Sandy Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandy Lake clear sky chart from 2005-11-06 to 2025-02-25.

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	13% 11% 4 10% 6% 5% 5% 4 6% 7% 29%	107904
	Night	11% 13% 5% 12% 7% 6% 6% 5% 5% 5% 23%	110972
Transparency	Day	54% 5% 12% 20% 8%	111273
	Night	62% 5% 10% 17% 6%	115772
Seeing	Day	19% 11% 30% 26% 11% 3	16377
	Night	19% 8% 29% 30% 12% 2	26178

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	6% 12% 6% 19% 11% 8% 7% 7% 5% 5% 14%	3178
	11th to 20th	Day	4 10% 5% 20% 11% 9% 8% 8% 10% 6% 10%	3152
	21st to end	Day	9% 17% 8% 17% 6% 6% 6% 5% 8% 8% 11%	3498
February	1st to 10th	Day	4 11% 5% 12% 8% 9% 9% 8% 9% 7% 19%	1694
	11th to 20th	Day	5% 10% 5% 9% 7% 7% 9% 8% 8% 8% 25%	1754
	21st to end	Day	3 10% 5% 12% 10% 7% 7% 7% 7% 7% 25%	1449
March	1st to 10th	Day	5% 13% 5% 12% 6% 6% 4 4 5% 7% 32%	3164
	11th to 20th	Day	12% 14% 5% 9% 8% 9% 7% 6% 4 5% 22%	3480
	21st to end	Day	8% 15% 4 10% 5% 6% 8% 6% 5% 5% 28%	3931
April	1st to 10th	Day	9% 11% 4 9% 6% 5% 5% 4 6% 8% 33%	2280
	11th to 20th	Day	13% 10% 4 9% 5% 4 5% 4 5% 7% 34%	2437
	21st to end	Day	10% 9% 3 8% 5% 5% 5% 4 6% 7% 39%	2608
May	1st to 10th	Day	16% 10% 4 8% 4 5% 4 3 6% 7% 32%	4428
	11th to 20th	Day	18% 14% 4 8% 5% 4 4 4 5% 7% 28%	4564
	21st to end	Day	23% 10% 3 6% 4 4 4 3 4 6% 33%	5223
June	1st to 10th	Day	19% 8% 3 8% 5% 5% 4 3 6% 7% 31%	3040
	11th to 20th	Day	17% 8% 3 7% 6% 5% 5% 5% 5% 7% 32%	3040
	21st to end	Day	15% 7% 3 7% 6% 5% 4 5% 7% 8% 34%	3040
July	1st to 10th	Day	15% 5% 2 7% 6% 6% 6% 5% 6% 7% 36%	4887
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 5% 6% 8% 40%	4750
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	5151
August	1st to 10th	Day	13% 4 2 5% 4 5% 5% 4 6% 9% 41%	4502
	11th to 20th	Day	11% 4 2 5% 4 4 5% 5% 7% 9% 45%	4201
	21st to end	Day	19% 8% 2 6% 4 5% 5% 4 6% 8% 32%	4532
September	1st to 10th	Day	18% 9% 3 7% 3 5% 4 4 5% 6% 35%	2290
	11th to 20th	Day	22% 11% 3 7% 4 4 4 3 4 5% 31%	2272
	21st to end	Day	21% 12% 3 8% 5% 4 4 3 4 5% 33%	1987
October	1st to 10th	Day	20% 17% 6% 7% 5% 3 5% 3 4 6% 23%	1900
	11th to 20th	Day	21% 20% 7% 10% 5% 4 3 3 4 5% 18%	1900
	21st to end	Day	19% 26% 7% 10% 4 4 4 2 4 5% 14%	1743
November	1st to 10th	Day	16% 20% 9% 16% 8% 6% 4 3 3 4 12%	1567
	11th to 20th	Day	15% 19% 9% 15% 9% 6% 5% 3 4 3 12%	1600
	21st to end	Day	13% 21% 6% 20% 9% 8% 4 2 4 3 9%	1390
December	1st to 10th	Day	6% 15% 9% 19% 12% 10% 8% 4 4 4 8%	1960
	11th to 20th	Day	7% 18% 6% 12% 8% 6% 9% 4 8% 6% 16%	2256
	21st to end	Day	9% 17% 11% 15% 7% 6% 5% 3 5% 6% 16%	3056
January	1st to 10th	Night	6% 12% 6% 18% 11% 11% 7% 6% 6% 4 13%	6787
	11th to 20th	Night	5% 9% 5% 22% 10% 8% 8% 9% 10% 4 11%	6753
	21st to end	Night	9% 16% 8% 17% 8% 8% 8% 5% 5% 5% 11%	7480
February	1st to 10th	Night	6% 12% 4 12% 10% 10% 10% 7% 7% 6% 17%	2705
	11th to 20th	Night	5% 10% 4 12% 9% 8% 9% 8% 9% 6% 20%	2533
	21st to end	Night	4 11% 5% 13% 8% 6% 7% 6% 6% 7% 26%	2100
March	1st to 10th	Night	10% 12% 5% 11% 7% 8% 7% 5% 5% 5% 25%	3705
	11th to 20th	Night	9% 17% 6% 11% 10% 7% 7% 5% 5% 4 20%	3479
	21st to end	Night	8% 14% 6% 9% 5% 7% 11% 5% 5% 5% 25%	3700
April	1st to 10th	Night	7% 9% 4 9% 5% 4 4 4 5% 6% 42%	1900
	11th to 20th	Night	11% 11% 4 9% 4 5% 4 3 5% 6% 38%	1691
	21st to end	Night	8% 12% 4 8% 5% 4 4 3 5% 5% 41%	1572
May	1st to 10th	Night	20% 13% 4 9% 5% 3 3 3 4 4 32%	2523
	11th to 20th	Night	17% 13% 3 7% 4 4 4 3 5% 7% 35%	2338
	21st to end	Night	24% 11% 3 7% 5% 4 4 3 4 5% 32%	2360
June	1st to 10th	Night	19% 8% 3 8% 6% 5% 5% 4 5% 6% 30%	1140
	11th to 20th	Night	20% 10% 3 8% 6% 5% 5% 3 5% 6% 29%	1140
	21st to end	Night	14% 5% 3 8% 5% 4 5% 5% 6% 8% 36%	1140
July	1st to 10th	Night	15% 4 2 4 4 4 5% 4 6% 8% 42%	2079
	11th to 20th	Night	12% 6% 2 7% 5% 4 5% 4 6% 9% 41%	2220
	21st to end	Night	10% 5% 2 6% 5% 5% 6% 5% 6% 10% 42%	2414
August	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 7% 9% 43%	2425
	11th to 20th	Night	11% 4 2 4 4 3 4 3 5% 8% 52%	2635
	21st to end	Night	18% 7% 2 6% 4 4 4 4 6% 9% 36%	3135
September	1st to 10th	Night	18% 7% 1 6% 4 4 4 3 6% 8% 39%	1890
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 5% 7% 32%	1905
	21st to end	Night	19% 10% 3 6% 5% 3 3 3 5% 7% 34%	2173
October	1st to 10th	Night	22% 15% 4 8% 4 4 5% 3 4 5% 26%	2280
	11th to 20th	Night	22% 18% 4 8% 5% 5% 4 3 4 6% 21%	2280
	21st to end	Night	21% 23% 7% 11% 5% 4 4 3 4 4 14%	2854
November	1st to 10th	Night	19% 20% 6% 14% 6% 4 3 2 4 5% 16%	2731
	11th to 20th	Night	16% 19% 8% 16% 9% 7% 5% 3 3 3 12%	2800
	21st to end	Night	12% 22% 10% 18% 10% 7% 5% 3 3 3 7%	3005
December	1st to 10th	Night	7% 19% 9% 21% 12% 9% 6% 4 3 3 7%	5230
	11th to 20th	Night	7% 14% 6% 14% 9% 10% 9% 6% 7% 5% 13%	5951
	21st to end	Night	8% 18% 9% 18% 7% 7% 5% 3 4 3 18%	7919

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	71% 5% 8% 11% 6%	3179
	11th to 20th	Day	69% 4 10% 14% 3	3180
	21st to end	Day	69% 4 5% 12% 9%	3498
February	1st to 10th	Day	58% 5% 10% 18% 9% 1	1694
	11th to 20th	Day	51% 5% 12% 22% 9%	1763
	21st to end	Day	54% 4 10% 19% 11% 1	1449
March	1st to 10th	Day	52% 4 11% 21% 12%	3166
	11th to 20th	Day	64% 3 8% 17% 6% 1	3480
	21st to end	Day	56% 4 10% 21% 8% 1	3931
April	1st to 10th	Day	49% 4 12% 22% 13% 1	2280
	11th to 20th	Day	50% 4 8% 22% 14% 1	2437
	21st to end	Day	42% 5% 10% 26% 15% 2	2608
May	1st to 10th	Day	50% 5% 13% 24% 7%	4440
	11th to 20th	Day	56% 7% 7% 18% 10% 1	4568
	21st to end	Day	53% 4 8% 19% 15% 1	5223
June	1st to 10th	Day	52% 6% 10% 18% 13%	3040
	11th to 20th	Day	50% 6% 10% 21% 12% 1	3040
	21st to end	Day	46% 7% 13% 20% 13% 1	3040
July	1st to 10th	Day	45% 7% 17% 24% 7%	4813
	11th to 20th	Day	41% 6% 17% 29% 5% 1	4669
	21st to end	Day	36% 8% 18% 33% 5%	4988
August	1st to 10th	Day	38% 7% 15% 33% 7%	4485
	11th to 20th	Day	34% 7% 19% 33% 7%	4243
	21st to end	Day	49% 6% 17% 19% 9%	4531
September	1st to 10th	Day	49% 7% 16% 21% 7% 1	2290
	11th to 20th	Day	56% 6% 15% 21% 2	2272
	21st to end	Day	54% 5% 14% 21% 6%	1987
October	1st to 10th	Day	57% 6% 13% 16% 7%	3160
	11th to 20th	Day	58% 5% 13% 18% 7%	3140
	21st to end	Day	69% 6% 10% 14% 1	2850
November	1st to 10th	Day	78% 4 9% 7% 1	1567
	11th to 20th	Day	77% 5% 7% 9% 1	1600
	21st to end	Day	82% 4 4 7% 3	1390
December	1st to 10th	Day	80% 3 9% 7% 2	1960
	11th to 20th	Day	67% 3 9% 14% 7%	2256
	21st to end	Day	69% 3 7% 14% 6%	3056
January	1st to 10th	Night	72% 4 6% 12% 6%	6787
	11th to 20th	Night	70% 4 9% 14% 3	6800
	21st to end	Night	74% 4 5% 10% 6%	7480
February	1st to 10th	Night	66% 5% 9% 15% 6%	2705
	11th to 20th	Night	59% 6% 10% 18% 7%	2534
	21st to end	Night	55% 4 10% 20% 10% 1	2100
March	1st to 10th	Night	59% 4 10% 18% 9%	3716
	11th to 20th	Night	67% 5% 7% 15% 4 1	3480
	21st to end	Night	59% 4 8% 18% 10% 1	3700
April	1st to 10th	Night	42% 4 9% 26% 17% 2	1900
	11th to 20th	Night	47% 4 9% 22% 16% 2	1691
	21st to end	Night	44% 5% 8% 23% 19% 2	1572
May	1st to 10th	Night	55% 4 8% 24% 8%	2530
	11th to 20th	Night	49% 5% 8% 23% 13% 2	2339
	21st to end	Night	57% 5% 4 19% 14% 1	2360
June	1st to 10th	Night	51% 6% 9% 17% 16%	1140
	11th to 20th	Night	56% 5% 7% 19% 12% 2	1140
	21st to end	Night	43% 7% 15% 21% 14% 1	1140
July	1st to 10th	Night	37% 8% 19% 24% 11% 1	2044
	11th to 20th	Night	40% 8% 15% 30% 6% 1	2178
	21st to end	Night	37% 7% 15% 32% 8%	2337
August	1st to 10th	Night	36% 7% 13% 34% 10%	2418
	11th to 20th	Night	30% 7% 19% 36% 8%	2659
	21st to end	Night	45% 6% 22% 22% 5%	3135
September	1st to 10th	Night	44% 6% 18% 24% 6% 1	1890
	11th to 20th	Night	52% 6% 16% 23% 2	1905
	21st to end	Night	50% 7% 16% 21% 6%	2173
October	1st to 10th	Night	62% 6% 12% 16% 5%	3800
	11th to 20th	Night	54% 5% 19% 15% 7%	3779
	21st to end	Night	71% 5% 9% 15% 1	4705
November	1st to 10th	Night	73% 5% 11% 9% 2	2731
	11th to 20th	Night	78% 4 8% 8% 2	2800
	21st to end	Night	82% 4 7% 5% 2	3005
December	1st to 10th	Night	83% 3 7% 5% 2	5230
	11th to 20th	Night	70% 4 9% 12% 5%	5950
	21st to end	Night	71% 3 6% 16% 4	7919

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	12% 9% 41% 27% 10% 1	300
	11th to 20th	Day	12% 7% 33% 27% 17% 4	300
	21st to end	Day	15% 11% 34% 28% 9% 2	330
February	1st to 10th	Day	9% 11% 39% 32% 8% 1	300
	11th to 20th	Day	10% 7% 39% 27% 14% 2	300
	21st to end	Day	6% 13% 39% 22% 18% 2	237
March	1st to 10th	Day	12% 9% 37% 25% 11% 6%	270
	11th to 20th	Day	11% 17% 37% 25% 7% 2	270
	21st to end	Day	15% 15% 34% 23% 10% 2	363
April	1st to 10th	Day	14% 10% 30% 30% 15% 2	460
	11th to 20th	Day	18% 11% 32% 24% 12% 2	635
	21st to end	Day	13% 11% 32% 29% 12% 4	650
May	1st to 10th	Day	22% 10% 30% 26% 9% 2	653
	11th to 20th	Day	21% 12% 29% 23% 12% 2	675
	21st to end	Day	23% 14% 31% 22% 9% 1	734
June	1st to 10th	Day	23% 12% 28% 22% 9% 5%	680
	11th to 20th	Day	20% 13% 30% 23% 10% 4	680
	21st to end	Day	18% 11% 28% 26% 12% 5%	680
July	1st to 10th	Day	15% 10% 28% 31% 14% 3	671
	11th to 20th	Day	18% 10% 26% 26% 15% 4	668
	21st to end	Day	15% 7% 24% 32% 15% 7%	739
August	1st to 10th	Day	17% 8% 24% 31% 15% 4	676
	11th to 20th	Day	14% 9% 22% 31% 18% 6%	665
	21st to end	Day	26% 7% 25% 27% 13% 2	748
September	1st to 10th	Day	23% 9% 33% 27% 7% 1	500
	11th to 20th	Day	26% 11% 27% 30% 6%	482
	21st to end	Day	27% 12% 34% 23% 4	300
October	1st to 10th	Day	28% 14% 31% 20% 6% 2	300
	11th to 20th	Day	27% 12% 30% 23% 5% 2	300
	21st to end	Day	36% 7% 30% 20% 7%	330
November	1st to 10th	Day	23% 15% 33% 23% 6% 1	300
	11th to 20th	Day	21% 18% 30% 20% 8% 3	300
	21st to end	Day	23% 19% 40% 14% 4	248
December	1st to 10th	Day	17% 10% 39% 27% 7% 2	200
	11th to 20th	Day	15% 14% 42% 23% 6% 2	200
	21st to end	Day	15% 7% 32% 27% 16% 3	233
January	1st to 10th	Night	14% 8% 35% 30% 11% 2	1000
	11th to 20th	Night	16% 7% 32% 31% 13% 2	1000
	21st to end	Night	16% 6% 33% 33% 10% 1	1100
February	1st to 10th	Night	12% 7% 34% 35% 10% 2	1000
	11th to 20th	Night	10% 7% 29% 33% 17% 4	979
	21st to end	Night	8% 7% 31% 34% 18% 2	808
March	1st to 10th	Night	15% 8% 29% 33% 14% 1	760
	11th to 20th	Night	17% 12% 32% 27% 12% 1	570
	21st to end	Night	15% 10% 28% 30% 12% 4	627
April	1st to 10th	Night	12% 6% 31% 37% 13% 2	570
	11th to 20th	Night	18% 6% 28% 34% 12% 3	563
	21st to end	Night	14% 10% 28% 34% 12% 2	570
May	1st to 10th	Night	25% 8% 26% 30% 10% 1	570
	11th to 20th	Night	21% 6% 30% 30% 11% 1	561
	21st to end	Night	27% 6% 29% 27% 10% 1	589
June	1st to 10th	Night	23% 7% 23% 31% 15% 2	380
	11th to 20th	Night	26% 6% 24% 30% 12% 2	380
	21st to end	Night	16% 4 25% 38% 17%	380
July	1st to 10th	Night	14% 6% 25% 37% 17%	466
	11th to 20th	Night	18% 6% 26% 32% 15% 2	552
	21st to end	Night	13% 3 22% 36% 21% 4	624
August	1st to 10th	Night	15% 5% 20% 42% 17% 1	567
	11th to 20th	Night	17% 5% 19% 36% 21% 2	564
	21st to end	Night	24% 6% 27% 31% 12% 1	627
September	1st to 10th	Night	19% 6% 34% 32% 9%	570
	11th to 20th	Night	28% 8% 23% 31% 9%	570
	21st to end	Night	26% 8% 30% 27% 8% 1	653
October	1st to 10th	Night	32% 9% 31% 22% 5% 1	760
	11th to 20th	Night	31% 11% 23% 25% 9% 2	760
	21st to end	Night	34% 8% 27% 22% 7% 1	983
November	1st to 10th	Night	30% 11% 27% 23% 8% 2	975
	11th to 20th	Night	26% 9% 32% 23% 8% 2	1000
	21st to end	Night	22% 14% 33% 22% 8% 1	1000
December	1st to 10th	Night	14% 8% 35% 29% 11% 2	1000
	11th to 20th	Night	14% 8% 34% 31% 12% 1	1000
	21st to end	Night	16% 7% 30% 32% 12% 3	1100

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