Sandy Flats Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandy Flats 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	14% 9% 3 7% 5% 5% 5% 4 5% 7% 35%	101542
	Night	15% 9% 3 8% 5% 4 4 4 5% 6% 36%	97509
Transparency	Day	46% 8% 13% 22% 11% 1	105127
	Night	46% 7% 13% 22% 11% 1	102012
Seeing	Day	20% 16% 31% 21% 9% 3	14471
	Night	19% 13% 31% 25% 10% 1	23723

By Month

Percentage Cloud Cover NE (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	15% 17% 5% 9% 4 4 4 3 4 5% 32%	2517
	11th to 20th	Day	11% 18% 7% 13% 6% 6% 5% 4 4 4 21%	2564
	21st to end	Day	19% 20% 6% 11% 5% 4 4 2 4 4 20%	3124
February	1st to 10th	Day	12% 16% 5% 11% 7% 5% 5% 4 5% 5% 25%	1710
	11th to 20th	Day	11% 13% 5% 12% 9% 5% 4 3 4 6% 28%	1663
	21st to end	Day	18% 13% 4 10% 7% 4 4 3 4 5% 27%	1494
March	1st to 10th	Day	18% 13% 3 7% 5% 4 5% 3 5% 6% 31%	3354
	11th to 20th	Day	10% 14% 4 9% 6% 5% 4 4 5% 6% 32%	3480
	21st to end	Day	13% 9% 2 5% 4 4 4 3 4 6% 45%	3868
April	1st to 10th	Day	18% 13% 3 8% 5% 4 4 3 5% 5% 32%	2280
	11th to 20th	Day	21% 8% 3 6% 4 3 4 2 4 6% 38%	2254
	21st to end	Day	20% 9% 3 7% 5% 5% 4 3 4 6% 33%	2460
May	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 6% 6% 35%	4263
	11th to 20th	Day	16% 9% 3 8% 4 4 5% 4 6% 8% 33%	4398
	21st to end	Day	12% 5% 2 6% 5% 4 4 4 6% 8% 44%	4837
June	1st to 10th	Day	16% 7% 2 6% 6% 4 5% 4 7% 8% 34%	2660
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 5% 7% 9% 40%	2660
	21st to end	Day	14% 7% 2 6% 4 4 5% 5% 7% 8% 37%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	4440
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 43%	4440
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 8% 10% 42%	4731
August	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 6% 7% 38%	4104
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 38%	4044
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 7% 10% 41%	4254
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 5% 7% 44%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 7% 50%	2269
	21st to end	Day	15% 7% 3 6% 5% 4 5% 3 5% 8% 40%	1990
October	1st to 10th	Day	17% 8% 3 7% 5% 5% 6% 4 5% 7% 35%	1900
	11th to 20th	Day	18% 9% 3 8% 7% 5% 4 4 4 7% 31%	1900
	21st to end	Day	23% 12% 3 7% 5% 5% 5% 4 4 5% 27%	2072
November	1st to 10th	Day	13% 10% 4 8% 5% 5% 4 3 5% 6% 37%	1634
	11th to 20th	Day	15% 11% 3 8% 6% 4 6% 4 6% 7% 30%	1520
	21st to end	Day	19% 15% 4 8% 6% 5% 5% 4 5% 6% 25%	1520
December	1st to 10th	Day	16% 16% 4 11% 6% 5% 5% 3 5% 5% 23%	1520
	11th to 20th	Day	17% 15% 4 9% 5% 4 4 3 5% 5% 29%	1906
	21st to end	Day	27% 16% 4 9% 4 4 3 3 4 5% 20%	2772
January	1st to 10th	Night	18% 14% 5% 9% 5% 5% 5% 4 5% 6% 25%	4429
	11th to 20th	Night	13% 13% 5% 12% 8% 6% 5% 3 5% 5% 24%	4394
	21st to end	Night	19% 15% 6% 10% 5% 4 5% 3 4 5% 25%	4532
February	1st to 10th	Night	12% 16% 4 10% 7% 5% 5% 4 5% 6% 26%	2470
	11th to 20th	Night	12% 13% 4 9% 6% 5% 4 5% 6% 5% 30%	2403
	21st to end	Night	13% 11% 4 9% 5% 5% 4 4 6% 7% 32%	1960
March	1st to 10th	Night	17% 10% 3 9% 5% 5% 5% 3 4 6% 32%	3483
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 4 6% 6% 33%	3479
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 47%	3764
April	1st to 10th	Night	17% 10% 3 7% 4 4 4 4 5% 5% 36%	1900
	11th to 20th	Night	18% 7% 2 7% 5% 4 4 3 4 6% 39%	1875
	21st to end	Night	17% 8% 2 7% 4 5% 4 5% 6% 7% 35%	1734
May	1st to 10th	Night	20% 8% 2 7% 5% 4 4 4 5% 6% 35%	2678
	11th to 20th	Night	15% 8% 2 7% 4 5% 4 4 6% 7% 38%	2495
	21st to end	Night	10% 6% 3 6% 4 3 5% 3 5% 7% 48%	2738
June	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 7% 7% 39%	1520
	11th to 20th	Night	9% 6% 2 7% 5% 4 4 4 7% 9% 44%	1520
	21st to end	Night	12% 6% 3 7% 6% 5% 4 3 6% 7% 41%	1520
July	1st to 10th	Night	7% 4 2 7% 5% 5% 4 5% 6% 9% 45%	2520
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	2520
	21st to end	Night	8% 5% 2 5% 4 3 4 4 6% 8% 52%	2820
August	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 6% 7% 48%	2825
	11th to 20th	Night	13% 6% 2 6% 3 3 3 4 4 8% 48%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 3 6% 8% 50%	3369
September	1st to 10th	Night	11% 5% 2 6% 4 5% 4 4 6% 8% 46%	1900
	11th to 20th	Night	11% 4 2 4 4 3 4 4 6% 7% 53%	1909
	21st to end	Night	16% 6% 3 6% 3 4 4 3 3 6% 45%	2190
October	1st to 10th	Night	16% 6% 3 6% 4 4 5% 3 5% 7% 40%	2280
	11th to 20th	Night	17% 8% 3 9% 6% 4 5% 4 5% 6% 34%	2280
	21st to end	Night	20% 9% 3 9% 5% 4 4 4 6% 7% 29%	2524
November	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 6% 6% 38%	2541
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 4 6% 7% 32%	2660
	21st to end	Night	19% 11% 3 7% 6% 5% 4 5% 6% 7% 27%	2660
December	1st to 10th	Night	18% 14% 4 8% 7% 5% 5% 4 5% 6% 23%	2660
	11th to 20th	Night	16% 15% 4 9% 5% 5% 5% 4 5% 5% 27%	3288
	21st to end	Night	27% 16% 4 9% 5% 4 4 4 4 6% 17%	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	54% 8% 15% 14% 7% 1	2518
	11th to 20th	Day	66% 7% 10% 10% 7%	2564
	21st to end	Day	68% 6% 9% 10% 5% 1	3124
February	1st to 10th	Day	60% 7% 11% 16% 5% 1	1710
	11th to 20th	Day	58% 6% 9% 15% 10% 1	1672
	21st to end	Day	60% 5% 10% 15% 10% 1	1494
March	1st to 10th	Day	53% 7% 13% 18% 10% 1	3357
	11th to 20th	Day	51% 7% 15% 16% 9% 2	3480
	21st to end	Day	40% 6% 9% 25% 18% 3	3868
April	1st to 10th	Day	53% 5% 10% 19% 11% 2	2280
	11th to 20th	Day	48% 5% 9% 21% 16% 1	2254
	21st to end	Day	51% 6% 9% 18% 13% 2	2460
May	1st to 10th	Day	47% 7% 10% 21% 12% 2	4275
	11th to 20th	Day	46% 9% 13% 20% 10% 1	4402
	21st to end	Day	36% 8% 14% 24% 15% 3	4837
June	1st to 10th	Day	45% 8% 12% 20% 14% 1	2660
	11th to 20th	Day	38% 8% 13% 25% 14% 1	2660
	21st to end	Day	42% 8% 13% 22% 13% 1	2660
July	1st to 10th	Day	35% 8% 15% 26% 14% 1	4370
	11th to 20th	Day	32% 9% 16% 34% 9%	4365
	21st to end	Day	33% 11% 13% 29% 13%	4580
August	1st to 10th	Day	43% 8% 14% 22% 12% 1	4091
	11th to 20th	Day	40% 7% 15% 29% 9%	4083
	21st to end	Day	35% 8% 13% 34% 10% 1	4254
September	1st to 10th	Day	38% 8% 13% 29% 12% 1	2280
	11th to 20th	Day	32% 8% 14% 31% 15% 1	2269
	21st to end	Day	43% 7% 14% 26% 9%	1990
October	1st to 10th	Day	38% 7% 14% 29% 12% 1	3160
	11th to 20th	Day	41% 8% 16% 21% 14% 1	3140
	21st to end	Day	48% 8% 15% 20% 9%	3398
November	1st to 10th	Day	46% 8% 14% 20% 10% 1	1634
	11th to 20th	Day	51% 8% 16% 15% 9% 1	1520
	21st to end	Day	58% 9% 12% 13% 7%	1520
December	1st to 10th	Day	63% 7% 11% 14% 5%	1520
	11th to 20th	Day	58% 9% 11% 13% 8%	1906
	21st to end	Day	67% 6% 11% 12% 4	2772
January	1st to 10th	Night	59% 9% 14% 13% 5%	4429
	11th to 20th	Night	60% 11% 11% 13% 4 1	4394
	21st to end	Night	62% 7% 12% 13% 6%	4532
February	1st to 10th	Night	58% 8% 11% 17% 5% 1	2470
	11th to 20th	Night	54% 7% 11% 19% 9%	2405
	21st to end	Night	50% 7% 13% 21% 9% 1	1960
March	1st to 10th	Night	53% 6% 10% 20% 10% 1	3493
	11th to 20th	Night	49% 8% 15% 17% 10% 1	3480
	21st to end	Night	37% 6% 9% 25% 21% 3	3764
April	1st to 10th	Night	48% 6% 9% 20% 15% 2	1900
	11th to 20th	Night	46% 5% 11% 19% 17% 2	1875
	21st to end	Night	45% 9% 11% 18% 16% 2	1734
May	1st to 10th	Night	47% 7% 13% 17% 14% 2	2685
	11th to 20th	Night	43% 10% 13% 22% 10% 2	2496
	21st to end	Night	34% 8% 13% 24% 20% 2	2739
June	1st to 10th	Night	41% 7% 12% 24% 14% 2	1520
	11th to 20th	Night	34% 8% 13% 21% 20% 3	1520
	21st to end	Night	42% 6% 13% 23% 14% 2	1520
July	1st to 10th	Night	33% 8% 13% 26% 20% 1	2480
	11th to 20th	Night	32% 6% 13% 33% 15%	2472
	21st to end	Night	30% 7% 16% 34% 13% 1	2730
August	1st to 10th	Night	34% 6% 13% 29% 16% 2	2814
	11th to 20th	Night	34% 6% 18% 29% 11% 2	2812
	21st to end	Night	30% 8% 12% 36% 13% 1	3369
September	1st to 10th	Night	35% 8% 15% 27% 14% 1	1900
	11th to 20th	Night	30% 7% 13% 32% 18% 1	1909
	21st to end	Night	42% 6% 11% 29% 13% 1	2190
October	1st to 10th	Night	33% 6% 10% 28% 21% 1	3800
	11th to 20th	Night	42% 6% 14% 28% 9%	3779
	21st to end	Night	44% 7% 14% 26% 8%	4148
November	1st to 10th	Night	44% 7% 14% 21% 12% 1	2541
	11th to 20th	Night	50% 8% 13% 18% 10% 2	2660
	21st to end	Night	54% 9% 16% 15% 5%	2660
December	1st to 10th	Night	60% 9% 12% 14% 5%	2660
	11th to 20th	Night	58% 7% 12% 15% 7% 1	3288
	21st to end	Night	68% 7% 10% 11% 4	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% 24% 34% 12% 4 1	180
	11th to 20th	Day	20% 33% 29% 13% 3 1	195
	21st to end	Day	23% 25% 33% 15% 3 1	297
February	1st to 10th	Day	26% 31% 30% 11% 2	270
	11th to 20th	Day	24% 30% 36% 10%	270
	21st to end	Day	33% 25% 33% 9%	306
March	1st to 10th	Day	23% 23% 34% 16% 4	460
	11th to 20th	Day	24% 23% 33% 16% 3	460
	21st to end	Day	32% 16% 34% 12% 5% 1	506
April	1st to 10th	Day	26% 21% 33% 13% 6%	460
	11th to 20th	Day	30% 19% 26% 14% 7% 4	452
	21st to end	Day	27% 20% 31% 15% 5% 1	460
May	1st to 10th	Day	23% 16% 29% 21% 6% 5%	463
	11th to 20th	Day	18% 12% 34% 23% 9% 4	487
	21st to end	Day	15% 13% 31% 25% 12% 4	529
June	1st to 10th	Day	17% 12% 35% 23% 11% 2	490
	11th to 20th	Day	13% 13% 33% 24% 14% 3	490
	21st to end	Day	20% 12% 30% 25% 10% 3	490
July	1st to 10th	Day	10% 8% 28% 33% 16% 5%	485
	11th to 20th	Day	15% 9% 28% 27% 15% 6%	484
	21st to end	Day	9% 11% 31% 33% 12% 4	532
August	1st to 10th	Day	11% 8% 30% 28% 17% 7%	486
	11th to 20th	Day	11% 5% 28% 29% 19% 8%	479
	21st to end	Day	12% 9% 29% 28% 14% 7%	539
September	1st to 10th	Day	12% 11% 29% 25% 15% 7%	490
	11th to 20th	Day	11% 8% 25% 24% 19% 13%	490
	21st to end	Day	15% 13% 30% 24% 12% 5%	490
October	1st to 10th	Day	19% 17% 28% 17% 13% 6%	490
	11th to 20th	Day	22% 20% 30% 20% 7% 1	490
	21st to end	Day	29% 20% 28% 18% 5%	529
November	1st to 10th	Day	23% 19% 27% 21% 8% 1	304
	11th to 20th	Day	23% 27% 34% 12% 2 1	180
	21st to end	Day	26% 22% 34% 14% 3	180
December	1st to 10th	Day	26% 24% 32% 13% 5%	180
	11th to 20th	Day	28% 28% 32% 10% 2 1	180
	21st to end	Day	32% 25% 32% 11%	198
January	1st to 10th	Night	26% 21% 29% 19% 5%	760
	11th to 20th	Night	27% 17% 30% 20% 6%	760
	21st to end	Night	21% 16% 33% 24% 5% 1	836
February	1st to 10th	Night	21% 20% 40% 17% 3	760
	11th to 20th	Night	21% 19% 37% 19% 4	743
	21st to end	Night	22% 20% 35% 19% 4	628
March	1st to 10th	Night	20% 19% 35% 21% 4	760
	11th to 20th	Night	21% 19% 36% 20% 4 1	760
	21st to end	Night	20% 14% 36% 22% 7% 1	836
April	1st to 10th	Night	22% 16% 34% 23% 5%	760
	11th to 20th	Night	21% 17% 31% 21% 9% 1	751
	21st to end	Night	20% 11% 32% 26% 10%	594
May	1st to 10th	Night	21% 7% 29% 29% 12% 2	480
	11th to 20th	Night	14% 7% 32% 34% 13%	374
	21st to end	Night	12% 9% 29% 35% 14% 1	411
June	1st to 10th	Night	13% 7% 34% 32% 13% 1	380
	11th to 20th	Night	11% 9% 26% 38% 14% 1	380
	21st to end	Night	13% 7% 29% 34% 15% 1	380
July	1st to 10th	Night	6% 5% 28% 43% 17% 1	370
	11th to 20th	Night	10% 3 27% 35% 24% 1	368
	21st to end	Night	8% 3 28% 42% 18% 2	461
August	1st to 10th	Night	11% 4 20% 37% 26% 2	567
	11th to 20th	Night	9% 5% 21% 36% 26% 3	564
	21st to end	Night	11% 5% 26% 34% 22% 2	768
September	1st to 10th	Night	12% 5% 26% 34% 20% 3	760
	11th to 20th	Night	11% 6% 22% 29% 26% 6%	760
	21st to end	Night	17% 10% 23% 29% 18% 4	760
October	1st to 10th	Night	19% 9% 29% 23% 17% 2	760
	11th to 20th	Night	22% 15% 30% 23% 8% 1	760
	21st to end	Night	26% 14% 32% 23% 5%	836
November	1st to 10th	Night	24% 13% 26% 27% 9% 1	760
	11th to 20th	Night	21% 19% 33% 20% 6% 2	760
	21st to end	Night	25% 16% 32% 21% 6%	760
December	1st to 10th	Night	26% 22% 31% 17% 4	760
	11th to 20th	Night	23% 19% 35% 18% 4	760
	21st to end	Night	30% 17% 33% 17% 3	836

Page generated at 2024-11-03 02:11:24. Version ac07

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