We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs .
Photometry has been obtained for 71 dwarfs and spectroscopy for 56 .
The sample comprises newly identified very red objects from the Sloan Digital Sky Survey ( SDSS ) and known dwarfs from the SDSS and the Two Micron All Sky Survey ( 2MASS ) .
Spectral classification has been carried out using four previously defined indices ( from Geballe et al .
2002 , G02 ) that measure the strengths of the near infrared water and methane bands .
We identify 9 new L8–9.5 dwarfs and 14 new T dwarfs from SDSS , including the latest yet found by SDSS , the T7 dwarf SDSS J175805.46 + 463311.9 .
We classify 2MASS J04151954 - 0935066 as T9 , the latest and coolest dwarf found to date .

We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system and with uniform spectroscopic classification .
We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L–T transition with predictions made by models of the distribution and evolution of photospheric condensates .
There is some scatter in the G02 spectral indices for L dwarfs , suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer .
The near-infrared colors of the L dwarfs also show scatter within a given spectral type , which is likely due to variations in the altitudes , spatial distributions and thicknesses of the clouds .
We have identified a small group of late L dwarfs that are relatively blue for their spectral type and that have enhanced FeH , H _ { 2 } O and K I absorption , possibly due to an unusually small amount of condensates .

The scatter seen in the H - K color for late T dwarfs can be reproduced by models with a range in surface gravity .
The variation is probably due to the effect on the K -band flux of pressure-induced H _ { 2 } opacity .
The correlation of H - K color with gravity is supported by the observed strengths of the J -band K I doublet .
Gravity is closely related to mass for field T dwarfs with ages > 10 ^ { 8 } yrs and the gravities implied by the H - K colors indicate that the T dwarfs in our sample have masses in the range 15 – 75 M _ { Jupiter } .
One of the SDSS dwarfs , SDSS J111010.01 + 011613.1 , is possibly a very low mass object , with log g \sim 4.2 – 4.5 and mass \sim 10 – 15 M _ { Jupiter } .