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 2025-10-04.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 35%	126840
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	122539
Transparency	Day	47% 7% 12% 21% 12% 1	130407
	Night	47% 7% 12% 21% 12% 1	127058
Seeing	Day	20% 17% 31% 20% 9% 3	15617
	Night	19% 14% 31% 25% 10% 1	24844

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	12% 13% 4 9% 4 4 5% 4 5% 6% 33%	3597
	11th to 20th	Day	8% 18% 8% 15% 7% 6% 5% 4 4 4 20%	3642
	21st to end	Day	14% 16% 7% 13% 6% 5% 5% 3 5% 5% 21%	4466
February	1st to 10th	Day	12% 16% 5% 12% 7% 5% 5% 4 5% 5% 25%	1800
	11th to 20th	Day	11% 13% 5% 12% 9% 5% 4 3 4 6% 27%	1753
	21st to end	Day	18% 14% 4 10% 7% 4 5% 3 4 5% 26%	1570
March	1st to 10th	Day	18% 12% 4 9% 5% 3 4 3 5% 6% 31%	4812
	11th to 20th	Day	10% 12% 4 10% 6% 5% 5% 4 5% 7% 32%	4980
	21st to end	Day	17% 11% 3 6% 4 4 4 3 4 5% 40%	5546
April	1st to 10th	Day	18% 12% 3 8% 5% 4 4 3 5% 6% 32%	2400
	11th to 20th	Day	21% 8% 3 7% 4 3 4 2 4 6% 38%	2374
	21st to end	Day	19% 9% 3 7% 5% 5% 4 3 4 6% 33%	2590
May	1st to 10th	Day	21% 9% 3 8% 5% 4 4 3 5% 6% 31%	6113
	11th to 20th	Day	15% 10% 3 10% 5% 4 5% 3 5% 6% 35%	6318
	21st to end	Day	18% 7% 2 7% 5% 4 4 3 5% 8% 36%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 7% 8% 34%	2800
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 5% 7% 9% 40%	2800
	21st to end	Day	14% 7% 2 6% 4 4 5% 5% 7% 8% 37%	2800
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	6360
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 46%	6360
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 10% 46%	6801
August	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 39%	4234
	11th to 20th	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	4174
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 7% 9% 42%	4388
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 5% 7% 44%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 51%	2388
	21st to end	Day	15% 7% 3 6% 5% 4 5% 3 5% 8% 40%	2094
October	1st to 10th	Day	17% 8% 3 7% 5% 5% 6% 4 5% 7% 36%	1930
	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% 36%	1683
	11th to 20th	Day	15% 11% 3 8% 6% 4 6% 5% 6% 7% 30%	1600
	21st to end	Day	19% 14% 4 8% 6% 5% 5% 4 5% 6% 24%	1600
December	1st to 10th	Day	17% 15% 5% 12% 7% 5% 5% 3 5% 6% 20%	2600
	11th to 20th	Day	15% 16% 5% 10% 5% 5% 4 4 4 4 27%	2986
	21st to end	Day	22% 15% 4 11% 5% 5% 4 3 4 5% 22%	3960
January	1st to 10th	Night	14% 12% 5% 11% 6% 5% 5% 4 5% 6% 26%	6349
	11th to 20th	Night	11% 13% 5% 12% 8% 7% 6% 4 6% 6% 21%	6241
	21st to end	Night	14% 13% 6% 12% 5% 4 5% 3 4 6% 28%	6490
February	1st to 10th	Night	12% 15% 4 10% 7% 5% 5% 4 5% 6% 26%	2600
	11th to 20th	Night	12% 13% 5% 10% 6% 5% 4 5% 6% 5% 29%	2533
	21st to end	Night	13% 11% 4 9% 5% 5% 4 4 6% 7% 31%	2060
March	1st to 10th	Night	15% 11% 3 10% 6% 5% 5% 4 5% 6% 31%	5025
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 5% 7% 7% 36%	4979
	21st to end	Night	19% 8% 3 6% 4 3 3 3 4 6% 41%	5386
April	1st to 10th	Night	17% 10% 4 7% 5% 4 4 4 5% 5% 35%	2000
	11th to 20th	Night	18% 8% 2 7% 5% 4 4 3 4 6% 39%	1975
	21st to end	Night	16% 8% 2 7% 5% 5% 4 5% 6% 7% 35%	1825
May	1st to 10th	Night	25% 8% 3 7% 4 4 4 4 5% 5% 30%	3828
	11th to 20th	Night	12% 8% 2 7% 4 5% 4 4 6% 7% 41%	3575
	21st to end	Night	18% 6% 3 7% 5% 4 5% 3 5% 6% 40%	3926
June	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 7% 7% 39%	1600
	11th to 20th	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 44%	1600
	21st to end	Night	12% 6% 3 7% 6% 5% 4 3 6% 7% 41%	1600
July	1st to 10th	Night	7% 5% 3 7% 5% 5% 5% 5% 7% 10% 43%	3600
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 3 5% 7% 51%	3600
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 8% 52%	4049
August	1st to 10th	Night	9% 5% 2 6% 4 4 5% 3 6% 7% 48%	2915
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 4 8% 49%	2875
	21st to end	Night	8% 5% 2 5% 4 4 5% 3 6% 8% 50%	3476
September	1st to 10th	Night	10% 5% 2 6% 4 5% 4 3 6% 8% 47%	2000
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 7% 53%	2010
	21st to end	Night	16% 6% 3 6% 3 4 4 3 3 6% 44%	2305
October	1st to 10th	Night	16% 6% 3 6% 4 4 5% 3 5% 7% 41%	2317
	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%	2612
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 4 6% 7% 32%	2800
	21st to end	Night	19% 11% 3 7% 6% 5% 4 5% 6% 7% 26%	2800
December	1st to 10th	Night	16% 15% 4 8% 6% 5% 5% 5% 7% 7% 22%	4580
	11th to 20th	Night	16% 14% 4 10% 5% 5% 5% 4 5% 6% 25%	5208
	21st to end	Night	24% 15% 4 11% 5% 4 4 4 4 5% 19%	6996

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	49% 8% 13% 19% 10% 1	3598
	11th to 20th	Day	67% 7% 9% 10% 7% 1	3672
	21st to end	Day	64% 7% 9% 10% 8% 1	4466
February	1st to 10th	Day	60% 7% 11% 16% 6% 1	1800
	11th to 20th	Day	59% 6% 9% 15% 10% 1	1762
	21st to end	Day	61% 5% 9% 15% 9% 1	1570
March	1st to 10th	Day	53% 6% 12% 14% 14% 1	4815
	11th to 20th	Day	50% 7% 17% 16% 9% 2	4980
	21st to end	Day	47% 5% 7% 22% 18% 2	5546
April	1st to 10th	Day	53% 5% 10% 20% 12% 2	2400
	11th to 20th	Day	48% 5% 9% 21% 16% 1	2374
	21st to end	Day	51% 6% 9% 18% 13% 3	2590
May	1st to 10th	Day	52% 6% 9% 18% 11% 4	6122
	11th to 20th	Day	48% 8% 10% 16% 17% 1	6322
	21st to end	Day	46% 7% 12% 22% 11% 2	6949
June	1st to 10th	Day	45% 8% 12% 20% 15% 1	2800
	11th to 20th	Day	38% 8% 13% 26% 13% 1	2800
	21st to end	Day	42% 8% 13% 22% 13% 1	2800
July	1st to 10th	Day	36% 8% 15% 27% 14% 1	6290
	11th to 20th	Day	31% 8% 14% 30% 16% 1	6285
	21st to end	Day	30% 9% 14% 34% 12%	6605
August	1st to 10th	Day	42% 8% 14% 23% 12% 1	4221
	11th to 20th	Day	40% 7% 15% 29% 9%	4213
	21st to end	Day	35% 8% 13% 34% 10% 1	4388
September	1st to 10th	Day	38% 7% 13% 30% 11% 1	2400
	11th to 20th	Day	31% 7% 14% 32% 16% 1	2388
	21st to end	Day	43% 7% 14% 26% 10%	2094
October	1st to 10th	Day	37% 7% 14% 29% 12% 1	3190
	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	47% 8% 14% 20% 10% 1	1683
	11th to 20th	Day	50% 8% 16% 15% 10% 1	1600
	21st to end	Day	59% 9% 12% 14% 7%	1600
December	1st to 10th	Day	64% 7% 14% 11% 4	2600
	11th to 20th	Day	59% 8% 10% 14% 9%	2986
	21st to end	Day	64% 7% 9% 12% 7%	3960
January	1st to 10th	Night	57% 8% 12% 14% 9%	6349
	11th to 20th	Night	59% 10% 12% 12% 5% 1	6286
	21st to end	Night	56% 7% 11% 12% 13% 1	6490
February	1st to 10th	Night	58% 8% 11% 17% 6% 1	2600
	11th to 20th	Night	55% 7% 11% 19% 9%	2535
	21st to end	Night	50% 8% 12% 21% 8% 1	2060
March	1st to 10th	Night	52% 6% 10% 16% 12% 4	5035
	11th to 20th	Night	43% 8% 18% 19% 10% 1	4980
	21st to end	Night	43% 5% 10% 22% 18% 2	5386
April	1st to 10th	Night	49% 6% 9% 20% 14% 2	2000
	11th to 20th	Night	46% 5% 11% 19% 17% 2	1975
	21st to end	Night	45% 9% 10% 18% 16% 2	1825
May	1st to 10th	Night	53% 7% 11% 14% 11% 4	3835
	11th to 20th	Night	40% 8% 14% 21% 15% 2	3576
	21st to end	Night	44% 7% 12% 21% 14% 2	3927
June	1st to 10th	Night	41% 8% 12% 24% 14% 2	1600
	11th to 20th	Night	35% 8% 13% 22% 20% 3	1600
	21st to end	Night	41% 6% 13% 23% 14% 2	1600
July	1st to 10th	Night	34% 9% 15% 23% 19% 1	3560
	11th to 20th	Night	31% 7% 13% 31% 17% 1	3552
	21st to end	Night	29% 7% 13% 34% 16% 1	3930
August	1st to 10th	Night	34% 7% 13% 29% 16% 2	2904
	11th to 20th	Night	34% 6% 18% 28% 12% 2	2902
	21st to end	Night	30% 8% 12% 36% 13% 1	3476
September	1st to 10th	Night	34% 7% 15% 29% 13% 1	2000
	11th to 20th	Night	29% 6% 13% 34% 17% 1	2010
	21st to end	Night	42% 6% 10% 28% 13% 1	2305
October	1st to 10th	Night	33% 6% 10% 28% 21% 1	3837
	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% 22% 12% 1	2612
	11th to 20th	Night	50% 8% 12% 19% 10% 2	2800
	21st to end	Night	54% 9% 16% 16% 5%	2800
December	1st to 10th	Night	56% 10% 15% 15% 4	4580
	11th to 20th	Night	57% 9% 12% 13% 8% 1	5208
	21st to end	Night	65% 7% 10% 11% 7%	6996

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	23% 25% 36% 12% 4 1	200
	11th to 20th	Day	20% 33% 29% 13% 3 1	217
	21st to end	Day	23% 28% 32% 14% 3 1	330
February	1st to 10th	Day	27% 31% 30% 11% 2	300
	11th to 20th	Day	27% 30% 33% 9%	300
	21st to end	Day	33% 23% 35% 8%	334
March	1st to 10th	Day	24% 23% 34% 15% 3	500
	11th to 20th	Day	24% 24% 33% 15% 3 1	500
	21st to end	Day	33% 16% 33% 12% 5% 1	550
April	1st to 10th	Day	27% 21% 33% 13% 6%	500
	11th to 20th	Day	30% 20% 27% 14% 6% 4	492
	21st to end	Day	27% 21% 32% 15% 5% 1	500
May	1st to 10th	Day	23% 17% 30% 20% 5% 5%	503
	11th to 20th	Day	18% 13% 35% 22% 8% 4	527
	21st to end	Day	17% 13% 31% 24% 11% 4	573
June	1st to 10th	Day	17% 13% 35% 23% 10% 2	530
	11th to 20th	Day	13% 14% 34% 24% 13% 3	530
	21st to end	Day	20% 13% 30% 25% 10% 3	530
July	1st to 10th	Day	10% 9% 30% 32% 15% 4	525
	11th to 20th	Day	14% 10% 28% 27% 15% 6%	524
	21st to end	Day	9% 11% 33% 31% 12% 4	576
August	1st to 10th	Day	10% 7% 29% 30% 17% 6%	526
	11th to 20th	Day	12% 5% 27% 30% 18% 8%	519
	21st to end	Day	13% 9% 29% 28% 15% 7%	583
September	1st to 10th	Day	12% 12% 28% 26% 15% 7%	530
	11th to 20th	Day	11% 8% 24% 24% 21% 13%	530
	21st to end	Day	15% 13% 31% 24% 12% 5%	530
October	1st to 10th	Day	19% 17% 27% 19% 13% 6%	502
	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	25% 20% 27% 20% 8% 1	317
	11th to 20th	Day	23% 28% 34% 12% 3 1	200
	21st to end	Day	27% 23% 34% 14% 3	200
December	1st to 10th	Day	27% 23% 32% 14% 5%	200
	11th to 20th	Day	30% 28% 30% 10% 2 1	200
	21st to end	Day	31% 23% 33% 12% 1	220
January	1st to 10th	Night	25% 22% 30% 18% 5%	800
	11th to 20th	Night	26% 17% 31% 20% 6%	800
	21st to end	Night	21% 17% 34% 24% 5% 1	880
February	1st to 10th	Night	21% 20% 40% 17% 2	800
	11th to 20th	Night	22% 20% 36% 18% 4	783
	21st to end	Night	22% 20% 36% 18% 3	660
March	1st to 10th	Night	21% 19% 35% 21% 4	800
	11th to 20th	Night	20% 19% 36% 20% 4 1	800
	21st to end	Night	20% 15% 35% 22% 7% 1	880
April	1st to 10th	Night	22% 17% 33% 23% 5%	800
	11th to 20th	Night	21% 18% 31% 20% 8% 1	791
	21st to end	Night	20% 11% 32% 26% 10%	625
May	1st to 10th	Night	22% 7% 28% 29% 11% 2	505
	11th to 20th	Night	14% 8% 32% 34% 12%	394
	21st to end	Night	12% 9% 29% 34% 14% 1	433
June	1st to 10th	Night	13% 8% 35% 32% 13% 1	400
	11th to 20th	Night	11% 10% 27% 37% 14% 1	400
	21st to end	Night	13% 8% 30% 34% 15% 1	400
July	1st to 10th	Night	6% 5% 28% 44% 16% 1	390
	11th to 20th	Night	10% 4 27% 35% 24% 1	388
	21st to end	Night	8% 3 29% 41% 17% 2	486
August	1st to 10th	Night	10% 4 20% 37% 27% 2	597
	11th to 20th	Night	9% 5% 20% 36% 26% 3	594
	21st to end	Night	11% 5% 27% 34% 21% 2	809
September	1st to 10th	Night	12% 5% 26% 34% 20% 3	800
	11th to 20th	Night	11% 6% 21% 31% 26% 6%	800
	21st to end	Night	17% 9% 23% 30% 17% 4	800
October	1st to 10th	Night	19% 9% 29% 24% 17% 2	773
	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% 14% 27% 26% 9% 1	780
	11th to 20th	Night	21% 19% 34% 20% 6% 2	800
	21st to end	Night	26% 16% 32% 21% 6%	800
December	1st to 10th	Night	26% 22% 32% 17% 4	800
	11th to 20th	Night	23% 20% 36% 17% 4	800
	21st to end	Night	30% 16% 33% 17% 3	880

Page generated at 2025-10-01 01:39:01. Version ac07

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