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 2024-11-11.

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% 10% 4 8% 5% 5% 4 4 5% 7% 32%	101738
	Night	16% 13% 4 9% 5% 4 4 3 5% 6% 30%	97477
Transparency	Day	49% 8% 13% 21% 9% 1	105290
	Night	53% 7% 12% 18% 8% 1	102015
Seeing	Day	20% 10% 27% 27% 13% 3	15158
	Night	22% 8% 29% 29% 11% 1	24184

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	16% 24% 9% 10% 5% 3 4 3 4 4 18%	2208
	11th to 20th	Day	11% 14% 7% 16% 7% 6% 5% 4 5% 6% 18%	2317
	21st to end	Day	17% 20% 7% 11% 7% 5% 5% 2 3 4 17%	3124
February	1st to 10th	Day	10% 19% 6% 13% 6% 5% 5% 6% 6% 5% 20%	1710
	11th to 20th	Day	13% 17% 5% 10% 7% 4 4 3 5% 6% 25%	1663
	21st to end	Day	17% 14% 4 11% 6% 6% 4 4 3 5% 25%	1437
March	1st to 10th	Day	20% 14% 4 8% 4 4 4 3 5% 6% 27%	3389
	11th to 20th	Day	10% 15% 5% 13% 7% 6% 5% 3 5% 6% 25%	3480
	21st to end	Day	12% 9% 6% 11% 6% 7% 5% 3 4 4 34%	3815
April	1st to 10th	Day	19% 12% 3 8% 5% 4 5% 3 5% 7% 29%	2245
	11th to 20th	Day	21% 10% 3 6% 5% 3 4 3 5% 6% 34%	2410
	21st to end	Day	22% 10% 3 7% 5% 3 3 3 4 5% 35%	2470
May	1st to 10th	Day	19% 9% 3 8% 5% 4 4 4 5% 6% 34%	4220
	11th to 20th	Day	13% 8% 2 8% 5% 5% 5% 5% 6% 7% 37%	4398
	21st to end	Day	18% 7% 2 6% 4 4 3 3 5% 7% 42%	4837
June	1st to 10th	Day	16% 8% 3 8% 6% 4 5% 4 6% 7% 33%	2828
	11th to 20th	Day	13% 6% 2 6% 6% 5% 5% 4 7% 7% 39%	2850
	21st to end	Day	15% 8% 3 7% 5% 5% 4 4 6% 7% 34%	2850
July	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 5% 6% 8% 35%	4760
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 7% 10% 40%	4760
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	4704
August	1st to 10th	Day	10% 6% 3 7% 4 5% 6% 4 7% 10% 37%	4104
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 9% 38%	4044
	21st to end	Day	9% 5% 2 7% 4 5% 5% 5% 6% 9% 43%	4450
September	1st to 10th	Day	15% 6% 3 7% 4 4 5% 4 5% 7% 41%	2280
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 4 5% 8% 42%	2157
	21st to end	Day	20% 8% 3 8% 4 5% 4 3 4 7% 34%	2062
October	1st to 10th	Day	18% 7% 3 7% 6% 6% 5% 4 4 6% 35%	1900
	11th to 20th	Day	24% 11% 4 9% 4 5% 5% 3 5% 6% 24%	1900
	21st to end	Day	27% 12% 3 10% 6% 4 5% 3 4 5% 21%	2038
November	1st to 10th	Day	21% 11% 4 8% 5% 4 4 3 6% 7% 28%	1590
	11th to 20th	Day	17% 16% 4 10% 7% 6% 6% 4 5% 5% 21%	1520
	21st to end	Day	27% 19% 5% 10% 5% 4 4 3 4 5% 14%	1520
December	1st to 10th	Day	18% 19% 7% 10% 7% 6% 5% 3 6% 6% 15%	1520
	11th to 20th	Day	17% 21% 4 9% 6% 4 3 3 4 5% 23%	1747
	21st to end	Day	21% 23% 7% 16% 5% 3 2 2 3 4 14%	2431
January	1st to 10th	Night	16% 18% 7% 13% 8% 6% 4 3 4 5% 16%	4748
	11th to 20th	Night	14% 15% 6% 15% 7% 5% 6% 4 5% 5% 20%	4644
	21st to end	Night	14% 24% 8% 12% 6% 5% 4 3 4 5% 16%	4532
February	1st to 10th	Night	12% 15% 6% 13% 7% 6% 5% 4 5% 6% 23%	2470
	11th to 20th	Night	14% 16% 6% 11% 7% 5% 5% 3 4 5% 24%	2403
	21st to end	Night	15% 18% 6% 9% 7% 5% 3 3 5% 5% 24%	2017
March	1st to 10th	Night	16% 14% 4 8% 5% 4 4 3 5% 5% 31%	3450
	11th to 20th	Night	13% 17% 5% 10% 5% 5% 4 3 5% 5% 28%	3479
	21st to end	Night	13% 10% 4 9% 7% 5% 4 3 5% 6% 34%	3817
April	1st to 10th	Night	19% 14% 4 7% 6% 4 4 2 4 7% 30%	1934
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 3 4 5% 37%	1735
	21st to end	Night	21% 10% 3 7% 4 4 5% 3 4 6% 32%	1710
May	1st to 10th	Night	16% 9% 3 8% 5% 4 3 4 5% 7% 36%	2719
	11th to 20th	Night	12% 8% 3 7% 5% 4 4 4 6% 8% 39%	2495
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 5% 7% 41%	2738
June	1st to 10th	Night	12% 10% 3 8% 7% 5% 5% 5% 6% 6% 36%	1348
	11th to 20th	Night	14% 6% 3 5% 3 4 4 4 6% 8% 44%	1330
	21st to end	Night	13% 8% 3 6% 4 4 4 5% 6% 9% 37%	1330
July	1st to 10th	Night	14% 7% 3 8% 3 4 5% 2 6% 7% 40%	2210
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 8% 42%	2210
	21st to end	Night	9% 6% 3 6% 4 4 4 4 5% 8% 49%	2845
August	1st to 10th	Night	12% 7% 2 7% 4 4 5% 4 6% 7% 42%	2825
	11th to 20th	Night	13% 7% 3 6% 5% 4 4 4 6% 7% 42%	2785
	21st to end	Night	13% 5% 2 6% 3 3 4 3 4 7% 49%	3174
September	1st to 10th	Night	16% 5% 2 5% 4 4 3 3 5% 7% 48%	1900
	11th to 20th	Night	12% 6% 3 5% 4 4 4 3 4 7% 47%	2019
	21st to end	Night	19% 7% 3 6% 3 3 3 3 5% 7% 41%	2118
October	1st to 10th	Night	18% 9% 4 8% 5% 4 4 4 5% 5% 35%	2280
	11th to 20th	Night	26% 10% 3 7% 5% 5% 4 3 4 6% 28%	2280
	21st to end	Night	26% 11% 4 9% 6% 4 4 3 4 5% 23%	2558
November	1st to 10th	Night	21% 11% 3 7% 4 4 4 3 5% 6% 32%	2707
	11th to 20th	Night	18% 14% 4 9% 6% 6% 5% 3 5% 6% 24%	2667
	21st to end	Night	24% 17% 4 9% 6% 4 4 3 6% 6% 18%	2660
December	1st to 10th	Night	20% 20% 6% 12% 5% 4 4 3 4 5% 17%	2660
	11th to 20th	Night	20% 18% 5% 10% 5% 4 4 3 5% 6% 20%	3444
	21st to end	Night	21% 23% 5% 15% 5% 4 3 2 3 3 15%	5236

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	69% 7% 12% 10% 2	2209
	11th to 20th	Day	65% 8% 12% 10% 5% 1	2317
	21st to end	Day	70% 7% 7% 11% 4	3124
February	1st to 10th	Day	63% 7% 11% 14% 5% 1	1710
	11th to 20th	Day	59% 7% 10% 14% 8% 1	1672
	21st to end	Day	62% 6% 8% 16% 8% 1	1437
March	1st to 10th	Day	55% 8% 13% 14% 9% 1	3392
	11th to 20th	Day	56% 9% 12% 16% 6% 1	3480
	21st to end	Day	49% 9% 11% 19% 10% 2	3815
April	1st to 10th	Day	53% 7% 13% 18% 9% 1	2245
	11th to 20th	Day	49% 6% 10% 22% 11% 1	2410
	21st to end	Day	50% 7% 8% 18% 13% 3	2470
May	1st to 10th	Day	45% 9% 11% 23% 10% 2	4232
	11th to 20th	Day	40% 10% 14% 26% 9% 1	4402
	21st to end	Day	41% 8% 14% 19% 17% 1	4837
June	1st to 10th	Day	47% 8% 12% 21% 11% 1	2828
	11th to 20th	Day	40% 9% 13% 23% 14% 1	2850
	21st to end	Day	47% 9% 14% 19% 11% 1	2850
July	1st to 10th	Day	44% 9% 16% 22% 8% 1	4686
	11th to 20th	Day	36% 8% 19% 30% 7%	4679
	21st to end	Day	33% 7% 15% 31% 13%	4554
August	1st to 10th	Day	38% 10% 13% 31% 8%	4091
	11th to 20th	Day	41% 8% 16% 25% 9%	4083
	21st to end	Day	35% 8% 14% 34% 9%	4450
September	1st to 10th	Day	41% 10% 14% 24% 10% 1	2280
	11th to 20th	Day	39% 9% 15% 28% 9%	2157
	21st to end	Day	51% 7% 13% 22% 6%	2062
October	1st to 10th	Day	44% 8% 13% 27% 8%	3160
	11th to 20th	Day	50% 8% 15% 20% 7%	3140
	21st to end	Day	64% 6% 14% 12% 4	3340
November	1st to 10th	Day	55% 8% 13% 18% 5%	1590
	11th to 20th	Day	64% 8% 11% 11% 6% 1	1520
	21st to end	Day	73% 6% 11% 7% 3	1520
December	1st to 10th	Day	69% 8% 11% 9% 3	1520
	11th to 20th	Day	65% 6% 11% 14% 4	1747
	21st to end	Day	76% 5% 6% 11% 2	2431
January	1st to 10th	Night	69% 9% 10% 8% 4	4748
	11th to 20th	Night	64% 10% 11% 11% 4	4644
	21st to end	Night	71% 6% 7% 10% 5%	4532
February	1st to 10th	Night	62% 6% 11% 14% 6% 1	2470
	11th to 20th	Night	62% 6% 12% 15% 6% 1	2405
	21st to end	Night	62% 6% 8% 14% 8% 1	2017
March	1st to 10th	Night	53% 7% 11% 16% 12% 1	3460
	11th to 20th	Night	55% 9% 12% 15% 8% 1	3480
	21st to end	Night	49% 8% 10% 18% 12% 3	3817
April	1st to 10th	Night	55% 5% 9% 18% 11% 2	1934
	11th to 20th	Night	50% 5% 8% 21% 15% 2	1735
	21st to end	Night	52% 7% 9% 14% 15% 3	1710
May	1st to 10th	Night	43% 10% 11% 22% 12% 2	2726
	11th to 20th	Night	38% 10% 15% 23% 13% 2	2496
	21st to end	Night	40% 9% 11% 22% 15% 4	2739
June	1st to 10th	Night	46% 7% 12% 21% 11% 3	1348
	11th to 20th	Night	37% 8% 12% 24% 18% 1	1330
	21st to end	Night	42% 9% 14% 21% 12% 2	1330
July	1st to 10th	Night	41% 7% 14% 24% 13% 2	2175
	11th to 20th	Night	40% 7% 16% 28% 8% 1	2168
	21st to end	Night	33% 7% 13% 30% 17% 1	2755
August	1st to 10th	Night	40% 7% 14% 27% 11% 1	2814
	11th to 20th	Night	39% 9% 17% 26% 8% 1	2812
	21st to end	Night	35% 6% 14% 32% 12% 1	3174
September	1st to 10th	Night	37% 7% 14% 29% 11% 2	1900
	11th to 20th	Night	37% 7% 15% 30% 10%	2019
	21st to end	Night	43% 6% 16% 25% 10%	2118
October	1st to 10th	Night	44% 8% 11% 27% 10%	3800
	11th to 20th	Night	48% 7% 13% 27% 5%	3779
	21st to end	Night	57% 7% 19% 12% 5%	4206
November	1st to 10th	Night	52% 7% 16% 19% 5%	2707
	11th to 20th	Night	60% 9% 12% 14% 5% 1	2667
	21st to end	Night	68% 7% 11% 10% 3	2660
December	1st to 10th	Night	70% 7% 10% 9% 3	2660
	11th to 20th	Night	64% 8% 9% 13% 5%	3444
	21st to end	Night	76% 4 6% 10% 3	5236

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	25% 18% 24% 23% 9% 1	180
	11th to 20th	Day	16% 14% 33% 25% 12% 2	200
	21st to end	Day	26% 7% 30% 24% 12% 2	297
February	1st to 10th	Day	23% 19% 33% 20% 4	270
	11th to 20th	Day	20% 16% 38% 22% 4	270
	21st to end	Day	28% 19% 32% 20%	225
March	1st to 10th	Day	20% 13% 37% 27% 4	428
	11th to 20th	Day	19% 12% 33% 30% 5% 1	460
	21st to end	Day	21% 13% 32% 26% 7% 1	506
April	1st to 10th	Day	22% 9% 32% 24% 11% 1	460
	11th to 20th	Day	28% 15% 23% 19% 11% 4	452
	21st to end	Day	27% 14% 25% 25% 8% 2	460
May	1st to 10th	Day	23% 8% 27% 27% 13% 2	463
	11th to 20th	Day	18% 9% 25% 27% 16% 4	487
	21st to end	Day	16% 9% 27% 30% 15% 3	529
June	1st to 10th	Day	17% 7% 30% 32% 12% 2	658
	11th to 20th	Day	14% 8% 30% 31% 13% 4	680
	21st to end	Day	19% 7% 26% 31% 15% 2	680
July	1st to 10th	Day	10% 5% 21% 41% 19% 3	671
	11th to 20th	Day	14% 6% 22% 31% 24% 4	668
	21st to end	Day	11% 5% 21% 33% 24% 5%	542
August	1st to 10th	Day	14% 6% 17% 33% 24% 6%	486
	11th to 20th	Day	14% 4 14% 32% 25% 11%	479
	21st to end	Day	14% 7% 24% 27% 21% 7%	539
September	1st to 10th	Day	18% 7% 22% 31% 16% 6%	490
	11th to 20th	Day	14% 6% 21% 28% 22% 9%	490
	21st to end	Day	24% 10% 19% 24% 18% 5%	490
October	1st to 10th	Day	21% 11% 27% 22% 16% 3	490
	11th to 20th	Day	24% 16% 31% 22% 7% 1	490
	21st to end	Day	31% 14% 31% 21% 2 1	487
November	1st to 10th	Day	29% 17% 27% 23% 3 1	213
	11th to 20th	Day	20% 21% 34% 19% 6%	180
	21st to end	Day	34% 14% 32% 16% 3 1	180
December	1st to 10th	Day	26% 14% 29% 27% 3	180
	11th to 20th	Day	27% 13% 37% 17% 5% 1	180
	21st to end	Day	30% 15% 32% 18% 5% 1	198
January	1st to 10th	Night	26% 9% 29% 28% 7% 1	950
	11th to 20th	Night	28% 10% 27% 28% 7%	920
	21st to end	Night	21% 6% 29% 31% 11% 2	836
February	1st to 10th	Night	17% 11% 38% 29% 5%	760
	11th to 20th	Night	22% 8% 37% 25% 8%	743
	21st to end	Night	22% 11% 32% 31% 4	628
March	1st to 10th	Night	21% 10% 37% 27% 5%	760
	11th to 20th	Night	20% 9% 36% 28% 6% 1	760
	21st to end	Night	19% 8% 35% 30% 8% 1	836
April	1st to 10th	Night	26% 9% 29% 29% 6%	760
	11th to 20th	Night	23% 8% 34% 24% 10%	611
	21st to end	Night	23% 8% 29% 29% 10% 1	570
May	1st to 10th	Night	20% 4 28% 35% 11% 1	508
	11th to 20th	Night	18% 5% 31% 32% 14% 1	374
	21st to end	Night	16% 6% 28% 37% 12%	411
June	1st to 10th	Night	15% 4 33% 36% 11% 1	380
	11th to 20th	Night	17% 6% 22% 40% 15% 1	380
	21st to end	Night	16% 5% 24% 36% 18% 1	380
July	1st to 10th	Night	10% 4 24% 47% 15% 1	370
	11th to 20th	Night	13% 4 20% 40% 22% 1	368
	21st to end	Night	12% 3 20% 42% 21% 1	484
August	1st to 10th	Night	16% 4 17% 40% 22% 2	567
	11th to 20th	Night	14% 4 18% 35% 26% 4	564
	21st to end	Night	18% 5% 23% 32% 20% 3	651
September	1st to 10th	Night	17% 5% 23% 31% 22% 2	760
	11th to 20th	Night	14% 8% 24% 29% 19% 6%	760
	21st to end	Night	22% 7% 21% 27% 20% 3	760
October	1st to 10th	Night	23% 8% 28% 27% 13% 2	760
	11th to 20th	Night	30% 11% 28% 24% 7% 1	760
	21st to end	Night	32% 12% 29% 23% 5%	836
November	1st to 10th	Night	27% 11% 29% 26% 6% 1	780
	11th to 20th	Night	25% 13% 32% 22% 7% 1	763
	21st to end	Night	34% 9% 31% 21% 4	760
December	1st to 10th	Night	30% 13% 30% 20% 6%	760
	11th to 20th	Night	28% 10% 32% 23% 5% 1	869
	21st to end	Night	30% 11% 28% 25% 6%	1045

Page generated at 2024-11-08 06:44:47. Version ac07

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