	Day	6% 8% 26% 30% 20% 10%	450
	21st to end	Day	4 2 16% 27% 30% 21%	450
July	1st to 10th	Day	2 2 12% 33% 35% 16%	446
	11th to 20th	Day	1 2 16% 29% 35% 17%	444
	21st to end	Day	1 2 10% 24% 37% 25%	487
August	1st to 10th	Day	4 3 10% 22% 36% 25%	447
	11th to 20th	Day	2 2 8% 24% 35% 29%	437
	21st to end	Day	4 4 16% 26% 30% 20%	495
September	1st to 10th	Day	4 3 14% 31% 31% 17%	450
	11th to 20th	Day	6% 4 17% 22% 32% 19%	450
	21st to end	Day	8% 9% 17% 27% 27% 11%	450
October	1st to 10th	Day	8% 5% 24% 26% 24% 13%	450
	11th to 20th	Day	10% 7% 18% 23% 28% 13%	422
	21st to end	Day	16% 8% 24% 29% 16% 7%	297
November	1st to 10th	Day	17% 10% 20% 32% 19% 3	270
	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 21% 35% 18% 3	728
	21st to end	Night	17% 3 21% 36% 20% 3	792
February	1st to 10th	Night	19% 5% 27% 33% 14% 2	720
	11th to 20th	Night	13% 5% 26% 35% 19% 2	704
	21st to end	Night	13% 9% 24% 36% 16% 2	592
March	1st to 10th	Night	13% 6% 25% 37% 16% 3	720
	11th to 20th	Night	11% 6% 24% 36% 21% 2	720
	21st to end	Night	13% 7% 28% 35% 15% 2	792
April	1st to 10th	Night	13% 6% 24% 34% 20% 3	720
	11th to 20th	Night	10% 8% 31% 30% 18% 4	711
	21st to end	Night	8% 6% 27% 36% 20% 3	534
May	1st to 10th	Night	6% 6% 31% 36% 19% 3	360
	11th to 20th	Night	8% 5% 20% 32% 31% 4	354
	21st to end	Night	7% 3 20% 38% 26% 6%	389
June	1st to 10th	Night	10% 3 20% 38% 28% 1	360
	11th to 20th	Night	7% 9% 26% 37% 19% 3	360
	21st to end	Night	6% 2 22% 41% 25% 4	360
July	1st to 10th	Night	3 2 16% 44% 33% 2	350
	11th to 20th	Night	3 3 20% 39% 32% 3	348
	21st to end	Night	1 2 11% 42% 40% 4	394
August	1st to 10th	Night	4 1 13% 35% 42% 6%	358
	11th to 20th	Night	3 1 13% 33% 41% 10%	462
	21st to end	Night	3 3 16% 34% 37% 6%	792
September	1st to 10th	Night	3 2 17% 42% 31% 6%	720
	11th to 20th	Night	6% 3 14% 31% 38% 8%	720
	21st to end	Night	6% 4 20% 34% 30% 5%	720
October	1st to 10th	Night	11% 3 18% 38% 26% 3	720
	11th to 20th	Night	10% 6% 21% 33% 25% 5%	720
	21st to end	Night	13% 5% 24% 35% 18% 4	792
November	1st to 10th	Night	17% 6% 20% 42% 13% 1	740
	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

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