Metcalf Nature Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Metcalf Nature Center clear sky chart from 2005-11-06 to 2025-04-26.

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	12% 9% 4 8% 5% 4 4 4 5% 7% 37%	113901
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 37%	114926
Transparency	Day	45% 6% 13% 24% 11% 1	117497
	Night	46% 5% 12% 23% 12% 1	119473
Seeing	Day	16% 11% 31% 26% 13% 3	16573
	Night	16% 9% 32% 32% 10% 1	26628

By Month

Percentage Cloud Cover SE (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	8% 13% 6% 20% 6% 6% 5% 3 4 6% 23%	3617
	11th to 20th	Day	5% 12% 7% 13% 7% 6% 9% 8% 8% 4 20%	3620
	21st to end	Day	7% 16% 7% 12% 5% 4 4 3 5% 6% 31%	4465
February	1st to 10th	Day	9% 11% 5% 11% 6% 6% 6% 5% 6% 6% 30%	1800
	11th to 20th	Day	8% 10% 6% 10% 6% 6% 4 4 5% 6% 34%	1754
	21st to end	Day	11% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	1566
March	1st to 10th	Day	12% 8% 3 6% 3 3 3 3 5% 7% 46%	4773
	11th to 20th	Day	10% 9% 3 7% 4 4 4 4 6% 8% 40%	4980
	21st to end	Day	22% 11% 4 9% 5% 4 4 3 5% 6% 26%	5569
April	1st to 10th	Day	17% 11% 4 7% 5% 4 4 3 5% 7% 34%	2400
	11th to 20th	Day	20% 12% 4 8% 5% 4 4 3 4 5% 32%	2374
	21st to end	Day	21% 12% 3 7% 5% 4 4 4 5% 6% 29%	2518
May	1st to 10th	Day	18% 6% 2 6% 4 4 4 3 5% 8% 40%	4296
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 5% 7% 44%	4399
	21st to end	Day	17% 8% 2 7% 5% 5% 4 3 5% 7% 36%	4824
June	1st to 10th	Day	12% 6% 2 4 4 4 5% 4 6% 9% 46%	2660
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	2660
	21st to end	Day	11% 5% 2 6% 6% 5% 6% 4 7% 9% 40%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 11% 37%	4440
	11th to 20th	Day	6% 3 2 3 3 3 4 3 5% 8% 61%	4440
	21st to end	Day	7% 4 2 6% 4 4 5% 5% 6% 9% 48%	4800
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	4104
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	4041
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 5% 7% 46%	4290
September	1st to 10th	Day	12% 6% 2 5% 5% 4 4 3 5% 8% 47%	2280
	11th to 20th	Day	12% 6% 1 5% 4 4 4 4 6% 8% 45%	2280
	21st to end	Day	14% 6% 3 6% 4 4 3 4 6% 7% 44%	1981
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 3 5% 6% 45%	1900
	11th to 20th	Day	14% 11% 2 6% 5% 4 5% 4 4 7% 38%	1900
	21st to end	Day	17% 13% 3 7% 3 4 4 3 4 6% 37%	2043
November	1st to 10th	Day	12% 13% 4 8% 5% 4 4 3 5% 5% 35%	1667
	11th to 20th	Day	12% 14% 7% 10% 7% 4 4 4 5% 6% 28%	1600
	21st to end	Day	13% 13% 4 9% 6% 3 3 3 5% 7% 34%	1600
December	1st to 10th	Day	8% 11% 5% 16% 12% 6% 5% 4 5% 6% 23%	2610
	11th to 20th	Day	11% 16% 7% 10% 8% 4 5% 3 5% 5% 27%	3008
	21st to end	Day	19% 21% 8% 18% 5% 4 3 2 3 3 14%	3982
January	1st to 10th	Night	7% 12% 6% 16% 9% 5% 4 3 5% 6% 28%	6348
	11th to 20th	Night	7% 12% 5% 12% 8% 7% 9% 9% 7% 5% 20%	6360
	21st to end	Night	9% 17% 7% 11% 5% 5% 4 3 5% 5% 31%	6510
February	1st to 10th	Night	6% 12% 5% 11% 7% 6% 5% 4 5% 6% 33%	2600
	11th to 20th	Night	7% 9% 4 9% 6% 6% 5% 4 5% 8% 38%	2533
	21st to end	Night	12% 11% 3 9% 6% 5% 5% 4 5% 6% 36%	2062
March	1st to 10th	Night	10% 6% 3 7% 4 4 4 3 5% 6% 46%	5111
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 4 7% 8% 39%	4980
	21st to end	Night	24% 12% 4 10% 4 3 3 3 4 4 29%	5358
April	1st to 10th	Night	16% 12% 3 6% 3 4 3 3 6% 6% 37%	2000
	11th to 20th	Night	20% 10% 2 7% 4 4 3 3 5% 5% 35%	1975
	21st to end	Night	18% 10% 3 7% 5% 3 4 3 5% 7% 35%	1775
May	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 41%	2606
	11th to 20th	Night	14% 5% 2 3 3 4 3 3 5% 7% 51%	2506
	21st to end	Night	12% 8% 3 7% 4 4 4 4 5% 7% 42%	2743
June	1st to 10th	Night	11% 5% 2 4 4 4 4 5% 7% 7% 48%	1520
	11th to 20th	Night	9% 5% 2 5% 5% 5% 5% 3 7% 9% 46%	1520
	21st to end	Night	7% 5% 2 4 4 4 5% 5% 6% 8% 51%	1520
July	1st to 10th	Night	11% 4 2 5% 4 4 4 4 6% 9% 48%	2530
	11th to 20th	Night	4 2 1 4 4 4 5% 3 7% 9% 58%	2530
	21st to end	Night	5% 4 2 4 3 4 3 3 6% 8% 59%	2750
August	1st to 10th	Night	12% 6% 3 7% 4 3 4 4 5% 7% 46%	2834
	11th to 20th	Night	14% 5% 2 4 3 3 4 4 5% 8% 50%	2798
	21st to end	Night	9% 6% 2 7% 4 5% 4 4 5% 7% 47%	3367
September	1st to 10th	Night	9% 4 2 5% 3 4 3 4 4 7% 54%	1900
	11th to 20th	Night	9% 5% 1 4 3 4 4 3 5% 7% 53%	1900
	21st to end	Night	13% 5% 2 4 4 4 3 2 5% 6% 51%	2176
October	1st to 10th	Night	14% 6% 3 6% 4 3 3 3 4 6% 50%	2280
	11th to 20th	Night	16% 9% 3 6% 4 3 3 3 5% 7% 42%	2280
	21st to end	Night	18% 11% 3 7% 3 3 4 2 5% 7% 37%	2553
November	1st to 10th	Night	13% 11% 3 6% 4 4 3 3 5% 6% 41%	2626
	11th to 20th	Night	13% 12% 5% 10% 6% 4 4 4 5% 5% 33%	2800
	21st to end	Night	13% 11% 4 9% 6% 4 4 3 4 7% 36%	2800
December	1st to 10th	Night	7% 10% 5% 14% 8% 8% 4 3 5% 7% 29%	4580
	11th to 20th	Night	10% 18% 7% 14% 6% 5% 5% 4 4 5% 23%	5199
	21st to end	Night	21% 16% 9% 16% 5% 3 4 2 4 4 17%	6996

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% 4 9% 19% 4	3618
	11th to 20th	Day	61% 5% 7% 17% 10%	3620
	21st to end	Day	55% 6% 16% 17% 7%	4465
February	1st to 10th	Day	52% 6% 14% 17% 9% 1	1800
	11th to 20th	Day	50% 5% 12% 25% 8% 1	1763
	21st to end	Day	51% 7% 13% 20% 10% 1	1566
March	1st to 10th	Day	37% 5% 18% 25% 12% 3	4773
	11th to 20th	Day	41% 4 16% 29% 10% 1	4980
	21st to end	Day	59% 5% 10% 18% 8% 1	5571
April	1st to 10th	Day	50% 5% 12% 21% 12% 1	2400
	11th to 20th	Day	54% 5% 11% 17% 11% 2	2374
	21st to end	Day	55% 6% 12% 18% 8%	2518
May	1st to 10th	Day	42% 6% 13% 25% 13% 2	4335
	11th to 20th	Day	38% 6% 13% 27% 14% 2	4403
	21st to end	Day	46% 7% 11% 24% 11% 1	4837
June	1st to 10th	Day	34% 8% 15% 29% 13% 2	2660
	11th to 20th	Day	42% 8% 15% 22% 13% 1	2660
	21st to end	Day	38% 9% 16% 26% 10% 2	2660
July	1st to 10th	Day	38% 9% 19% 23% 10% 1	4371
	11th to 20th	Day	21% 7% 17% 35% 19% 1	4365
	21st to end	Day	30% 9% 15% 28% 17% 1	4649
August	1st to 10th	Day	38% 7% 13% 28% 13% 1	4090
	11th to 20th	Day	36% 6% 13% 34% 10%	4080
	21st to end	Day	36% 6% 17% 27% 13% 1	4289
September	1st to 10th	Day	35% 6% 15% 30% 13%	2280
	11th to 20th	Day	36% 8% 16% 31% 9%	2280
	21st to end	Day	39% 7% 12% 31% 11%	1981
October	1st to 10th	Day	29% 4 8% 35% 23%	3160
	11th to 20th	Day	39% 5% 12% 26% 17% 1	3140
	21st to end	Day	43% 4 15% 28% 9%	3345
November	1st to 10th	Day	50% 5% 13% 18% 12% 2	1667
	11th to 20th	Day	56% 7% 12% 16% 8% 1	1600
	21st to end	Day	50% 6% 17% 19% 7% 1	1600
December	1st to 10th	Day	61% 6% 15% 13% 5%	2610
	11th to 20th	Day	59% 5% 13% 18% 5%	3005
	21st to end	Day	76% 4 8% 8% 4	3982
January	1st to 10th	Night	58% 4 10% 20% 8%	6348
	11th to 20th	Night	61% 6% 10% 13% 9%	6360
	21st to end	Night	55% 4 11% 22% 7%	6510
February	1st to 10th	Night	51% 6% 12% 21% 9% 1	2600
	11th to 20th	Night	45% 5% 15% 25% 8% 1	2534
	21st to end	Night	49% 5% 12% 21% 11% 1	2062
March	1st to 10th	Night	38% 4 15% 23% 17% 3	5111
	11th to 20th	Night	40% 5% 17% 24% 14% 1	4980
	21st to end	Night	59% 4 10% 17% 10%	5358
April	1st to 10th	Night	48% 5% 9% 23% 15% 1	2000
	11th to 20th	Night	50% 5% 9% 19% 14% 2	1975
	21st to end	Night	49% 5% 10% 21% 14% 2	1775
May	1st to 10th	Night	42% 7% 12% 18% 20% 1	2632
	11th to 20th	Night	32% 6% 11% 28% 20% 4	2507
	21st to end	Night	40% 7% 12% 24% 16% 1	2751
June	1st to 10th	Night	33% 6% 14% 28% 17% 2	1520
	11th to 20th	Night	34% 8% 14% 23% 19% 2	1520
	21st to end	Night	29% 8% 15% 30% 15% 3	1520
July	1st to 10th	Night	32% 6% 22% 25% 13% 1	2490
	11th to 20th	Night	20% 7% 15% 36% 20% 2	2482
	21st to end	Night	23% 7% 16% 31% 21% 3	2662
August	1st to 10th	Night	38% 6% 11% 28% 14% 3	2825
	11th to 20th	Night	33% 6% 14% 32% 14% 1	2825
	21st to end	Night	36% 7% 16% 28% 12% 2	3367
September	1st to 10th	Night	29% 6% 13% 34% 17% 1	1900
	11th to 20th	Night	30% 6% 16% 31% 16%	1900
	21st to end	Night	34% 5% 14% 31% 15% 1	2176
October	1st to 10th	Night	25% 3 7% 35% 26% 4	3800
	11th to 20th	Night	34% 5% 12% 25% 22% 3	3779
	21st to end	Night	45% 5% 11% 26% 12%	4201
November	1st to 10th	Night	44% 6% 12% 23% 14% 1	2626
	11th to 20th	Night	53% 6% 12% 18% 10% 1	2800
	21st to end	Night	50% 5% 16% 22% 7% 1	2800
December	1st to 10th	Night	55% 6% 17% 18% 4	4580
	11th to 20th	Night	64% 5% 11% 15% 5% 1	5201
	21st to end	Night	72% 4 9% 12% 3	6996

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	13% 13% 35% 27% 11% 2	300
	11th to 20th	Day	17% 14% 41% 22% 5%	300
	21st to end	Day	14% 11% 42% 23% 8% 2	330
February	1st to 10th	Day	14% 17% 44% 20% 4 1	300
	11th to 20th	Day	14% 16% 42% 21% 6% 1	300
	21st to end	Day	18% 14% 41% 25% 2	249
March	1st to 10th	Day	17% 13% 44% 19% 6%	300
	11th to 20th	Day	19% 14% 38% 21% 7% 1	300
	21st to end	Day	28% 14% 26% 22% 7% 3	373
April	1st to 10th	Day	23% 18% 33% 21% 5%	500
	11th to 20th	Day	29% 15% 24% 25% 7% 1	492
	21st to end	Day	28% 13% 29% 21% 9% 2	658
May	1st to 10th	Day	20% 16% 31% 23% 9% 1	653
	11th to 20th	Day	18% 13% 30% 23% 13% 3	675
	21st to end	Day	18% 11% 29% 24% 16% 3	734
June	1st to 10th	Day	11% 9% 27% 33% 17% 4	680
	11th to 20th	Day	13% 12% 34% 28% 11% 2	680
	21st to end	Day	12% 9% 31% 27% 16% 5%	680
July	1st to 10th	Day	9% 3 22% 32% 24% 10%	671
	11th to 20th	Day	8% 5% 29% 29% 21% 7%	668
	21st to end	Day	5% 4 26% 37% 20% 7%	739
August	1st to 10th	Day	11% 5% 23% 33% 22% 6%	676
	11th to 20th	Day	14% 5% 20% 33% 24% 5%	665
	21st to end	Day	11% 6% 24% 33% 20% 5%	610
September	1st to 10th	Day	14% 7% 30% 28% 17% 5%	490
	11th to 20th	Day	13% 5% 28% 30% 22% 3	490
	21st to end	Day	15% 18% 28% 22% 12% 5%	300
October	1st to 10th	Day	17% 16% 25% 27% 11% 4	300
	11th to 20th	Day	18% 20% 36% 20% 7%	300
	21st to end	Day	23% 16% 39% 15% 6% 1	330
November	1st to 10th	Day	21% 16% 33% 20% 9% 1	300
	11th to 20th	Day	19% 19% 40% 18% 3	300
	21st to end	Day	23% 16% 38% 19% 4	300
December	1st to 10th	Day	14% 12% 40% 26% 8% 1	300
	11th to 20th	Day	20% 16% 39% 19% 6%	300
	21st to end	Day	27% 14% 39% 16% 3 1	330
January	1st to 10th	Night	11% 13% 37% 31% 8% 1	1000
	11th to 20th	Night	17% 10% 37% 30% 6% 1	1000
	21st to end	Night	18% 10% 36% 29% 6% 1	1100
February	1st to 10th	Night	13% 12% 40% 28% 7% 1	1000
	11th to 20th	Night	12% 11% 43% 26% 7% 1	979
	21st to end	Night	17% 13% 34% 28% 7%	742
March	1st to 10th	Night	15% 10% 38% 28% 8% 1	694
	11th to 20th	Night	17% 12% 38% 27% 6%	600
	21st to end	Night	22% 12% 29% 25% 11% 1	660
April	1st to 10th	Night	22% 13% 35% 27% 5%	600
	11th to 20th	Night	25% 9% 33% 27% 6%	593
	21st to end	Night	21% 9% 33% 28% 8% 1	587
May	1st to 10th	Night	20% 10% 32% 31% 6% 1	570
	11th to 20th	Night	17% 9% 30% 33% 10% 1	561
	21st to end	Night	14% 8% 25% 38% 14%	617
June	1st to 10th	Night	12% 5% 28% 41% 13% 1	570
	11th to 20th	Night	11% 4 33% 43% 9%	570
	21st to end	Night	9% 4 24% 46% 15% 2	570
July	1st to 10th	Night	5% 3 21% 42% 27% 2	555
	11th to 20th	Night	5% 4 25% 44% 21% 2	552
	21st to end	Night	6% 3 22% 46% 22% 2	624
August	1st to 10th	Night	6% 4 17% 50% 21% 1	567
	11th to 20th	Night	13% 3 18% 36% 27% 2	564
	21st to end	Night	9% 4 25% 37% 23% 2	627
September	1st to 10th	Night	10% 6% 23% 42% 16% 4	570
	11th to 20th	Night	10% 7% 29% 35% 18% 2	570
	21st to end	Night	14% 9% 30% 30% 16% 1	660
October	1st to 10th	Night	17% 8% 28% 32% 14% 1	760
	11th to 20th	Night	21% 11% 32% 29% 7% 1	760
	21st to end	Night	25% 12% 26% 30% 7%	836
November	1st to 10th	Night	19% 10% 34% 30% 7% 1	870
	11th to 20th	Night	20% 12% 35% 26% 7% 1	1000
	21st to end	Night	18% 14% 35% 25% 7%	1000
December	1st to 10th	Night	15% 9% 38% 29% 7% 1	1000
	11th to 20th	Night	19% 9% 39% 27% 6%	1000
	21st to end	Night	24% 11% 34% 25% 5%	1100

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