1. **Stating the problem:** We have a thermometer showing a temperature between -10°C and -20°C, closer to -10°C, approximately at -13°C. 2. **Understanding the thermometer scale:** The thermometer scale is marked in increments of 10 degrees Celsius, with 0, 10, 20, 30, 40, 50 above zero and -10, -20 below zero. 3. **Reading the temperature:** Since the mercury level is between -10°C and -20°C, closer to -10°C, we estimate the temperature as approximately -13°C. 4. **Final answer:** The thermometer shows a temperature of **-13°C**.