Herridge Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Herridge Lake clear sky chart from 2005-11-06 to 2025-09-19.

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	16% 12% 4 9% 5% 4 4 3 5% 7% 29%	126689
	Night	16% 14% 5% 10% 6% 5% 4 3 5% 6% 28%	122455
Transparency	Day	54% 6% 12% 19% 8% 1	130204
	Night	57% 6% 11% 17% 8% 1	127028
Seeing	Day	23% 16% 31% 20% 7% 2	16308
	Night	24% 13% 31% 24% 8% 1	25253

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	11% 20% 9% 16% 8% 9% 5% 3 4 5% 11%	3158
	11th to 20th	Day	10% 18% 9% 17% 7% 6% 3 3 4 6% 18%	3156
	21st to end	Day	12% 18% 8% 14% 6% 5% 4 3 6% 6% 17%	4444
February	1st to 10th	Day	8% 18% 5% 14% 9% 7% 6% 3 5% 6% 19%	1800
	11th to 20th	Day	10% 20% 6% 11% 7% 5% 4 3 5% 5% 24%	1753
	21st to end	Day	15% 18% 5% 12% 7% 5% 4 4 4 5% 23%	1493
March	1st to 10th	Day	19% 16% 5% 11% 5% 4 4 3 4 6% 24%	4860
	11th to 20th	Day	16% 14% 4 13% 7% 4 4 3 5% 6% 24%	4980
	21st to end	Day	14% 16% 6% 11% 5% 4 4 2 3 5% 29%	5465
April	1st to 10th	Day	19% 13% 5% 8% 5% 4 4 3 5% 7% 26%	2372
	11th to 20th	Day	22% 11% 3 7% 4 4 4 3 4 5% 33%	2545
	21st to end	Day	22% 13% 3 7% 4 4 3 3 4 5% 33%	2600
May	1st to 10th	Day	19% 11% 4 7% 4 4 4 3 4 6% 35%	6111
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 3 5% 7% 38%	6318
	21st to end	Day	21% 10% 3 6% 4 4 3 3 5% 7% 35%	6995
June	1st to 10th	Day	19% 8% 3 7% 5% 4 4 4 6% 7% 31%	3000
	11th to 20th	Day	14% 6% 2 7% 5% 6% 5% 5% 7% 8% 36%	3000
	21st to end	Day	19% 9% 3 7% 6% 5% 5% 4 5% 8% 30%	3000
July	1st to 10th	Day	16% 7% 3 7% 5% 4 5% 5% 8% 9% 31%	6819
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 7% 9% 39%	6820
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 7% 10% 41%	6937
August	1st to 10th	Day	11% 6% 2 8% 5% 5% 6% 5% 8% 10% 35%	4234
	11th to 20th	Day	19% 7% 2 6% 4 4 5% 4 6% 9% 35%	4174
	21st to end	Day	12% 6% 3 7% 5% 4 5% 4 6% 8% 40%	4609
September	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 5% 8% 36%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 3 4 7% 39%	2261
	21st to end	Day	22% 10% 3 7% 5% 4 3 3 5% 6% 32%	2054
October	1st to 10th	Day	21% 11% 3 7% 5% 4 3 3 5% 7% 31%	1900
	11th to 20th	Day	25% 16% 4 9% 5% 5% 4 3 4 4 22%	1900
	21st to end	Day	29% 18% 4 9% 5% 4 4 3 4 5% 16%	2026
November	1st to 10th	Day	21% 17% 6% 8% 5% 3 4 3 4 5% 25%	1566
	11th to 20th	Day	19% 18% 5% 14% 6% 5% 4 3 3 6% 17%	1600
	21st to end	Day	26% 20% 5% 13% 6% 5% 4 2 3 4 13%	1600
December	1st to 10th	Day	15% 24% 10% 16% 8% 5% 4 3 3 3 10%	2600
	11th to 20th	Day	17% 21% 6% 13% 7% 4 2 2 3 4 22%	2663
	21st to end	Day	20% 23% 8% 16% 7% 4 2 2 4 3 11%	3476
January	1st to 10th	Night	11% 17% 8% 17% 10% 6% 5% 3 4 4 14%	6808
	11th to 20th	Night	10% 18% 7% 14% 8% 6% 5% 4 5% 4 18%	6746
	21st to end	Night	9% 19% 9% 14% 7% 6% 5% 3 5% 4 18%	6511
February	1st to 10th	Night	11% 15% 5% 12% 10% 6% 4 4 5% 5% 22%	2600
	11th to 20th	Night	12% 17% 7% 12% 8% 5% 4 4 5% 4 23%	2533
	21st to end	Night	11% 18% 5% 12% 7% 6% 5% 3 5% 5% 22%	2135
March	1st to 10th	Night	17% 16% 4 10% 4 4 4 4 5% 5% 26%	4980
	11th to 20th	Night	16% 16% 5% 11% 6% 5% 5% 4 5% 5% 22%	4979
	21st to end	Night	17% 13% 5% 11% 5% 5% 3 3 4 5% 30%	5467
April	1st to 10th	Night	20% 15% 4 9% 5% 3 4 3 4 5% 29%	2027
	11th to 20th	Night	20% 11% 3 7% 4 4 4 3 4 6% 33%	1822
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 6% 7% 32%	1800
May	1st to 10th	Night	16% 11% 3 8% 5% 4 4 3 4 5% 36%	3828
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 5% 8% 42%	3575
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	3865
June	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 3 6% 6% 36%	1400
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 6% 9% 41%	1400
	21st to end	Night	16% 7% 3 7% 5% 4 4 5% 7% 7% 36%	1400
July	1st to 10th	Night	15% 8% 2 7% 4 5% 5% 5% 5% 6% 37%	3160
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 7% 43%	3160
	21st to end	Night	11% 6% 2 5% 4 4 4 3 6% 9% 46%	3914
August	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 4 7% 8% 41%	2915
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 5% 7% 41%	2875
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 7% 46%	3256
September	1st to 10th	Night	17% 6% 2 5% 3 4 4 3 7% 7% 43%	2000
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 4 6% 47%	2093
	21st to end	Night	20% 9% 3 6% 4 3 3 3 5% 6% 39%	2125
October	1st to 10th	Night	19% 11% 3 8% 5% 4 4 3 4 5% 35%	2280
	11th to 20th	Night	27% 14% 3 6% 4 4 5% 3 5% 5% 24%	2280
	21st to end	Night	24% 17% 5% 9% 5% 3 3 3 3 4 24%	2570
November	1st to 10th	Night	20% 14% 4 8% 5% 4 3 4 4 6% 28%	2731
	11th to 20th	Night	19% 16% 5% 12% 7% 4 5% 3 5% 5% 19%	2800
	21st to end	Night	22% 20% 6% 11% 6% 4 3 3 4 4 16%	2800
December	1st to 10th	Night	15% 23% 7% 15% 8% 6% 5% 3 4 4 11%	4580
	11th to 20th	Night	15% 21% 5% 13% 7% 5% 4 3 4 5% 17%	5538
	21st to end	Night	20% 22% 8% 14% 6% 3 3 2 3 3 14%	7502

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	76% 6% 9% 7% 3	3159
	11th to 20th	Day	70% 4 8% 9% 9%	3183
	21st to end	Day	68% 5% 7% 12% 7%	4444
February	1st to 10th	Day	67% 5% 9% 12% 7% 1	1800
	11th to 20th	Day	63% 5% 9% 12% 9% 1	1762
	21st to end	Day	64% 5% 7% 15% 8% 1	1493
March	1st to 10th	Day	61% 6% 7% 13% 12% 2	4863
	11th to 20th	Day	61% 6% 11% 13% 8% 1	4980
	21st to end	Day	58% 5% 9% 17% 10% 1	5465
April	1st to 10th	Day	57% 4 11% 18% 8% 1	2372
	11th to 20th	Day	53% 5% 8% 21% 11% 1	2545
	21st to end	Day	54% 6% 7% 17% 14% 2	2600
May	1st to 10th	Day	49% 6% 9% 20% 14% 4	6120
	11th to 20th	Day	44% 8% 12% 22% 12% 2	6322
	21st to end	Day	49% 6% 10% 20% 13% 1	6995
June	1st to 10th	Day	50% 7% 11% 20% 11% 1	3000
	11th to 20th	Day	43% 8% 12% 24% 12% 1	3000
	21st to end	Day	51% 9% 11% 19% 9% 1	3000
July	1st to 10th	Day	45% 9% 16% 25% 5%	6746
	11th to 20th	Day	38% 8% 20% 29% 5%	6739
	21st to end	Day	36% 7% 19% 30% 8%	6738
August	1st to 10th	Day	41% 9% 15% 28% 6%	4221
	11th to 20th	Day	44% 8% 15% 24% 8% 1	4213
	21st to end	Day	40% 7% 14% 30% 8% 1	4609
September	1st to 10th	Day	47% 7% 14% 23% 9%	2400
	11th to 20th	Day	44% 8% 15% 26% 7%	2261
	21st to end	Day	52% 6% 15% 20% 7%	2054
October	1st to 10th	Day	50% 6% 11% 26% 6%	3160
	11th to 20th	Day	56% 6% 11% 19% 8%	3140
	21st to end	Day	68% 5% 15% 9% 4	3315
November	1st to 10th	Day	61% 6% 13% 14% 5%	1566
	11th to 20th	Day	69% 6% 9% 12% 4 1	1600
	21st to end	Day	77% 5% 7% 8% 3	1600
December	1st to 10th	Day	81% 4 7% 6% 2	2600
	11th to 20th	Day	68% 4 7% 17% 4	2663
	21st to end	Day	78% 4 4 11% 4	3476
January	1st to 10th	Night	71% 7% 9% 9% 3	6808
	11th to 20th	Night	68% 7% 9% 9% 7%	6794
	21st to end	Night	68% 5% 5% 10% 11%	6511
February	1st to 10th	Night	63% 6% 9% 14% 7% 1	2600
	11th to 20th	Night	65% 5% 10% 14% 6% 1	2535
	21st to end	Night	62% 6% 8% 15% 8% 1	2135
March	1st to 10th	Night	58% 6% 8% 13% 11% 4	4990
	11th to 20th	Night	62% 7% 11% 14% 6% 1	4980
	21st to end	Night	56% 6% 8% 16% 12% 3	5467
April	1st to 10th	Night	58% 5% 8% 16% 11% 1	2027
	11th to 20th	Night	52% 6% 7% 19% 14% 2	1822
	21st to end	Night	51% 6% 12% 14% 15% 2	1800
May	1st to 10th	Night	47% 7% 7% 19% 17% 2	3835
	11th to 20th	Night	39% 8% 13% 21% 16% 3	3576
	21st to end	Night	45% 7% 11% 21% 14% 3	3866
June	1st to 10th	Night	47% 7% 11% 22% 11% 2	1400
	11th to 20th	Night	40% 6% 12% 24% 16% 2	1400
	21st to end	Night	45% 8% 13% 20% 13% 1	1400
July	1st to 10th	Night	45% 8% 11% 28% 8% 1	3125
	11th to 20th	Night	38% 7% 17% 29% 9%	3118
	21st to end	Night	34% 7% 14% 32% 13%	3799
August	1st to 10th	Night	38% 9% 16% 29% 8%	2904
	11th to 20th	Night	41% 7% 17% 28% 7%	2902
	21st to end	Night	38% 6% 16% 27% 11% 1	3256
September	1st to 10th	Night	39% 8% 17% 26% 10% 2	2000
	11th to 20th	Night	37% 7% 15% 31% 10%	2093
	21st to end	Night	45% 6% 16% 25% 8%	2125
October	1st to 10th	Night	49% 7% 11% 24% 9%	3800
	11th to 20th	Night	53% 6% 13% 23% 5%	3779
	21st to end	Night	58% 7% 18% 13% 5%	4231
November	1st to 10th	Night	55% 8% 16% 16% 5%	2731
	11th to 20th	Night	65% 7% 11% 12% 5% 1	2800
	21st to end	Night	71% 6% 11% 9% 3	2800
December	1st to 10th	Night	77% 6% 7% 8% 2	4580
	11th to 20th	Night	69% 6% 8% 10% 8%	5537
	21st to end	Night	76% 4 5% 12% 4	7502

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	26% 19% 38% 13% 4 1	200
	11th to 20th	Day	20% 15% 41% 16% 8%	205
	21st to end	Day	24% 15% 33% 21% 6% 1	330
February	1st to 10th	Day	27% 29% 33% 8% 3	300
	11th to 20th	Day	24% 27% 36% 11% 1	300
	21st to end	Day	31% 26% 33% 10%	249
March	1st to 10th	Day	23% 22% 35% 18% 3	465
	11th to 20th	Day	22% 20% 37% 17% 3 1	500
	21st to end	Day	26% 23% 31% 15% 3 1	550
April	1st to 10th	Day	26% 20% 29% 17% 7% 1	500
	11th to 20th	Day	35% 16% 24% 15% 8% 2	492
	21st to end	Day	33% 19% 29% 12% 5% 2	500
May	1st to 10th	Day	26% 13% 31% 20% 8% 2	503
	11th to 20th	Day	21% 15% 34% 19% 8% 2	527
	21st to end	Day	24% 13% 31% 22% 8% 2	593
June	1st to 10th	Day	20% 16% 35% 23% 5% 1	730
	11th to 20th	Day	17% 18% 31% 26% 7% 2	730
	21st to end	Day	22% 12% 30% 26% 8% 1	730
July	1st to 10th	Day	13% 15% 33% 29% 9% 1	721
	11th to 20th	Day	16% 11% 33% 25% 12% 3	718
	21st to end	Day	14% 11% 31% 28% 12% 4	620
August	1st to 10th	Day	16% 10% 30% 28% 13% 3	526
	11th to 20th	Day	17% 7% 27% 27% 15% 7%	519
	21st to end	Day	18% 12% 30% 23% 13% 4	583
September	1st to 10th	Day	24% 11% 30% 22% 10% 3	530
	11th to 20th	Day	17% 10% 27% 30% 10% 6%	522
	21st to end	Day	26% 13% 25% 21% 13% 3	490
October	1st to 10th	Day	29% 15% 25% 20% 10% 1	490
	11th to 20th	Day	28% 22% 34% 11% 4 1	490
	21st to end	Day	35% 17% 32% 15% 1	475
November	1st to 10th	Day	37% 22% 30% 11% 2	200
	11th to 20th	Day	27% 25% 32% 14% 3 1	200
	21st to end	Day	37% 19% 30% 12% 3 1	200
December	1st to 10th	Day	28% 20% 31% 18% 4	200
	11th to 20th	Day	31% 17% 32% 15% 5% 1	200
	21st to end	Day	30% 21% 35% 10% 3 1	220
January	1st to 10th	Night	25% 14% 37% 18% 5%	1000
	11th to 20th	Night	27% 15% 31% 22% 5%	1000
	21st to end	Night	21% 13% 32% 23% 10% 1	888
February	1st to 10th	Night	18% 19% 37% 20% 4 1	800
	11th to 20th	Night	23% 15% 38% 19% 4	783
	21st to end	Night	22% 20% 35% 19% 4	660
March	1st to 10th	Night	20% 21% 34% 20% 4 1	800
	11th to 20th	Night	24% 19% 34% 18% 6%	800
	21st to end	Night	21% 15% 36% 22% 5% 1	880
April	1st to 10th	Night	28% 16% 29% 23% 5%	800
	11th to 20th	Night	27% 17% 27% 20% 8% 1	638
	21st to end	Night	25% 14% 32% 21% 8% 1	600
May	1st to 10th	Night	22% 9% 30% 30% 8% 1	505
	11th to 20th	Night	19% 11% 31% 29% 8% 1	394
	21st to end	Night	19% 10% 35% 30% 6%	433
June	1st to 10th	Night	16% 12% 33% 31% 8% 1	400
	11th to 20th	Night	18% 11% 29% 31% 11%	400
	21st to end	Night	19% 8% 27% 32% 14% 1	400
July	1st to 10th	Night	13% 6% 35% 36% 10% 1	390
	11th to 20th	Night	13% 8% 28% 36% 14%	388
	21st to end	Night	11% 7% 29% 38% 15% 1	469
August	1st to 10th	Night	14% 6% 27% 37% 16% 2	597
	11th to 20th	Night	15% 7% 24% 33% 19% 3	594
	21st to end	Night	20% 8% 26% 29% 14% 2	680
September	1st to 10th	Night	21% 6% 29% 27% 16% 1	800
	11th to 20th	Night	19% 9% 25% 27% 18% 2	793
	21st to end	Night	24% 7% 24% 29% 13% 2	760
October	1st to 10th	Night	24% 13% 28% 26% 8% 1	760
	11th to 20th	Night	35% 12% 27% 18% 7% 1	760
	21st to end	Night	34% 14% 28% 21% 3 1	836
November	1st to 10th	Night	29% 12% 34% 20% 5%	780
	11th to 20th	Night	28% 17% 31% 18% 6%	800
	21st to end	Night	34% 15% 30% 18% 4	800
December	1st to 10th	Night	32% 17% 30% 16% 6% 1	800
	11th to 20th	Night	27% 15% 35% 17% 5% 1	965
	21st to end	Night	31% 16% 28% 20% 5%	1100

Page generated at 2025-09-16 01:49:16. Version ac07

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