Martens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martens Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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	11% 8% 4 7% 6% 4 5% 5% 6% 7% 37%	77972
	Night	10% 9% 4 8% 6% 4 5% 5% 5% 7% 37%	81369
Transparency	Day	45% 7% 15% 23% 9% 1	77660
	Night	46% 7% 15% 22% 10% 1	81204
Seeing	Day	15% 8% 30% 29% 14% 3	15104
	Night	15% 8% 32% 32% 12% 1	25249

By Month

Percentage Cloud Cover SW (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	6% 10% 6% 11% 11% 5% 7% 6% 6% 7% 25%	2218
	11th to 20th	Day	4 8% 5% 11% 16% 8% 12% 10% 7% 4 16%	2391
	21st to end	Day	6% 12% 6% 15% 9% 4 6% 5% 5% 6% 26%	2794
February	1st to 10th	Day	9% 12% 6% 9% 7% 4 7% 6% 5% 5% 27%	1662
	11th to 20th	Day	6% 10% 5% 11% 8% 5% 9% 8% 7% 5% 27%	1664
	21st to end	Day	9% 8% 6% 12% 9% 5% 6% 5% 4 7% 30%	1493
March	1st to 10th	Day	10% 11% 7% 11% 8% 5% 6% 5% 6% 7% 24%	1833
	11th to 20th	Day	10% 10% 4 9% 8% 3 4 6% 7% 8% 31%	1980
	21st to end	Day	15% 11% 4 10% 7% 3 4 5% 5% 7% 29%	2178
April	1st to 10th	Day	11% 11% 4 9% 7% 3 6% 4 6% 6% 34%	2163
	11th to 20th	Day	16% 12% 4 7% 5% 4 6% 5% 4 5% 33%	2309
	21st to end	Day	20% 9% 3 6% 5% 3 5% 5% 5% 7% 32%	2340
May	1st to 10th	Day	17% 8% 3 5% 5% 3 4 5% 5% 6% 38%	2340
	11th to 20th	Day	17% 7% 3 4 4 3 5% 5% 7% 8% 38%	2531
	21st to end	Day	17% 7% 3 6% 5% 3 5% 5% 5% 8% 37%	2918
June	1st to 10th	Day	14% 6% 3 5% 4 3 5% 5% 7% 9% 40%	2700
	11th to 20th	Day	15% 5% 3 5% 5% 3 5% 5% 6% 8% 40%	2700
	21st to end	Day	12% 6% 2 5% 6% 4 6% 6% 6% 8% 40%	2700
July	1st to 10th	Day	7% 3 2 5% 5% 3 5% 5% 6% 9% 50%	2700
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 5% 7% 8% 47%	2700
	21st to end	Day	5% 3 1 3 4 2 4 6% 7% 9% 55%	2795
August	1st to 10th	Day	9% 5% 2 5% 5% 3 5% 7% 6% 9% 45%	2388
	11th to 20th	Day	11% 3 2 5% 4 3 5% 5% 6% 9% 48%	2301
	21st to end	Day	9% 5% 2 4 4 3 4 4 5% 7% 52%	2574
September	1st to 10th	Day	12% 4 2 4 5% 3 3 4 5% 7% 50%	2192
	11th to 20th	Day	13% 6% 2 6% 4 3 5% 4 6% 7% 44%	1980
	21st to end	Day	14% 5% 2 5% 4 3 3 4 6% 8% 45%	1980
October	1st to 10th	Day	18% 8% 3 6% 4 3 4 4 4 6% 40%	1864
	11th to 20th	Day	14% 10% 3 5% 6% 3 4 4 4 6% 40%	1774
	21st to end	Day	13% 10% 5% 8% 6% 4 4 4 5% 7% 33%	1782
November	1st to 10th	Day	13% 11% 6% 8% 5% 4 5% 4 4 5% 36%	1590
	11th to 20th	Day	10% 14% 7% 10% 8% 3 6% 4 5% 5% 28%	1520
	21st to end	Day	11% 13% 6% 11% 7% 4 4 3 4 6% 30%	1463
December	1st to 10th	Day	9% 10% 5% 12% 11% 5% 7% 5% 4 6% 26%	1330
	11th to 20th	Day	9% 13% 6% 10% 9% 4 6% 6% 6% 7% 24%	1683
	21st to end	Day	13% 13% 5% 10% 8% 5% 5% 4 4 5% 27%	2442
January	1st to 10th	Night	6% 11% 6% 11% 9% 5% 7% 6% 6% 6% 28%	4737
	11th to 20th	Night	5% 10% 6% 12% 11% 7% 10% 8% 7% 5% 18%	4576
	21st to end	Night	7% 12% 7% 14% 9% 4 5% 5% 4 5% 28%	4873
February	1st to 10th	Night	8% 11% 5% 11% 8% 4 6% 5% 5% 6% 31%	2517
	11th to 20th	Night	7% 9% 4 8% 8% 4 7% 6% 5% 6% 34%	2403
	21st to end	Night	7% 10% 5% 10% 6% 3 6% 4 4 7% 37%	1873
March	1st to 10th	Night	10% 11% 5% 10% 7% 4 6% 5% 6% 7% 29%	2125
	11th to 20th	Night	9% 9% 3 7% 7% 3 5% 4 5% 7% 40%	1980
	21st to end	Night	12% 10% 4 8% 5% 3 5% 5% 5% 8% 35%	2178
April	1st to 10th	Night	10% 7% 3 6% 6% 3 5% 6% 5% 8% 41%	1792
	11th to 20th	Night	15% 12% 4 6% 5% 3 4 4 5% 5% 37%	1599
	21st to end	Night	16% 9% 4 6% 4 3 5% 4 5% 7% 37%	1620
May	1st to 10th	Night	15% 8% 3 6% 4 2 4 4 5% 7% 41%	1620
	11th to 20th	Night	16% 6% 2 4 4 2 4 5% 5% 9% 43%	1359
	21st to end	Night	14% 5% 3 4 5% 3 3 4 5% 8% 45%	1358
June	1st to 10th	Night	10% 4 1 4 4 3 4 6% 7% 8% 49%	1260
	11th to 20th	Night	11% 6% 2 4 4 3 4 5% 5% 10% 46%	1260
	21st to end	Night	7% 3 2 4 5% 3 5% 6% 6% 9% 51%	1260
July	1st to 10th	Night	4 3 2 4 4 3 3 5% 7% 8% 59%	1260
	11th to 20th	Night	4 3 1 4 4 2 4 5% 5% 9% 59%	1260
	21st to end	Night	4 2 1 4 3 3 4 5% 5% 10% 58%	1520
August	1st to 10th	Night	4 3 2 4 5% 3 4 4 6% 7% 58%	1549
	11th to 20th	Night	7% 4 1 3 3 3 4 4 7% 10% 55%	1600
	21st to end	Night	8% 4 1 5% 4 2 3 5% 5% 7% 57%	1782
September	1st to 10th	Night	8% 5% 2 4 3 2 4 4 5% 8% 55%	1766
	11th to 20th	Night	10% 4 1 4 4 2 4 5% 5% 7% 54%	1980
	21st to end	Night	14% 5% 2 4 4 3 4 4 4 7% 49%	1980
October	1st to 10th	Night	16% 6% 2 5% 5% 3 4 5% 5% 6% 43%	2078
	11th to 20th	Night	14% 10% 3 5% 5% 3 5% 4 5% 8% 40%	2184
	21st to end	Night	14% 10% 3 6% 4 3 5% 4 5% 7% 39%	2574
November	1st to 10th	Night	11% 8% 4 7% 6% 4 5% 6% 7% 7% 34%	2485
	11th to 20th	Night	8% 13% 6% 10% 7% 4 5% 5% 5% 5% 32%	2660
	21st to end	Night	12% 10% 5% 9% 6% 3 5% 4 5% 6% 34%	2712
December	1st to 10th	Night	8% 13% 5% 10% 9% 4 6% 5% 6% 6% 28%	2850
	11th to 20th	Night	10% 13% 6% 12% 8% 4 6% 5% 5% 6% 25%	3514
	21st to end	Night	15% 13% 4 9% 8% 4 5% 3 4 5% 29%	5225

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	56% 9% 14% 17% 4	2219
	11th to 20th	Day	64% 9% 14% 10% 4	2391
	21st to end	Day	58% 6% 10% 20% 6%	2794
February	1st to 10th	Day	56% 5% 14% 16% 8% 1	1662
	11th to 20th	Day	53% 7% 14% 17% 8% 1	1673
	21st to end	Day	55% 6% 12% 19% 7% 1	1493
March	1st to 10th	Day	59% 8% 14% 15% 5%	1833
	11th to 20th	Day	49% 6% 15% 22% 7%	1980
	21st to end	Day	54% 7% 16% 18% 5%	2178
April	1st to 10th	Day	49% 7% 13% 20% 9% 1	2163
	11th to 20th	Day	52% 5% 14% 17% 11% 1	2312
	21st to end	Day	51% 6% 16% 19% 6% 1	2340
May	1st to 10th	Day	45% 6% 14% 24% 11% 1	2340
	11th to 20th	Day	43% 7% 13% 26% 11% 1	2535
	21st to end	Day	44% 7% 16% 22% 10% 1	2918
June	1st to 10th	Day	39% 8% 19% 26% 8%	2700
	11th to 20th	Day	39% 9% 17% 24% 10% 1	2700
	21st to end	Day	39% 8% 17% 27% 8% 1	2700
July	1st to 10th	Day	27% 8% 18% 33% 13% 1	2626
	11th to 20th	Day	32% 9% 18% 29% 12% 1	2619
	21st to end	Day	23% 8% 18% 35% 14% 1	2640
August	1st to 10th	Day	34% 9% 17% 28% 11% 1	2372
	11th to 20th	Day	32% 8% 18% 31% 10% 1	2301
	21st to end	Day	30% 7% 16% 33% 14% 1	2574
September	1st to 10th	Day	34% 5% 16% 31% 13% 1	2192
	11th to 20th	Day	38% 7% 18% 26% 10% 1	1980
	21st to end	Day	37% 6% 17% 32% 7%	1980
October	1st to 10th	Day	46% 7% 15% 23% 9%	1864
	11th to 20th	Day	46% 5% 14% 25% 10% 1	1774
	21st to end	Day	51% 8% 16% 19% 7%	1782
November	1st to 10th	Day	49% 7% 15% 17% 10% 1	1590
	11th to 20th	Day	58% 6% 14% 15% 7% 1	1520
	21st to end	Day	56% 6% 14% 16% 7%	1463
December	1st to 10th	Day	60% 7% 14% 15% 5%	1330
	11th to 20th	Day	58% 7% 14% 14% 6%	1680
	21st to end	Day	59% 6% 10% 18% 6%	2442
January	1st to 10th	Night	55% 8% 15% 18% 5%	4737
	11th to 20th	Night	62% 8% 12% 13% 4	4576
	21st to end	Night	58% 6% 11% 20% 6%	4873
February	1st to 10th	Night	53% 5% 13% 18% 9%	2517
	11th to 20th	Night	47% 7% 14% 19% 12%	2404
	21st to end	Night	48% 5% 12% 24% 10% 1	1873
March	1st to 10th	Night	53% 7% 15% 18% 7%	2125
	11th to 20th	Night	44% 6% 13% 26% 11% 1	1980
	21st to end	Night	48% 7% 16% 18% 10% 1	2178
April	1st to 10th	Night	41% 7% 14% 24% 12% 2	1792
	11th to 20th	Night	49% 4 11% 19% 14% 2	1599
	21st to end	Night	47% 5% 14% 23% 8% 2	1620
May	1st to 10th	Night	42% 5% 14% 24% 14% 2	1620
	11th to 20th	Night	38% 6% 14% 24% 15% 3	1360
	21st to end	Night	37% 6% 16% 26% 14% 1	1358
June	1st to 10th	Night	30% 8% 20% 30% 10% 2	1260
	11th to 20th	Night	34% 8% 15% 28% 14% 2	1260
	21st to end	Night	29% 8% 21% 32% 7% 2	1260
July	1st to 10th	Night	20% 7% 17% 36% 17% 2	1225
	11th to 20th	Night	22% 9% 19% 34% 14% 3	1218
	21st to end	Night	21% 8% 16% 34% 20% 2	1436
August	1st to 10th	Night	24% 7% 18% 32% 16% 3	1541
	11th to 20th	Night	25% 7% 22% 31% 14% 1	1600
	21st to end	Night	26% 6% 15% 32% 20% 1	1782
September	1st to 10th	Night	27% 7% 15% 33% 17% 1	1766
	11th to 20th	Night	28% 6% 19% 31% 14% 1	1980
	21st to end	Night	35% 6% 13% 31% 14% 1	1980
October	1st to 10th	Night	41% 7% 15% 24% 12% 1	2078
	11th to 20th	Night	43% 6% 13% 27% 11% 1	2184
	21st to end	Night	44% 7% 17% 23% 8% 1	2574
November	1st to 10th	Night	47% 7% 15% 19% 11% 2	2485
	11th to 20th	Night	53% 6% 15% 16% 8% 1	2660
	21st to end	Night	50% 7% 17% 18% 8%	2712
December	1st to 10th	Night	56% 6% 16% 16% 5%	2850
	11th to 20th	Night	59% 7% 16% 14% 4	3516
	21st to end	Night	58% 5% 12% 16% 8%	5225

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	11% 8% 34% 33% 11% 2	270
	11th to 20th	Day	12% 10% 33% 34% 10% 1	270
	21st to end	Day	16% 6% 36% 30% 10% 2	297
February	1st to 10th	Day	18% 12% 35% 27% 6% 1	270
	11th to 20th	Day	10% 13% 41% 24% 10% 2	270
	21st to end	Day	9% 7% 42% 34% 8% 1	210
March	1st to 10th	Day	15% 13% 28% 33% 10% 2	240
	11th to 20th	Day	15% 9% 28% 33% 12% 4	240
	21st to end	Day	14% 8% 30% 32% 11% 4	264
April	1st to 10th	Day	19% 14% 34% 23% 9% 1	423
	11th to 20th	Day	25% 10% 29% 23% 11% 2	590
	21st to end	Day	23% 10% 25% 27% 12% 3	600
May	1st to 10th	Day	20% 10% 27% 28% 12% 3	603
	11th to 20th	Day	20% 6% 29% 29% 13% 3	625
	21st to end	Day	18% 7% 25% 33% 15% 3	679
June	1st to 10th	Day	15% 7% 28% 28% 16% 6%	630
	11th to 20th	Day	17% 8% 35% 25% 13% 3	630
	21st to end	Day	14% 6% 28% 30% 16% 6%	630
July	1st to 10th	Day	8% 3 30% 32% 20% 7%	621
	11th to 20th	Day	11% 6% 30% 31% 18% 4	618
	21st to end	Day	5% 4 27% 38% 20% 6%	684
August	1st to 10th	Day	7% 4 26% 34% 24% 6%	626
	11th to 20th	Day	13% 4 21% 34% 20% 8%	615
	21st to end	Day	11% 6% 27% 28% 22% 5%	693
September	1st to 10th	Day	13% 8% 30% 32% 14% 3	482
	11th to 20th	Day	15% 5% 33% 30% 14% 1	270
	21st to end	Day	20% 8% 37% 25% 8% 2	270
October	1st to 10th	Day	21% 10% 30% 26% 11% 3	270
	11th to 20th	Day	14% 9% 39% 29% 8% 1	270
	21st to end	Day	21% 12% 37% 21% 8% 1	297
November	1st to 10th	Day	23% 9% 28% 29% 11%	270
	11th to 20th	Day	17% 12% 37% 26% 8%	270
	21st to end	Day	17% 13% 39% 24% 6%	270
December	1st to 10th	Day	12% 10% 36% 30% 11% 1	270
	11th to 20th	Day	19% 12% 34% 29% 6%	270
	21st to end	Day	17% 8% 36% 28% 9% 2	297
January	1st to 10th	Night	14% 11% 34% 29% 12% 1	950
	11th to 20th	Night	14% 8% 36% 33% 9%	950
	21st to end	Night	15% 9% 36% 31% 8% 1	1045
February	1st to 10th	Night	13% 11% 35% 31% 9% 1	950
	11th to 20th	Night	13% 9% 37% 29% 10% 3	929
	21st to end	Night	13% 8% 34% 30% 11% 2	653
March	1st to 10th	Night	16% 7% 33% 30% 10% 2	685
	11th to 20th	Night	14% 8% 37% 30% 10% 1	540
	21st to end	Night	18% 6% 31% 29% 14% 3	594
April	1st to 10th	Night	14% 9% 35% 33% 10%	540
	11th to 20th	Night	20% 7% 29% 32% 11%	533
	21st to end	Night	21% 7% 25% 34% 12% 1	540
May	1st to 10th	Night	19% 6% 31% 33% 11% 1	540
	11th to 20th	Night	19% 5% 28% 34% 13% 1	531
	21st to end	Night	17% 6% 30% 33% 13%	584
June	1st to 10th	Night	13% 6% 25% 40% 15% 1	540
	11th to 20th	Night	14% 6% 33% 32% 14% 1	540
	21st to end	Night	7% 3 27% 44% 18% 1	540
July	1st to 10th	Night	6% 3 26% 42% 22% 1	525
	11th to 20th	Night	7% 2 32% 42% 16% 1	522
	21st to end	Night	5% 2 25% 46% 20% 2	591
August	1st to 10th	Night	6% 3 24% 46% 20% 1	537
	11th to 20th	Night	10% 2 27% 40% 20% 2	534
	21st to end	Night	9% 4 27% 39% 21%	594
September	1st to 10th	Night	11% 4 28% 38% 18% 2	540
	11th to 20th	Night	10% 5% 29% 39% 16% 1	540
	21st to end	Night	15% 6% 31% 32% 14% 1	540
October	1st to 10th	Night	19% 6% 34% 31% 8%	638
	11th to 20th	Night	20% 8% 35% 28% 9% 1	744
	21st to end	Night	20% 11% 35% 25% 9%	990
November	1st to 10th	Night	16% 10% 37% 26% 11% 1	925
	11th to 20th	Night	15% 10% 34% 29% 11% 1	950
	21st to end	Night	17% 14% 38% 24% 7% 1	950
December	1st to 10th	Night	14% 10% 35% 30% 9% 1	950
	11th to 20th	Night	19% 10% 33% 29% 8% 1	950
	21st to end	Night	24% 7% 32% 27% 9% 1	1045

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