Annual Report 1974-75

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Global lake responses to climate change

Climate change is one of the most severe threats to global lake ecosystems. Lake surface conditions, such as ice cover, surface temperature, evaporation and water level, respond dramatically to this threat, as observed in recent decades. In this Review, we discuss physical lake variables and their responses to climate change. Decreases in winter ice cover and increases in lake surface temperature modify lake mixing regimes and accelerate lake evaporation. Where not balanced by increased mean precipitation or inflow, higher evaporation rates will favour a decrease in lake level and surface water extent. Together with increases in extreme-precipitation events, these lake responses will impact lake ecosystems, changing water quantity and quality, food provisioning, recreational opportunities and transportation. Future research opportunities, including enhanced observation of lake variables from space (particularly for small water bodies), improved in situ lake monitoring and the development of advanced modelling techniques to predict lake processes, will improve our global understanding of lake responses to a changing climate

MEASUREMENT OF THE STARK EFFECT IN A FLYGARF.-RALLE MICROWAVE SPECTROMETER

$^{\ast}$ Work supported by NSF and PRF. $^{1}$ T. J. Balle and W. H. Flygare, Rev. Sci. Inst. 52. 35 (1981).Author Institution: Noyes Chemical Laboratory, University of IllinoisIn the Flygare-Balle Fourier transform spectrometer a microwave pulse is applied to a Fabry-Perot cavlty. synchronized with an expanding jet of a gas mixture from a pulsed supersonic $nozzle.^{1}$ Measurement of the Stark effect in this type of spectrometer has been hampered by the difficulty of producing an adequately homogeneous electric field without degrading spectrometer performance. The common approach to the production of a homogeneous electric field is to use large. parallel metal plates, spaced as closely as possible. This method works poorly in the present case because the plates disturb both the microwave field and the gas expansion. We have built a device which generates a good electric field (line broadening $<0.5^{\ast}$ of the Stark shift) over a volume of $-8 \times B \times 10^{\prime\prime}$, but does not disturb the gas expansion significantly. It consists of two square Plexiglass frames $(13 \times 13^{\prime/\prime} o.d.. 11 \times 11^{\prime\prime} i.d.$ and $1^{\prime\prime}$ thick), joined at the corners by four aluminum rods 2*** long. forming a structure similar to a box kite. Twelve 24 gauge wires are stretched 1*** apart along each of the four long faces. At one end, each pair of adjacent wires is connected by a 5 Mohn resistor and the four corner wires to external terminals. The orientation of the electric field is selected by the connections of the terminals to the HV power supply. The microwave field is unaffected by the assembly at frequencies above 10 GHz. tolerable at 9 GHz, but unusable below 8 GHz. Examples of performance will be presented. Modifications to allow operation at lower frequencies are under consideration

The rotational spectrum, structure and dynamics of a benzene dimer

The low J (2 to 7) rotational spectrum of a symmetric-top benzene dimer has been obtained with a Balle/Flygare Fourier transform microwave spectrometer. Each transition is a symmetrical quartet with two J- and K-dependent tunneling splittings of 30 to 400 kHz. Rotational constants B̅, D<SUB>J</SUB>, and D<SUB>JK</SUB> were determined to be 427.76(2) MHz, 7.2(3) kHz, and 0.869(5) MHz. The dimer is T-shaped with a benzene c.m. to c.m. distance of 4.96 Å