Atmospheric Moisture Chapter 5 Weather & Climate Mrs. Schwartz

The Hydrologic Cycle  Earth has been called the “water planet” as over ¾ of the Earth’s surface is covered by water.  Water is unique in that it can simultaneously exist in all three states (solid, liquid, gas) at the same temperature and shift between states easily.  The hydrologic cycle refers to the cycle of water through the earth and atmosphere.

The Hydrologic Cycle

Water Vapor and Liquid Water  Evaporation and Condensation Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates. Why?

Water Vapor and Liquid Water Evaporation and Condensation Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation. There is an equilibrium between evaporation and condensation during saturation. Upon saturation, condensation begins and water returns to the surface. The air/atmosphere does not “hold” water vapor. Water vapor exists in gas form just like any other atmospheric gas; oxygen, nitrogen, etc.

Water Vapor and Liquid Water  Evaporation and Condensation

Water Vapor and Liquid Water  Sublimation and Deposition Water vapor (gas) can change directly into ice or snow (solid) during the deposition phase change. Ice or snow (solid) can turn directly into water vapor (gas) during the sublimation phase change.

Indices of Water Vapor Content  Introduction Humidity is the amount of water vapor in air. Humidity expressed through a few ways. Each has advantages and disadvantages, depending upon use. All humidity refers to water vapor and NOT solid or liquid states of water, as in ice, snow, or rain.

Amount of moisture  Saturation – air that contains as much water vapor as possible (at a given temperature) such that additional water vapor would result in condensation  Unsaturated – air that contains less water vapor (at a given temperature) than possible  Supersaturation – air that contains more water vapor than possible (at a given temperature)

Indices of Water Vapor Content  Vapor Pressure  Absolute Humidity  Specific Humidity  Mixing Ratio  Relative Humidity  Dew point

Indices of Water Vapor Content  Vapor Pressure Vapor pressure is the amount of pressure exerted on the atmosphere by water vapor. Vapor pressure is dependent upon both density and temperature of the vapor.

Indices of Water Vapor Content  Vapor Pressure Saturation vapor pressure is the vapor pressure of the atmosphere when it is saturated. The movement of water vapor molecules exerts vapor pressure on surfaces.

Indices of Water Vapor Content  Vapor Pressure Saturation vapor pressure is temperature dependent. At low temperatures the saturation vapor pressure increases slowly but it increases rapidly at higher temperatures. It is not a linear increase. Non linear increase in saturation vapor pressure with increase in temperature.

Indices of Water Vapor Content  Absolute Humidity Absolute humidity is the density of water vapor expressed as the number of grams of water vapor per cubic meter of air. (g/m 3 ) The absolute humidity value changes as air volume expands and contracts.

Indices of Water Vapor Content  Specific Humidity Specific humidity represents a given mass of water vapor per in a given mass of air. Saturation specific humidity is the specific humidity of the atmosphere when it is saturated.

Indices of Water Vapor Content  Mixing Ratio Mixing ratio is a measure of atmospheric moisture; the mass of water vapor per unit mass of dry air, usually expressed in grams per kilograms (g/kg). Mixing ratio is very similar to specific humidity in that it expresses the mass of water vapor relative to air mass. Maximum mixing ratio is the saturation mixing ratio.

Indices of Water Vapor Content  Relative Humidity Relative humidity (RH) is the amount of water vapor relative to the maximum that can exist at a particular temperature. RH = (specific humidity/saturation specific humidity) x 100% Relative humidity describes the amount of water vapor relative to a saturation point. The saturation point, or the relative humidity term, is relative to air temperature and total water vapor.

Indices of Water Vapor Content  Relative Humidity The highest RH occurs in the morning during the coolest time of the day. The lowest RH occurs in late afternoon, during the warmest time of the day. Because of temperature dependency the term cannot be used to compare moisture content at different locations having different temperatures.

Indices of Water Vapor Content The relationship between RH and temperature. Relative Humidity

Indices of Water Vapor Content  Dew Point The dew point temperature is the temperature at which saturation occurs in the air and is dependent upon the amount of water vapor present. High dew points indicate abundant atmospheric moisture. Dew points can be only equal or less than air temperatures. If saturation is reached and air temperatures cool further, water vapor is removed from the air through condensation. When air reaches saturation at temperatures below freezing the term frost point is used

Indices of Water Vapor Content  Dew Point Dew point/temperature relationships in (a) unsaturated air and saturated air (b and c).

Measuring Humidity  Humidity is measured by the use of a sling psychrometer.  Consists of a pair of thermometers (wet bulb and dry bulb), one of which has a wetted cotton wick attached to the bulb ( wet bulb ).  By swinging the sling psychrometer, evaporation occurs from the wet bulb which cools the bulb.  The wet bulb temperature eventually stabilizes allowing for comparison with the dry bulb temperature.  Charts gauge the amount of atmospheric humidity using the depression.  Aspirated and hair hygrometers are alternatives.

Measuring Humidity

Distribution of Water Vapor  Water vapor is supplied to the atmosphere by local evaporation or horizontal transport (advection).

Processes That Cause Saturation  Air may become saturated through 3 ways: 1. the addition of water vapor 2. mixing cold air with warm air 3. moist air—by cooling the air to dew point  If enough vapor is added to the air to saturate it, a precipitation fog forms beneath the cloud (#1 listed above)

Factors Affecting Saturation  Ice Nuclei Atmospheric water does not normally freeze at 0°C (32°F). Supercooled water refers to water having a temperature below the melting point of ice but nonetheless existing in a liquid state. Ice crystal formation requires ice nuclei, a rare temperature dependent substance similar in shape to ice (6-sided). Examples: clay, ice fragments, bacteria, etc. Ice nuclei become active at temperatures below -4°C

Between -10° and -30°C (14°–22°F ), saturation may lead to ice crystals, supercooled drops, or both. Below -30°C, clouds are composed solely of ice crystals. At or below -40°C (-40°F) spontaneous nucleation, the direct deposition of ice with no nuclei present, occurs.

Condensation in the Atmosphere  Cloud- and fog-forming condensation results from cooling in two forms  Diabatic cooling – heat is removed from the air by its surroundings (example – nighttime cooling of surface air)  Adiabatic cooling – no heat is exchanged between the air and its surroundings (example – rising air)

 The word “adiabatic” literally means “without heat.” This means that if a parcel of air were to rise and cool, it would reach different temperatures for a certainlevel if no heat were exchanged with the environment (adiabatic) or if heat were exchanged (diabatic)

Forms of Condensation  Dew Dew is liquid condensation on surface often occurring during the early morning hours. Loss of longwave radiation at night can cause the surface to cool diabatically. Surface air becomes saturated and condensation forms on objects acting as condensation nuclei.

Forms of Condensation  Frost Frost is similar to dew except that it forms when surface temperatures are below freezing. Deposition occurs instead of condensation. May be referred to as white frost or hoar frost.

Forms of Condensation  Frozen Dew Frozen dew occurs when normal dew formation processes occur followed by a drop in temperature to below freezing. Ensures a tight bond between ice and the surface. Causes “black ice” on roadways.

Dew (a), Frost (b), and frozen dew (c). Forms of Condensation

Fog  Radiation fog – fog that forms overnight due to the cooling of air in contact with the ground  Associated with temperature inversions  Advection fog – fog that forms when warm, moist air moves over a cool surface and cools

Advection Fog

Fog  Upslope fog – fog that forms due to the cooling of air as it rises up a gentle slope  Steam fog – fog that forms when warm, moist air mixes with cooler air  Precipitation fog – fog that forms when rain evaporates and adds water vapor to ambient air, which then condenses

Steam Fog

Distribution of Fog Different types of fog found throughout the U.S.

Formation and Dissipation of Cloud Droplets  Clouds are mainly associated with adiabatic cooling of rising air.  Approximately 50m above the lifting condensation level, all condensation nuclei have condensed water attached, which leads to additional growth of those drops over the creation of new drops.  Process soon stops leaving drops to slowly evaporate or sublimate.

High Humidity and Human Discomfort  High temperature extremes are compounded by humidity (and other factors such as wind and intensity of sunlight).  The effect of humidity and high temperatures can be expressed in a heat index.  Humans are cooled by the release of perspiration which cools the body by evaporating into air.  When the atmosphere has a high moisture content, the rate of evaporation is reduced.  The apparent temperature of the air increases leading to heat related health risks.  Muscle cramps, heat exhaustion, heat stroke (potentially fatal).