Sheridan Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheridan Lake clear sky chart from 2005-11-06 to 2024-11-30.

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	9% 11% 4 9% 6% 5% 5% 4 6% 7% 33%	105652
	Night	9% 13% 5% 10% 6% 6% 5% 4 6% 7% 29%	102523
Transparency	Day	46% 10% 15% 19% 9% 2	105401
	Night	51% 10% 15% 16% 7% 1	102420
Seeing	Day	15% 13% 26% 25% 16% 5%	15971
	Night	16% 12% 24% 30% 15% 2	24162

By Month

Percentage Cloud Cover NW (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	7% 19% 7% 18% 7% 5% 5% 4 5% 6% 16%	2219
	11th to 20th	Day	8% 14% 5% 11% 5% 4 4 4 10% 10% 24%	2199
	21st to end	Day	5% 15% 5% 14% 9% 8% 6% 5% 6% 7% 21%	2740
February	1st to 10th	Day	9% 21% 6% 12% 6% 6% 5% 4 4 4 22%	1567
	11th to 20th	Day	7% 17% 7% 10% 6% 5% 5% 4 5% 7% 26%	1665
	21st to end	Day	6% 15% 7% 13% 7% 8% 5% 4 6% 6% 23%	1475
March	1st to 10th	Day	3 14% 6% 13% 8% 7% 7% 5% 6% 7% 25%	3138
	11th to 20th	Day	3 10% 4 10% 6% 6% 7% 5% 8% 9% 33%	3415
	21st to end	Day	6% 11% 4 12% 7% 7% 6% 5% 6% 8% 29%	4058
April	1st to 10th	Day	7% 17% 5% 12% 7% 7% 6% 4 6% 7% 21%	2280
	11th to 20th	Day	8% 16% 6% 10% 8% 6% 7% 5% 7% 6% 23%	2298
	21st to end	Day	9% 13% 4 10% 8% 6% 6% 4 6% 7% 25%	2660
May	1st to 10th	Day	9% 11% 4 9% 5% 5% 5% 4 6% 8% 35%	4420
	11th to 20th	Day	9% 10% 3 9% 6% 7% 6% 5% 8% 9% 28%	4377
	21st to end	Day	15% 15% 4 10% 6% 6% 6% 4 6% 7% 21%	5077
June	1st to 10th	Day	16% 10% 4 9% 6% 5% 6% 4 6% 7% 26%	3040
	11th to 20th	Day	18% 10% 4 10% 7% 6% 6% 5% 6% 6% 21%	3040
	21st to end	Day	11% 8% 3 8% 7% 6% 6% 4 7% 7% 33%	3040
July	1st to 10th	Day	6% 4 1 5% 3 4 4 3 5% 7% 58%	4914
	11th to 20th	Day	5% 3 2 4 3 3 3 3 5% 6% 63%	4740
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 6% 8% 48%	4898
August	1st to 10th	Day	5% 3 2 5% 4 5% 4 5% 6% 8% 52%	4401
	11th to 20th	Day	4 4 2 6% 5% 6% 6% 5% 8% 11% 42%	4278
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4352
September	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 6% 7% 47%	2280
	11th to 20th	Day	13% 8% 2 7% 5% 5% 5% 3 6% 6% 40%	2280
	21st to end	Day	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	2029
October	1st to 10th	Day	11% 13% 4 8% 5% 5% 5% 4 5% 8% 32%	3180
	11th to 20th	Day	16% 12% 3 8% 4 5% 4 3 5% 6% 34%	3103
	21st to end	Day	10% 12% 5% 10% 7% 6% 5% 4 6% 7% 29%	3080
November	1st to 10th	Day	12% 19% 5% 12% 6% 5% 4 4 7% 5% 21%	1568
	11th to 20th	Day	11% 14% 4 12% 7% 6% 6% 5% 7% 6% 22%	1600
	21st to end	Day	9% 18% 5% 11% 6% 6% 6% 5% 6% 7% 21%	1444
December	1st to 10th	Day	9% 15% 5% 14% 9% 7% 6% 5% 6% 5% 19%	1264
	11th to 20th	Day	6% 17% 7% 14% 8% 6% 7% 6% 7% 5% 15%	1442
	21st to end	Day	7% 14% 5% 11% 9% 7% 6% 5% 7% 5% 24%	2091
January	1st to 10th	Night	8% 19% 7% 17% 7% 6% 5% 5% 5% 6% 16%	4754
	11th to 20th	Night	6% 15% 6% 13% 7% 5% 5% 4 10% 6% 23%	4699
	21st to end	Night	7% 15% 5% 12% 7% 7% 7% 5% 7% 8% 21%	4914
February	1st to 10th	Night	8% 21% 7% 13% 7% 6% 5% 3 5% 6% 21%	2610
	11th to 20th	Night	7% 16% 6% 10% 7% 5% 4 4 5% 6% 29%	2402
	21st to end	Night	6% 15% 5% 12% 8% 5% 6% 4 6% 6% 28%	1979
March	1st to 10th	Night	3 13% 6% 11% 5% 6% 7% 6% 8% 9% 27%	3736
	11th to 20th	Night	2 9% 4 9% 5% 5% 7% 5% 8% 8% 37%	3561
	21st to end	Night	5% 14% 5% 10% 6% 7% 5% 4 7% 6% 31%	3595
April	1st to 10th	Night	5% 14% 5% 12% 7% 6% 6% 5% 8% 7% 26%	1900
	11th to 20th	Night	6% 14% 4 9% 8% 7% 7% 5% 7% 7% 28%	1828
	21st to end	Night	8% 12% 5% 10% 7% 6% 5% 4 6% 7% 31%	1520
May	1st to 10th	Night	9% 13% 4 10% 6% 5% 4 4 5% 7% 33%	2540
	11th to 20th	Night	14% 10% 4 9% 5% 6% 5% 5% 6% 7% 30%	2516
	21st to end	Night	21% 15% 5% 9% 6% 4 4 4 6% 5% 20%	2492
June	1st to 10th	Night	17% 10% 4 11% 5% 6% 6% 4 5% 7% 26%	1140
	11th to 20th	Night	20% 12% 4 9% 8% 5% 5% 4 5% 8% 19%	1140
	21st to end	Night	11% 9% 3 9% 5% 5% 6% 5% 6% 7% 34%	1140
July	1st to 10th	Night	9% 5% 2 5% 4 4 3 3 4 6% 54%	2043
	11th to 20th	Night	6% 3 2 4 4 4 5% 4 6% 8% 55%	2220
	21st to end	Night	11% 7% 3 8% 5% 4 6% 3 5% 7% 40%	2653
August	1st to 10th	Night	5% 5% 2 7% 5% 5% 6% 4 6% 7% 48%	2529
	11th to 20th	Night	6% 6% 3 7% 5% 6% 5% 4 8% 10% 38%	2549
	21st to end	Night	11% 7% 3 8% 5% 6% 5% 4 6% 8% 37%	3320
September	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 5% 7% 47%	1900
	11th to 20th	Night	11% 8% 2 6% 4 5% 4 4 6% 9% 41%	1900
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 3 6% 6% 38%	2148
October	1st to 10th	Night	14% 13% 4 10% 6% 5% 5% 3 5% 7% 30%	3800
	11th to 20th	Night	11% 10% 3 9% 5% 5% 5% 4 6% 9% 32%	3832
	21st to end	Night	10% 13% 5% 10% 7% 5% 4 4 5% 7% 30%	4476
November	1st to 10th	Night	15% 15% 5% 10% 6% 5% 6% 4 6% 7% 22%	2730
	11th to 20th	Night	10% 15% 4 11% 7% 6% 5% 5% 6% 6% 26%	2800
	21st to end	Night	9% 19% 6% 11% 7% 7% 5% 5% 6% 5% 21%	2930
December	1st to 10th	Night	10% 15% 5% 11% 7% 7% 6% 4 6% 6% 23%	2911
	11th to 20th	Night	8% 19% 7% 15% 6% 6% 5% 5% 5% 6% 16%	3750
	21st to end	Night	5% 16% 6% 13% 7% 7% 6% 5% 7% 7% 22%	5566

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% 13% 12% 10% 1	2219
	11th to 20th	Day	52% 9% 19% 8% 5% 7%	2220
	21st to end	Day	58% 15% 15% 10% 2	2740
February	1st to 10th	Day	65% 8% 11% 10% 6% 1	1567
	11th to 20th	Day	56% 9% 15% 12% 8% 1	1674
	21st to end	Day	59% 9% 13% 13% 5%	1475
March	1st to 10th	Day	53% 10% 13% 15% 10%	3138
	11th to 20th	Day	42% 10% 20% 23% 4	3415
	21st to end	Day	49% 11% 13% 17% 9%	4058
April	1st to 10th	Day	58% 11% 12% 13% 5%	2280
	11th to 20th	Day	56% 8% 13% 14% 7% 1	2298
	21st to end	Day	55% 9% 12% 16% 7%	2660
May	1st to 10th	Day	43% 9% 15% 23% 10%	4420
	11th to 20th	Day	46% 11% 15% 21% 6%	4380
	21st to end	Day	58% 12% 13% 13% 4	5078
June	1st to 10th	Day	55% 8% 11% 14% 9% 3	3040
	11th to 20th	Day	60% 10% 13% 13% 4	3040
	21st to end	Day	47% 11% 15% 16% 10% 1	3040
July	1st to 10th	Day	25% 7% 10% 22% 23% 12%	4839
	11th to 20th	Day	22% 6% 11% 29% 25% 7%	4659
	21st to end	Day	31% 7% 16% 25% 14% 6%	4743
August	1st to 10th	Day	26% 8% 12% 33% 20% 1	4384
	11th to 20th	Day	30% 11% 21% 28% 9% 1	4320
	21st to end	Day	36% 9% 26% 23% 5% 1	4351
September	1st to 10th	Day	35% 8% 19% 26% 9% 3	2280
	11th to 20th	Day	43% 8% 13% 24% 9% 2	2280
	21st to end	Day	48% 9% 16% 16% 10% 1	2029
October	1st to 10th	Day	48% 9% 20% 17% 5%	3180
	11th to 20th	Day	49% 8% 16% 23% 4	3103
	21st to end	Day	52% 9% 20% 17% 2	3080
November	1st to 10th	Day	62% 9% 10% 13% 5%	1568
	11th to 20th	Day	59% 12% 14% 12% 3 1	1600
	21st to end	Day	60% 12% 16% 8% 4	1444
December	1st to 10th	Day	64% 12% 10% 11% 3	1264
	11th to 20th	Day	63% 14% 13% 8% 3	1444
	21st to end	Day	56% 13% 15% 13% 3	2091
January	1st to 10th	Night	64% 12% 13% 8% 2	4754
	11th to 20th	Night	57% 8% 15% 10% 6% 5%	4740
	21st to end	Night	56% 19% 13% 9% 3	4914
February	1st to 10th	Night	64% 7% 11% 11% 6% 1	2610
	11th to 20th	Night	54% 9% 14% 15% 7% 2	2404
	21st to end	Night	55% 8% 16% 17% 4	1979
March	1st to 10th	Night	46% 12% 13% 16% 12%	3736
	11th to 20th	Night	37% 12% 19% 24% 8%	3562
	21st to end	Night	47% 13% 14% 14% 12%	3595
April	1st to 10th	Night	52% 11% 14% 16% 6%	1900
	11th to 20th	Night	50% 7% 15% 20% 9%	1828
	21st to end	Night	50% 10% 11% 17% 10% 1	1520
May	1st to 10th	Night	47% 8% 12% 21% 10% 1	2540
	11th to 20th	Night	47% 11% 16% 19% 6%	2516
	21st to end	Night	61% 9% 13% 12% 4	2492
June	1st to 10th	Night	54% 9% 10% 18% 7% 3	1140
	11th to 20th	Night	60% 10% 14% 11% 5%	1140
	21st to end	Night	46% 10% 16% 18% 8% 2	1140
July	1st to 10th	Night	30% 6% 9% 17% 24% 12%	2009
	11th to 20th	Night	23% 7% 11% 27% 28% 3	2178
	21st to end	Night	39% 8% 12% 20% 13% 8%	2569
August	1st to 10th	Night	31% 9% 11% 29% 19% 1	2521
	11th to 20th	Night	36% 11% 19% 24% 8% 2	2573
	21st to end	Night	41% 8% 25% 18% 6% 1	3320
September	1st to 10th	Night	34% 9% 16% 25% 14% 2	1900
	11th to 20th	Night	38% 9% 15% 23% 11% 4	1900
	21st to end	Night	44% 8% 17% 19% 11% 1	2148
October	1st to 10th	Night	52% 8% 18% 18% 4	3800
	11th to 20th	Night	48% 7% 16% 24% 5%	3832
	21st to end	Night	52% 7% 18% 19% 3	4476
November	1st to 10th	Night	58% 12% 13% 12% 4 1	2730
	11th to 20th	Night	57% 12% 13% 12% 5% 1	2800
	21st to end	Night	62% 10% 12% 10% 5% 1	2930
December	1st to 10th	Night	60% 9% 16% 11% 4	2911
	11th to 20th	Night	67% 12% 11% 8% 2	3747
	21st to end	Night	57% 12% 15% 12% 3	5566

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	18% 17% 37% 16% 7% 6%	180
	11th to 20th	Day	11% 21% 30% 22% 13% 3	180
	21st to end	Day	16% 23% 30% 19% 10% 2	198
February	1st to 10th	Day	21% 21% 31% 21% 5% 1	227
	11th to 20th	Day	15% 16% 31% 25% 12% 1	365
	21st to end	Day	11% 18% 34% 22% 12% 3	337
March	1st to 10th	Day	13% 20% 30% 23% 12% 1	460
	11th to 20th	Day	12% 23% 31% 19% 12% 3	460
	21st to end	Day	17% 20% 28% 18% 12% 4	506
April	1st to 10th	Day	17% 22% 28% 21% 9% 3	460
	11th to 20th	Day	13% 24% 26% 21% 10% 5%	452
	21st to end	Day	16% 15% 27% 25% 12% 4	460
May	1st to 10th	Day	13% 17% 29% 23% 14% 3	463
	11th to 20th	Day	14% 10% 25% 29% 18% 5%	488
	21st to end	Day	18% 11% 21% 28% 18% 5%	676
June	1st to 10th	Day	19% 13% 30% 23% 14% 2	870
	11th to 20th	Day	20% 11% 24% 28% 15% 2	870
	21st to end	Day	13% 10% 22% 30% 19% 6%	870
July	1st to 10th	Day	12% 6% 26% 30% 21% 6%	779
	11th to 20th	Day	10% 8% 22% 30% 24% 6%	668
	21st to end	Day	7% 5% 22% 32% 26% 7%	600
August	1st to 10th	Day	7% 6% 16% 31% 29% 11%	487
	11th to 20th	Day	7% 7% 19% 32% 23% 12%	477
	21st to end	Day	14% 9% 23% 25% 20% 9%	539
September	1st to 10th	Day	17% 9% 23% 29% 14% 8%	490
	11th to 20th	Day	16% 12% 20% 26% 18% 8%	490
	21st to end	Day	21% 14% 26% 24% 11% 3	490
October	1st to 10th	Day	21% 11% 24% 20% 15% 8%	490
	11th to 20th	Day	23% 17% 23% 20% 13% 3	457
	21st to end	Day	21% 18% 30% 19% 10% 2	325
November	1st to 10th	Day	22% 23% 29% 17% 8% 3	200
	11th to 20th	Day	22% 19% 23% 22% 12% 3	200
	21st to end	Day	17% 17% 26% 23% 13% 5%	199
December	1st to 10th	Day	11% 11% 29% 24% 18% 6%	180
	11th to 20th	Day	14% 16% 29% 23% 13% 4	180
	21st to end	Day	16% 14% 35% 23% 10% 2	198
January	1st to 10th	Night	22% 13% 33% 22% 8% 1	865
	11th to 20th	Night	16% 13% 28% 29% 13% 1	850
	21st to end	Night	14% 14% 30% 29% 11% 2	848
February	1st to 10th	Night	18% 14% 28% 27% 11% 2	760
	11th to 20th	Night	15% 11% 26% 38% 10% 1	744
	21st to end	Night	13% 13% 24% 31% 17% 2	628
March	1st to 10th	Night	13% 15% 28% 28% 13% 3	760
	11th to 20th	Night	10% 19% 30% 27% 13% 2	760
	21st to end	Night	11% 17% 23% 28% 18% 3	836
April	1st to 10th	Night	11% 13% 26% 29% 17% 3	760
	11th to 20th	Night	11% 11% 30% 31% 15% 2	700
	21st to end	Night	11% 14% 33% 27% 13% 2	380
May	1st to 10th	Night	16% 11% 25% 29% 17% 1	380
	11th to 20th	Night	15% 7% 25% 37% 15% 2	374
	21st to end	Night	18% 7% 23% 27% 24% 2	411
June	1st to 10th	Night	19% 12% 24% 33% 12% 1	380
	11th to 20th	Night	21% 8% 24% 31% 14% 3	380
	21st to end	Night	11% 8% 21% 34% 22% 4	380
July	1st to 10th	Night	16% 4 21% 33% 25% 2	370
	11th to 20th	Night	11% 5% 20% 36% 25% 4	368
	21st to end	Night	9% 4 18% 36% 29% 4	416
August	1st to 10th	Night	8% 3 11% 36% 37% 6%	378
	11th to 20th	Night	8% 5% 11% 36% 35% 6%	402
	21st to end	Night	12% 7% 18% 35% 25% 4	723
September	1st to 10th	Night	13% 7% 17% 36% 24% 3	760
	11th to 20th	Night	14% 6% 14% 30% 30% 5%	760
	21st to end	Night	18% 11% 22% 30% 15% 4	760
October	1st to 10th	Night	18% 13% 18% 31% 18% 2	760
	11th to 20th	Night	19% 15% 21% 29% 15% 2	760
	21st to end	Night	21% 14% 27% 26% 10% 2	836
November	1st to 10th	Night	23% 17% 24% 27% 7% 2	780
	11th to 20th	Night	21% 19% 24% 24% 10% 1	800
	21st to end	Night	19% 16% 26% 26% 9% 2	930
December	1st to 10th	Night	15% 14% 26% 31% 13% 1	979
	11th to 20th	Night	18% 15% 27% 30% 9% 1	1040
	21st to end	Night	16% 17% 30% 28% 8%	1144

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