Summit Weather Instruments

Believe it or not, the weather observers on Mount Washington may determine what you decide to wear today. Our observers, as well as hundreds of other weather stations throughout the country (including many which are totally automated) are the basic link in producing forecasts. If we look at the readings from stations all over the United States, we can see patterns of pressure, temperature, wind, etc. By watching these over a period of hours or days, we can make predictions about where the weather is going to head next. This is the essence of forecasting.
 

When heated, most substances expand. Mercury or alcohol, which expand at a known rate, expand and contract in a glass vacuum tube - a thermometer. In an unconfined space, this warmer air rises and expands; as it expands it cools: 5.4 degrees F for every 1000 feet in dry air. This property, called the Adiabatic Lapse Rate, explains why mountaintops are colder than the lowlands.

The sling psychrometer uses evaporation to cool a thermometer below the actual (or, "ambient") air temperature. The larger the difference between this "wet bulb" temperature and the ambient temperature, the less moisture in the air. We can express this amount in two ways: dew point and relative humidity.

Dew Point = the temperature to which a parcel of air can be cooled to where it reaches saturation, and clouds form.

Relative Humidity = the percentage of moisture in an air mass, compared to what the air mass will actually hold at saturation (100% relative humidity)
 

As you have probably noticed, one way to get out of the wind is to get close to the ground. Irregularities on the ground break up the flow of wind, decreasing its strength (surface friction). For this reason, the most accurate wind readings are taken 10 meters (not quite 33 feet) above ground level. Our instrument tower on top of Mount Washington is 6,310 feet above sea level, more than 40 feet tall, and even 22 feet higher than the actual summit.
 

Wind is defined as the horizontal movement of air. Wind will flow from areas of high pressure toward low pressure; the bigger the pressure differential, the stronger the wind. However, many other factors such as terrain and weather system circulation significantly alter wind. For example, the global circulation of air in the region of the United States is mainly south to north. The spin of the earth causes the winds in the Northern Hemisphere to turn to the right (the Coriolis Effect), giving us predominantly west winds.
 

The Windchill Equivalency Temperature (WET) was derived by scientists at the South Pole to describe how exposed flesh would feel during various wind speeds. However, the actual wind chill reading is often misrepresented; no matter how strong the wind, the temperature of a dry object will never fall below the actual air temperature due to the wind alone. Wind Chill Chart

The 70 miles of air over your head at sea level is made up of tiny particles-- molecules. These molecules, added up, have measurable weight. In fact, an inch square tube all the way up through the atmosphere would hold 14.7 pounds of air. This pressure is enough to push a column of mercury almost 30 inches up a glass vacuum tube. This is exactly how the barometer works. However, since the air density (number of air molecules packed into a certain space) changes, we can measure the differences by watching the height of the mercury move up and down in the column.