Collections Menu

Date: 1909

Inventory Number: 1998-1-1051f

Classification: Meteorological Chart

Subject:

Maker: Blue Hill Meteorological Observatory (1885-present)

Cultural Region:

Place of Origin:

Dimensions:

56.2 x 81.2 cm (22 1/8 x 31 15/16 in.)

Material:

DescriptionGraphs of altitude and direction of a pilot balloon ascent from Blue Hill Observatory on October 7, 1909. Graph in upper left tracks the altitude by minute; graph in lower right tracks the direction by mile. Graph paper is made of large vertical rectangles bounded by thick lines, containing 25 smaller rectangles bounded by semi-thick lines, which in turn contain four rectangles bounded by thin lines.

The vertical axis of the altitude graph ascends from sea level to 6300 meters in intervals of 900 at every three semi-thick lines. On the other side of the axis, the altitude is marked in miles. The horizontal axis, located above the graph, ascends from 0 to 50 in numbered intervals of 5, each minute demarcated by one of the fine lines. The line on the graph begins four fine-line spaces from the right at a circled point. The altitude at each minute is marked by an x. The minutes are written at each point, with 11, 21, 31, and 50 written in larger font.

The horizontal axis of the direction graph ascends from 0 to 9600 meters and 0 to 7 miles. Above and to the right of this graph is a compass with east pointing upward. The line on the graph begins at the same origin, tracks to the "north" briefly, and then begins to curve around to head in a southerly direction until it begins to drift southwest. The minutes are written at each point, with 11, 21, 31, and 50 in the larger font.

The vertical axis of the altitude graph ascends from sea level to 6300 meters in intervals of 900 at every three semi-thick lines. On the other side of the axis, the altitude is marked in miles. The horizontal axis, located above the graph, ascends from 0 to 50 in numbered intervals of 5, each minute demarcated by one of the fine lines. The line on the graph begins four fine-line spaces from the right at a circled point. The altitude at each minute is marked by an x. The minutes are written at each point, with 11, 21, 31, and 50 written in larger font.

The horizontal axis of the direction graph ascends from 0 to 9600 meters and 0 to 7 miles. Above and to the right of this graph is a compass with east pointing upward. The line on the graph begins at the same origin, tracks to the "north" briefly, and then begins to curve around to head in a southerly direction until it begins to drift southwest. The minutes are written at each point, with 11, 21, 31, and 50 in the larger font.

Signedon chart: PILOT BALLOON ASCENT / FROM / BLUE HILL OBSERVATORY / OCTOBER 7, 1909

Inscribedupper graph, middle: ALTITUDE

upper graph, vertical axis, left side: METRES / 6300 / 5400 / 4500 / ... / 900 / SEA LEVEL

upper graph, vertical axis, right side: 4 MILES / 3 MILES / 2 MILES / 1 MILE

upper graph, horizontal axis (top): 0 5 10 ... 50

lower graph, upper middle: DIRECTION

lower graph, horizontal axis (bottom), upper line: 1 / MILE 2 / MILES 3 / MILES / ... / 7 MILES

lower graph, horizontal axis (bottom), lower line: 0 2400 4800 7200 9600 METRES

upper graph, vertical axis, left side: METRES / 6300 / 5400 / 4500 / ... / 900 / SEA LEVEL

upper graph, vertical axis, right side: 4 MILES / 3 MILES / 2 MILES / 1 MILE

upper graph, horizontal axis (top): 0 5 10 ... 50

lower graph, upper middle: DIRECTION

lower graph, horizontal axis (bottom), upper line: 1 / MILE 2 / MILES 3 / MILES / ... / 7 MILES

lower graph, horizontal axis (bottom), lower line: 0 2400 4800 7200 9600 METRES

FunctionPilot balloons were used to measure velocity and direction of wind currents in higher layers of the atmosphere. They did not carry instruments and were therefore cheaper than larger ballon-sondes and could be used more frequently. The balloon would be tracked with one or two theodolites. The one-theodolite method used an assumed velocity of ascent, while the two-theodolite method would allow the altitude to be determined through triangulation. The one-theodolite method was sometimes advantageous, because it could be set up more quickly when conditions became favorable, and it actually proved superior in some cases where the balloon was too distant to triangulate accurately.

The lift is given for each balloon flight, as it was dependent on the amount of hydrogen put in it, and therefore roughly constant if leakage and tension in the rubber of the balloon was assumed to be zero. The velocity function could be given as a function of the lift and the density of the air. As the balloon ascended, the ratio of the density of the hydrogen in the balloon and the density of the surrounding air would be fairly constant, as the balloon's radius would increase with the decrease in air pressure during the ascent. The velocity of the balloon would vary inversely with the density of the air, so the balloon would increase in vertical velocity as it ascended. This allowed meteorologists to use a velocity function to calculate height when using the one-theodolite method.

Given the variation in the change in altitude of the balloon, it is most likely that the two-theodolite method was used to measure the height. Unlike the others in this group, this chart shows both the change in altitude and the change in direction of the balloon, which could then be used to determine the wind direction at different heights in the atmosphere. It might also be used to determine wind velocity along a horizontal trajectory, but as there is no vertical axis, only one component of the velocity vector could be calculated.

The lift is given for each balloon flight, as it was dependent on the amount of hydrogen put in it, and therefore roughly constant if leakage and tension in the rubber of the balloon was assumed to be zero. The velocity function could be given as a function of the lift and the density of the air. As the balloon ascended, the ratio of the density of the hydrogen in the balloon and the density of the surrounding air would be fairly constant, as the balloon's radius would increase with the decrease in air pressure during the ascent. The velocity of the balloon would vary inversely with the density of the air, so the balloon would increase in vertical velocity as it ascended. This allowed meteorologists to use a velocity function to calculate height when using the one-theodolite method.

Given the variation in the change in altitude of the balloon, it is most likely that the two-theodolite method was used to measure the height. Unlike the others in this group, this chart shows both the change in altitude and the change in direction of the balloon, which could then be used to determine the wind direction at different heights in the atmosphere. It might also be used to determine wind velocity along a horizontal trajectory, but as there is no vertical axis, only one component of the velocity vector could be calculated.

Primary SourcesFor reference to these balloon flights, see Alexander McAdie, *The Principles of Aërography* (Rand McNally & Company, 1917): 18, available here.

ProvenanceFrom the Blue Hill Meteorological Observatory.

Related WorksFor more on soundings of pilot balloons in the atmosphere, see Charles John Philip Cave, *The Structure of the Atmosphere in Clear Weather: A Study of Soundings with Pilot Balloons* (Cambridge: The University Press, 1912), available here.

Copyright © 2017 The President and Fellows of Harvard College | Privacy | Accessibility | Report Copyright Infringement