Mark Slade Remote Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mark Slade Remote Observatory clear sky chart from 2005-11-06 to 2024-12-06.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	101397
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	98270
Transparency	Day	38% 7% 12% 23% 17% 4	105040
	Night	38% 6% 11% 23% 18% 4	102766
Seeing	Day	18% 11% 26% 25% 14% 5%	15120
	Night	17% 9% 29% 30% 13% 1	24764

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% 6% 2 4 3 3 3 3 5% 6% 48%	2718
	11th to 20th	Day	15% 9% 2 7% 5% 4 4 3 5% 6% 41%	2840
	21st to end	Day	18% 9% 2 7% 4 5% 4 3 4 5% 38%	3124
February	1st to 10th	Day	13% 9% 2 7% 4 5% 4 4 5% 7% 40%	1710
	11th to 20th	Day	18% 7% 2 6% 3 3 3 3 5% 7% 42%	1699
	21st to end	Day	18% 8% 3 7% 5% 4 5% 4 5% 6% 36%	1578
March	1st to 10th	Day	21% 8% 2 6% 5% 4 4 3 4 6% 36%	3435
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 5% 7% 50%	3480
	21st to end	Day	20% 7% 2 6% 4 4 4 4 6% 6% 38%	3834
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2280
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 48%	2254
	21st to end	Day	17% 7% 2 6% 4 5% 4 4 6% 6% 41%	2280
May	1st to 10th	Day	19% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	4041
	11th to 20th	Day	23% 7% 2 6% 4 4 4 3 6% 8% 33%	4144
	21st to end	Day	11% 4 1 5% 4 4 5% 4 7% 10% 44%	4823
June	1st to 10th	Day	11% 5% 2 4 5% 5% 5% 4 6% 10% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 6% 5% 4 4 6% 9% 42%	2660
	21st to end	Day	8% 4 2 5% 5% 5% 6% 4 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 4 6% 7% 5% 9% 11% 39%	4440
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 5% 10% 12% 36%	4129
	21st to end	Day	11% 7% 2 8% 6% 6% 6% 4 7% 9% 36%	4466
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 11% 32%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 6% 5% 5% 8% 9% 38%	3894
	21st to end	Day	10% 4 1 5% 4 4 4 3 5% 9% 49%	4180
September	1st to 10th	Day	14% 4 2 4 5% 4 5% 4 5% 7% 46%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 9% 46%	2275
	21st to end	Day	22% 6% 2 5% 3 4 4 4 5% 6% 38%	1985
October	1st to 10th	Day	16% 5% 2 5% 4 4 3 3 4 6% 47%	1900
	11th to 20th	Day	16% 4 2 5% 3 3 3 3 4 5% 51%	1900
	21st to end	Day	13% 6% 2 5% 4 3 3 3 4 6% 51%	2090
November	1st to 10th	Day	12% 5% 2 4 4 3 4 3 5% 6% 53%	1922
	11th to 20th	Day	15% 6% 1 4 3 3 3 4 6% 5% 50%	1770
	21st to end	Day	16% 5% 2 4 3 4 4 3 4 5% 51%	1600
December	1st to 10th	Day	16% 6% 2 6% 3 3 4 4 6% 7% 43%	1560
	11th to 20th	Day	17% 7% 2 4 3 4 3 2 4 6% 49%	1910
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 8% 38%	2772
January	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 4 6% 49%	4223
	11th to 20th	Night	18% 8% 2 5% 3 3 3 3 5% 5% 47%	4120
	21st to end	Night	19% 8% 3 6% 4 3 3 2 4 5% 43%	4532
February	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 6% 6% 42%	2470
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 4 6% 44%	2365
	21st to end	Night	16% 8% 3 6% 4 4 3 4 5% 6% 43%	1874
March	1st to 10th	Night	21% 7% 2 6% 4 4 4 3 5% 5% 39%	3418
	11th to 20th	Night	13% 4 2 4 4 3 3 3 5% 7% 50%	3480
	21st to end	Night	14% 7% 2 5% 4 4 3 3 5% 6% 48%	3797
April	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 6% 45%	1900
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 4 8% 48%	1875
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 45%	1900
May	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 6% 7% 32%	2883
	11th to 20th	Night	24% 8% 2 6% 4 4 4 3 5% 7% 33%	2734
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 3 7% 10% 43%	2732
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 9% 44%	1520
	11th to 20th	Night	12% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	1520
	21st to end	Night	8% 5% 2 5% 4 4 5% 5% 8% 10% 43%	1520
July	1st to 10th	Night	6% 6% 2 7% 5% 5% 5% 5% 8% 10% 41%	2520
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 6% 10% 37%	2827
	21st to end	Night	9% 7% 3 8% 6% 7% 6% 5% 7% 8% 33%	3087
August	1st to 10th	Night	16% 6% 2 7% 5% 5% 6% 4 7% 9% 32%	2825
	11th to 20th	Night	11% 7% 3 9% 5% 5% 5% 4 7% 8% 35%	2903
	21st to end	Night	11% 6% 2 5% 3 4 3 4 5% 8% 48%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 45%	1900
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 5% 7% 49%	1901
	21st to end	Night	21% 7% 3 6% 3 3 3 3 5% 6% 39%	2194
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 6% 46%	2280
	11th to 20th	Night	15% 6% 1 6% 3 3 4 3 5% 5% 50%	2280
	21st to end	Night	14% 4 2 5% 3 4 4 3 5% 6% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 6% 53%	2375
	11th to 20th	Night	14% 7% 2 6% 4 3 3 2 5% 6% 48%	2625
	21st to end	Night	13% 4 2 4 4 3 4 4 5% 7% 51%	2800
December	1st to 10th	Night	16% 7% 3 6% 4 3 3 3 5% 6% 43%	2737
	11th to 20th	Night	18% 6% 2 6% 3 4 4 3 4 6% 45%	3285
	21st to end	Night	17% 7% 2 5% 4 4 5% 4 5% 8% 40%	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	36% 5% 8% 31% 16% 5%	2719
	11th to 20th	Day	44% 5% 8% 19% 17% 6%	2840
	21st to end	Day	47% 6% 9% 21% 15% 2	3124
February	1st to 10th	Day	43% 6% 10% 25% 13% 2	1710
	11th to 20th	Day	42% 5% 8% 23% 18% 4	1708
	21st to end	Day	47% 6% 11% 14% 18% 4	1578
March	1st to 10th	Day	48% 6% 8% 20% 13% 5%	3435
	11th to 20th	Day	31% 6% 8% 24% 24% 6%	3480
	21st to end	Day	46% 7% 10% 19% 15% 4	3834
April	1st to 10th	Day	36% 6% 12% 21% 18% 8%	2280
	11th to 20th	Day	36% 6% 11% 20% 22% 6%	2254
	21st to end	Day	42% 7% 10% 23% 13% 5%	2280
May	1st to 10th	Day	47% 11% 11% 19% 11% 1	4078
	11th to 20th	Day	47% 8% 12% 23% 9% 1	4148
	21st to end	Day	31% 11% 15% 22% 19% 2	4837
June	1st to 10th	Day	35% 9% 14% 26% 15% 1	2660
	11th to 20th	Day	36% 8% 15% 23% 16% 2	2660
	21st to end	Day	33% 10% 14% 25% 17% 1	2660
July	1st to 10th	Day	34% 11% 20% 18% 11% 6%	4370
	11th to 20th	Day	34% 10% 18% 27% 9% 1	4059
	21st to end	Day	44% 8% 11% 21% 16% 1	4324
August	1st to 10th	Day	43% 8% 17% 25% 7%	4091
	11th to 20th	Day	38% 9% 13% 25% 15% 1	3933
	21st to end	Day	32% 7% 15% 27% 17% 2	4180
September	1st to 10th	Day	38% 9% 11% 18% 20% 5%	2280
	11th to 20th	Day	34% 6% 9% 27% 20% 3	2275
	21st to end	Day	46% 7% 10% 18% 17% 3	1985
October	1st to 10th	Day	36% 4 7% 18% 30% 5%	3160
	11th to 20th	Day	26% 3 6% 24% 29% 11%	3140
	21st to end	Day	21% 3 10% 28% 24% 12%	3428
November	1st to 10th	Day	32% 4 10% 25% 24% 5%	1922
	11th to 20th	Day	35% 5% 10% 22% 22% 7%	1770
	21st to end	Day	35% 6% 10% 25% 22% 4	1600
December	1st to 10th	Day	39% 7% 11% 25% 13% 5%	1560
	11th to 20th	Day	38% 4 7% 31% 14% 6%	1906
	21st to end	Day	43% 8% 11% 26% 11% 1	2772
January	1st to 10th	Night	36% 5% 9% 25% 20% 5%	4223
	11th to 20th	Night	39% 4 8% 23% 20% 5%	4120
	21st to end	Night	45% 4 7% 25% 17% 2	4532
February	1st to 10th	Night	41% 6% 10% 24% 16% 2	2470
	11th to 20th	Night	43% 4 8% 21% 18% 7%	2367
	21st to end	Night	41% 5% 10% 20% 17% 6%	1874
March	1st to 10th	Night	45% 7% 8% 23% 15% 2	3418
	11th to 20th	Night	31% 6% 8% 23% 26% 5%	3480
	21st to end	Night	38% 6% 9% 22% 20% 5%	3797
April	1st to 10th	Night	38% 7% 10% 17% 22% 5%	1900
	11th to 20th	Night	34% 7% 12% 21% 21% 6%	1875
	21st to end	Night	38% 6% 11% 26% 13% 6%	1900
May	1st to 10th	Night	50% 9% 10% 19% 10% 2	2911
	11th to 20th	Night	49% 9% 12% 19% 9% 1	2735
	21st to end	Night	34% 10% 16% 25% 13% 2	2739
June	1st to 10th	Night	34% 9% 15% 27% 15% 2	1520
	11th to 20th	Night	37% 9% 16% 23% 14% 2	1520
	21st to end	Night	32% 11% 17% 19% 18% 3	1520
July	1st to 10th	Night	36% 9% 22% 17% 12% 3	2480
	11th to 20th	Night	40% 9% 20% 22% 8% 1	2774
	21st to end	Night	43% 10% 14% 19% 13%	2988
August	1st to 10th	Night	46% 9% 15% 22% 7% 1	2814
	11th to 20th	Night	44% 8% 12% 22% 14% 1	2930
	21st to end	Night	34% 7% 12% 29% 17% 1	3476
September	1st to 10th	Night	38% 8% 11% 20% 16% 6%	1900
	11th to 20th	Night	34% 7% 10% 24% 22% 3	1901
	21st to end	Night	46% 6% 10% 19% 16% 3	2194
October	1st to 10th	Night	42% 4 9% 19% 19% 7%	3800
	11th to 20th	Night	25% 4 9% 22% 31% 10%	3779
	21st to end	Night	22% 4 8% 31% 26% 10%	4120
November	1st to 10th	Night	33% 5% 9% 24% 23% 6%	2375
	11th to 20th	Night	37% 4 8% 22% 22% 7%	2625
	21st to end	Night	33% 5% 10% 25% 22% 5%	2800
December	1st to 10th	Night	42% 5% 10% 24% 16% 4	2737
	11th to 20th	Night	40% 5% 9% 21% 20% 5%	3288
	21st to end	Night	40% 8% 13% 23% 14% 1	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	24% 22% 29% 21% 4	238
	11th to 20th	Day	27% 28% 24% 18% 3	270
	21st to end	Day	23% 24% 36% 12% 4	297
February	1st to 10th	Day	22% 27% 31% 16% 4	270
	11th to 20th	Day	33% 24% 31% 11% 1	306
	21st to end	Day	29% 19% 30% 19% 3	382
March	1st to 10th	Day	22% 20% 34% 19% 5%	460
	11th to 20th	Day	22% 23% 32% 18% 6%	460
	21st to end	Day	24% 17% 34% 18% 6% 1	506
April	1st to 10th	Day	23% 21% 29% 20% 6% 1	460
	11th to 20th	Day	17% 18% 29% 21% 12% 3	452
	21st to end	Day	22% 14% 33% 22% 9% 1	460
May	1st to 10th	Day	20% 15% 32% 19% 11% 3	463
	11th to 20th	Day	22% 9% 30% 25% 13% 3	487
	21st to end	Day	18% 5% 25% 26% 21% 5%	529
June	1st to 10th	Day	12% 6% 27% 31% 18% 7%	490
	11th to 20th	Day	11% 6% 27% 30% 20% 7%	490
	21st to end	Day	9% 5% 25% 36% 19% 5%	490
July	1st to 10th	Day	10% 3 17% 33% 27% 11%	485
	11th to 20th	Day	7% 1 15% 38% 27% 12%	484
	21st to end	Day	12% 4 18% 32% 24% 9%	532
August	1st to 10th	Day	11% 4 20% 27% 28% 10%	486
	11th to 20th	Day	12% 2 18% 31% 27% 10%	479
	21st to end	Day	11% 3 18% 29% 24% 15%	539
September	1st to 10th	Day	16% 4 18% 28% 18% 16%	490
	11th to 20th	Day	15% 3 18% 30% 23% 11%	490
	21st to end	Day	28% 9% 23% 24% 12% 4	490
October	1st to 10th	Day	21% 5% 23% 26% 17% 7%	490
	11th to 20th	Day	17% 13% 23% 27% 17% 3	490
	21st to end	Day	15% 12% 34% 28% 8% 4	539
November	1st to 10th	Day	16% 10% 32% 26% 13% 3	478
	11th to 20th	Day	19% 10% 37% 21% 11% 2	370
	21st to end	Day	24% 13% 34% 20% 10% 1	200
December	1st to 10th	Day	19% 21% 35% 18% 6% 2	190
	11th to 20th	Day	21% 16% 34% 21% 9%	180
	21st to end	Day	26% 18% 28% 18% 9% 1	198
January	1st to 10th	Night	21% 17% 39% 20% 4	760
	11th to 20th	Night	21% 15% 33% 27% 4	760
	21st to end	Night	19% 16% 38% 25% 2	836
February	1st to 10th	Night	20% 13% 38% 26% 3	760
	11th to 20th	Night	25% 18% 38% 18% 1	743
	21st to end	Night	22% 16% 38% 21% 3	628
March	1st to 10th	Night	19% 16% 37% 25% 3	760
	11th to 20th	Night	21% 16% 37% 21% 5% 1	760
	21st to end	Night	17% 16% 38% 23% 5%	836
April	1st to 10th	Night	17% 17% 38% 22% 5%	760
	11th to 20th	Night	14% 13% 35% 28% 9%	751
	21st to end	Night	19% 9% 33% 31% 8%	760
May	1st to 10th	Night	20% 8% 32% 27% 12% 1	615
	11th to 20th	Night	20% 5% 29% 33% 14%	522
	21st to end	Night	14% 3 20% 39% 23% 1	411
June	1st to 10th	Night	10% 3 22% 45% 19% 1	380
	11th to 20th	Night	14% 3 19% 42% 21% 1	380
	21st to end	Night	9% 3 22% 46% 19% 1	380
July	1st to 10th	Night	11% 2 19% 35% 29% 3	370
	11th to 20th	Night	7% 1 13% 39% 34% 5%	540
	21st to end	Night	9% 2 17% 40% 29% 3	624
August	1st to 10th	Night	12% 2 12% 42% 30% 3	567
	11th to 20th	Night	12% 3 13% 42% 27% 3	630
	21st to end	Night	12% 3 13% 37% 29% 6%	836
September	1st to 10th	Night	16% 3 15% 34% 29% 5%	760
	11th to 20th	Night	13% 3 17% 34% 30% 3	760
	21st to end	Night	24% 4 19% 32% 19% 1	760
October	1st to 10th	Night	20% 4 20% 31% 21% 3	760
	11th to 20th	Night	17% 8% 29% 29% 16% 1	760
	21st to end	Night	17% 8% 33% 29% 13% 1	836
November	1st to 10th	Night	14% 9% 33% 30% 13% 1	780
	11th to 20th	Night	18% 11% 35% 28% 9%	800
	21st to end	Night	15% 11% 34% 29% 9% 1	800
December	1st to 10th	Night	20% 10% 37% 23% 9%	783
	11th to 20th	Night	19% 9% 39% 28% 5%	760
	21st to end	Night	21% 11% 33% 31% 4	836

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