The Instrument Shelter, also known at our latitude as the Cotton Region Shelter (or CRS for short), and as the Stevenson Screen; named after its British inventor, Thomas Stevenson in 1800s.
The general purpose of an instrument shelter is to protect the monitoring equipment inside–which may consist of any number of items such as thermometers, hygrometers, psychrometers et cetera–from direct sunlight, wind & precipitation.
Base of shelter is set 5.5′ above ground level; louvered on all sides; double roof; always located in direct sunlight; door always opens in the direction where sunlight will not enter the shelter; Our station is located north of 23° 30′ north latitude, therefore, our shelter is situated to face north. (Stations south of 23° 30′ south latitude must face south, and stations between these 2 latitudes must be mounted on a rotating base to accommodate changing sun angles); some instrument shelters are hung freely onboard ships and remains vertical regardless of the motions of the vessel. In polar regions during 24-hour sunlight periods, observers can set the shelter up on a rotating base or in any direction, but must take care to shade the instruments within when opening the shelter door to conduct an observation. Students take their observations from the shelter at the same time every day, marking down the temperature extremes, current soil temperature and taking dew point & relative humidity readings on paper forms. We are one of the very few stations left in Los Angeles that still utilizes a cotton region shelter.
Siting of the instrument shelter is extremely important. In addition to a weather station being at least 100 feet away from any artificial heating or reflective source, it is important instrument shelters at these stations be:
- …set up far from buildings, trees or other obstructions. A good rule of thumb is to set the shelter up at least twice the distance horizontally as the vertical height of its nearest obstruction (such as a building, tree, telephone pole etc.)
- …set up over natural landscaping; do not set up a shelter on or near artificial ground cover such as blacktop, cement, or any other man-made surface; including, but not limited to rooftops, artificial turf and wood patio decks. They should be placed over a natural surface representative of the natural landscape (or what’s left of it).
- …set at a standard height; the World Meteorological Organization (WMO) agreed standard for the height of the thermometers is between 1.25 m (4 ft 1 in) and 2 m (6 ft 7 in) above the ground. At Pierce, we have had our shelter set up at 5 feet 6 inches above ground level since 1949.
- …kept in its original position. Never relocate or change the height of a shelter once data collection has begun, unless a new nearby structure justifies its move; for instance, a new building is erected nearby.
- …kept clean and painted white. Keep your shelter painted white, but not any old white paint will do. Though white paint generally reflects the visible spectrum of light, minimizing heat absorption, the molecular properties of different white paints can absorb infrared wavelengths of the spectrum at different rates. Though the National Weather Service recommends using a latex semi-gloss white paint, it is better to stick with what has been used for centuries; whitewash, or calsomine (also spelled kalsomine), which is a very inexpensive ‘paint” made from calcium hydroxide and chalk. Unlike semi-gloss latex, whitewash does NOT absorb infrared wavelengths (heat energy) as readily. Whitewash is less a paint and more a kind of super-thin plaster.
As far as I’m aware, you can’t buy whitewash anywhere, however it is very simple to make from items available at your local hardware store. Here’s a link that explains how you can make it.
Making your own whitewash.
If you plan to set up your own shelter, we highly recommend you cover it in whitewash, as latex tends to record slightly higher maximum temperatures than what is actual.
Whatever paint you use, be sure to keep your shelter painted white to prevent excess heat absorption by the wood. This is an ongoing endeavor, and you may find yourself reapplying paint throughout the year. Fading paint exposes the wood frame, allowing heat radiation to be absorbed into the shelter unit & causing temperature readings inside the box to be higher than what is actual ambient air temperature.
Instruments found in Weather Shelter include…
- U.S.N.W.S. Official Maximum and Minimum Air Thermometers
- Soil Temperature Gauge
- Wet and Dry Bulb Thermometers (psychometric readings)
Within the weather shelter is the thermograph, max and min thermometers, soil temperature sensor (in white box beneath shelter), dry and wet bulb thermometers, and an aspirating motor (top right side of shelter on the outside of unit). Be sure the motor is mounted on the outside of the shelter and blows the air away from the unit.
Beneath the shelter is a box protecting the soil temperature gauge from direct solar radiation. The temperature tube is shaded to avoid false readings.
Above is a closer look at what is contained within our weather shelter. The top horizontal thermometer is the alcohol minimum thermometer. The thermometer beneath it is the mercurial maximum thermometer. The round dial on the back wall is the readable end of the platinum soil temperature probe buried beneath the shelter at a depth of 20 centimeters. Front and center–the beige box with two pen arms traversing a graph-wrapped cylinder–is the thermograph. The top pen records temperature continuously, the bottom pen measures relative humidity continuously. On the right I have devised a system for measuring RH. The two dangling thermometers (side-by-side) hang in front of an orifice which is the opposite end of a tube connected to the aspirating motor (via a pre-heater hose to a car) connected outside of the shelter (outside to radiate heat from the motor away from the measuring devices.) I ran an underground conduit to the weather shelter from a nearby electrical source and put a switch to activate the aspirator inside the shelter. The cylindrical chrome device to the far left is the psychrometer which has a built in motor which draws air past its two thermometers. The switch operates both devices simultaneously, and the result is a high level of accuracy. Extra pen ink and distilled water can be seen on the lower right hand corner.
~ Steve Woodruff and Devin Lussier