The formation of small disturbances in cold air outbreaks over the north-east Atlantic is shown to be due to baroclinic instability. A case study of two such disturbances shows their growth rate, wavelength and phase speed to be very close to the predictions of the baroclinic theory. Surface friction is shown to have a very important effect; strong surface winds enhancing the friction and severely reducing the growth rate. Heating from the sea surface also reduces the growth rate. It is concluded that these lows will develop only when the surface winds are light and where the air flow is not parallel to sea-surface isotherms, thus explaining their rarity compared to that of cold air outbreaks and providing more discriminating criteria for forecasting their development.