In summertime some people go to the mountains to escape the heat. The higher they go, the cooler it gets. Scientists have measured the rate of cooling with altitude. It averages about 1 degree Celsius for every 160 meters. This change is called the normal lapse rate. The troposphere is warmest near the Earth’s surface because most of the sun’s radiation is absorbed at the surface. Heat is transferred from the surface to the air just above by conduction and carried aloft by convection. The rising air in the convection currents cools. The rising air cools from expansion. Rising air expands because it meets lower pressure at higher altitudes. The lower pressure allows the air molecules to move farther apart. As molecules move farther apart, they collide less often and transfer less energy, which in turn produces less heat.
Outside of clouds, rising air cools at a rate of 1 degree Celsius for every 100 meters. When air sinks, its molecules are squeezed closer together by the increased pressure. The molecules gain energy. The temperature rises at a rate of 1 degree Celsius for every 100 meters in clean air. The word adiabatic describes the temperature change caused by expansion or compression.
The rate of temperature change of rising or sinking air outside of clouds is called the dry-adiabatic lapse rate. In contrast, the normal lapse rate is the average temperature change with height. This average includes all air-clear or cloudy, moving or still.
Normally the air gets colder with height in the troposphere. Sometimes, however, the air at the surface is colder than the air above it. This upside-down temperature condition is called a temperature inversion. Temperature inversions form during clear, dry nights. On these nights, the ground and the air near the ground cool rapidly by radiation from the surface. Since the ground cools faster than the air, the air near the ground cools still more due to contact with the cooler ground. The wind mixes the cold air upward in a layer a few hundred meters deep. This bottom layer is cooler than the air above it. Cold air is heavier than warmer air, so smoke and other pollution are trapped beneath the inversion. If the sky remains clear, sunlight soon warms the ground and lower atmosphere. The low-level inversion is destroyed by late morning. Mixing of the air by strong winds can also destroy temperature inversions or prevent their formation. Below is an example of a temperature inversion.
The temperature varies with the seasons because the sun’s rays do not heat Earth’s surface evenly. Because Earth is round, the sun’s rays strike the surface at angles ranging from 0 to 90. When the sun is directly overhead, the angle of insolation is 90-. The sun’s rays are vertical, and Earth’s surface gets all the energy possible. As the angle of insolation decreases, the energy of the rays is spread out over a large area. Also, the distance that sunlight travels through the atmosphere increases. More sunlight is absorbed or reflected before it reaches the surface. Both factors reduce the amount of solar energy reaching the surface.
Places near the equator get nearly vertical rays all through the year. Thus, these areas have hot climates. Places in the middle latitudes (like most of the United States) get near-vertical rays in summer. Their summers are hot. The angle of the rays is less vertical in the winter, so winters in middle latitudes are cold. Places in high latitudes (near the poles, for example) never get rays striking the surface at near-vertical angles. These areas may even have no sun at all for part of the year. They are cold all year round.
Varying insolation also changes the temperature during the day. The highest temperature is not at noon, however, when the sunlight is strongest. Instead, the warmest hour of a sunny day is usually in the afternoon. For several hours after noon, the lower air still receives more heat from the sun and the ground than it loses. Thus, its temperature keeps rising until well into the afternoon. The coldest hour usually comes just before sunrise because the lower air loses heat all through the night.
The temperature range is the difference between the highest and coldest temperatures. The temperature range for one day is called the daily temperature range. For example, a high of 35 degrees Celsius and a low of 10 degrees Celsius give a range of 25 degrees Celsius. The daily temperature range is variable. It is usually large when skies are clear. The clear skies allow strong heating by day. At night they allow rapid loss of heat by radiation. The daily temperature range is small on cloudy days. The clouds keep out sunshine by day, so the air hardly warms up. At night the clouds keep the air from radiating its heat out into space. This blanket effect keeps the air from cooling much at night. The average temperature of a day is the sum of the high and low temperatures divided by two. This daily average is then used to compute the average, or mean, temperatures for months, years, or other periods of time.
Like the day’s highest temperature, the year’s highest temperature occurs after the time of strongest sunlight. In middle latitudes of the Northern Hemisphere, June 21 is the time of strongest sunlight. However, July is usually the warmest month. Similarly, December 21 is the time of weakest sunlight, but January is usually the coldest month. In the Southern Hemisphere, the warmest and coldest months are the reverse. The annual temperature range for an area is the difference between the average temperatures of the warmest and coldest months. Oceans have small annual temperature ranges. They are relatively cool in summer, and warm in the winter. Continents and large landmasses have large annual temperature ranges. They are relatively hot in summer and cool in winter.
The reason why the daily and annual temperature ranges are larger over continents than over oceans is because that water and land warm up and cool off at different rates. Water warms much more slowly than land for many reasons.
Unlike water, land has many kinds of surface materials. Some of these absorb the sun’s rays better than others. Dark soils and rocks absorb more energy than light-colored ones. Rough surfaces absorb more energy than smooth ones. The temperature of dry ground increases faster than wet ground. Meadows warm up more quickly than forests. Pavements get warms long before grassy lawns. Snow and ice reflect sunlight and remain cold. Surfaces that warm up faster usually also cool off faster. They are warmer in sunshine and cooler at night.
