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Weather! What is weather? Weather is the condition of the Earth’s atmosphere at a certain place and time. The elements of weather: Temperature Humidity Precipitation Air Pressure Wind Visibility Temperature What is temperature? Temperature is a measure of how hot or cold something is. A thermometer measures temperature. In science, we use degrees Celsius. A liquid, usually alcohol or mercury, is sealed in a glass tube. When its heated, it expands, (thermal expansion) rising higher in the tube. When it cools, it contracts, filling less of the tube. Humidity What is humidity? Humidity is the amount of water vapor in the air. As water evaporates, it changes from liquid to gas, from liquid water to water vapor. That amount of water vapor in the air is humidity. As more water evaporates and turns to water vapor, humidity increases. Warm air can “hold” more water vapor than cold air. Relative Humidity What is relative humidity? Relative humidity is the amount of water vapor in the air compared to the total amount of water vapor needed to reach saturation. Saturation occurs when the rates of evaporation and condensation are equal. Relative humidity is 100%. Air can become saturated when evaporation adds water vapor to the air, or when the air is cooled to its dew point. A psychrometer measures relative humidity. Dew Point What is dew point? Dew point is the temperature at which more condensation occurs than evaporation in air. The condensation can form dew, fog and clouds. evaporation evaporation evaporation Saturated condensation condensation condensation Unsaturated Dew Point Precipitation What is Precipitation? Precipitation is any form of water that falls to the Earth’s surface from clouds. Types of Precipitation: Rain – water droplets; the most common type of precipitation Sleet – rain falling through cold air forms ice pellets Hail – layered lumps of ice Snow – ice crystals in the form of flakes Freezing Rain – liquid droplets that land and freeze when they hit surface temperatures of less than 0°C Sleet vs. Freezing Rain Sleet Freezing Rain Air Pressure What is Air pressure? Air pressure is the force of air molecules pushing on an area. Air pressure and density decrease as altitude increases. Lower pressure Less dense Higher pressure More dense Wind What is Wind? Wind is air that is moving horizontally, parallel to the ground. It is caused by the uneven heating of the Earth’s surface by the Sun This causes pressure differences in different areas Air moves from high to low pressure Visibility What is visibility? Visibility is a measure of the transparency of the atmosphere. It describes how far we can see. Instruments for Measuring Weather Thermometer – Measures temperature Barometer – Measures air pressure Anemometer – Measures wind speed Psychrometer – Measures relative humidity Wind Vane Shows wind direction
