Donald C. Martin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Donald C. Martin 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	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	101195
	Night	16% 9% 3 7% 4 4 4 3 5% 7% 37%	97792
Transparency	Day	46% 3 10% 24% 15% 2	104924
	Night	46% 3 9% 23% 16% 3	102202
Seeing	Day	21% 12% 25% 22% 14% 6%	14959
	Night	20% 9% 23% 29% 16% 2	25017

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	18% 18% 5% 11% 5% 4 3 3 5% 7% 23%	2836
	11th to 20th	Day	17% 18% 8% 13% 7% 4 3 2 3 4 22%	2840
	21st to end	Day	24% 18% 5% 7% 4 3 3 3 4 5% 23%	3124
February	1st to 10th	Day	21% 13% 4 9% 4 4 3 3 4 4 30%	1710
	11th to 20th	Day	22% 11% 4 9% 5% 4 4 3 5% 5% 28%	1837
	21st to end	Day	22% 14% 4 6% 4 4 3 3 4 6% 31%	1570
March	1st to 10th	Day	22% 15% 4 7% 4 4 4 3 3 5% 29%	3334
	11th to 20th	Day	16% 7% 3 6% 4 4 4 3 5% 6% 42%	3480
	21st to end	Day	18% 8% 2 7% 5% 4 4 3 6% 7% 34%	3815
April	1st to 10th	Day	15% 9% 3 7% 4 5% 5% 3 4 7% 38%	2090
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 5% 7% 41%	2372
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 5% 7% 38%	2470
May	1st to 10th	Day	20% 10% 3 8% 5% 5% 4 4 6% 6% 29%	4082
	11th to 20th	Day	19% 9% 3 6% 4 4 4 4 5% 6% 34%	4081
	21st to end	Day	10% 6% 2 7% 4 6% 6% 5% 7% 9% 37%	4476
June	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 8% 9% 41%	2470
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 6% 10% 42%	2470
	21st to end	Day	9% 6% 2 5% 5% 6% 5% 5% 7% 9% 41%	2470
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 4 4 7% 9% 43%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	4120
	21st to end	Day	17% 8% 3 8% 6% 6% 6% 4 6% 7% 28%	4466
August	1st to 10th	Day	9% 6% 3 6% 5% 5% 6% 5% 7% 11% 37%	4104
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 10% 39%	4044
	21st to end	Day	5% 4 1 4 3 4 4 3 6% 9% 56%	4391
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 7% 48%	2090
	11th to 20th	Day	11% 5% 2 7% 4 4 4 4 6% 8% 47%	2090
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 5% 42%	2090
October	1st to 10th	Day	13% 5% 1 5% 3 4 4 3 5% 7% 49%	2024
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 5% 6% 48%	1900
	21st to end	Day	20% 9% 2 6% 3 3 3 2 4 6% 42%	2090
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 6% 5% 51%	1732
	11th to 20th	Day	15% 11% 3 6% 5% 4 4 2 5% 5% 40%	1710
	21st to end	Day	17% 10% 2 6% 4 4 4 3 5% 5% 40%	1710
December	1st to 10th	Day	25% 13% 3 8% 5% 4 4 3 4 5% 26%	1710
	11th to 20th	Day	20% 13% 5% 8% 4 4 3 3 4 6% 30%	2153
	21st to end	Day	20% 18% 4 9% 5% 5% 5% 4 5% 6% 19%	3124
January	1st to 10th	Night	17% 19% 5% 10% 6% 5% 4 3 4 5% 23%	4109
	11th to 20th	Night	22% 14% 5% 10% 4 3 3 3 4 4 28%	4120
	21st to end	Night	25% 14% 4 8% 4 4 4 3 4 4 25%	4532
February	1st to 10th	Night	20% 12% 4 8% 5% 4 4 3 5% 5% 29%	2470
	11th to 20th	Night	23% 10% 4 8% 5% 4 4 4 4 5% 29%	2227
	21st to end	Night	24% 14% 3 7% 5% 3 4 4 4 5% 28%	1884
March	1st to 10th	Night	23% 11% 3 7% 4 4 4 3 4 5% 32%	3519
	11th to 20th	Night	14% 9% 3 6% 4 4 5% 3 5% 7% 41%	3480
	21st to end	Night	14% 8% 2 6% 5% 5% 5% 4 6% 7% 39%	3817
April	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 5% 7% 39%	2090
	11th to 20th	Night	15% 7% 3 6% 4 3 4 4 6% 8% 41%	1770
	21st to end	Night	16% 6% 3 6% 4 5% 4 3 7% 6% 39%	1710
May	1st to 10th	Night	23% 9% 3 8% 5% 4 5% 3 5% 7% 28%	2813
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 8% 37%	2812
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 6% 9% 42%	3079
June	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 7% 10% 43%	1710
	11th to 20th	Night	7% 4 3 7% 4 5% 5% 4 6% 10% 45%	1710
	21st to end	Night	10% 5% 2 4 4 4 5% 5% 8% 10% 45%	1710
July	1st to 10th	Night	8% 6% 2 6% 4 4 5% 5% 8% 10% 42%	2840
	11th to 20th	Night	8% 6% 2 7% 5% 5% 5% 3 6% 8% 47%	2840
	21st to end	Night	12% 9% 4 8% 6% 6% 5% 3 7% 7% 34%	3087
August	1st to 10th	Night	11% 7% 3 8% 5% 4 5% 5% 6% 9% 38%	2825
	11th to 20th	Night	10% 6% 3 7% 5% 4 5% 4 6% 7% 43%	2785
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 9% 57%	3214
September	1st to 10th	Night	11% 4 2 5% 4 3 5% 4 6% 8% 49%	2090
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 6% 53%	2090
	21st to end	Night	13% 7% 2 6% 4 4 4 3 6% 6% 44%	2090
October	1st to 10th	Night	11% 4 2 6% 4 3 4 3 4 7% 51%	2156
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 6% 5% 46%	2280
	21st to end	Night	19% 6% 2 5% 4 4 4 2 5% 5% 43%	2508
November	1st to 10th	Night	12% 6% 2 4 4 4 5% 3 4 5% 51%	2441
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 5% 7% 42%	2470
	21st to end	Night	17% 11% 3 6% 4 4 4 3 4 5% 40%	2470
December	1st to 10th	Night	23% 12% 4 9% 5% 4 4 3 4 5% 29%	2470
	11th to 20th	Night	17% 14% 6% 8% 4 4 4 3 4 6% 31%	3042
	21st to end	Night	20% 15% 4 10% 6% 5% 5% 3 5% 5% 22%	4532

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% 2 10% 16% 8% 1	2837
	11th to 20th	Day	70% 1 6% 18% 5% 1	2840
	21st to end	Day	66% 1 6% 21% 5% 1	3124
February	1st to 10th	Day	59% 3 9% 18% 11% 1	1710
	11th to 20th	Day	59% 3 6% 20% 11% 2	1847
	21st to end	Day	56% 2 7% 21% 11% 2	1570
March	1st to 10th	Day	61% 1 6% 21% 9% 2	3334
	11th to 20th	Day	45% 1 8% 27% 17% 1	3480
	21st to end	Day	50% 2 7% 26% 14% 1	3815
April	1st to 10th	Day	47% 2 10% 23% 14% 4	2090
	11th to 20th	Day	45% 3 8% 23% 18% 4	2372
	21st to end	Day	47% 3 11% 24% 12% 3	2470
May	1st to 10th	Day	56% 3 9% 20% 11% 1	4120
	11th to 20th	Day	52% 3 9% 24% 12% 1	4085
	21st to end	Day	43% 3 12% 21% 17% 4	4489
June	1st to 10th	Day	38% 4 14% 27% 16% 1	2470
	11th to 20th	Day	37% 5% 15% 23% 17% 2	2470
	21st to end	Day	38% 6% 14% 24% 15% 3	2470
July	1st to 10th	Day	37% 6% 15% 26% 14% 2	4055
	11th to 20th	Day	35% 5% 11% 34% 13% 1	4051
	21st to end	Day	53% 5% 10% 17% 14% 2	4324
August	1st to 10th	Day	39% 6% 14% 30% 11% 1	4091
	11th to 20th	Day	39% 5% 10% 30% 16% 1	4083
	21st to end	Day	26% 4 14% 33% 21% 2	4391
September	1st to 10th	Day	36% 4 10% 23% 21% 6%	2090
	11th to 20th	Day	37% 3 9% 27% 22% 2	2090
	21st to end	Day	45% 3 9% 22% 20% 2	2090
October	1st to 10th	Day	28% 3 7% 27% 33% 2	3362
	11th to 20th	Day	28% 2 10% 31% 24% 5%	3140
	21st to end	Day	33% 2 10% 30% 23% 3	3428
November	1st to 10th	Day	35% 3 8% 28% 22% 3	1732
	11th to 20th	Day	47% 1 7% 22% 20% 3	1710
	21st to end	Day	47% 2 9% 23% 17% 1	1710
December	1st to 10th	Day	62% 3 8% 21% 6% 1	1710
	11th to 20th	Day	57% 2 9% 18% 13% 1	2150
	21st to end	Day	68% 2 8% 18% 5%	3124
January	1st to 10th	Night	66% 1 7% 17% 8% 1	4109
	11th to 20th	Night	62% 1 6% 21% 9% 1	4120
	21st to end	Night	64% 1 7% 20% 6% 1	4532
February	1st to 10th	Night	58% 2 8% 20% 11% 1	2470
	11th to 20th	Night	58% 3 6% 19% 13% 1	2228
	21st to end	Night	61% 2 8% 16% 10% 4	1884
March	1st to 10th	Night	57% 2 4 22% 14% 1	3519
	11th to 20th	Night	45% 2 7% 28% 17% 1	3480
	21st to end	Night	46% 2 8% 26% 17% 1	3817
April	1st to 10th	Night	45% 4 9% 21% 19% 3	2090
	11th to 20th	Night	42% 2 10% 23% 16% 7%	1770
	21st to end	Night	45% 4 11% 20% 18% 2	1710
May	1st to 10th	Night	57% 3 10% 18% 10% 2	2840
	11th to 20th	Night	48% 4 10% 23% 13% 1	2813
	21st to end	Night	40% 4 11% 25% 17% 4	3087
June	1st to 10th	Night	36% 6% 12% 25% 18% 4	1710
	11th to 20th	Night	35% 5% 15% 19% 22% 4	1710
	21st to end	Night	33% 7% 14% 25% 17% 3	1710
July	1st to 10th	Night	35% 6% 14% 22% 18% 4	2795
	11th to 20th	Night	35% 6% 12% 28% 16% 2	2786
	21st to end	Night	48% 5% 10% 19% 13% 4	2988
August	1st to 10th	Night	41% 6% 14% 26% 12% 1	2814
	11th to 20th	Night	40% 5% 9% 26% 20% 1	2812
	21st to end	Night	24% 4 13% 32% 24% 3	3214
September	1st to 10th	Night	33% 5% 13% 22% 19% 8%	2090
	11th to 20th	Night	33% 3 10% 25% 25% 4	2090
	21st to end	Night	40% 3 10% 21% 23% 2	2090
October	1st to 10th	Night	29% 2 7% 25% 32% 4	3598
	11th to 20th	Night	29% 2 8% 26% 25% 9%	3779
	21st to end	Night	32% 2 6% 26% 28% 6%	4120
November	1st to 10th	Night	37% 2 8% 25% 24% 5%	2441
	11th to 20th	Night	44% 2 7% 22% 21% 4	2470
	21st to end	Night	48% 2 9% 24% 16% 2	2470
December	1st to 10th	Night	60% 3 8% 20% 8% 1	2470
	11th to 20th	Night	57% 2 8% 22% 11% 1	3044
	21st to end	Night	66% 2 7% 18% 7%	4532

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	27% 21% 36% 10% 4 1	270
	11th to 20th	Day	36% 21% 31% 9% 2	270
	21st to end	Day	32% 26% 27% 12% 4	297
February	1st to 10th	Day	31% 22% 31% 10% 6%	270
	11th to 20th	Day	45% 21% 26% 4 2 1	270
	21st to end	Day	41% 16% 34% 8% 1	225
March	1st to 10th	Day	30% 18% 30% 17% 5% 1	393
	11th to 20th	Day	27% 21% 31% 15% 5%	460
	21st to end	Day	27% 17% 33% 19% 4	506
April	1st to 10th	Day	26% 17% 29% 19% 7% 1	460
	11th to 20th	Day	22% 19% 31% 16% 9% 3	452
	21st to end	Day	24% 16% 33% 19% 6% 2	460
May	1st to 10th	Day	24% 14% 33% 16% 8% 4	463
	11th to 20th	Day	22% 10% 26% 23% 13% 6%	487
	21st to end	Day	15% 9% 21% 29% 17% 9%	529
June	1st to 10th	Day	14% 7% 23% 29% 19% 8%	490
	11th to 20th	Day	13% 6% 21% 30% 21% 10%	490
	21st to end	Day	11% 7% 31% 22% 19% 9%	490
July	1st to 10th	Day	14% 3 18% 26% 23% 17%	485
	11th to 20th	Day	8% 2 22% 33% 21% 13%	484
	21st to end	Day	14% 5% 18% 28% 24% 12%	532
August	1st to 10th	Day	14% 3 17% 31% 25% 12%	486
	11th to 20th	Day	15% 4 16% 31% 24% 11%	479
	21st to end	Day	8% 4 15% 31% 27% 14%	539
September	1st to 10th	Day	13% 2 15% 30% 27% 12%	490
	11th to 20th	Day	13% 3 11% 30% 30% 13%	490
	21st to end	Day	24% 8% 18% 26% 17% 7%	490
October	1st to 10th	Day	14% 7% 23% 27% 21% 7%	490
	11th to 20th	Day	16% 13% 29% 23% 14% 4	490
	21st to end	Day	24% 11% 28% 25% 11% 1	539
November	1st to 10th	Day	17% 12% 32% 26% 9% 4	306
	11th to 20th	Day	27% 17% 27% 19% 9% 1	270
	21st to end	Day	24% 16% 33% 20% 6% 1	270
December	1st to 10th	Day	31% 20% 27% 14% 6% 1	270
	11th to 20th	Day	27% 19% 33% 14% 5% 1	270
	21st to end	Day	33% 19% 31% 13% 3 1	297
January	1st to 10th	Night	28% 18% 29% 19% 5%	760
	11th to 20th	Night	33% 16% 28% 19% 4	760
	21st to end	Night	26% 16% 32% 22% 3	836
February	1st to 10th	Night	27% 13% 31% 27% 2	760
	11th to 20th	Night	31% 17% 30% 19% 2	743
	21st to end	Night	32% 15% 33% 18% 2	628
March	1st to 10th	Night	26% 15% 28% 26% 5%	760
	11th to 20th	Night	25% 15% 31% 21% 7% 1	760
	21st to end	Night	24% 12% 32% 25% 7%	836
April	1st to 10th	Night	21% 14% 31% 26% 8%	760
	11th to 20th	Night	17% 11% 30% 28% 12% 1	606
	21st to end	Night	20% 9% 35% 28% 7% 1	570
May	1st to 10th	Night	18% 8% 28% 31% 14% 2	570
	11th to 20th	Night	20% 7% 23% 32% 15% 2	561
	21st to end	Night	12% 4 20% 33% 27% 4	617
June	1st to 10th	Night	11% 3 18% 40% 25% 3	570
	11th to 20th	Night	8% 2 16% 43% 26% 5%	570
	21st to end	Night	12% 4 15% 38% 29% 2	570
July	1st to 10th	Night	13% 2 11% 33% 33% 8%	555
	11th to 20th	Night	6% 2 14% 38% 37% 4	552
	21st to end	Night	12% 2 12% 36% 33% 5%	624
August	1st to 10th	Night	12% 2 9% 38% 34% 5%	567
	11th to 20th	Night	11% 3 10% 36% 33% 7%	564
	21st to end	Night	10% 2 10% 34% 37% 8%	646
September	1st to 10th	Night	13% 1 12% 33% 35% 6%	760
	11th to 20th	Night	11% 3 12% 31% 36% 6%	760
	21st to end	Night	17% 5% 18% 32% 25% 4	760
October	1st to 10th	Night	13% 4 18% 33% 28% 3	760
	11th to 20th	Night	18% 8% 25% 31% 17% 2	760
	21st to end	Night	22% 8% 23% 30% 15% 1	836
November	1st to 10th	Night	15% 7% 27% 32% 18% 1	760
	11th to 20th	Night	21% 13% 26% 28% 11%	760
	21st to end	Night	23% 11% 23% 34% 8% 1	760
December	1st to 10th	Night	32% 12% 26% 25% 5%	760
	11th to 20th	Night	24% 15% 29% 26% 6%	760
	21st to end	Night	30% 14% 25% 27% 4	836

Page generated at 2024-11-03 02:11:22. Version ac07

Copyright © 2024 Attilla Danko.