David Reain Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the David Reain Memorial Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	15% 11% 4 9% 5% 5% 4 4 5% 7% 32%	127358
	Night	16% 13% 4 10% 5% 5% 4 3 5% 6% 29%	123431
Transparency	Day	50% 7% 12% 21% 9% 1	130890
	Night	54% 7% 11% 18% 9% 1	127988
Seeing	Day	19% 10% 26% 28% 14% 3	16510
	Night	22% 8% 28% 29% 11% 1	25544

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% 8% 14% 7% 5% 5% 3 5% 5% 18%	3158
	11th to 20th	Day	11% 16% 7% 16% 6% 6% 5% 3 6% 7% 17%	3308
	21st to end	Day	13% 19% 8% 14% 7% 5% 5% 3 3 5% 20%	4466
February	1st to 10th	Day	11% 19% 6% 13% 6% 5% 5% 6% 6% 5% 20%	1800
	11th to 20th	Day	12% 17% 5% 10% 7% 4 4 3 5% 6% 25%	1753
	21st to end	Day	18% 14% 4 11% 6% 6% 4 4 4 5% 24%	1510
March	1st to 10th	Day	20% 15% 4 7% 4 4 4 3 5% 6% 27%	4875
	11th to 20th	Day	13% 12% 5% 12% 6% 5% 5% 4 6% 7% 24%	4980
	21st to end	Day	15% 12% 6% 11% 6% 6% 4 3 4 4 31%	5465
April	1st to 10th	Day	18% 12% 3 8% 5% 4 5% 3 5% 7% 29%	2363
	11th to 20th	Day	21% 11% 3 6% 5% 3 4 3 5% 6% 34%	2539
	21st to end	Day	22% 10% 3 7% 5% 4 3 3 4 5% 35%	2600
May	1st to 10th	Day	18% 8% 3 9% 5% 5% 4 3 5% 6% 34%	6042
	11th to 20th	Day	12% 8% 3 8% 5% 4 4 4 5% 7% 40%	6318
	21st to end	Day	20% 9% 3 7% 4 3 3 3 4 6% 36%	6949
June	1st to 10th	Day	16% 8% 3 8% 6% 4 5% 4 6% 7% 34%	2977
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 7% 39%	3000
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	3000
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 9% 35%	6819
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	6820
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 10% 47%	6760
August	1st to 10th	Day	10% 6% 3 7% 4 5% 6% 4 7% 10% 38%	4234
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 9% 38%	4174
	21st to end	Day	9% 5% 2 7% 4 5% 5% 5% 6% 9% 43%	4591
September	1st to 10th	Day	15% 6% 3 7% 4 4 5% 4 5% 8% 40%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 5% 8% 43%	2270
	21st to end	Day	20% 9% 3 8% 4 4 4 3 5% 7% 34%	2170
October	1st to 10th	Day	17% 7% 3 7% 5% 6% 5% 4 5% 6% 36%	2000
	11th to 20th	Day	24% 11% 4 9% 5% 5% 5% 3 5% 5% 25%	2000
	21st to end	Day	26% 12% 3 10% 6% 4 4 3 4 5% 22%	2145
November	1st to 10th	Day	21% 12% 4 8% 5% 4 4 3 5% 7% 28%	1671
	11th to 20th	Day	16% 17% 4 10% 7% 6% 5% 4 5% 5% 21%	1680
	21st to end	Day	26% 19% 5% 10% 5% 4 4 3 3 5% 14%	1680
December	1st to 10th	Day	16% 20% 7% 13% 7% 5% 5% 4 5% 5% 11%	2616
	11th to 20th	Day	16% 18% 5% 13% 6% 4 3 3 4 4 23%	2749
	21st to end	Day	23% 20% 6% 15% 5% 4 3 3 3 5% 13%	3476
January	1st to 10th	Night	13% 17% 7% 14% 9% 6% 5% 3 4 5% 18%	6808
	11th to 20th	Night	13% 17% 7% 15% 7% 5% 5% 3 4 4 20%	6586
	21st to end	Night	11% 21% 8% 13% 5% 6% 5% 3 5% 5% 18%	6490
February	1st to 10th	Night	12% 15% 6% 13% 7% 6% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	13% 17% 6% 11% 7% 5% 4 3 4 5% 24%	2533
	21st to end	Night	14% 18% 6% 9% 8% 5% 4 3 5% 5% 24%	2119
March	1st to 10th	Night	18% 14% 4 9% 5% 4 4 4 5% 5% 28%	4964
	11th to 20th	Night	14% 15% 5% 10% 6% 5% 5% 4 6% 6% 25%	4979
	21st to end	Night	18% 12% 4 8% 6% 4 4 3 4 5% 30%	5467
April	1st to 10th	Night	19% 14% 4 8% 6% 4 4 2 4 6% 30%	2036
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 3 4 5% 37%	1827
	21st to end	Night	21% 10% 3 7% 5% 4 5% 4 4 6% 32%	1800
May	1st to 10th	Night	16% 9% 3 8% 5% 4 3 3 5% 7% 36%	3897
	11th to 20th	Night	10% 8% 3 6% 5% 4 4 4 5% 9% 42%	3575
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 36%	3926
June	1st to 10th	Night	11% 10% 3 8% 6% 5% 5% 5% 6% 6% 35%	1419
	11th to 20th	Night	14% 6% 3 5% 3 4 4 4 6% 8% 44%	1400
	21st to end	Night	13% 8% 3 7% 4 4 4 5% 6% 9% 37%	1400
July	1st to 10th	Night	15% 8% 3 8% 4 4 5% 3 6% 8% 37%	3160
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 46%	3160
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 49%	4088
August	1st to 10th	Night	12% 7% 2 7% 4 4 5% 4 7% 7% 42%	2915
	11th to 20th	Night	13% 7% 3 6% 5% 4 4 4 6% 7% 42%	2875
	21st to end	Night	13% 5% 2 6% 3 4 4 3 4 7% 49%	3274
September	1st to 10th	Night	16% 5% 2 5% 4 4 3 3 5% 7% 47%	2000
	11th to 20th	Night	12% 6% 3 5% 4 4 4 3 4 7% 48%	2125
	21st to end	Night	18% 7% 3 6% 3 3 3 3 5% 7% 41%	2230
October	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 5% 6% 36%	2400
	11th to 20th	Night	26% 9% 3 7% 5% 5% 4 3 4 6% 28%	2400
	21st to end	Night	25% 11% 4 9% 6% 4 4 3 4 5% 24%	2693
November	1st to 10th	Night	20% 11% 3 7% 4 4 5% 3 5% 6% 31%	2846
	11th to 20th	Night	18% 14% 4 9% 6% 6% 5% 3 4 6% 24%	2940
	21st to end	Night	23% 18% 4 10% 6% 5% 4 3 6% 6% 17%	2940
December	1st to 10th	Night	17% 21% 5% 12% 7% 6% 5% 4 4 5% 14%	4609
	11th to 20th	Night	18% 17% 5% 13% 6% 5% 4 3 5% 6% 19%	5448
	21st to end	Night	21% 22% 6% 15% 5% 4 3 2 3 4 15%	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	66% 7% 11% 13% 3	3159
	11th to 20th	Day	65% 6% 10% 10% 8%	3335
	21st to end	Day	67% 5% 7% 12% 8%	4466
February	1st to 10th	Day	62% 7% 10% 14% 5% 1	1800
	11th to 20th	Day	59% 7% 10% 14% 8% 1	1762
	21st to end	Day	62% 6% 8% 16% 8% 1	1510
March	1st to 10th	Day	56% 6% 10% 12% 13% 2	4878
	11th to 20th	Day	55% 8% 14% 14% 8% 1	4980
	21st to end	Day	53% 7% 11% 18% 9% 2	5465
April	1st to 10th	Day	53% 7% 12% 18% 9% 1	2363
	11th to 20th	Day	49% 6% 9% 22% 12% 1	2539
	21st to end	Day	50% 7% 8% 18% 13% 3	2600
May	1st to 10th	Day	47% 8% 10% 20% 12% 4	6051
	11th to 20th	Day	40% 9% 13% 22% 14% 2	6322
	21st to end	Day	47% 7% 11% 19% 14% 2	6949
June	1st to 10th	Day	47% 8% 12% 21% 12% 1	2977
	11th to 20th	Day	40% 9% 13% 24% 14% 1	3000
	21st to end	Day	47% 9% 14% 19% 11% 1	3000
July	1st to 10th	Day	43% 9% 17% 25% 7%	6746
	11th to 20th	Day	33% 7% 17% 36% 6%	6739
	21st to end	Day	31% 7% 18% 32% 11%	6565
August	1st to 10th	Day	38% 10% 14% 31% 8%	4221
	11th to 20th	Day	40% 8% 16% 25% 9%	4213
	21st to end	Day	35% 8% 15% 33% 9%	4591
September	1st to 10th	Day	41% 10% 15% 23% 10% 1	2400
	11th to 20th	Day	38% 9% 15% 28% 10%	2270
	21st to end	Day	50% 7% 12% 23% 7%	2170
October	1st to 10th	Day	43% 8% 13% 28% 8%	3260
	11th to 20th	Day	50% 8% 14% 21% 7%	3240
	21st to end	Day	64% 6% 13% 12% 4	3447
November	1st to 10th	Day	55% 8% 13% 19% 5%	1671
	11th to 20th	Day	64% 7% 11% 11% 6% 1	1680
	21st to end	Day	73% 6% 10% 7% 3	1680
December	1st to 10th	Day	73% 8% 11% 6% 2	2616
	11th to 20th	Day	65% 6% 8% 16% 5%	2749
	21st to end	Day	75% 5% 6% 11% 4	3476
January	1st to 10th	Night	67% 8% 9% 12% 4	6808
	11th to 20th	Night	66% 8% 10% 8% 8%	6634
	21st to end	Night	68% 6% 6% 10% 10% 1	6490
February	1st to 10th	Night	62% 6% 11% 14% 6% 1	2600
	11th to 20th	Night	62% 5% 11% 14% 6% 1	2535
	21st to end	Night	62% 6% 8% 15% 8% 1	2119
March	1st to 10th	Night	56% 6% 9% 13% 12% 4	4974
	11th to 20th	Night	57% 7% 13% 15% 7% 1	4980
	21st to end	Night	55% 7% 8% 17% 11% 2	5467
April	1st to 10th	Night	56% 5% 8% 18% 11% 2	2036
	11th to 20th	Night	50% 5% 8% 21% 15% 2	1827
	21st to end	Night	53% 7% 8% 14% 15% 3	1800
May	1st to 10th	Night	44% 8% 9% 21% 15% 3	3904
	11th to 20th	Night	36% 8% 14% 21% 17% 3	3576
	21st to end	Night	46% 8% 10% 20% 14% 3	3927
June	1st to 10th	Night	46% 7% 12% 21% 11% 3	1419
	11th to 20th	Night	37% 8% 12% 24% 18% 1	1400
	21st to end	Night	42% 9% 14% 21% 12% 1	1400
July	1st to 10th	Night	43% 7% 14% 25% 10% 1	3125
	11th to 20th	Night	37% 6% 14% 32% 10% 1	3118
	21st to end	Night	33% 7% 13% 33% 13%	3969
August	1st to 10th	Night	39% 7% 15% 27% 11% 1	2904
	11th to 20th	Night	40% 9% 17% 26% 8% 1	2902
	21st to end	Night	35% 6% 14% 33% 12% 1	3274
September	1st to 10th	Night	37% 7% 14% 29% 11% 2	2000
	11th to 20th	Night	37% 7% 14% 31% 10%	2125
	21st to end	Night	43% 6% 15% 25% 10%	2230
October	1st to 10th	Night	43% 8% 11% 28% 10%	3920
	11th to 20th	Night	48% 7% 13% 27% 5%	3899
	21st to end	Night	57% 7% 19% 12% 5%	4341
November	1st to 10th	Night	53% 7% 16% 18% 5%	2846
	11th to 20th	Night	60% 8% 12% 14% 5% 1	2940
	21st to end	Night	69% 7% 11% 10% 3	2940
December	1st to 10th	Night	72% 8% 10% 8% 2	4609
	11th to 20th	Night	66% 8% 8% 10% 9%	5448
	21st to end	Night	74% 5% 6% 11% 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	23% 18% 24% 27% 9% 1	200
	11th to 20th	Day	16% 13% 30% 27% 11% 2	223
	21st to end	Day	25% 7% 30% 25% 12% 2	330
February	1st to 10th	Day	23% 18% 34% 20% 5%	300
	11th to 20th	Day	20% 15% 39% 23% 4	300
	21st to end	Day	28% 18% 31% 22% 2	249
March	1st to 10th	Day	21% 12% 36% 27% 4	467
	11th to 20th	Day	19% 12% 32% 31% 5% 1	500
	21st to end	Day	21% 13% 32% 25% 7% 1	550
April	1st to 10th	Day	22% 10% 32% 25% 11% 1	500
	11th to 20th	Day	27% 14% 23% 19% 12% 4	492
	21st to end	Day	27% 13% 24% 26% 8% 2	500
May	1st to 10th	Day	23% 7% 27% 29% 13% 2	503
	11th to 20th	Day	18% 9% 25% 28% 17% 4	527
	21st to end	Day	17% 9% 25% 29% 15% 3	573
June	1st to 10th	Day	17% 7% 29% 34% 12% 2	707
	11th to 20th	Day	14% 7% 29% 32% 14% 4	730
	21st to end	Day	19% 7% 25% 33% 14% 2	730
July	1st to 10th	Day	10% 5% 21% 41% 20% 3	721
	11th to 20th	Day	14% 5% 22% 32% 23% 4	718
	21st to end	Day	10% 5% 21% 34% 24% 5%	586
August	1st to 10th	Day	14% 5% 16% 33% 26% 6%	526
	11th to 20th	Day	13% 4 14% 32% 26% 11%	519
	21st to end	Day	13% 7% 23% 28% 21% 7%	583
September	1st to 10th	Day	17% 7% 22% 31% 16% 7%	530
	11th to 20th	Day	13% 6% 19% 28% 23% 11%	530
	21st to end	Day	23% 10% 19% 24% 20% 4	530
October	1st to 10th	Day	19% 11% 25% 23% 17% 4	530
	11th to 20th	Day	24% 15% 30% 23% 7% 1	530
	21st to end	Day	31% 13% 29% 22% 4 1	528
November	1st to 10th	Day	29% 16% 27% 22% 4 1	234
	11th to 20th	Day	20% 20% 35% 20% 5%	220
	21st to end	Day	31% 16% 31% 18% 3	220
December	1st to 10th	Day	25% 14% 28% 28% 4	204
	11th to 20th	Day	26% 13% 36% 18% 7% 1	200
	21st to end	Day	31% 14% 29% 19% 7% 1	220
January	1st to 10th	Night	26% 8% 29% 30% 7% 1	1000
	11th to 20th	Night	28% 10% 26% 28% 8% 1	968
	21st to end	Night	20% 6% 29% 32% 11% 1	880
February	1st to 10th	Night	17% 10% 38% 30% 5%	800
	11th to 20th	Night	22% 8% 36% 26% 8%	783
	21st to end	Night	21% 11% 31% 32% 5%	660
March	1st to 10th	Night	21% 10% 36% 27% 6%	800
	11th to 20th	Night	20% 9% 36% 29% 6% 1	800
	21st to end	Night	20% 8% 34% 29% 9% 1	880
April	1st to 10th	Night	26% 9% 30% 29% 6%	800
	11th to 20th	Night	23% 8% 33% 24% 11% 1	643
	21st to end	Night	23% 8% 29% 30% 10% 1	600
May	1st to 10th	Night	20% 4 28% 37% 11% 1	535
	11th to 20th	Night	17% 4 30% 35% 13% 1	394
	21st to end	Night	17% 5% 27% 36% 13%	433
June	1st to 10th	Night	14% 5% 32% 37% 12% 1	400
	11th to 20th	Night	17% 5% 23% 40% 15% 1	400
	21st to end	Night	16% 6% 24% 37% 17% 1	400
July	1st to 10th	Night	11% 3 23% 47% 15% 1	390
	11th to 20th	Night	13% 4 21% 40% 21% 1	388
	21st to end	Night	12% 3 20% 44% 21% 1	510
August	1st to 10th	Night	15% 4 16% 40% 23% 2	597
	11th to 20th	Night	13% 4 18% 35% 26% 4	594
	21st to end	Night	17% 5% 23% 31% 21% 3	685
September	1st to 10th	Night	17% 5% 23% 31% 22% 2	800
	11th to 20th	Night	14% 7% 23% 30% 20% 7%	800
	21st to end	Night	21% 7% 21% 28% 20% 3	800
October	1st to 10th	Night	22% 8% 27% 28% 14% 2	800
	11th to 20th	Night	30% 11% 27% 24% 7% 1	800
	21st to end	Night	31% 12% 28% 24% 5%	880
November	1st to 10th	Night	27% 11% 29% 26% 6% 1	820
	11th to 20th	Night	24% 13% 32% 22% 8%	840
	21st to end	Night	33% 11% 32% 20% 4	840
December	1st to 10th	Night	30% 13% 30% 21% 7%	809
	11th to 20th	Night	28% 10% 32% 24% 5% 1	915
	21st to end	Night	30% 11% 27% 25% 6%	1100

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